II

BASIC ELECTRONICS

Part 6

A Course of Training Developed for

THE UNITED STATES NAVYby the New York firm of

Management Consultants and Graphiological Engineers

VAN VALKlENBURGH, NOOGER & NEVILLE, INC. j^WIGANCENTRALLIBRARY

Adapted to British and Commonwealth Usage

by a Special Electronics Training Investigation Team of

the Royal Electrical & Mechanical Engineers

©LONDON

THE TECHNICAL PRESS, LTDNEW YORK

THE BROLET PRESS

British and Commonwealth Edition first published 1959

©Copyright 1959 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

New York, U.S.A.

All rights reserved

American Edition first published 1959

©Copyright 1959 by

VAN VALKENBURGH, NOOGER & NEVILLE, INC.

New York, U.S.A.

U.S. Library of Congress Catalog Card No. 55-6984

All rights reserved

WiGANPUBLIC

LIBRARIES

7^/33fcftl 3S 1

Made andprinted in Great Britain by

William Clowes and Sons, Limited, London and Beccles

PREFACE

THIS SIXTH Part of BASIC ELECTRONICS, which deals in its first half with

the fundamentals of Frequency Modulation and in its second half with the

comparatively recent (but immensely significant) discovery of the Transistor, repre-

sents the first important addition to an Illustrated Course of Elementary Technician

Training—carefully planned, brilliantly simplified, and radically new—which was

developed some years ago at the request of the United States Navy by a distinguished

New York firm, VAN VALKENBURGH, NOOGER & NEVILLE, INC., manage-

ment consultants and method engineers.

This Illustrated Training Course, consisting of the material contained in the five

Parts ofBASIC ELECTRICITY and in the first five Parts ofBASIC ELECTRONICS,has become, since its first adoption in 1953, a standard text in U.S. Navy Training

Schools. More than 100,000 men have taken it as an essential part of their training

to technician level in 14 different Navy trades; their average training time has been

cut by half; and supplies of Course materials are now held as part of the U.S. Navy's

official War Mobilization Stores.

The text of the Course was subsequently released in a condensed form to the

general public in the United States, where it has proved an outstanding success. In

addition to large sales to individuals, to schools and to technical institutions of all

kinds, more than a score of world-famous companies have taken the published

Manuals for use in their apprentice training schemes, and have found that they

enable them to turn out qualified technicians both faster and at less cost than did the

old methods of text-book and lecture. Several American trade unions (who take a

keen interest in the "up-grading" of their members to more skilled and better-paid

jobs) have chosen the Manuals as the best available training materials for their

purpose. Spanish, Dutch and Portuguese translations of the Manuals have been

published.

While negotiations with the American authors were in progress in the latter months

of 1957, word reached the British publishers of the Manuals that there had recently

been set up, under command of Training Headquarters, Royal Electrical and Mechan-

ical Engineers, at Arborfield in Berkshire, a special "Electronics Training Investiga-

tion Team" whose task was to devise solutions for some of the training problems

which would face the British Army when National Service ended, and when the

Army's increasingly elaborate electrical and electronics gear would have to be mannedand serviced by recruits entering the Army with none of the technical knowledge

which many National Servicemen had hitherto brought with them into the Forces.

It seemed possible that most of the REME requirements for a new-style, yet

technically sound, instructional approach could be met by a suitably edited British

version of the"WN & N" Manuals. A visit to Arborfield was accordingly arranged,

where the reception given to the Manuals, with their attractive appearance and

proved record of success, was enthusiastic; and after a careful evaluation of their

merits and potential suitability had been made, War Office consent was secured to

a proposal that the work of adapting text and illustrations to British notation and

terminology should be undertaken by the Electronics Team at Arborfield.

Later, while this work was still proceeding, a decision was reached to adopt the

revised Manuals as basic texts for the training of future REME technicians, and anorder for large numbers of complete sets of the Manuals was placed. Early interest

was also shown by several other branches of the Armed Forces; and the Military

Advisers to the High Commissioners of at least six leading Member Nations of the

Commonwealth submitted early proofs of the English edition to their respective

Ministries of Defence.

At the time of publication of this sixth Part of BASIC ELECTRONICS, sub-

stantial orders for earlier Parts in the Series have been supplied to: REMETraining HQ.; the Royal Corps of Signals; the School of Anti-Aircraft Artillery;

the Army Apprentices School (Arborfield); Technical Training Command of the

Royal Air Force; the Armies of New Zealand and the Federation of Malaya; the

South African Air Force; the Royal Ceylon Navy; and the Jordan Arab Army. TheMinistries of Defence and Education of the Republic of India are among manyothers who have the question of adopting the Manuals under urgent consideration.

Non-Services purchasers include: the Atomic Weapons Research Establishment(Aldermaston); the United Kingdom Atomic Energy Authority (Dounreay); the

U.K.A.E.A. Industrial Group (Capenhurst); the Uganda Electricity Board; the

Underwood Business Machines Corporation; British Nylon Spinners; Mullard Ltd.;

Richard Thomas & Baldwins; and British Thomsom-Houston.Like all its predecessors, this sixth Part of BASIC ELECTRONICS has been

adapted to British usage by the Electronics Team at Arborfield.

* * *

The original U.S. Navy Course was based on a novel technique of teaching

developed by the Authors after extensive research and practical experience withthousands of students. Immense pains were taken to identify and present only the

essential facts about each new concept or piece of equipment. These facts werethen explained in the simplest possible language, one at a time; and each was illus-

trated by a cartoon-type drawing. Nearly every page in every one of the Manualscarries one or more ofthese brilliantly simple "visualizations" ofthe concept described.

The approach throughout is strictly non-mathematical. Only the simplest

equations needed for working with the fundamental laws of electricity are employed.Yet there has been no shirking of essentials, even when they are difficult; andstudents with higher qualifications and educational background find nothing in the

Manuals to irritate or slow them down. They merely pass on to the next subject

quicker than the rest.

-* * * *

Despite their Services background, the Manuals have been proved suitable for

civilian .use. Their purpose, however, is limited to the training of practical tech-

nicians, not of engineers. They aim to turn out men capable of operating, main-taining, and carrying out routine repairs to the equipment described—not men capableof inventing or improving it.

They present a unique simplification of an ordinarily complex set of subjects—soplanned, written and illustrated as to become the best and quickest way to teach orlearn BASIC ELECTRICITY and BASIC ELECTRONICS that has ever beendevised.

In these Manuals, first things come first—and only the essentials come anywhere.It is already becoming clear that their accuracy and thoroughness, combined withtheir extreme lucidity, has made their publication a landmark in technical education

in Britain and the Commonwealth.

TABLE OF CONTENTS

FREQUENCY MODULATION

Section Page

1 The Fundamentals of Frequency Modulation 6.2

2 FM Transmitters 6.6

3 FM Receivers 6.27

TRANSISTORS

1 Introduction to "Solid-State" Electronics 6.56

2 Semi-Conductor Diodes 6.67

3 Transistor Construction and Operation 6.81

4 Transistor Characteristics 6.94

5 Transistor Circuits 6.98

6 The Transistorized Superheterodyne Receiver 6.113

7 Fault-finding on a Transistor Superhet 6.119

8 General Review of Transistors 6.125

9 Introduction to Synchros and Servomechanisms 6.127

Index 6.129

Cumulative Index (Parts 1-6) 6.131

f2 LIORAHt *'*

This Course in

BASIC ELECTRONICS

comprises 6 Parts

This is PART 6

It is preceded by a Course in

BASIC ELECTRICITY

comprising 5 Parts

all uniform with this volume.

Part 1 explained the General Principles of Electricity.

Part 2 described and discussed D.C. and D.C. Circuits.

Parts 3 and 4 described and discussed A.C. and A.C. Circuits.

Part 5 described and discussed A.C. and D.C. Machines.

BASIC ELECTRONICS

will be followed by a further Course in

BASIC SYNCHROS & SERYONECHANISNS

in two Parts

also uniform with this volume

« § I : THE FUNDAMENTALS OF FREQUENCY

MODULATION

What Frequency Modulation Is

You will recall that in Parts 4 and 5 of Basic Electronics three basic methods were

described by which a message can be superimposed on the r.f. carrier signal of a

radio transmitter, and then received by a receiver. You were also told that a fourth

basic method existed, of which you would learn later in your Course.

Let us first briefly review the three basicmethods about which you have already learnt.

1. In continuous wave (CW) transmission, the carrier signal is interrupted, or

turned on and off, with a key. This method is used primarily for long-distance com-

munication. A communications receiver incorporating a Beat Frequency Oscillator

(BFO) is used to receive these signals.

2. In modulated continuous wave (MCW) transmission, an audio-frequency

signal ofconstant amplitude is superimposed on the carrier. The carrier is then turned

on and off with-a key, just as in CW transmission. Any receiver with the appropriate

B M ^%B A M.

- - - - MCWTRANSMISSION!\

nterrupted (Keyed)

V-dulated RF Signal*

N''

,!!. ill] li'l ill!

1o-

l

RFTransmitter

Audio 1

-1 Oscillator

j

1

;|||

|!|i!j"i|"-

Re

Drr Drr. n

ceiver ~\~)Phones

1 V**£V .;.. J";-",,. \ ; _ ^^^^^BgH^^^BB '-r f^if^^^^S;

§11

What Frequency Modulation Is (continued)

6.3

3. In amplitude modulated (AM) transmission, the amplitude of the carrier is

varied at a rate dependent on the frequency of an audio signal, and to an extent

dependent on the amplitude of this audio signal.

A fourth method by which a message can be carried by an r.f. carrier signal,

and then received, is called Frequency Modulation. In FM, the frequency of the r.f.

carrier signal is shifted, or deviated, to a higher or lower number of cycles per

second, at a rate dependent on the frequency of the audio signal. The extent ofthe deviation is made to depend on the amplitude of the audio signal.

The first half of this Part of Basic Electronics will be devoted to a description ofthe methods by which voice and other sound signals can be transmitted, and then

received, by means of FM.

Modulated RFUnmodulatedRF Carrier

TRANSMISSION

6.4

Applications of Frequency Modulation

Si

HIGH FIDELITYHOME RECEPTION

The outstanding advantage of FM is that it permits reception which is free from

interference and noise.

There are many occasions on which it is obviously important that signals be trans-

mitted and received free from interfering noise. In military, naval or air tactics, for

instance, efficient intercommunication demands clear reception of messages without

danger of their content being obscured by noise. This is especially important in the

case of intercommunication between moving vehicles—especially tanks—in which

very high interference levels are produced by the variety of electrical equipment

carried.

Again, freedom from interference and noise is essential in the high-speed, high-

quality facsimile transmission of detailed maps, photographs and printed informa-

tion of all kinds. In such applications, FM provides faithful reproduction of the

wide band of video frequencies required, and prevents noise from blotting out the

fine details.

§1]- 6.5

Applications of Frequency Modulation (continued)

FM is also, of course, used in the high-fidelity radio receiver you may have in

your home, in which it provides excellent noise-free reception of voice and music.

It has been said that the outstanding advantage of FM is that it permits reception

which is free from interference and noise. This in turn, of course, permits the re-

production of an extremely wide range of audio frequencies.

Remember that the audio frequency limitations of AM are in no way inherent in

the transmitter. It is interference and noise which cause the amplitude of the desired

r.f. signal in the receiver to vary; and attempts to reduce the level of noise affect the

amplitude of the required output signal.

FM receivers, on the other hand, can be so designed that noise is eliminated without

affecting the amplitude of the required output signal.

In the medium- and long-wave broadcasting bands, regulations restrict transmitter

bandwidth to prevent interference between stations. In an FM system the carrier-

frequency must deviate over a wide band of frequencies (often ± 75 kc/s). FM is

therefore not used in the medium- and long-wave bands; and FM transmitters and

receivers are normally designed for operation in the VHF band.

In Part 4 of Basic Electronics you learnt that the VHF band extends from 30 to 300

Mc/s. The BBC FM broadcasts are in the 88-100 Mc/s range.

In this frequency band FM faces the limitations which are well known to all

television viewers. Reception is essentially limited to locations which are in "line-

of-sight" of the transmitting aerial, and there is a "fringe" area in which reception

is unreliable.

This limitation ofFM can be an advantage if you visualize a network of permanent

FM broadcasting stations in which the same frequencies can be used to provide

different programmes for different localities.

In military operations involving large numbers of mobile transmitters, however,

transmitter frequencies must be allocated with great care if interference between

stations is to be prevented.

6.6 § 2 : FM TRANSMITTERS

Simplest Form of an FM Transmitter

You will most quickly understand the nature of FM if you consider how the

simplest possible form of FM transmitter would work.

Picture to yourselfan r.f. oscillator (such as a Hartley oscillator) coupled to an aerial.

Omit, for reasons of simplicity, the buffer amplifiers, the frequency doubters and the

power amplifiers which you would usually find between the oscillator and the aerial.

You will remember from your previous study of transmitters that the signal sent out

by this arrangement will be an r.f. sine wave carrier signal of constant amplitude. Thefrequency of this transmitted signal will be determined by the setting of the variable

capacitor. The frequency increases as the capacitor is set towards its minimumcapacitance, and the frequency decreases as the capacitor is set towards its maximumcapacitance.

§2] 6.7

Simplest Form of an FM Transmitter (continued)

Assume that when the variable capacitor is set to the centre of its range, the

frequency generated is 1,000,000 c/s (1 Mc/s). Now if (using either your hand or any

suitable vibrator mechanism) you turn the variable capacitor shaft rapidly back and

forth about its centre position, you will obtain the effect of frequency modulation.

If the shaft is mechanically oscillated about its centre position at a rate of 40

cycles per second, an FM receiver tuned to the centre frequency will produce a

40-c/s tone from its loudspeaker. If the shaft is turned at a rate of 400 or 4,000

cycles per second, the receiver loudspeaker will produce tones of 400 or 4,000 c/s

respectively. The frequency of the output from the FM receiver loudspeaker is

always the same as the rate at which the carrier frequency is shifted or deviated. This

is true regardless of the magnitude of the deviation.

The transmitter capacitor shaft may be turned by only a small amount to either side

of its centre position—varying the frequency 1,000 c/s above and below the

1,000,000-c/s centre frequency. Or the capacitor shaft may be turned at the same

rate but by a larger amount—varying the frequency 10,000 c/s above and below the

1,000,000-c/s centre frequency. In either case the same tone will come out of the

FM receiver loudspeaker.

But the loudness of the tone will vary, the rule being that the loudness of the tone

increases with the magnitude of the frequency deviation. Thus the tone produced

by the ± 1,000-c/s shift will be quite low in volume, while the same tone produced by

the ± 10,000-c/s shift will be much louder.

You can say then that: The frequency and the amplitude of the output from the

FM receiver are determined respectively by the rate and the magnitude of the trans-

mitter frequency shift.

40c/s/,TonelvAAA

; i Tx

©Motor Sp«ed 40rpsCnpocitor shoft oscillates at 4Q c/s

-

Shaft deviating IO° from centre position

in each direction..

4kc/s/Tone [mmm

i' Tx Rx

.€ Tx -I H Rx

0''Motor Speed 40r.ps.

Capacitor shaft oscillates at 40c/s -

Shaft deviating IO° from centre position

jn y;fif-h direction

40c/sTone

£ Tx Rx

® Motor Speed 4COOrp.s.

rfflMrttflT A*** ^filiates at 4QOOc/s -

Shaft deviating IO° from centre position

in each direction.

®*' Motor Speed 40 r.p.s.

Capacitor shaft oscillates at 40c/s -

Shnft deviating ^0° from centre position

in each direction.

6.8 [§2

Simple Reactance Valve FM Transmitter

Now it is obvious that the method of frequency-modulating a carrier wave bymechanical means described on the last page cannot be used to transmit voice or

other complex sound signals.

What is needed to accomplish this is that the output from, say, a microphone used

in conjunction with an audio amplifier must cause deviations in the carrier frequency

of the transmitter. The rate of this frequency shift must be equal to the frequency

of the sound going into the microphone, and the magnitude of the frequency shift

must be in proportion to the loudness of the sound.

One very widely used method of accompUshing these desired results is based onthe use of a reactance valve. The basic arrangement of a reactance valve FM trans-

mitter is shown in the block diagram below. Note that the frequency multipliers andpower amplifier normally following the oscillator are omitted, for simplicity. In the

arrangement shown, a microphone drives an audio amplifier which, by means of a

reactance valve, shifts the frequency of the oscillator.

s^

§*J 6.9

Bask Reactance Valve Circuit

The circuit diagram on this page shows a basic reactance valve circuit. Thiscircuit injects the effect of a varying capacitance into the oscillator tuned circuit.

The rate and amplitude of the capacitance change are determined by the frequencyand amplitude of the applied audio signal.

To understand how this effect is achieved, consider first the

voltage-current relationship in a capacitive circuit. You will

remember that the current through a capacitor leads the voltage

across it by 90°. This is illustrated by the vector diagram(a) opposite.

Consider now the circuit shown below: Ec

Ic

n (a)

HT+

Oscillator

TunedCircuit

The tuned circuit, L3 and C3 , is part of an oscillator stage.

C2 couples the oscillator circuit to the anode of the reactance

valve. When the oscillator is oscillating, the r.f. voltage across

the tuned circuit, Eot is applied to the series circuit Cx and Rx .

The value of Cx is such that its reactance is much greater than is

the resistance of Ru so the circuit is capacitive.

That is to say, the current through this series circuit, Ix, leads

E by very nearly 90° (see diagram (b) opposite).

M

Eo

Ix

a Eg W

Ix

-Eg

i,ia

Eo

M

6.10 02

Basic Reactance Valve Circuit {continued)

The voltage Eg across Rt will be in phase with the current

through Ru i.e. with lx . So Eg leads E by very nearly 90°—see

diagram (c). This voltage Eg is applied to the grid of the

reactance valve.

You have learnt that the current change in a valve is in phase

with the voltage change at its control grid. In other words, that

as the grid voltage goes positive, so the current through the

valve increases. Since the r.f. component of the current through

the valve (ia) is in phase with Eg, it must also be very nearly 90°

leading with respect to E .

If you compare vector diagram (d) with diagram (a), you will

see that the reactance valve circuit is behaving like a capacitor

connected across the tuned circuit of the oscillator stage. The

value of ia is determined by the r.f. voltage applied to the grid

Eg, and by the mutual conductance (gm) of the valve.

Now consider the effect of applying an audio voltage (as well

as the r.f. voltage present) to the grid of the reactance valve.

This audio voltage will have the effect of changing the mutual

conductance of the valve, and therefore of changing the r.f.

component (Q of the current through the valve.

It has been said that the reactance valve circuit behaves like a capacitor connected

across the oscillator tuned circuit. Now the current through a capacitor depends

on the voltage applied, on the frequency of this voltage, and on the capacitance

of the capacitor. In other words, ia=E 2ir/C, where C is the capacitance presented

by the reactance valve circuit.

E and / are fixedly the oscillator circuit of which C3 and L3 are part when no

audio voltage is applied, so if ia changes when an audio voltage is applied to the

grid of the reactance valve, the capacitance presented by the reactance valve circuit

must also have changed.

In this way, the audio voltage applied to the grid of the reactance valve, by changing

the effective capacitance of the reactance valve circuit which is connected across the

oscillator tuned circuit, changes the oscillator frequency.

Eo

§*] 6.11

Basic Reactance Valve Circuit (continued)

When you connect capacitance across the oscillator tuned circuit, you lower theoscillator frequency in proportion to the amount of capacitance introduced. Theamount of capacitance that is injected into the oscillator tuned circuit is equal tothe mutual conductance (gm) of the reactance valve, multiplied by the product of theresistance, Ru and the capacitance, Cx (C injected =*gm xRx x Cx).

Reactance valve circuits generally employ pentode valves such as the 6AC7 or6SJ7. The value of Cx is generally between 20 ^iF and 30 [XfxF, and the value ofRtis usually between 500 ohms and 1,000 ohms. With such an arrangement, an audiosignal offrom 1 to %volts peak-to-peak on the grid will cause the capacitance injectedinto the oscillator tuned circuit to shift from approximately 100 (jtfjtF to 300 fifjtF.

Remember that the reactance valve circuit which has been considered above is

only one of the four basic types which can be used. If Rx and Cx are interchangedin the circuit, a variable inductance will be injected into the oscillator tuned circuit;and this will cause the desired frequency changes as effectively as did the variablecapacitance.

And if a coil Lx be substituted for Cx in the original arrangement, the circuit willinject a variable inductance into the oscillator tuned circuit; while interchangingLi and Rx will inject variable capacitance. Any of these arrangements can be usedin an FM transmitter.

6.12

The Complete Basic FM Transmitter

IS *

HIGH POWER HIGHFREQUENCY FM

HIGHFREQUENCY FM

LOW FREQUENCY FM

Reactance

Valve

1_I

PowerAmplifier

J Intermediate #

\ Power\

j Amplifier \

t

±Oscillator

Amplified Audio Varying

Injected

Capacitance

Once the operation of the reactance valve is understood, there is little difference

between a basic FM transmitter and the AM transmitter which was described in

Part 4. The diagram on this page shows a basic FM transmitter; and you can see

that it is very like the AM transmitter you already know.

To review the operation of the system, consider the function of each stage, be-

ginning with the microphone. The microphone converts sound waves into low-

voltage electrical signals having the same frequency and amplitude characteristics

as the sound wave. This low-voltage audio signal is fed to the audio amplifier,

which steps up the signal voltage to a level suitable for driving the reactance valve.

Without the reactance valve the oscillator generates an r.f. signal of constant

frequency and amplitude. With the reactance valve in the circuit, however, the

oscillator frequency is shifted at a rate equal to the frequency of the audio signal;

and the magnitude of the frequency shift is in proportion to the amplitude of the

audio signal.

§2] 6.13

The Complete Basic FM Transmitter (continued)

The purpose of the intermediate power amplifier (IPA) is to isolate the oscillator

for improved frequency stability, and to amplify the r.f. signal in order to drive the

power amplifier efficiently. The IPA is also used as a frequency multiplier. Thispermits the transmitter to send out the desired frequency while the oscillator operates

at a much lower frequency, where it can maintain stable operation more easily.

Note that in addition to multiplying the oscillator signal frequency, the IPA also

multiplies the oscillator signal frequency deviation. Thus, if the oscillator centre

frequency be 30 Mc/s with a frequency deviation of ±25,000 c/s, and the IPA beoperated as a frequency tripler, the IPA output centre frequency will be 90 Mc/swith a frequency deviation of ± 75,000 c/s. Consequently, the use of frequency

multipliers permits the required frequency deviation to be obtained while leaving the

oscillator to operate at a lower, more stable frequency and with a smaller, moreeasily obtainable frequency deviation.

The purpose of the power amplifier is to increase the power level of the IPA out-

put signal. The power amplifier usually has its input and output signals at the samefrequency in order to obtain maximum operating efficiency.

The aerial converts the signal delivered by the power amplifier into electromag-

netic waves.

BASIC

HIGH POWER HIGHFREQUENCY FM

Xi]

TRMtswrntR

HIGHFREQUENCY FM

LOW FREQUENCY FM

r%.™k

Reactance

Valve

4 • ^wPower

Amplifier

IntermediatePower

Amplifier

HIX"\*A Audio « . j Reactance s . j ~ ... . 2

i Jfrl a ,.,. f*\ ir i f*-\ Oscillator \

A \AmPllfier JiS Valve # i J ;

Audio Amplified Audio Varying Injected

Capacitance

6.14

The Automatic Frequency Controlled FM Transmitter

[§2

Varying Low-

Frequency Signal

Crystal

Oscillator

FrequencyCorrecting

Voltage

AudioAmplifier

FrequencyConverter

ConstantFrequency

High FrequencySignal

High Frequency

FM Signal

y\t/

PowerAmplifier

il

The FM transmitter just described is subject to oscillator frequency drift. Themost important reasons for this drift are the changes in value which take place in

the oscillator coil and capacitor by reason of temperature variations within the

transmitter itself. In AM transmitters the most reliable method of preventing this

drift is to use a crystal oscillator.

In this type of FM transmitter, however, it is necessary (in order to obtain

the required frequency deviation while maintaining the necessary frequency stability)

to make use of a crystal oscillator in an indirect manner. The method used is shownin the block diagram above. The FM transmitter previously described is used as

the major part of the system. To this arrangement are added a crystal oscillator, a

frequency converter (mixer), and a discriminator.

The frequency converter receives signals from both the power amplifier and the

crystal oscillator. The frequency of the mixer output is equal to the difference

between the two input signals. When the transmitted frequency is the same as the

crystal oscillator frequency, there is no output from the frequency converter; and the

mixer output frequency increases and decreases as the transmitted frequency drifts.

The discriminator output is a "frequency correcting voltage" which rises and falls

in accordance with the drift of the power amplifier signal. The operation of

the discriminator will be described in detail in the section on FM receivers. Thefrequency correcting voltage is applied to the reactance valve to cause it to bring

the oscillator back to the predetermined centre frequency, thus correcting any drift

in the oscillator frequency.

§2] 6.15

Pre-emphasis

It has been said that an important advantage ofFM is that it can be used to trans-

mit a wide range of audio signals—frequencies up to 15,000 cycles or even higher.

Now the frequencies above 5,000 cycles contain mainly the upper harmonics of the

fundamental frequencies in voice or music. These upper harmonics are of low

amplitude; but if they can be reproduced at the receiver, they give the unusually fine

quality that is characteristic of FM reception.

Because these upper harmonics are low in amplitude, there is a danger that these

signals will be lost in the system. Pre-emphasis is a method of improving the re-

production of these higher audio frequency signals.

The method used is simply to increase the high frequency gain of the transmitter

audio amplifier, as illustrated by the response curve below. Since the higher audio

frequencies are of low amplitude, it is necessary that the amplifier gain shall be

increased as the frequency rises.

The simplest method of obtaining the desired gain characteristic in the audio

amplifier is to use the RC network shown in the diagram.

At low frequencies, C has such a high impedance that the voltage division is de-

termined only by Rt and R2. As the frequency increases, the impedance of C de-

creases rapidly. Eventually the impedance of C is much lower than Ru and the

voltage division is determined only by C and R2.

The result of the voltage dividing action is that the high-frequency signals are

attenuated much less than are the low-frequency signals, and the desired frequency

response characteristics have been introduced into the amplifier.

6.16 [§2

The Phase-modulated FM Transmitter

The FM transmitter considered up to this point is known as the "direct" type.

In this transmitter the r.f. carrier signal is frequency modulated in the oscillator

—

the original source ofthe carrier signal. The oscillator frequency is deviated in accord-

ance with the frequency and amplitude of the audio signal; and equivalent, but

multiplied, frequency deviation takes place in the stages which follow.

Another basic type of FM transmitter is known as the "indirect" type, because

the frequency deviation is not introduced at the source of the r.f. carrier signal (the

oscillator). Instead, the oscillator frequency remains constant, and the frequency

deviation is introduced in one of the stages following the oscillator. The basic

advantage of this method is that the oscillator can be of the crystal-controlled type,

which will maintain a stable centre frequency without the need for a separate auto-

matic frequency control circuit. Frequency deviation is not introduced in the same

manner as in "direct" FM transmitters; a method known as "phase modulation" is

used instead.

It should be clearly understood that the final output signal of an "indirect" FMtransmitter is the same as that produced by a "direct" FM transmitter. The differ-

ences are confined to the methods used to obtain the final output signal; they do not

lie in the characteristics of the output signal itself.

The phase-modulated transmitter consists of a crystal oscillator, frequency multi-

pliers (IPA), a power amplifier (PA) and an aerial; plus a microphone, audio amplifier,

audio correction network and phase modulator. The r.f. carrier signal from the

crystal oscillator is stepped up in frequency by the frequency multipliers, and the

power amplifiers increase the signal power. The audio signal from the microphone

goes into the audio amplifier, and the signal is amplified to a level suitable to operate

the audio correction network and phase modulator. All but these last two circuits

have been described already.

pHASE-MODULATED _^-^

fm TRANSMITTER

PowerAmplifier

§2]

The Phase-modulated FM Transmitter (continued)

When the frequency of an oscillator

is deviated, phase modulation takes

place; and when the phase of the

oscillator signal is shifted, frequency

modulation takes place. A simplified

graphic presentation of what happens

is given on this page.

The first diagram shows a sine wave

(drawn in solid line), a sine wave which

lags 45 degrees behind the solid curve

(drawn in dotted line) and a sine wavewhich leads the solid curve by 45

degrees (drawn in dot-dash line). These

curves show that the peaks of a sine

wave can be advanced or retarded in

time by means of phase shifting.

The second diagram shows the result

of using a variable phase-shifting net-

work to shift the phase of the sine wavesmoothly while it is being generated.

When the phase is smoothly shifted

so that it lags the original wave, the

peaks occur later; this is equivalent

to increasing the wavelength or lower-

ing the frequency.

6.17

r— Leads solid curve by 45°

I r Lags solid curve by 45°

W, ,m.ww

If the phase is smoothly shifted so

that it leads the original wave (see the

third diagram), the peaks occur sooner;

this is equivalent to shortening the

wavelength or increasing the frequency.

It will be seen, then, that a smoothshifting of the phase of a signal is

equivalent to a smooth shifting of its

frequency.

PHASE LKAI) SIM

DKCREAMM, WAVF

horiginal •

wave length-d increasing lead

«r-JL ahead of

/ /»\ s solid UneI If I \ /

6.18 in

Basic Phase Modulator

The effects of a basic phase-shifting

network were described in detail in

Part 4 of Basic Electricity, page 4,26.

The circuit consists of a capacitive (or

inductive) reactance connected in series

with a resistor. When a signal of

constant frequency is connected across

the series combination, the signal out-

put across the resistor is shifted in

phase with respect to the input signal.

If the resistance is varied, the phase

shift also varies.

When the resistance is more than

10 times larger than the reactance,

there is almost no phase shift. As the

resistance approaches one tenth the

value of the reactance, the phase-shift

approaches 90 degrees.

Thus the constant-frequency r.f. sig-

nal from a crystal oscillator can be

applied to such a network, and its phase

can be shifted by varying the resistance.

By replacing the resistor with a

valve, the resistance in the phase shift

network can be varied by an audio

signal applied to the grid of the valve.

As the audio signal voltage increases

and decreases in amplitude, the anode

current rises and falls. Rises in the

anode current are equivalent to lower-

ing the anode resistance of the valve,

and this causes a maximum shift in

the phase of the input r.f. signal. De-

creases in the anode current are

equivalent to raising the anode re-

sistance of the valve, and this causes

a minimum shift in the phase of the

input r.f. signal.

Thus an r.f. signal of constant

frequency, such as that coming from a

crystal oscillator, can be phase-shifted

in accordance with the amplitude of

an audio signal at a rate dependent

on the frequency of the audio signal.

There are many variations of this

arrangement, but they all involve

changing either the reactance or the

resistance in a circuit of ttnYtype.

Basic

Phase

Shifter

Phase

Shifter

§2] 6.19

Audio Correction Network

The phase modulator described on the previous page, however, can produce only

limited phase deviation without introducing distortion.

In the circuit there shown, the rate of phase change, and the equivalent frequency

deviation, rises as the audio frequency rises. Assume, for instance, that a 100-c/s

audio signal of 1-volt amplitude causes an equivalent 1,000-c/s frequency deviation

in the r.f. carrier. If the audio frequency rises to 1,000 c/s and the amplitude

remains at 1 volt, there will be an equivalent frequency deviation of 10,000 c/s in

the r.f. carrier.

Such an effect is completely contrary to the working principles ofFM transmission.

According to the requirements of FM, the r.f. carrier frequency deviation must beproportional only to the amplitude of the audio signal, and only the rate of carrier

frequency swing is in proportion to the frequency of the audio signal.

The basic method of eliminating the effect of increasing equivalent frequency

deviation with increasing audio frequency is to decrease the amplitude of the audio

signal in proportion to the frequency rise. The fundamental circuit for achieving

this effect is the RC divider network shown in the diagram below.

The reactance of a capacitor decreases as the applied frequency increases, but the

effect of the resistor remains constant at all frequencies. Consequently, the ampli-

tude of the audio output signal decreases with increasing frequency. The values of

R and C are selected so as to make the audio signal amplitude decrease exactly

counteract the undesired frequency deviation increase.

AUDIOCORRECTIONNETWORK

6.20 [§2

The Link Phase Modulator

A practical form of phase modulator is the link phase modulator, the circuit for

which is shown below.

The r.f voltage is applied to the grid circuit of the modulator valve Vit through the

coupling capacitor C\. The r.f. can reach the anode of Vx by either of two paths.

One is through the grid-anode capacitance of the valve shown dotted in the circuit

as Cga . The other is through the valve acting as an amplifier in the normal way.The anode voltage of an amplifier is out of phase with the grid voltage. Since, how-

ever, the voltage fed to the anode through Cga is in phase with the grid voltage, it is

theoretically 180° out of phase with the amplified voltage. (In practice, these twovoltages are sometimes less than 180° out of phase.)

If the valve is operated as a normal high-gain voltage amplifier, the amplified anodevoltage is much larger than is the voltage fed to the anode by the grid-anode capaci-

tance. When a large value of cathode resistor (R5) is used, however, the operating

point of the valve can be such that the amplification is greatly reduced.

Moreover, if this cathode resistor is not decoupled, variations of anode current

cause negative feed-back (that is to say, the varying voltage at the cathode acts in

opposition to the grid input voltage). This negative feed-back reduces the amplifica-

tion of the stage even more.

§2] 6.21

The Link Phase Modulator (continued)

In this way, the r.f. anode voltage component arising from amplification can be

reduced until it is of the same magnitude as the component arising from the grid-anode

capacitance.

Resistors Ru R2 and R3 form the grid circuit of the modulator. The modulating

a.f. voltage is fed to the grid of the modulator through C2 . The H.T. voltage is

applied to the modulator valve through the coil Lu which is broadly tuned with stray

capacitances to the resonant frequency of the r.f. applied to the grid. Capacitor

C3 and resistor R2 form the audio correction network.

The vector diagrams below illustrate the relative values and phase-angle relation-

ships of the amplified component (Ea) of the anode voltage, and of the component

(Ec) of the anode voltage which is fed through the inter-electrode capacitance Cga .

The instantaneous a.c. anode voltage (Ep) is the vector sum of Ea and Ec, as shown.

If the anode current is varied at an audio rate by the modulating signal, the ampli-

fied component of the anode voltage varies in amplitude. As the audio signal

becomes positive, the anode voltage component is decreased in amplitude; but the

r.f. signal coupled through Cga does not change in either amplitude or phase. The

vector Ec remains constant, as shown at (b).

Since the amplified voltage Ea changes, and the voltage Ec does not, the amplitude

and phase-angle of the resultant, Ep, must change.

When the grid of the valve is made negative by the applied audio voltage, the

amplified anode voltage increases. The resultant voltage Ep changes in amplitude

and phase, in a direction opposite to its change when the grid swings positive. This

is shown at (c).

The phase of the r.f. output signal from the modulator valve therefore varies in

accordance with the amplitude of the audio signal input to the modulator valve.

The rate of change of phase is dependent on the frequency of the audio signal

input to the modulator valve.

You learnt on page 6.17 that a change of phase is equivalent to a change of

frequency. So the r.f. output from this circuit is being deviated at a rate dependent

on the frequency of the audio signal, and to an extent dependent on the amplitude of

the audio signal.

These are just the conditions needed for FM.

Ea due toamplification

6.22

The FM Phase-modulated Transmitter

B*

CRYSTAL PHASE '„.,«.„OSCILLATOR MODULATOR BUFFER

AUDIO AMPLIFIERS

The circuit above shows an FM transmitter using phase modulation. (For

greater clarity of illustration, the power supply unit and its connections have been

omitted.)

This FM transmitter has a -crystal oscillator, V\. The crystal acts as a tuned

parallel resonant circuit connected between the grid and anode through a d.c. blocking

capacitor, C2. Grid-leak bias is provided by the combination Cy-R^ H.T. is supplied

through R2, and the output voltage of the oscillator is developed across R2. The

coupling capacitor C3 couples the output voltage to the grid of the phase modulator

stage V2. The phase modulator V2 has r.f. and a.f. applied to its grid, and operates

in the manner described on pages 6.20 and 6.21.

Audio voltages are introduced through the audio correction network R\6-C2q>which produces the necessary a.f. frequency response required for the generation of

good-qualityFM signals. The high-cathode bias employed for this type ofmodulator

is provided by JR5, its value being large enough to operate the valve near cut-off

so that only a small anode current flows through the valve.

The frequency modulated output is developed at the anode of V2.

V6 and V7 together form a two-stage RC coupled a.f. amplifier. Included in the

input circuit of V6 is the pre-emphasis circuit.

The voltage supplied by the microphone is applied to transformer Tu where the

a.f. voltage is increased by the step-up ratio of the transformer. Resistor Rn serves

to damp the impedance of the secondary winding of Tt so that the pre-emphasis

network Rt2-L6 can function correctly. The remainder of the circuitry of V6 and

§ 2] 6.23

The FM Phase-modulated Transmitter {continued)

V7 is conventional, RVi controlling the gain of the two-stage amplifier, and thereforethe deviation of the carrier. The output from the a.f. amplifier stages is fed to theaudio-correction network Ri6-C2o through C22 .

The frequency-modulated signal from the anode of V2 passes through capacitorC4, and develops a voltage across R6 in the grid of V3 . This stage is a class A bufferamplifier which isolates the modulator and associated circuits from a frequencymultiplier stage which follows.

V3 employs cathode bias provided by Rs and decoupled by C6. The output is

developed across the tuned circuit Li-C7 . Screen voltage is applied through R7l

and C5 acts as a decoupling capacitor. The voltage developed across Lx can be eitherat the fundamental crystal frequency or at harmonics of the crystal frequency, de-pendent on the setting of the tuning capacitor C7. The output from the anodetuned circuit of V3 is transformer-coupled to the tuned cathode circuit of V4 (L2-C9).You have already learnt that FM equipments are usually designed to operate in

the VHF band. For this reason, the circuits for frequency multiplier and poweramplifier stages in FM transmitters differ from those described in Part 4 of BasicElectronics. The circuits in this part of the transmitter are typical, and are known as"grounded grid" circuits.

V4 is a grounded-grid multiplier stage. The output from the buffer stage is trans-

former-coupled to the tuned cathode circuit, the grid of V4 being connected to earth(from an r.f. point of view) through C10 . The grid is therefore, effectively, an earthedscreen between the input and output of the stage which considerably reduces theeffect of the interelectrode capacitances of the valve. Bias for the stage is developedin a way similar to that used in oscillator circuits, grid current flowing when thecathode circuit is driven sufficiently negative.

A Ac. micro-ammeter, Mu is connected in series with grid resistor R9 and acts

as a tuning indicator, since maximum grid current will flow only when the anodecircuit of V3 and the cathode circuit of V4 are tuned to the same frequency.

This stage supplies excitation, or "drive," for the final power amplifier.

The final power amplifier stage is similar to the frequency-multiplier stage, exceptthat no frequency multiplying takes place. Bias is derived in a similar way, andM2 is a micro-ammeter for use as a tuning indicator.

The output from the FPA is fed to a transmission line through the impedancematching network Cl4-Cls-L5. Cl4 and C15 can be adjusted so as to achievematching with a variety of transmission lines.

H.T. is fed to the final stage through RFCU and C17 is a blocking capacitor whichprevents the H.T. voltage from reaching the aerial.

[§*6.24

The Ferrite Reactor Modulator

A typical circuit for a "ferrite reactor" FMIn this circuit, the a.f. modulating signal is

valve, and causes a change in the inductances

of the reactor is part of the tuned circuit of the oscillator; and when its inductance

changes, so the frequency of the oscillator output changes

HT +

modulator is illustrated below,

applied to the grid of the modulator

of the ferrite reactor. The secondary

FerriteReactor

ModulatorValve

IHo

a.f.

Modulating

Signal

Oscillator

Output

You have already learnt that one of the fattors affecting the inductance of a coil

is the nature of the material composing its c|ore. Core materials which are easily

magnetized increase the inductance of a coil. Such materials are said to have high

"permeability." Ferrites, which are alloys Of iron, have very high permeability.

The permeability of a material may be considered as the measure of the ease with

which magnetic lines of force can pass through it. Clearly, a change of permeability

of the core material of a coil will cause a change in the inductance of the coil. When

a d.c. current is passed through one of the windings of a transformer having a ferrite

core, the permeability of the core material, and hence the inductances of the coils

wound on the core, change. As d.c. current increases, so permeability decreases.

In the circuit shown above, the "steady" or "no-signal" current through the modu-

lator valve acts as a polarizing current. Whe^n an audio signal is applied to the grid

of the modulator valve, the d.c. current through the transformer primary is varied

at the frequency of the applied audio voltage, thus causing variations in the perme-

ability of the core and hence in the inductance^ of the primary and secondary.

These inductances vary at a rate dependent on the frequency of the applied signal,

and to an extent dependent on the amplitude

And since the secondary of the reactor forms part of the tuned circuit of the

oscillator stage, the oscillator frequency is also varied; and its output becomes

frequency-modulated.

of the audio signal.

§2] 6.25

REVIEW of FM Transmitters

Frequency Modulation, In frequency

modulation the frequency of the r.f.

carrier signal is deviated at a rate and to

an extent determined by the frequency

and loudness of the audio signal being

transmitted.

Reactance Valve. The reactance

valve achieves frequency modulation by

injecting the effects of varying capaci-

tance or inductance into the transmitter

oscillator circuit. This is known as

"direct" frequency modulation. Thereactance injected into the oscillator

varies at a rate and to an extent de-

termined by the frequency and amplitude

of the applied audio signal. *"—

*

Basic FM Transmitter. Except for

the addition of a reactance valve circuit,

a basic FM transmitter is essentially

the same as an AM transmitter with

modulation introduced into the oscil-

lator. Pre-emphasis is achieved by in-

creasing the high-frequency gain of the

audio amplifier by a high-pass filter.

This increases the amplitude of the high-

frequency audio signals.

^ "SI

"5]£ntcQUKNCY m-EL

low nmucHCY r»

w>r3

AmpUflwl A«4o V*iyl«t

Ifl(ect»d

CipKltinc*

Ferrite Reactor Modulator. In this

circuit, the a.f. modulating signal is

applied to the grid of a modulator valve,

and causes a change in the inductances

of a ferrite reactor. The secondary of

the reactor is part of the tuned circuit

of an oscillator. When the inductance

of the reactor secondary changes, the

frequency of the oscillator output

changes.

6.26

REVIEW ofFM Transmitters (continued)

Indirect Frequency Modulation. FMtransmission can be achieved "indi-

rectly." The oscillator frequency re-

mains constant, and the required

frequency deviation is introduced in one

of the stages following the oscillator.

The advantage of this method is that a

crystal-controlled oscillator can be used,

and a stable centre frequency can be

achieved without the use of frequency-

correcting circuits.

B*

D>SS&H&fe

Phase Modulation. Indirect fre-

quency modulation can be achieved by

the use of a phase modulator which

shifts the phase of the r.f. signal in

accordance with the variations of the

audio signal. The transmitted signal is

exactly equivalent to an FM transmitted

signal produced "directly."

Audio Correction Network. When a

phase modulator is used, the equivalent

carrier frequency deviation is in pro-

portion to the amplitude of the audio

signal. Increases in the audio signal

frequency cause additional undesired

phase-shift of the r.f. carrier signal.

The undesired increase in phase-shift is

eliminated by using an RC network to

decrease the amplitude of the audio signal

in proportion to the frequency rise.

Corrected Audio Signal

To Phase Modulator

i»

§3: FN RECEIVERS 6.27

Introduction

The block diagram for an FM superhet receiver is very simitar to that of an AMsuperhet; but there are important differences within the i.f. amplifier and detector

sections.

The design and construction of the r.f. amplifier and oscillator stages also require

special consideration if stable and reliable operation is to be achieved, because FMreceivers normally operate in the VHF band. The tendency towards instability

arises largely from the effects of stray capacitances, which have to be minimized byusing specially designed components and by careful lay-out of wiring and components.

Furthermore, in FM sets which are to be used to receive high-fidelity radio broad-casts, the audio stages must be eapable of reproducing the wide range of audiofrequencies used to modulate the transmitter.

In AM receivers, for instance, the audio stages need only (by reason of broad-casting regulations) be capable of reproducing audio signals up to 7 kc/s. In FMreceivers the audio stages are required to reproduce up to 15 kc/s.

In the following pages, the differences between the i.f. amplifier and detector sections

ofAM and FM receivers will be described; but the differences in the lay-out and con-struction of the r.f., oscillator and audio,stages will not be considered.

6.28

FM I.F. Amplifiers

[§3

In an AM receiver the i.f. amplifiers are tuned to a fairly sharp peak. This permits

high gain and good selectivity to be obtained With only two, rarely more than three,

simply designed stages. Since the audio signal modulating the r.f. signal does not

normally exceed 5,000 c/s, the mixing of the r.f. and local oscillator signals will

result in an i.f. signal (usually of 465 kc/s) ha\)ing a bandwidth of 10,000 c/s.

The bandwidth is 10,000 c/s because modulating the amplitude of an r.f. signal with

an audio signal of 5,000 c/s results in sidebands both 5,000 c/s above and 5,000 c/s

f. amplifier can have a bandwidth as

attenuating modulating signals with

below the r.f. carrier frequency. Thus an i.

narrow as 10,000 c/s without significantly

frequencies up to 5,000 c/s.

The situation in FM receivers is quite different. Yorf have already learnt that an

FM transmitter could well have a frequency deviation of 75,000 c/s above and

below its centre frequency. When the received FM signal is mixed with the fixed-

frequency oscillator signal, the resulting i.f. [signal deviates 75,000 c/s above and

below its centre frequency. This means that t50,000 c/s is the minimum bandwidth

with which the i.f. amplifier has to deal. $But, in order to pass the sidebands necessary to give a 150,000-c/s deviation, it

is desirable that the i.f. amplifier of an FM receiver be so designed that its frequency

response is essentially flat over a bandwith of £s much as 200 kc/s.

The i.f. amplifier of an FM receiver normally has a centre frequency of 10-7 mega-

cycles. It would, therefore, be ideal for the amplifier to have a frequency response

which is perfectly flat between 10-6 and 10-8 ntegacycles—with a perfectly sharp cut-

offjust outside this frequency range.

The reason why the response curve should be perfectly flat over the bandwidth

is that, if it is not, variations in gain at different frequencies will be introduced, and

will produce the effect of amplitude modulation of the signal. This effect is un-

desirable.

Although the ideal frequency response mentioned above is never achieved in

practice, threemethods ofobtaining a reasonable approximation^) it are in general use.

§3] 6.29

FM I.F. Amplifiers (continued)

One method of obtaining a reasonably flat response curve in an i.f. amplifier is

known as "staggered tuning."

Three i.f. amplifier stages are used, tuned to 10-6, 10-7 and 10-8 megacycles res-pectively. The overall frequency response of this combination is as shown in thediagram below. Note that at the intersection of the individual response curves oftwo adjacent amplifiers, the gain of the two amplifiers adds up to produce a total

which is about equal to the maximum gain of a single stage.

Although this method produces a fairly satisfactory frequency response curve, theoverall gain produced by the three stages is no higher than that of a single stage.

One way of counteracting this is to take special care to design the individual stagesto have very high gain. Another way is to add a second group of three similarly

tuned i.f. amplifiers—giving six stages in all.

Neither of these solutions is economical; and special alignment techniques arerequired.

Because of these two disadvantages, the "staggered tuning" method of achievinga good frequency response is not used as often as are the two others which are de-scribed in the following pages.

10.6 10.7 10.8 Frequency

6.30

FM I.F. Amplifiers (continued)

[§3

Another method of obtaining the desired broad frequency response is to use

three i.f. amplifiers all tuned to the same centre frequency.

Normally, this would give a very sharply tuned frequency response curve; but a

broader response is obtained in each stage either by damping the windings of the

transformers, or by using transformer coils with low Q. (Transformer windings are

damped by connecting resistors of suitable values across them.)

By reason of the broad frequency response of each of the amplifiers used in this

method, the gain of each stage is low. The overall gain of the three stages, however,

is sufficient to give the required signal amplification.

Although this method produces only an approximation of the desired response

curve, the overall gain is adequate; and neither the design nor the alignment pro-

duces great problems. The method is frequently used in low-cost FM receivers.

Moreover, in recent years valves have been evolved capable of producing unusually

high gain at the i.f. frequencies which are used in FM. When such Valves are used

in conjunction with careful circuit and component design, good gain and frequency

§3] 6.31

FM I.F. Amplifiers (continued)

response characteristics can be obtained with only two broadly tuned i.f. stages of the

type described.

A third method of achieving good i.f. frequency response also makes use of three

i.f. amplifier stages tuned to the same centre frequency. The first and third stages

are designed and tuned as in the second method; but the transformer of the second

stage is "over-coupled"—that is to say, it is so designed as to produce a double-

peaked frequency response curve.

When the individual response curves of the three stages are combined, the double

peaks of the second stage have a significant effect in broadening the frequency

response, and so producing a good approximation of the desired response curve.

This method gives a flat frequency response curve with adequate gain; but align-

ment is more difficult than is the case in the other methods.

Use of the new high-gain valves in this method gives good gain and response

characteristics.

There are other methods of obtaining the required response with adequate over-

all gain; but they are usually combinations of the three methods mentioned.

When quality rather than cost is the prime consideration, one or more i.f. stages

may be added—and the result will be a better response characteristic with higher gain.

First IF

Second IF

Third IF

Overall IF

ALTERNATE SINGLE-AND-DOUBLE- PEAKED STAGES

6.32

The Llmiter and the Discriminator

[§3

•Ae^

A

To Audio\mplifier

: Input Signal I.imiter Output Sign

Amplitude Has No Amplitude

nations Variations

AAA/b etor Output Smnai

Two diflferent types of detector arrangement are in common use in FM receivers;

and these must both be described in detail.

The first type makes use of two stages with different functions: a limiter and a dis-

criminator. The limiter eliminates all amplitude variations in the received signal,

so that the detector receives a signal which is varying in frequency only.

The discriminator converts the FM signal output of the limiter to an audio signal,

the frequency of which is equal to the frequency of the carrier signal swing, and the

amplitude of which is in proportion to the magnitude of the frequency variation.

§3]

Hie Limiter and the Discriminator (continued)

6.33

The Elimination Of

Amplitude Variations

Results In TheElimination Of Noise

/

\l

' ' / ,/

/

non-ideal RF and IF

Amplifier Responselightning

electrical

equipment

/ ''/'i \ ' f

\\v

defective neon signs temporary fading

The purpose of the limiter is to eliminate all amplitude variations in FM signals.

This elimination is necessary because the discriminator is sensitive to amplitudevariations and will reproduce them as signal distortion and noise in its audio output

signal.

There are two main reasons for the amplitude variations which exist in FM signals.

First, the r.f. and i.f. stages do not have a frequency response which is perfectly flat

across the top and with sharp cut-offs. Any variations from this response cause

different amounts of amplification for different signal frequencies.

The second cause of amplitude variations is the interference signals caused byelectrical equipment, lightning flashes, atmospheric disturbances, neon signs and awide variety of other causes. In vehicles there is also signal fading when hills,

power lines, steel bridges and other large objects temporarily obstruct the signal

path between the transmitter and receiver.

If these variations reach the discriminator, they will produce noise or fading in

the audio signal output. It is the function of the limiter to eliminate these variations,

and so to give FM receivers their well-known freedom from noise.

6.34 [§3

The Limiter and the Discriminator (continued)

The diagram of a limiter circuit given below shows a close resemblance both to an

i.f. amplifier and to the grid-leak detector described in Part 5. The circuit limits

the peak-to-peak voltage of the output signal to a fixed and pre-determined value

for all normal input signal levels received from the i.f. amplifier.

Examination of a typical limiter circuit shows that the valve develops its grid bias

by means of a grid resistor and of a fixed capacitor in the grid circuit. The valve is

of the sharp cut-off type, and is operated with low anode voltage. The circuit

diagram shows a triode for purposes of simplicity, although a pentode is generally

used.

Note that the grid circuit arrangement is basically the circuit of a diode detector.

The control grid acts as the diode anode, and the grid-leak resistor, Rlt acts as the

diode load. The grid capacitor, Clt serves both to couple signal to grid and to take

part in developing the grid bias.

When a positive signal peak from the i.f. amplifier reaches the grid, the grid be-

comes positive and attracts electrons from the cathode. The flow of electrons

through the grid-leak resistor to earth produces a voltage drop across that resistor,

and the flow is in such a direction as to make the grid negative with respect to the

cathode.

During the first few positive signal peaks from the i.f. amplifier, electrons also

accumulate on the capacitor plate next to the grid. Sufficient electrons accumulate

on that plate to maintain a stable current flow through the resistor during all parts

of the cycle.

Thus the magnitude of the grid bias is determined by the magnitude of the positive

signal peak from the i.f. amplifier. The greater the amplitude of the input signal,

the greater is the negative grid bias.

LIMITER| HT+

Discrim-inator

§3] 6.35

The Limiter and the Discriminator (continued)

To see the effect of this negative grid bias, examine the IJVg curve shown in the

illustration.

An input signal of low amplitude develops the low negative bias illustrated by line

X. This bias voltage is less than the voltage of the positive signal peak. Conse-

quently, the uppermost portion of the positive signal peak will drive the grid positive,

and the grid will draw current.

This current flow overloads the i.f. amplifier output transformer, and the current

flow through the internal impedance of the transformer causes a drop in the output

signal voltage. Thus the positive peak of the signal is clipped at the limiter grid,

so that the peak can rise only a slight amount above zero volts.

The negative peak of the grid signal drives the limiter almost to the point of cut-

off, just up to the point where clipping of the negative peak begins.

Anode C

LIMITER OPERATIONAnode Current Curve

Cutoff

Grid Current Flows

6.36 [§3

The Limiter and the Discriminator (continued)

When the input signal is of greater amplitude, a large negative bias is developed,

as illustrated by line Y in the diagram below. Large as this bias is, it is less than the

voltage of the positive signal peak. So again the upper section of the signal peakdrives the grid positive, causes grid current flow, and results in the clipping of the

positive peak.

The negative peak of the grid signal is now of sufficient amplitude to drive the valve

beyond cut-off, and the negative peak is clipped as shown.

Thus any signal which has an amplitude greater than the signal shown will haveboth its positive and negative peaks clipped. The limiter output will be the same for

all such signals. Weaker signals will not have their negative peaks clipped, and there

will be a small amount of amplitude variation. While the clipping action does dis-

tort the shape of the output signal, the discriminator is not sensitive to such dis-

tortion, and the desired frequency variations are preserved.

For a limiter stage to act as a limiter, however, grid current must flow so that at

least the positive peaks of the input signal are clipped.

§3]

The Limiter and the Discriminator (continued)

6.37

DISCRIMINATOR

limiterAudio

Amplifier

Discriminator Circuit

Although there are a number of discriminator circuits in use today, they have all

been developed from the basic circuit described here.

In this circuit the final i.f. transformer has a centre-tapped secondary winding, and

each half of this secondary winding has its own tuning capacitor. To understand

the operation of the circuit, it is essential to know exactly how the various windings

are tuned.

The transformer primary is tuned to the centre frequency of the i.f. signal. Secon-

dary winding Lx is tuned to a frequency 75 kc/s higher than the centre frequency,

and secondary winding Ly is tuned to a frequency 75 kc/s lower than the centre

frequency. Thus these two Windings are tuned to the extreme ends of the maximumfrequency deviation of the i.f. signal.

Low-voltage signals are developed across each transformer secondary winding

when the incoming signal is at the centre frequency. As the incoming signal swings

towards the resonant frequency of either tuned secondary winding, an increasingly

large signal is developed across that winding.

The signal appearing across each transformer is rectified by a separate diode

rectifier. Thus, d.c. voltages are developed across resistors Rx and Ry, and each

voltage is proportional to the amplitude of the signal appearing across the associated

transformer secondary winding. Current flow through each resistor is in the di-

rection shown by the arrows, and the voltages developed across the two resistors are

in opposition.

The resultant of these two voltages is the signal applied to the audio amplifier.

6.38 B3The Limiter and the Discriminator (continued)

The resonance curves shown below indicate how an audio signal results from the

FM signal coming out of the i.f. amplifier. (You must remember that the frequency

of the i.f. amplifier swings at a rate equal to the transmitted audio signal, and the

deviation of the frequency is in proportion to the amplitude of the audio signal.)

Assume that the transmitted signal is a 1,000-c/s note at low volume. In this

case the i.f. signal will deviate, say, 15 kc/s to either side of its centre frequency, andthe frequency swing will be at a rate of 1,000 times per second.

When the i.f. signal swings to 15 kc/s higher than its centre frequency, a certain

d.c. voltage is developed across Rx and a lower d.c. voltage is developed across Ry .

These two voltages are opposed to each other; but since the voltage across Rx is the

larger, a positive signal is applied to the audio amplifier input.

As the i.f. signal swings back towards its centre frequency, the voltage across Rxbecomes lower; and the voltage across Ry becomes larger. At the centre frequency

the two voltages are equal and opposite, and the voltage applied to the audio

amplifier is zero. Then, as the i.f. signal swings towards 15 kc/s lower than its centre

frequency, the voltage across Ry becomes larger, while that across Rx becomes lower.

The result is that the voltage applied to the audio amplifier input becomes increasingly

negative.

In this way, since the i.f. frequency swings back and forth at a rate of 1,000 cycles

per second, a 1,000-c/s signal of low amplitude is connected to the audio amplifier

input.

If the frequency deviation increases, as it would with a louder transmitted audiosignal, larger voltages are developed across Rx and Ry at the extremes of the frequencyswing. The result is that an audio signal of high amplitude is connected to the

audio amplifier input.

Produces Negative Voltage

Resonance

'urve of Ly

TunedCircuit

Resonance

Curve or- L^

i Tuned

FREQUENCY 10.625 [ 10.7 10.75

Produces Positive Voltage

/ Across Rx

>

\ + Output /

\ Across Rx /\l"I^Low Amplitude \

Input Signal \vOutput

1

Across Ry

^High Amplitude

Input Signal

\l

§31"9

The Foster-Seeley Discriminator

Although the discriminator described on the previous pages is the basic type from

which a number of other discriminators have been evolved, it is itself seldom used.

One reason is because the circuit is difficult to align, on account of the two separate

tuning frequencies of the secondary windings of the transformer. Another is because

the transformer it uses is more costly to make than is the ordinary i.f. transformer,

on account of the difficulties of designing and manufacturing the unusual secondary

arrangements.

The Foster-Seeley discriminator now to be described is a variation on this basic

circuit which is very widely used in modern FM receivers. The major physical

difference is that the transformer secondary is centre-tapped, rather than split; and

that a single tuning capacitor is used in the transformer secondary circuit. The diodes

operate in the same manner as in the basic discriminator circuit.

Both the primary and secondary windings are tuned to resonate at the centre

frequency of the i.f. signal. Regardless of the frequency swing of the i.f. signal, the

signal voltage across the upper half of the transformer secondary, Lx, is always

equal to the signal voltage across the lower half of the secondary, Lr The signal

voltage developed across the transformer primary is fed to the r.f. choke, L2, through

the coupling capacitor, Ca.

The voltage across Lz adds both to the voltage across Lx and to the voltage across

Ly, as will be shown.

With this arrangement, the phase relationships between the voltages across Lxt

L and Lz will vary as the i.f. frequency deviates; and an audio signal voltage is

developed across the discriminator output.

FOSTER

SEELEYDISCRIMINATOR

_f

6.40[§3

TTie Foster-Seeley Discriminator {continued)

If you warnine the circuit diagram on page 6.39, you will see that vectorialaddition of the voltages across Ly and L2 will produce the voltage across diode Dyand its load Ry . Similarly, vectorial addition of the voltages across Lx and Lz willproduce the voltage across diode Dx and its load Rx .

When the i.f. signal is at its centre frequency, the signal voltages across Lx andLyare equal, and 180° out of phase with each other. They are also 90° out of phasewith the signal voltage across Lz (forLz is connected to the centre-tap). The voltagesacross diode Dx and diode Dy are therefore equal (see vector diagram (a) below),and the output to the audio amplifier is zero.

When the i.f. signal rises, the reactance in the secondary winding becomes in-creasingly inductive. Although the signal voltages across Lx and Ly remain equaland 180° out of phase with each other, they are no longer 90° out of phase with thevoltage across Lz . The resultant voltage across diode Dy is higher than that acrossdiode Dx (see vector diagram (b) below), and a negative voltage is applied to the audioamplifier.

When the i.f. signal falls below that of the centre frequency, the reactance in thesecondary winding becomes increasingly capacitive. The voltages add as shown invector diagram (c) below, and that across diode Dx becomes higher than that acrossdiode Dy. Thus a positive voltage is delivered to the audio amplifier.

So, as the i.f. frequency swings back and forth, an audio signal is developed tocorrespond to the frequency and amplitude of the i.f. frequency deviation.You can now see why a limiter stage is used with such discriminators. If the

amplitude of the i.f. signal applied to the discriminator is varying because of noise andinterference, the voltage across Lz will also vary—and the audio signal output willbe affected.

There are thus two factors which could affect the amplitude of the audio outputvoltage—changes in i.f. signal frequency (which convey intelligence), and changesin i.f. signal amplitude (which are unwanted).A limiter stage always precedes discriminators of this kind, therefore, with the

object of eliminating variations in amplitude.

Voltages across componentswith if signal at centre

frequency

NO AUDIO SIGNAL OUTPUT

Ly

W ^ Lz

Lx

Diode Dy&Ry

Lx Diode Dx & Cx

Voltages across components with

if. signal above centre frequency.

Negative Voltage to Audio Amplifier

Diode Dx & Cx

Ly

vcDiode Dy& Ry

•.LzM

l_x —Diode Dx&Cx

Voltages across components with

i f. signal below centre frequencyPositive. Voltage to Audio Amplifier

IF[Amplifier

RATIO DETECTOR

You saw on the last page that something is needed in discriminator circuits toeliminate variations in the amplitude of the i.f. signal; and that in some discrimina-tors a limiter is used for that purpose.

In order to provide FM detection without the need for a limiter stage, a circuitcalled a ratio detector has been developed. Since it permits the elimination of onestage in the FM receiver, it is in widespread use today.

The diodes in this circuit are connected in series, the individual diode audio signalvoltages being developed across Rx andR2. The detector output voltage is developedacross jR3, which is in the circuit of both diodes.

Because of the way in which the diodes are connected, the incoming i.f. signal is

rectified in such a manner that the upper part of the RC network—Ru R2 and Cy—is charged positively, while the lower part of that network is charged negatively.The voltage developed across the RC network is determined by the amplitude of theaverage i.f. signal voltage.

The time constant of the network is generally of the order of 0-1 seconds. Withsuch a time constant short-term amplitude variations of the i.f. signal caused by noisewill have no effect in changing the voltage across the network.Long-term increases and decreases in i.f. signal level cause corresponding increases

and decreases across the RC network.

6.42 t§3

The Ratio Detector (continued)

It is very important in the operation of this circuit to establish a stable voltage

across C2 . As the i.f. signal deviates about its centre frequency, the voltages across

both Cx and Cy will vary. But the sum of these voltages will always be equal to that

across C2, which thus remains constant.

The tuned circuits in the ratio detector operate in exactly the same way as they do

in the Foster-Seeley discriminator. The phase-shift diagrams for that discriminator

are also true for the ratio detector.

When the i.f. signal is at its centre frequency, the transformer secondary is at

resonance. The voltages across Cx and Cy are therefore equal, but opposite in

polarity. The voltage across output resistor R$ is zero, and no signal is applied to

the audio amplifier.

When the i.f. signal increases in frequency, the phase relationships are as shown

in diagram (b) below; and the resultant voltage across diode Dy is higher than

that across diode Dx. When the i.f. signal frequency decreases, the resultant voltage

across diode Dx is higher than that across diode Dy. In both cases the voltages

across Cx and Cy are unequal, but their sum always equals the voltage across C2 .

The differences between Cx and Cy cause the development of an audio frequency

voltage across i?3 , and this output signal is applied to the audio amplifier.

The purpose of inserting R4 and R5 in the circuit is to improve the ability of the

circuit to reject signals of varying amplitude.

hVolumes ar r< ;ss components

JT Diode Dy & Cv

with I F simKil at centre ^y^frequency |^—

-

Ia

"

NO AUDIO SIGNAL OUTPUT ^\\* Diode Dx <vCx

(b)

-y

t ^Diode Dy&C, Diode Dy&Cj

a: Diode DX&CX

(c)

Diode Dv&C,

Voltages across components with | Voltages across components with

IF signal above centre frequency I IF signal below centre frequency

Negative Voltage to Audio AmplifierJ

Positive Voltage to Audio Amplifier

§ 3J 6.43

The Advantages and Disadvantages ofFM Detectors

Both the limiter-discriminator and ratio detector circuits have advantages and disad-vantages. Each type is preferred by some designers, and it is essentially a matter ofpersonal preference as to which is used.

The important advantage of the liniiter^scriminator arrangement is that it is

a relatively simple matter to balance the two sides of the discriminator, and so toobtain excellent reproduction of the audio-frequency signal. One disadvantage is

that the limiter does not operate unless the incoming signal has sufficient amplitudeto cause the limiting action to take place. When limiting action does not take place,the amplitude variations in the signal result in interfering noise and signal dis-tortion.

This means that high-gain r.f. and i.f. stages must be used to boost the signalamplitude into the limiter. Alternatively, in a number of FM receivers two limitersare used in a cascade arrangement to assure that adequate limiting action will takeplace; but even under these conditions signals of very low amplitude may result in a"noisy" output from the discriminator.

The important advantage of the ratio detector is that it is not sensitive to ampli-tude variations in the incoming signal. Hence, the ratio detector circuit has no needfor a limiter stage, or stages, and does not depend on the use of r.f. and i.f. stages ofunusually high gain. One disadvantage of the ratio detector is that special caremust be taken to balance the two sides of the detector; otherwise some of the in-sensitivity to amplitude modulation will be lost, and noise will accompany weaksignals. Another is that the ratio detector is more liable to produce distortion inthe audio signal output when the i.f. amplifier has an insufficiently broad frequencyresponse range.

O Detector Comparison

LIMITERDISCRIMINATOR

ADVANTAGES DISADVANTAGES

Simple balancing Weak signals subject to

interference

RATIODETECTOR

Weak signals not

subject to

interference

Distortion and noise rise

with detector unbalance and

decreasing bandwidth

6.44 [§*

FM Receiver—Automatic Gain Control

Because of the transmission charac-

teristics of FM radio waves, signals

from distant stations are subject to

significant variations in signal strength.

Moreover, the difference between signal

levels from near and distant stations is

greater in an FM system than it is in

existing AM systems, because FMsystems work in the VHF band.

Although FM receivers are largely

immune from trouble caused by differ-

ences in signal amplitude, it is still important that the signal level at the detector

section should be of sufficient strength for correct operation ofthe limiter-nliscrimina-

tor or ratio detector stages.

As in AM superheterodyne receivers, the negative AGC voltage in an FM receiver

varies the gain of the i.f. stages in accordance with the amplitude of the received

signal. The gain is reduced by the negative AGC voltage when there are strong

incoming signals, and the gain is reduced to a lesser degree in the case of weak in-

coming signals. This results in a stable "signal amplitude at the detector.

When limiter-discriminator stages are used, AGC voltage can be obtained from

the resistor in the limiter grid-to-cathode circuit. Remember that the capacitor

plate next to the grid retains a negative charge. If two resistors rather than one

are placed in the grid-to-cathode circuit, a voltage divider arrangement is formed; and

AGC voltage is available at the junction of the two resistors (see diagram above).

Since the negative voltage on the grid increases with increasing i.f. signal level, the

requirements for AGC voltage are met.

In the ratio detector circuit the voltage across the Ru R2,< C2 network increases

and decreases with corresponding changes in the i.f. signal level. Since a negative

voltage is available, this is a convenient source of AGC voltage.

AGC FROMRATIO DETECTOR

- AGCVOLTAGE

§3] 6.45

FM Receiver—De-emphasis

The purpose of pre-emphasis was explained in the section dealing with FM trans-

mitters. It is a method of improving the reproduction of high-frequency audiosignals; and the method involves increasing the high-frequency gain of the trans-

mitter audio amplifier according to the frequency response curve shown below.

To bring the amplitude of the entire range of audio frequencies back to the distri-

bution found in the speech and music originally used to modulate the carrier waveput out by the transmitter, it is necessary to use "de-emphasis" to counteract the

added high-frequency gain.introduced in the transmitter.

The method of accomplishing de-emphasis is to use an RC network which reducesthe high-frequency audio signal by exactly the same amount as that by which it wasincreased in the transmitter. The basic circuit is shown in the diagram below.

The circuit is a low-pass filter which operates on the principle of a voltage divider.

The capacitor is a small one, in the order of 0001 microfarads. At low frequencies

the capacitor has a very high impedance, and most of the audio signal voltage is

applied' to the grid. As the audio frequency rises, the impedance of the capacitor

decreases; and increasingly less signal voltage is applied to the grid. The result is

that the de-emphasis network reduces the high-frequency gain of the amplifier andcounterbalances the effects of pre-emphasis.

The de-emphasis circuit is normally to be found between the output of the de-

tector and the input to the first audio amplifier.

- Pre-emphasis

Curve

Resultant

Frequency

Response

- De-emphasisCurve

FREQUENCY

6.46

The Complete FM Tuner

[§3

RFAMP6AM6

FREQUENCYCHANGERI2AH8

IF AMP6BA6

IF AMP6BA6

RATIODETECTOR

6ALS

To receive FM broadcasts, you must use an FM wireless receiver or an FM tuner

with a separate a.f. amplifier. The a.f. amplifier might well be part of anAM wireless

receiver with switching facilities such that the output of the FM tuner, or of the AMdetector, or of a gramophone pick-up can be connected to the input of the a.f.

amplifier.

The circuit of the complete FM tuner shown above is typical of those in widespread

use to-day. (See also page 6.51.)

An FM tuner is an FM receiver which is complete except for the audio amplifier

stages. In some cases tuners have their own power supply—others obtain their sup-

plies from a separate source, as is the case here.

The FM tuner above has five valves. The circuit consists of an r.f. amplifier, a

frequency changer, a two-stage i.f. amplifier and a ratio detector. The tuning range

is from 88 to 100 Mc/s, the i.f. frequency is 10-7 Mc/s. Examination of the com-

plete circuit will show you that there is very little in it which you have not met before.

FM tuners require a dipole aerial mounted as high as possible, either horizontally

or vertically depending on the polarization of the transmitter aerial serving the area.

The aerial should be connected to the timer by an 80-ohm screened feeder cable.

The input circuit of the r.f. amplifier in the tuner is broadly tuned so as to allow

amplification over the frequency band without the need for manual tuning. Incoming

signals from the aerial are coupled to a 6AM6 (rX amplifier valve) by means of the r.f.

transformer 7V The secondary of 7\ is not tuned by a variable capacitor; instead,

the inter-electrode capacitance between grid and cathode of Vlt together with stray

capacitances, broadly tune the secondary ofT\ over the range 88-100 Mc/s. Although

gain is lost by broad tuning, this is compensated by using high-gain amplifier valves

in the r.f. amplifier and in the stages that follow.

The signal developed across Lx in the anode circuit of the 6AM6 is coupled to the

grid of V2> which is the frequency changer—in this case a triode-hexode 12AH8.

The hexode section of this valve amplifies the incoming signal frequency, and com-

bines it with the oscillator frequency produced by the triode section of the valve.

§3]

The Complete FM Tuner (continued)

6.47

FREQUENCYRFAMR CHANGER6AM6 I2AH8

Rl«

47KJ

mo:22K-

3-5L_Pp. i'5 p

,

04^015^

CB-700P

-nnnRp

—

L2

The difference between the two, the i.f., is developed across the primary of the i.f.

transformer T3 in the anode circuit of the hexode section, and coupled by the secon-dary of T3 to the succeeding stages.

The oscillator is an Armstrong type, and the tuning is such that the oscillator

frequency is always 10-7 Mc/s lower than the signal frequency—the result being a10-7 Mc/s i.f. signal.

The choke L2 is connected in the heater circuits of Vx and V2 in order to preventfeed-back affecting the stability of the tuner.

To form the two i.f. amplifier stages, two 6BA6 pentode valves are used. Con-ventional i.f. amplifier circuits, as described in Part 5 of Basic Electronics, are used.The i.f. transformers are permeability-tuned to the centre frequency 10-7 Mc/s. Thefrequency response of the i.f. transformers is broad enough to allow the full deviationof ± 75 kc/s in the i.f. signal to be passed to the following stage.

6.48

The Complete FM Toner (continued)

[§3

RATIOIF AMP IF AMP DETECTOR6BA6 6BA6 6AL5

The output of the i.f. amplifier stages is coupled to a double diode, 6AL5, connected

as a ratio detector. This valve operates as described on pages 6.41-6.42, save that

the transformer Ts has a third winding L2t instead of a separate r.f. choke.

This "tertiary" winding, as it is called, consists of a few turns closely coupled to

the lower end of the primary winding. The voltage induced in it from the primary is

180° out of phase with that in the primary. The voltages across the two halves of thesecondary are both 90° out of phase with that in the primary, and the same amountout of phase with the reference voltage across the "tertiary" winding.

The purpose of the tertiary is to permit the use of a high-impedance primary in the

anode circuit of the last i.f. transformer, while making the construction of the trans-

former simpler.

The a.f. output of the ratio detector is developed across C2St and fed via the de-

emphasis network R1S and C26 to the output socket. This output should then befed through a screened cable into a good-quality a.f. amplifier whose frequencyresponse is reasonably flat over the range 40 c/s to 15,000 c/s.

§3]

REVIEW ofFM Receivers

FM Receiver Block Diagram. Theblock diagrams of an FM receiver and

of an AM superheterodyne receiver are

the same. But there are differences

inside every one of the stages—the most

important being in the i.f. amplifier and

the detector.

6.49

LF. Amplifier Bandwidth. For effec-

tive amplification of the receiver carrier

and side-band signals, it is desirable

that the i.f. amplifier should have a

flat response for 100 kc/s both above

and below the centre frequency.

Staggered-tuned LF. This type of

i.f. amplifier may contain three stages

tuned to 10-6, 10*7 and 10-8 megacycles,

respectively. Characteristics are a good

frequency response curve, an overall

gain equal to that of a single stage, and

the need for special alignment tech-

niques.

STAGGEREDTUNING

10.1 Frequency

Centre-tuned Broad Band LF. In

this type of i.f. amplifier all stages em-ploy transformers whose windings are

damped so as to gire a low Q. All

stages are tuned to the centre frequency.

Adequate gain can be achieved, fre-

quency response is acceptable and the

alignment procedure is simple.

£mrt?"i -»

10. « 10.7 10.8

6.50

REVIEW of FM Receivers (continued)

Double-peakedI.F. Stage. The fre-

quency response carve obtainable with

centre-frequency tuning can be

improved by using an over-coupled i.f.

transformer in one stage. The double-

peaked response curve produced

broadens the overall response of the

complete i.f. amplifier. The align-

ment procedure becomes only slightly

more complex.

[§3

Limiter Stage. The limiter clips

the positive and negative peaks of the

i.f. amplifier output signal, and thus

eliminates amplitude variations. Oneor more limiting stages are required

if a discriminator type of detector is

used.

LIMITER»HT*

Arrows snow electron current flow

Discriminator. The discriminator

converts the limiter output signal to

an audio signal. The frequency of

the audio signal is equal to the fre-

quency of the carrier signal swing, and

the amplitude is in proportion to the

magnitude of the frequency variation.

The discriminator is sensitive to ampli-

tude variations, and must be preceded

by a limiter.

Ratio Detector. The ratio detector

provides FM detection without the

need for a limiter. The audio out-

put of this detector is a function of the

ratio of the voltages across C% and

Cy. The circuit is not sensitive to

amplitude variations in the i.f. signal.

§3] 6.51

mZDH

£

Z

fe

6.53

WARNING

Certain of the processes, devices and circuit arrangements described

in the following Sections of "Basic Electronics" Part 6, are pro-

prietary. The information given is presentedfor educational purposes

only; and the fact that it is included does not imply that it is freely

available for use in design, manufacture or sale. No person or body

having any part in the preparation, printing or publication of this

volume will be responsible for any liability resulting from unlicensed

use of the material in question.

The Authors and Publishers wish to extend their special appreciation

to the Radio Corporation of America, and to the Heath Company {a

subsidiary of Daystrom Incorporated), for authority to reproduce

some of their designs.

6 56 §| ; INTRODUCTION TO "SOLID-STATE"

ELECTRONICS

A Brief History of Transistors and Semi-conductor Diodes

Two important developments in present-day electronics are the "semi-conductordiode" and the "transistor." These two devices are the first commercially available

representatives of a vast new field of study. It is called "solid-state" electronics.

The semi-conductor diode is a development of the crystal detector used in the early

radio receivers. This new device can detect, mix and rectify alternating currentsignals with excellent efficiency, and has a wide variety of important new applications.

The transistor was discovered in 1948 as the result of extensive studies of the operationof semi-conductor diodes. Its name was derived from the words "transfer resistor"

and is descriptive ofa phenomenon which enables a completely solid device to amplifyelectrical signals.

Within the next few years these two devices will extensively replace valves in manytypes of existing equipment, and they may be employed in a wide variety of domestic,transportation, industrial, scientific and military equipments which do not employelectronics at present.

Nor is this their only significance. They are the precursors of other new solid-

state electronic devices, now in various stages of development, which may bring great

advances in domestic and industrial illumination, electric power generation, con-version of eiSctiicnower to mechanical motion, computer memory storage, ultra-

Mgh-soeeo^dat^ transmission, detection and measurement of physical and chemicalchanges, electronic ignitio%and inmanyother aspects ofour domesticand industrial life.

§]A Brief History of Transistors and Semi-conductor Diodes (continued)

6.57

The early discoveries in the field of electricity made by Volta, Ampere, Gauss,

Faraday, Hertz and others raised fundamental problems concerning the nature of

matter; and the first investigations into matter itself raised more problems than they

provided answers.

The first real break-through to the modern concept of matter came in 1897, whenSir J. J. Thompson discovered the electron while studying electric discharges through

rarified gasses. Thompson's discovery was rapidly verified by other investigators.

In 1913 Bohr evolved the basic theory of atomic structure, and that theory has been

developed to our present-day concept of the nature of matter.

According to the atomic theory, all materials consist of various combinations of

about one hundred different types of atoms. The atom is defined as the smallest

unit into which an element may be divided before it loses its physical and chemical

identity.

All atoms consist of a positively charged nucleus, around which one or morenegatively charged electrons rotate. The electrons rotate in orbits which make uprings or shells located at varying distances from the nucleus. The number of electrons

in the various rings or shells determine the specific element to which the atom belongs.

Electrons in the inner rings or shells have nothing to do with the ability of an atom

to enter into chemical combinations with other atoms, or to exhibit its various

electrical characteristics; that is determined only by the electrons in the outer ring.

The electrical characteristics of an atom are determined by how tightly the nucleus

holds on to its outer electrons. If the outer electrons are easily stripped off the atom

by a weak electric field, the material will conduct easily; and the material is knownas a "conductor." If a very strong electric field is required to strip the electrons

off the atom, the material is known as an "insulator."

6.58 [§|

A Brief History of Transistors and Semi-conductor Diodes {continued)

The material which is used in transistors and semi-conductor diodes is known as

"semi-conductor" material. This term is a general one, and applies to all materials

having a greater electrical resistance to current flowing through them in one direction

than they have to currentflowing through them in the opposite direction. The materials

which are of greatest use in present-day transistor applications are germanium andsilicon, the characteristics of which will be considered shortly.

Interest in semi-conductors began back in 1873, when it was discovered that rodsand wires of selenium acquired decreased electrical resistance when struck by sun-light. It was demonstrated that this was due to the presence of light; not to heating,

which normally results in an increase in electrical resistance.

Later investigators found similar effects in other materials, but the change in re-

sistance was so small that no practical applications could be found.

The next significant development was in 1906. At that time a variety of crystalline

semi-conductors were used as detectors of radio signals. Materials so used included

galena (lead sulphide), silicon, iron pyrites and carborundum. The most commondetector arrangement consisted of a piece of crystalline galena in contact with ashort length of flexible wire (which was called a "cat's whisker").

Such a device, which was known as the "crystal detector," would have been usedin the circuit arrangement shown in the diagram below. This circuit made possible

the reception of radio signals; for the crystal acted as a rectifier allowing easy current

flow in one direction only.

§ I] 6.59

A Brief History of Transistors and Semi-conductor Diodes (continued)

The success of the crystal detector was short-lived. The thermionic valve, knownas the "vacuum tube" in the United States, was developed, and it served as a muchmore reliable detector than the crystal arrangement. Valves had the additional

advantage of being able to amplify the detector output signal to an amplitude and

power level sufficiently high to drive a loudspeaker.

During World War II radar was in a continuous state of development. One of the

most important problems was the detection of its extremely high-frequency radio

signals. Improvement in locating small targets demanded an increase in radar

frequency, and every increase in frequency caused new problems in the mixer stage.

New types of valves were developed for the purpose, but eventually a frequency limit

was reached beyond which valve-type mixers would not operate.

Crystal mixers were then tried; and the silicon semi-conductor type was found to

be the most successful. Improved types of this mixer are widely used to-day in micro-

wave radars.

While crystal mixers were being developed, a variety of semi-conductor materials

were also investigated. Of these materials, silicon and germanium were found to

have very interesting properties, which were systematically investigated on a large

scale as soon as the War was over.

6.60 [§l

A Brief History of Transistors and Semi-conductor Diodes (continued)

One of the first developments was a diode detector made of germanium. This

detector was used in radio, television and miscellaneous electronics applications;

but there was only a limited need for a detector of this type.

During the development of germanium detectors, however, a very importantdiscovery was made. It was found that when two very close electrical contacts are

made with a piece of germanium, the current flow through one of the contacts affects

the amount of current flow through the other contact. A close similarity wasobserved between this effect and the signal amplification which occurs in a valve,

except that no heated cathode and no vacuum was required.

An enormous amount of work was done at the Bell Telephone Laboratories, in

the United States, on developing this effect; and in 1948 the first solid-state amplifying

device, the transistor, was announced.

The discovery of the transistor led to a new interest in semi-conductor diodes, andsubsequently to the development of a wide variety of important new uses.

§1]

A Vast New Field of Study Opens

6.61

,~***

AUTOMATICDATA TRANSMISSION

The result of these discoveries, made during the last decade, is that there are

opening to-day enormous opportunities for interesting and important work in the

fields of transistor and semiconductor diode applications. And these opportunities

will be vastly multiplied in the field of solid-state electronics which is just beginning

to be developed.

The most important characteristic of transistors to-day is their ability to replace

valves in a wide variety of applications. About 65 per cent of present-day transistors

are used in the field of entertainment and domestic applications, about 25 per cent in

the industrial and commercial fields, and about 10 per cent goes to military applica-

tions.

But in a few years' time, there can be little doubt that transistors will become well

established in fields in which even valves have to-day only a tentative foothold. These

will include industrial control systems, complete automation systems, computers andautomatic data transmission systems.

It is also expected that there will soon be an enormous expansion of the use of

transistors in military applications.

6.62 [§ I

The Advantages of Transistors

Why is it that transistors are now being so widely accepted as replacements for

valves?

Transistors have five basic advantages over valves; and these must now be briefly

discussed.

First, transistors are extremely small in

size, ranging from the dimensions of sub-

miniature valves down to less than a quarter

of that size. This small size makes it mucheasier to produce miniature equipment, l^'VI I

Subminiaturewhich is a great convenience in the enter- kVf ^mII Valve

tainment and industrial fields and of vital Hv! /1 I aimportance in military applications. ^^ J B *W

Transistor

SMALL SIZE

Second, transistors are inher-

ently capable of performing their

function for an indefinite period

of time before their operating

characteristics deteriorate. Valves

wear out much more quickly.

Third, transistors consume much less

power than do valves. They operate with-

out any of the need for a heated cathode

which is a feature of valves—and rememberthat this heating of the cathode accounts for

a large proportion of the power which valve-

type equipment consumes.

It is these large power needs that make it

difficult to produce portable battery-powered

valve-type equipment with a reasonably

long operating life. Transistors enable the

same equipment to be made both lighter andsmaller; and also give it an operating life

of the order of five times longer.

Sv^&^^&Sww

wevk

Battery for

!&&>TwTivAVAV>v Valvevy.\VAVCAvr,

.\v.

Radio

Battery for

Transistor Radio

LOW POWERCONSUMPTION

§1]

The Advantages of Transistors (continued)

Fourth, transistors require lower power

supply voltages: from about 4*5 to 75 volts,

compared with 75 to 350 volts generally re-

quired by valves. Use ofthese lower voltages

reduces the filtering, screening and voltage

rating requirements of the power supply.

The lower insulation requirements permit

the use of R, L and C components signifi-

cantly smaller in size than those used in con-

ventional valve equipment—further facilitat-

ing the miniaturisation of equipment.

6.63

Fifth, a transistor circuit is generally

simpler, and needs fewer components than

does an equivalent valve circuit. When this

feature is combined with the small size and

low power requirements of transistor circuit

components, it permits the construction of

subminiature equipment and subminiature

assemblies for larger equipment systems.

Since mass-production can be used to provide

such assemblies at low cost, they can be madeavailable to simplify the task of keeping

equipment in operation by making possible

speedy and economical replacement of com-plete sections of the equipment.

:L\1PLJHLD REPAIR

\

Finally, a possible advantage lies in the future. Transistors tend at

present to be more expensive than are valves doing the same sort of job;

but as the demand for transistors reaches heights which allow certain

types of them to be mass-produced, it is hoped that prices can be brought

down to between a quarter and a third of present levels. Such a reduction

would, of course, make some transistors competitive with valves even

in price.

6.64

Semi-conductor Materials

E§

I I I I

I I I I

lVAVAVJ(VAVAI

WJVAVl

LvxlvxlvAvXvi

XvXyAVjAVA*

WAVA^VAvAvAvi

VAVAVAVAW.W.

To understand how transistors and semi-conductor diodes operate, you must first

learn a little about the basic materials used, and about the modified forms of themwhich it has proved desirable to introduce.

The basic materials in commercial use are purified germanium and silicon, bothspecially processed until they are in a crystalline state. These materials are excellentinsulators, because their crystalline structure effectively bonds in place all of theouter electrons which would normally be free to enter into current flow.

The diagram shows a simplified picture of a pure crystalline semi-conductormaterial, such as germanium or silicon. Every atom has four outer electrons, whichare shown as small minus signs. The inner electrons which are bound to the nucleus,and the nucleus itself, are shown as a single solid black dot.

The crystalline structure forces the nuclei into a symmetrical arrangement, withevery outer electron sharing an orbit with one outer electron fr«m a neighbouringatom. It is this orbit-sharing arrangement which effectively locks each electron inplace—not any unusually strong bond between the electron and its nucleus.For an applied voltage to cause electron current flow, it would have to be sufficiently

high to break the electron bonds before those electrons would be free to flow towardsthe positive voltage terminal. In breaking the bonds the voltage would also destroythe crystalline structure.

5 '] 6.65

Semi-conductor Materials (continued)

Since electric current will not easily flow through the pure crystalline materialdescribed, that material must be modified before a controllable amount of currentflow can be obtained.

One method of obtaining current flow is to add a small number of atoms whichhave five outer electrons. Atoms suitable for this purpose include phosphorus,antimony and arsenic. These atoms are distributed through the pure basic materialas it is being processed into the crystal state, and the resulting structure is shown inthe diagram below. The proportion of impurity atoms added is in the order of onepart per hundred million. A larger proportion would allow a current flow whichwas not precisely controllable.

The impurity atom enters into the structure in the same manner as do the atoms ofthe basic material. The important difference is that the extra outer electron of everyimpurity atom remains unbonded to the crystal structure. If a d.c. voltage is con-nected across the ends of a piece of such material, these unbonded electrons are freeto flow through the crystal structure towards the positive terminal. The totalnumber of unbonded electrons in the crystal always remains the same—every electronthat leaves the crystal at the positive terminal is replaced by one entering at thenegative terminal. Consequently, there is a continuous flow of current.

Since the current flow in this material consists ofexcess negative particles (electrons),the material is known as an "excess" or "N-type" semi-conductor.

V^tS Electa N-TYPE

SEMICONDUCTOR

!£S?S«—:

ClliliKST FLOW INN-TYPL SEMICONDUCTOR

gniritt

Atone

(Arrows show direction piElectron Current Flow) \

- . 1 +

- * Electron

6.66 Bl

Semi-conductor Materials (continued)

There is another method of modifying the pure basic crystalline material in order

to obtain a controllable amount of current flow. During the processing of the basic

material, impurity atoms such as aluminium, boron or indium can be added in small

amounts. These impurity atoms have only three outer electrons, and they enter

into the crystalline structure as shown in the diagram below.

Comparison of the diagram with that of the pure basic material shows that the

modified structure has one missing electron for every impurity atom. The space in

the structure caused by the missing electron is known as a "hole."

Note that the hole is not necessarily located in the immediate vicinity of the im-

purity atom. During processing the impurity atom attracts a nearby outer electron

to fill the gap in the surrounding crystal structure, and the hole "moves" elsewhere.

A succession of outer electrons may leave their nuclei to fill the gap, and the hole

may move a considerable distance before it reaches a state of equilibrium.

If a d.c. voltage is connected across the ends of a piece of such material, the hole

acquires the characteristics of a positive charge, and flows towards the negative

terminal of the voltage source. The total number of holes in the crystal always re-

mains the same. Every hole reaching the negative end of the crystal is neutralized

by an electron which leaves the negative terminal and enters the crystal. This gives

the crystal an excess negative charge.

A neutral charge is regained by the crystal when it discharges an electron to the

positive voltage terminal and creates another hole. The new hole flows towards

the negative terminal, and the result is the continuous flow of holes through the

crystal, and a continuous flow of electrons through the connecting wires.

Since the current flow in this material is caused by defects (holes) in the crystal

structure, and since these defects act in the same way as would positive charges, the

material is known as a "defect" or "P-type" semi-conductor.

P-TYPE SEMICONDUCTORDirection of Electron Slow ^

• • • • • • • • m [

3;

f$?~ ^tHfection ol Hole flow'v*'

; *J;v -;

'>

" •. + * Hole

CURKENT FLOW IN

I'Y PL SEMICONDUCTOR

§2 : SEMI-CONDUCTOR DIODES 6.67

Basic Construction—Junction Diode

Cut Germanium BarGrown N-Type Germanium RodGrown P-Type Germanium Rod

Wire L

N-Ivpr' P-Tvpt-

SIMPLIFILD DIAGRAM OhGROWN JUNCTION IAUUE

Pulling andRotating Force

SealedContainer

MoltenGermanium

j3**Induction10 Heating

Coil

A semi-conductor diode consists essentially of P- and N-type semi-conductor

materials in close contact with one another.

There are two basic types of semi-conductor diodes in use to-day—the junction andthe point-contact types.

Two different types ofjunction constructions are in common use. In one type the

junction is "grown," and in the second type the junction is formed by diffusion.

A simplified diagram is shown of the arrangement for making a grown junction.

A crucible containing pure germanium is suspended inside a sealed container, whichcan be either evacuated or filled with inert gas. An induction heating coil is used to

heat the germanium to melting point.

To begin the formation of the diode, an N-type impurity is added and diffuses

itself throughout the molten germanium. A small bar cut from single-crystal ger-

manium is dipped down to touch the surface of the melted germanium, and is thenslowly withdrawn and rotated. The molten germanium solidifies at the point ofcontact with the solid bar, and the repeated dipping and withdrawal process causesthe growth of a rod of N-type germanium at the end of the bar. This rod is actually

a single perfect crystal with a diameter in the order of one inch.

The junction is formed after the rod has grown to the length of about half-an-inch,

by adding enough P-type impurity first to neutralize the N-type impurity in themolten germanium, and then to change the latter over to P-type. The dipping andwithdrawal process is then continued, with the result that the remainder of the rodis of P-type germanium.

The entire rod is a single crystal of germanium, the only difference being the typeof impurity in the two halves.

Lastly, the P-N junction region is cut out of the rod, and diced up into as many asa hundred or more small junctions. Every piece then has wire leads fused or soldered

6.68

Basic Construction—Junction Diode (continued)

m

to it; and the assembly is mounted in a container which gives mechanical protection,

and which also shields it from atmospheric contamination.

There are several methods of making junction diodes by diffusion. The "alloy-

junction" method, as it is called, has been widely adopted because it lends itself to

product uniformity, and also to quantity production techniques.

In this method a small disc of P-type material (indium) is placed on a somewhat

larger flat plate of N-type germanium. The materials are placed in a graphite holder

and heated to a temperature of about 500° Centigrade. The indium disc melts at

about 155° Centigrade; and as the temperature rises further, it dissolves away some

of the germanium beneath it. A germanium-indium alloy is formed.

In the molten region the indium first neutralizes the N-type impurities in the ger-

manium, and then leaves an excess of P-type impurities.

After the disc and plate have been subjected to heat for several minutes, an equi-

librium condition is reached, and no more dissolving action takes place.

The amount of P-type germanium formed is determined only by the temperature

reached, and by the size of the original indium disc. The time spent in the furnace

is not important—a fact which helps greatly in achieving product uniformity.

Once equilibrium has been established, the assembly is allowed to cool very slowly.

The dissolved P-type germanium begins to recrystallize out of the alloy on to the

N-type germanium base. The recrystallization follows the same atomic arrange-

ment as that in the N-type germanium base, and a uniform P-N junction is formed.

After the assembly has cooled, electrical connections are bonded to the germanium

base and to the indium disc. The assembly is mounted in a small container, and

the alloy-junction semi-conductor diode is complete.

§2] 6.69

Basic Construction—Point-contact Diode

The point-contact method of construction resembles that of the crystal detector

used in early radio receivers. It consists of a pointed wire pressed into contact with

a small plate of semi-conductor material. The assembly is sealed in a small con-

tainer, in much the same way as are the junction types.

Platinum alloy, tungsten, phosphor-bronze and other types of wire are used to

make the contact. Several bends in the wire give it a spring-like shape which presses

its point against the semi-conductor surface. The flexible nature of this wire is the

reason why it is called a "cat's whisker." The pressure applied must be sufficient

to hold the point in place.

The semi-conductor plate usually consists of either P-type silicon or N-type ger-

manium.

It has been said that a semi-conductor diode consists basically of a junction be-

tween P- and N-type semi-conductors—and on first examination there certainly

appears to be no P-N junction in the point-contact construction. To be quite

frank about the matter, the exact method of operation of the point-contact diode is

not well understood. A number of fairly involved theories on the subject are too

complex to be reviewed here; but they all boil down to the undoubted fact that there

is something in the point-contact region which works in the same way as does a

P-N junction!

One possible clue may be the fact that N-type germanium diodes using this con-

struction generally operate better after "forming." Forming consists ofpassing a large

pulse or current through the diode. After forming, the point of the "cat's whisker"

is found to be bonded to the semiconductor plate. The heavy current apparently

melts the semi-conductor material in the region of the point; and the rapid melting,

and then cooling, seem to cause a localized conversion of N-type material to P-type

material. A P-N junction is thus formed.

Why this conversion takes place is, as has been said, not easy to explain; but

exacting tests prove that it does. What you should remember are the basic principles

of how a P-N junction is formed; for the method of operation of all semi-conductor

devices can be explained on such a foundation of knowledge.

SIMPLIFIED CONSTRUCTION OF POINT- CONTACT DIODE

|Tjconnectingwire

Plate of N-Type Germaniumor P-Type Silicon ;

6.70

The Operation of a P-N Junction

B2

IHi§iS^rviSiP

RRENT FLOW IN A DIODE VALVE

= electron = hole flowflow

- = electron + = hole

I

4^=-

N-TypeSemiconductorI

t

£-P-TypeSemiconductor

Semi-conductor diodes consist basically of junctions between N and P semi-

conductors. The effects which take place at the junction are in practice equivalent

to the results produced by a diode valve. This can be demonstrated by comparing

the results of connecting a d.c. voltage across a diode valve, and then across a P-Njunction. In the valve, electrons flow from the negative voltage terminal to the

cathode, through the vacuum to the anode, and on to the positive voltage terminal.

In the diagram of the P-N junction shown above, electrons are shown as minus

signs, and holes are shown as positive signs. Holes in the P-type material flow

away from the positive voltage terminal towards the negative terminal, and electrons

in the N-type material flow away from the negative voltage terminal towards the

positive voltage terminal. There thus arrives at the junction a continuous flow

of holes from one direction and a continuous flow of electrons from the other di-

rection.

When the electrons meet the holes at the junction, they neutralize each other's

charge. This permits the formation of more holes at the positive end of the P-type

material, and the entry of more electrons at the negative end of the N-type material.

All the requirements of a continuous current flow are met; and a continuous current

flow does in fact take place.

The direction of current flow in the connecting leads of the semi-conductor diode

is the same as it is in those of the valve. The polarity used for connecting the applied

voltage is known as the "forward bias."

§2] 6.71

The Operation of a P-N Junction (continued)

When the positive and negative terminals of the voltage source are reversed with

respect to those of the previous arrangement, a different set of conditions is created.

In the valve circuit the anode is negative with respect to the cathode. Since the

electrons emitted by the cathode are negatively charged, they are repelled by the nega-

tively charged anode. No current flow takes place in the connecting wires.

In the P-N junction diode, holes in the P-type material are attracted towards the

negative voltage terminal, and electrons in the N-type material are attracted towards

the positive voltage terminal. This biasing arrangement, therefore, does not facilitate

the flow of current carrying holes or electrons to the junction; and in theory no

current flow can take place in the connecting wires.

In practice, however, a very small amount of current does flow through the con-

necting wires. The reason is that N-type material does contain a small number

of holes, and P-type material a small number of electrons. These charges are able

to flow in the direction required to maintain a steady current flow, such as was

described on the previous page.

The existence of these stray charges is not due to a defect in the manufacturing

process, but is caused by the breakdown of a few bonds in the crystal structure

under the stress of thermal agitation. As the temperature increases, so the number

of these stray charges—and hence the current—also increases.

Since the polarity used for connecting the applied voltage in the condition described

above is opposite to that used in the forward bias condition, this method ofconnection

is known as "reverse bias."

,6.72

Characteristics of Semi-conductor Diodes

[§2

It has been shown that the amount of current flow through a semi-conductor diodedepends upon the polarity of the biasing voltage. It is now necessary to find outother details of the relationship between current flow and biasing voltage. A com-parison with the corresponding valve characteristic will again help to clarify thecurrent flow characteristics of a semi-conductor diode.

The IaVa characteristic of a typical diode valve is illustrated above. Also shownis the circuit for obtaining this characteristic.

You will remember that in a diode valve the heated cathode emits electrons, whichcollect in a space charge around the cathode. When the anode is made negativewith respect to the cathode, no current flows from the cathode to the anode becausethe negative anode repels the electrons. Current cannot flow from the anode tothe cathode since the anode does not emit electrons.

When the anode and cathode are at the same potential, the anode neither attractsnor repels electrons; the current can be considered zero. When the anode is madeslightly positive with respect to the cathode, a small portion of the electrons areattracted out of the space charge, and flow to the anode and through the outsidecircuit. As the anode is made increasingly positive, so the current flow becomeslarger.

Eventually the current flow is so large that electrons are attracted to the anode asfast as the cathode can emit them. Further increase in anode voltage causes nofurther flow of anode current, and a state of saturation is reached.

§ 2] 6.73

Characteristics of Semi-conductor Diodes (continued)

When the same procedure is used to study the voltage and current characteristicsof a semi-conductor diode, somewhat different results are obtained. Shown belowis a typical voltage-current curve for a junction diode. Examination shows that it is

quite different from the curve of a typical diode valve; but the same general type ofrectifying action nevertheless takes place.

Consider the characteristic curve of the junction diode. When voltage is appliedin the forward direction, the current varies as shown by the solid-line curve. Notethat only a few tenths of a volt are required to cause a current flow of the order of100 milliamperes. Further increase in forward voltage causes a current rise that is

almost linear in relationship to the applied voltage, and the maximum rated currentis reached before one volt is applied.

When voltage is applied in the reverse direction, the current varies as shown bythe dotted-line curve. Large increases in voltage cause only very small rises incurrent. In fact„ the current rise is so small that a different set of graph scales areneeded to show the change. The currenrflow is extremely small, because there arevery few current carriers under reverse bias conditions; and once all of these currentcarriers are flowing, a state of saturation exists.

This state of saturation does not continue indefinitely. Eventually, a condition isreached where the diode resistance drops very rapidly, and a very large reversecurrent increase takes place with no further increase in reverse voltage. This largecurrent increase is capable of destroying the junction itself.

The reverse voltage at which this effect takes place is called the "Zener" voltage,named after the man who predicted the effect.

The Zener effect is of practical importance only in certain special applications inwhich precautions have to be taken to conduct away the heat generated by the cur-rent, and to limit the reverse voltage.

6.74 B2

Characteristics of Semi-conductor Diodes (continued)

Shown below is the voltage-current curve of a typical point-contact diode. The

curve has many similarities to that of the junction diode considered on the last page;

but there are several significant differences to be observed.

Firsts the rated current flow in the forward direction is only a small fraction of

that obtainable from the junction diode. The reason is that the active junction area

in the point-contact construction is much smaller than is that in a junction diode.

Second, the reverse current flow is several times larger than is that of the junction

diode. In addition, the reverse current increases steadily with reverse voltage, and

there is no sharp saturation effect as in the case of the junction diode.

Third, a different effect is obtained as the reverse voltage is increased. Instead

of the Zener effect described on the last page, there is a "turnover" effect. At the

turnover voltage the internal resistance of the junction appears to become negative,

instead of merely dropping to zero. Therefore, the current increases very rapidly,

and continues to rise even though the reverse voltage is lowered.

No satisfactory explanation has been put forward for this effect; and it is not

useful in practical applications, since the diode is destroyed when the effect takes

place.

+2

VOLTS

§2]

Commercial Semi-conductor Diodes

6.75

TYPICAL CONSTRUCTIONSCOMMERCIAL

SEMICONDUCTORDIODES

Illustrated above are a number of various types of semi-conductor diodes avail-able from commercial sources. It can be seen that there is a wide variety of physicalconstructions available. Included are ceramic jackets with metal ends, glass tubeswith metal ends, all-glass jackets, plastic cases, plastic-coated metal cases and metaljackets with a screw mounting.

Some ofthese outward variations arise mainly from the preferences of the individualmanufacturer. Others have a specific function, such as the screw-mounting whichmay be employed to dissipate the heat generated by power rectifiers.

Although it is not always obvious without close examination, many semi-conductordiode cases are marked with an arrow. The arrow shows the direction of easy cur-rent flow as indicated by a d.c. milliammeter. Alternatively, the "anode" of thediode may be marked with a red spot.

The reason for these markings is that they give technicians and repairmen a reliablemethod ofchecking the connections required.

6.76

Applications of Semi-conductor Diodes

[§2

RECTIFIER-TYPEAC VOLTMETERSTWO RECTIFIERS

c=4H_ftFOUR RECTIFIERS

Semi-conductor diodes can be used in most applications for which metal rectifiers

or diode valves are employed. The advantage of using a semi-conductor diode as a

replacement is that it is generally smaller and more efficient, and that it operates at

significantly higher frequencies than do the valve or metal rectifier. Nor is any

filament power required as it is in the case of the valve.

The most elementary semi-conductor diode circuit is one you met in your study of

a.c. meters in Part 3 of Basic Electricity. This circuit makes it possible for a basic

d.c. voltmeter circuit to be used to measure a.c. voltage.

The simplest arrangement contains a resistor, a rectifier and a d.c. meter movement.

Electron flow (indicated by the black arrows) passes through the meter movement

and causes the pointer to move up-scale. This electron flow results from one half-

cycle of the a.c. voltage. The electron flow resulting from the alternate half-cycle

of the a.c. voltage is shown by the white arrows.

Although only pulses of current flow through the movement, the pointer cannot

move rapidly enough to follow the rise and fall; and the average value of the current

pulses is indicated. The resistor is often made adjustable so that the scale can be

calibrated. If a semi-conductor diode is used as the rectifier, the meter can be

calibrated at mains frequencies and will give accurate voltage readings, without a

correction factor, over a wide frequency range.

The a.c. voltmeter circuit considered above presents a low resistance to one half-

cycle of the applied voltage and a high resistance to the alternate half-cycle. This is

of no consequence in measuring voltages in power circuits. In a.f. and r.f. circuits,

however, this lack of uniform loading may cause inaccurate readings and disturb the

operation of the circuit.

By the addition of a second rectifier to the circuit, however, the halfoycle which is

not being used is afforded a low-resistance path around the meter, aiid fairly uniform

loading is achieved.

A bridge circuit of four rectifiers can also be used, as shown in the diagram, so that

both half-cycles of the a.c. current flow through the meter in the same direction.

This results in a balanced load to both half-cycles of current.

§2] 6.77

Applications of Semi-conductor Diodes {continued)

Other applications of semi-conductor diodes include their use in power supplycircuits. In such applications semi-conductor diodes have the advantage of beingrobust, long-lived, small in size and capajble of large current output. They are often

preferred to metal rectifiers because, although they are smaller and more efficient,

there is little difference in cost.

Half-wave and bridge rectifier circuits employing semi-conductor diodes may beused in power supply units. A resistor is usually connected in series with the semi-conductor diodes, to prevent the excessive current flow which could occur in the eventof a short circuit in the equipment to which the p.s.u. is connected. Either an RCor LC filter would be placed between the rectifier and the load.

Also available for use is the voltage doubler circuit shown in the diagram. Thiscircuit was explained in detail in Part 1 of Basic Electronics, and only a brief reviewis needed here. The voltage doubler consists of two half-wave rectifier circuits.

During one half-cycle of the mains voltage the upper diode conducts, and chargesthe upper capacitor to peak mains voltage. During the alternate half-cycle the lowerdiode conducts, and charges the lower capacitor to peak mains voltage.

Since the two capacitors are connected in series across the d.c. output terminals,

the d.c. output voltage is equal to twice the peak of the mains voltage. With a240-volt mains supply, the d.c. output voltage is in fact about 680 volts.

^^^^ electron flow for

one half-cycle of line voltage

£ electron flow for

alternate half-cycle of line voltage

240 V. AC -4—* (34-OV. Peak) ^— J340V

I

J ^2 ? VW\r-t_ _ui

-1—OHT-J

DC output(approx. 680V.)

6.78 [§2

Applications of Semi-conductor Diodes (continued)

In receiver circuits, the semi-conductor diode can be used efficiently either as a

mixer or as a detector.

Shown in the diagram below is a simple type of semi-conductor diode mixer.

Although this type of mixer can in theory be used in either the H.F., V.H.F., U.H.F.

or S.H.F. bands, it is not often used in practice in the broadcast or television bands,

since its gain is less than one; and significant gain can be obtained by means of valve

or transistor mixers. At microwave frequencies, however, the semi-conductor

diode mixer operates efficiently where other circuits fail.

When the circuit is in operation, the local oscillator applies a constant voltage to

the rectifier. The result is a constant flow of current through the semi-conductor

mixer—a current flow consisting of uni-directional pulses at the frequency of the

local oscillator. Also applied to the mixer is the incoming r.f. signal from the aerial.

Heterodyning action takes place just as in a standard mixer circuit, and the output

of the mixer consists of many different frequencies: including the frequency of the

r.f. signal from the aerial, the local oscillator frequency, the sum of these incoming

signals and the difference between these incoming signals. As in the case of a

standard mixer, the i.f. transformer is tuned to the difference frequency only; and

amplification of the modulated signal takes place at this i.f. frequency.

In detector applications, the semi-conductor diode circuit is essentially similar

to the diode valve detector circuit. When the amplitude-modulated i.f. signal is

rectified, the result is a pulsating uni-directional current which carries both an i.f.

signal component and an audio signal component. The i.f. signal component is by-

passed to earth by a capacitor which is too small to bypass the audio signal component.

The result is that the audio signal component is applied to the input of the audio

amplifier, and detection has taken place.

§2]

REVIEW of Semi-conductor Diodes

Sena-conductor Materials. Puri-

fied crystalline germanium and silicon

are the basic materials commonly

used in semi-conductor diodes and

transistors. These materials are ex-

cellent insulators because their

crystalline structure bonds all the

outer electrons in place.

6.79

N-Type Semi-conductor. Semi-

conductor material can be made to

conduct by adding impurity atoms

which enter the crystalline structure,

but which have excess outer electrons

not bonded to the structure. Current

flow is conducted by the excess nega-

tively charged electrons flowing

through the crystal to the positively

charged terminal.

N-TYPE

SEMICONDUCTOR

P-Type Semi-conductor. Conduc-

tion can also be obtained by adding

impurity atoms which lack sufficient

outer electrons to fill all the crystal

bonds. The unfilled spaces or de-

fects are known as "holes," and

have the characteristics of positive

charges. An applied voltage causes

the holes to flow through the crystal

to the negatively charged terminal.

P-TYPE SEMICONDUCTOR

Junction Diode. A junction diode

consists of P- and N-type semi-con-

ductor materials in close contact

The junction can either be formed

during the crystal growing process

[grown junction], or by a dissolving

and recrystallization process [alloy-

junction].

Cut Germanium BarGrown N-Tjpe Germanium HodGrown P-T>pe Germanium Rod

Pulling andRotating Force

6.80

REVIEW of Semi-conductor Diodes (continued)

[§2

Point-contact Diode, A point-con-

tact diode consists of a plate of N- or

P-type semi-conductor material in

contact with a pointed metal wire.

The contact region can be regarded

as a P-N junction.

Plate of N-Type Gormamuuior P-Type Silicon

Forward Bias. The P-N junction

biasing arrangement shown is knownas "forward bias." Only a small

voltage is required to cause all holes

and excess electrons to flow to the

junction, and so give maximum rated

current flow.

\

1

5: jru

t

N-Type\

Semiconductor—

*

I*.P-TypeSemiconductor

Reverse Bias. When the junction

biasing connections are the reverse

of forward bias, nearly all holes and

excess electrons flow away from the

junction, and so do not enter into a

continuous current flow. Only stray

holes and electrons can enter into a

continuous current flow. High volt-

ages are required to enable them to

do so, however, and the maximumcurrent is only a small fraction of that

obtained with forward bias.

Stray Hole Flow

N-Type Semiconductor

Stray Electron Flow

P-Type Semiconductor

Semi-conductor Diode Applica-

tions. Semi-conductor diodes can be

used in most applications for which

vacuum valves or metal rectifiers are

also suitable. Such circuits with

which you are familiar include meter

rectifiers, power supply circuits,

mixers and detectors.

§ 3 : TRANSISTOR CONSTRUCTION ANDOPERATION

Basic Construction—Point-contact Type

6.81

SIMPLIFIED CONSTRUCTION DIAGRAMOF POINT-CONTACT TRANSISTOR

PROTECTIVE CASE

CAT WHISKER

BASE

Emitter

Emitter Base CollectorAAAELECTRODES

"Collector

ELECTRICAL S^ MBOL

Transistors to-day are oftwo basic types—the "point-contact" type of construction,and the "junction" type. Both of these types have a number of variations; but onlythe basic construction of each is discussed below—the details given being sufficientlyfundamental to cover any production differences introduced by various manu-facturers.

The point-contact construction is the earliest, but is no longer in widespread use.The arrangement is similar to that of a point-contact diode, but with a second "cat'swhisker" in contact with the germanium block. The point-contacts of these two"cat's whiskers" have to be kept separated a few thousandths ofan inch apart, other-wise the transistor will not work.As in the case of the point-contact diode, you must assume (as you quite validly

can) that there is a P-N junction in the region of each "cat's whisker" point.The germanium block is known as the "base," since it is the foundation of the

transistor. The base is exceedingly small, all its dimensions being of the order of afew hundredths of an inch only. The material of which it is made is almost alwaysN-type. The use of P-type material is theoretically possible, but has never been verysuccessful in practice.

One of the contact wires is known as the "emitter," the other as the "collector."The names are derived from the functions of the two wires at their points of contactwith the base. When proper voltages are applied, the emitter causes the generationof current-carrying charges at its contact point. The collector accumulates current-carrying charges at its contact point, and both provide terminals for conductingelectric current through the outside circuit.

Also shown in the diagram above is the electrical symbol used at present torepresent a transistor.

6.82

Basic Construction—Junction Types

The junction-type transistor also consists of a

base, an emitter and a collector.

The two basic forms of it which are in general

use are made in much the same way as were the

"grown-junction" and the "alloy-junction" types

of junction semi-conductor diodes. In both

cases the result of the manufacturing process is

essentially the same—two P-N junctions are

formed, and located some thousandths of an inch

apart.

The diagrams on this page illustrate "grown"

and "alloy" junction transistors. Note that in the

grown-junction type the semi-conductor materials

may be arranged either in a P-N-P or in an

N-P-N sequence.

Transistor manufacturers are continuously trying

to achieve greater product uniformity, plus speed

and economy of production. Present efforts are

concentrated on methods of producing P-N

junctions of easily controlled size and spacing.

The alloy junction method shows great promise,

and automatic machinery is being developed to

control precisely and speed up all stages of the

manufacturing process.

Another method under development starts with

the electrolytic etching away of two spots on

opposite sides of an N-type germanium plate

until the etched surfaces are only some ten-

thousandths of an inch apart. Further electro-

lytic action is then used to plate an indium spot

on to each etched surface. The result is a "sur-

face-barrier" transistor in which an indium

collector and an indium emitter are separated by

several ten-thousandths of an inch, of N-type

germanium.

Although a complex "surface-barrier" theory is

used to explain the operation of this type of

transistor, it is simpler to realize that the contact

area between the indium and the germanium has

characteristics almost identical to those of the

contact-area of a P-N junction; for the method

of operation of this transistor can be quite validly

explained on that basis.

(§3

SIMPLIFIEDCONS TRUCTION DIAGRAMSOF JUNCTION TRANSISTORS

GROWN-JUNCTIONTRANSISTORS

C"V.Base "^CollectorEmitter

Plated IndiumEmitter

M.'RFACE-BARRILR'RANSI.s I'OR

Emitter.

PlatedIndium "»

Collector

Collector

§3]

Operating Principles—N-P-N Transistor

You have already seen that two

different basic arrangements of

semi-conductor material can be

used in transistor construction.

There can either be a sequence in

which P-material is located be-

tween surfaces of N-material, in

an N-P-N arrangement; or there

can be N-material located be-

tween surfaces of P-material, in a

P-N-P arrangement. In either

case the transistor is made up of

two junctions of N- and P-type

semi-conductors situated very

close to one another.

6.83

TRIODE CIRCUIT

Either of these arrangements can be usedto amplify an electrical signal. The operation

of an N-P-N transistor will be described first,

since its operation most closely resembles

that of the triode valve you learnt about in

Part 2 of Basic Electronics. In the explana-

tions that follow it is important to note that,

whereas triode operation is under the control

of the signal voltage applied to its input, andthere is no grid current flow under ordinary

conditions of operation, transistor operation

depends on signal current flow through its

input circuit.

The circuits used to compare valve andtransistor operation are both shown on this

page. Two voltage sources, Vx and V2, are

connected across the elements of the valve

and of the transistor, and appropriate vol-

tage and current meters are connected into

the circuit to measure the results.

N-P-N TRANSISTOR

CIRCUIT

N

N

fS

Collector

Base

-Emitter

-JrV,

6.84 [§3

Operating Principles—N-P-N Transistor (continued)

In the triode circuit, electrons flow in the direction shown below. They flow from

the negative cathode, through the retarding negative electric field of the grid, to the

positive anode, and through the outside circuit back to the cathode. The flow of

electrons through the outside circuit can be increased by making the grid less nega-

tive, thus reducing the effectiveness of the grid in retarding the flow of electrons from

cathode to anode. Similarly the flow of electrons through the outside circuit can be

decreased by making the grid more negative, thus increasing the effectiveness of the

grid in retarding the cathode-to-anode electron flow.

Amplification is obtained because a very small change in grid voltage causes a

change in anode current. The anode current can be passed through a large resistor,

and the change in anode current will cause a large change in the voltage drop across

the anode resistor. Thus, a small change in grid voltage produces a much larger

change in anode voltage, and the result is signal voltage amplification.

Amplification can also be obtained by passing the anode current through a step-

up transformer. In this case the change in anode current can be used to produce a

large signal voltage at the output terminals of the transformer secondary winding.

In a Class C valve power amplifier, the grid bias is such that the input signal can

drive the grid positive for part of the signal cycle. Since a positive grid attracts

electrons, there is a flow of current in the grid circuit; and power is consumed from

the source supplying the signal. The circuit is known as a power amplifier, because

a small amount of input power can be used to control a large amount of output

power.

This latter type of valve operation more closely resembles that of a transistor.

HighAnodeVoltage '^m^KEh

Direction of Electron Flow +—

Highly Negative Grid= Small Electron Flow= High Anode Voltage

• - ••• «„*>

Slightly Negative Grid= Large Electron Flow« Low Anode Voltage

§3]

Operating Principles—N-P-N Transistor (continued)

6.85

:&S&S*V" _

Consider now an N-P-N transistor circuit, and examine what happens when theemitter-to-base variable resistor is set to the "off" position. Now, only the collector-

to-base voltage is applied across the transistor. And a look at the polarity of thevoltage across the P-N junction between the base and the collector will show youthat it is connected in the manner of a semi-conductor diode biased in the reversedirection. The positive terminal of the voltage source attracts the negatively chargedelectrons in the N-type collector, and the negative terminal of the voltage sourceattracts the positively charged holes in the P-type base.

None of these current carriers can combine at the junction, as they do in the caseof the forward biased semi-conductor diode. The result is that the only currentflow is that caused by the stray holes in the collector and the stray electrons in thebase, just as it is in the case of the reverse biased semi-conductor diode. Under theseconditions the current indicated by current meters A2 and A3 will be very low,say about 0-01 mA for example.

Now consider what happens when V2 is disconnected, and the variable resistor inthe emitter-base circuit is set to the position of highest resistance. The emitter andthe base are now connected as is a semi-conductor diode biased in the forwarddirection. The electrons in the emitter and the holes in the base are attracted to-wards the junction, where they combine to maintain an appreciable current flow.As much as 0-1 mA may be indicated on both A t and A2 with the variable resistor

set to maximum resistance. When the variable resistor is set towards minimumresistance, the current flow through A x and A2 may increase to 1 mA.

6.86

Operating Principles—N-P-N Transistor (continued)

B3

= small electron flow

<3 = small hole flow

^1= large electron flow

/ 1 = large hole flow

The conditions change significantly when both voltage sources are connected

simultaneously. If the variable resistor is set so that A\ indicates 1 mA, A?, will

indicate approximately 0-98 mA, and A2 will indicate approximately 0-02 mA.There has been no increase in the voltage applied across the base and collector.

With no current flow in the emitter-to-base circuit, A$ indicated 0-01 mA; but, with

current flowing in the emitter-to-base circuit, the current through A3 is almost

identical to that flowing through A\.

The reason for this can be seen by examining the electrical conditions in the region

of the base. Because of the forward bias conditions between the emitter and the

base, there are a large number of free electrons in that region. Because the base is

so thin—only some thousandths of an inch thick—electrons penetrate through

the base structure and come under the influence of the positively charged collector

before they can combine with the holes in the base. Once the electrons are in the

collector, they rapidly flow to the positive terminal of V2 and on through the out-

side circuit.

Since a small portion of the electrons from the emitter do combine with the holes

in the base, however, there is a small base-to-emitter current. If the base were

thicker, nearly all the emitter electrons would combine with the base holes; and the

result would be a large base-to-emitter current and a small base-to-collector current.

The general rule is that the emitter electrons are divided into the two current flows

shown, and the proportions of the division are determined essentially by the thick-

ness of the base and by the base-to-collector voltage.

§33 6.87

Operating Principles—N-P-N Transistor (continued)

You must now meet a new technical term. In transistor circuits, "current gain"

or "alpha" (a) is defined as "the output current change divided by the input curreltt;

change, with the collector voltage kept constant."

Now you saw that in the transistor operation described on the last page, 1 mA of

current change in the input circuit (emitter-to-base) was required to produce 0-97 mA(i.e. 0-98 minus 0*01) of current change in the output circuit (collector-to-base). In

other words, the current gain was 0-97, or less than unity. This is true of junction

transistors in general; alpha falls in the range 0-95-0-99, and the current gain for this

type of circuit is always less than 1.

You may now ask what is the point of a method of transistor operation (such as

was described on the last page) which produces no useful current amplification. Theanswer is that it produces instead very significant gains in voltage and power.

Look at the current and resistance conditions in the input and output circuits.

You saw that the current in the two circuits is almost identical; but the resistances

in the two circuits are enormously different. The bias across the emitter and base is in

the forward direction, giving the junction between them a low resistance—suchresistances generally range from 40 to 800 ohms. But the bias across the base andcollector is in the reverse direction, giving the junction between them a high resistance

—such resistances generally range from 100,000 ohms to 1 megohm.In Basic Electricity you learnt that the voltage developed across a resistance is

equal to the current multiplied by the resistance (E=IR), and you also learnt that the

power developed in that arrangement is equal to the square of the current multiplied

by the resistance. Since almost identical currents flow in the input and output cir-

cuits, and since the output circuit resistance is in the order of a thousand times higher

than the input circuit resistance, it follows that voltage and power gains in the region

of a thousand times have been produced.

0.98 mA

0.02 mA

1.0 mAfa

-ir-V,

6.88 [§3

N-P-N Transistor—Voltage and Power Gains

A simple calculation will show how voltage and power gain are produced, and will

reveal also some useful relationships and fundamental terms.

The voltage gain in any amplifying device is:

VOLTAGE GAIN =Output Voltage

Input Voltage

iilsflimmmm

I out x R out

I in x R in

Since /out/^in (current gain) has already been denned as alpha (a):

iVOLTAGE GAIN = a x

R out

R in i

Thus a typical transistor, connected in the manner shown, can achieve a voltage

gain of about 2,000 times. This gain is not due to any current amplification. It is

entirely due to the high resistance in the output circuit compared with the low re-

sistance in the input circuit. Amplification is achieved because the semi-conductor

arrangement has transferred a current, with almost no loss, from a low resistance

circuit to a high resistance circuit.

This transfer through resistance is the reason why these devices are known as

"transfer resistors," or "transistors."

§3] 6.89

N-P-N Transistor—Voltage and Power Gains (continued)

A similar calculation shows how power gain is achieved by the transistor. Thepower gain in any amplifying device is:

= .0604 x 2000

= 1020.8 Times

Since the ratio between output and input resistance (i?out/^in) is so frequently usedin voltage and power calculations, this ratio is sometimes called the "resistance

gain" (Rg). In junction-type transistors, the resistance gain falls within the rangeof 500-5,000 times.

Additional useful relationships can be learnt from direct use of the current andresistance gains

:

6.90 [§3

Operating Principles of the P-N-P Transistor

PENT FLOW IN COMPLETE-P TitANSISTOR CIRCUIT

All you have just learnt about the N-P-Ntransistor applies equally to the P-N-Ptransistor. The magnitudes of the emitter,

base and collector currents are the same; and

t\^jy ^ the same relationships exist for the current,

0.98 mA ^ voltage, resistance and power gains.

The major difference to be noted is that in

the case of the N-P-N transistor, the major

part of the current flow through the unit is

caused by the movement of electrons; while

in the case of the P-N-P transistor, the major

part of the current flow is caused by the

movement of positively charged holes.

Because of this difference, it is necessary

to reverse the connections of the voltage

sources in the P-N-P transistor, to obtain

the desired current flow.

The diagram shows a P-N-P transistor

connected in an arrangement equivalent to

that already considered. At the junction

between the emitter and the base, the bias is

in the direction of easy hole current flow, and

holes flow into the base with very little

resistance. At the junction between the base

and the collector, however, the junction is biased in the reverse direction, and there

is a very high resistance to the flow of the free current carriers which are normally

present in the base and collector.

However, since the thickness of the base is only some thousandths of an inch, many

of the holes from the emitter penetrate through the base before they can combine

with the free electrons in the base. Once these free holes come under the influence of

the collector, they are attracted towards the negative terminal of the collector voltage

source.

In practice, about 98 per cent of the holes from the emitter flow into the collector,

and only about 2 per cent of the holes are "caught," as it were, in the base. Except

for the reversal of the voltage sources and current flows, therefore, the same relation-

ships exist as for the N-P-N transistor already considered.

Note that all commercially available point-contact transistors operate according

to the principles described for the P-N-P transistor. An unexpected feature of these

point-contact transistors, however, is that actual current gains (a) of up to 3 times

or more are commonly achieved. No satisfactory explanation for this has been

found, and the various theories are too complex to be gone into here. In any event,

although the current gain is of some minor assistance, the resistance gain remains

by far the major factor in determining the voltage and power gains.

In a typical point-contact transistor, the resistance gain is between 65 and 70; and

typical voltage and power gains are approximately 175 and 400 respectively.

» = small electron flow

O = small hole flow

^is large electron flow

£ZZ3 = large nole flow

§ 33 6.91

Transistor Current Amplification

The circuits just described have shown how voltage and power amplification can 4

be achieved by means of a transistor. Current amplification also can be obtained,and the general method used to accomplish this will now be described.You will recall that, in considering transistor operation, it is often helpful to com-

pare the emitter, base and collector with the cathode, grid and anode, respectively,of a triode valve. The circuit you have just been studying was one in which currentchange through the collector was caused by changing the current flow through theemitter. Such a circuit is called a "common-base" or "grounded-base" circuit.

In a triode valve, this would correspond to changing the anode current by changingthe cathode current.

With a junction transistor, however, it becomes possible to change the collectorcurrent by changing the current flow in the base circuit. (In terms of a triode valvecircuit, this corresponds to varying anode current by changes in the grid current.)Now you saw that, in the last circuit you studied, the base current generally amounts

to less than 5 per cent of the total current through the emitter and collector circuits.So, by increasing or decreasing the current through the base circuit alone (i.e. throughA2 in the P-N-P transistor diagram below), it becomes possible to obtain much largercurrent changes in the collector circuit. The reason is that the flow of holes throughthe base-emitter junction depends on the existence of a forward bias in that region.If there is no current flow in the base circuit (through meter A2), there can be no for-ward bias at the base-emitter junction; and there will be no current flow in the emittercircuit. When the current flow through A2 increases, however, this indicates thepresence of an increasingly strong forward bias at the base-emitter junction; and boththe base and emitter currents will increase greatly. (Except for the change in thedirection of the bias, the same conditions apply to the N-P-N transistor.)

The type of current gain produced in this manner is known as "beta" OS), or a',

current gain, p can be defined as "the change in collector current divided by thechange in base current, with the collector voltage kept constant." Values forbeta are from 25 to 100; and high beta values are always associated with high alphavalues.

A small change inemitter-base current.

^ » small electron flow

O = small hole flow

^1= large electron flow

<Q I = large hole flow

. causes a largecurrent change in

N emitter-collectorcircuit—

t

6.92

What Transistors Look Like

»3

•615

MAZDAXAIIIXAIB

475

SEMCONDUCTORSSBIOSBKOSBKD3

MULLARDOC44OC45OC70OC7IOC75

•600

•470"

PHLCO2N223

5K>

•6QO

The illustrations on this page show the shape and dimensions of a number of

transistors, the internal construction and basic operating characteristics of which

have already been described. On every drawing, one dimension is indicated in inches,

so as to give you some idea of the relative sizes.

You will see that there exists a variety of shapes, sizes and arrangements of ter-

minals and leads. Except for the fact that those intended for power amplification

are larger, and sometimes have flanged bases for conducting heat away, the variations

in external characteristics depend largely on the preferences of individual manu-

facturers.

The way to identify the connecting leads is explained later on, in the Section dealing

with fault-finding in transistor circuits.

§3]

Care and Handling of Transistors

6.93

PROTECT TRANSISTORS FROM.

Excessive Voltage *\^ '/»>«

Bright Lights

Moisture IncorrectBias Connections

You must now learn the most important rules for the care and handling of tran-

sistors. Failure to follow these rules may make a transistor fail in operation, or

may considerably alter its characteristics.

1. Unless a transistor is of the hermetically sealed type, it should not be used (or

stored) in a damp place.

2. Do not drop transistors, or subject them to unnecessary mechanical shock.

Although transistors will withstand considerable vibration and shock whenmounted in equipment, rough handling can often damage them.

3. Some semi-conductors are sensitive to light. The fact that some transistors maybe encased in a transparent casing does not necessarily mean that they will not be

damaged if exposed to strong light. If such units must be used in brilliant light,

they should be shielded by a covering of black tape or of other suitable material.

4. Before installing a transistor in any circuit, check the manufacturer's data sheet.

Be sure to identify the emitter, base and collector terminals. Check the bias and

other operating requirements, and make sure that the maximum limits will not

be exceeded in the circuit.

5. Always switch off the power before making or breaking transistor circuit con-

nections. This precaution is not only for your personal safety, but also for that

of the transistor. The application of voltage to one or two terminals before it

is applied to the others may damage the transistor.

6. Always check the voltage and polarity of the circuit bias supplies before con-

necting a transistor into the circuit. Previous changes in the circuit may apply

excess or incorrectly polarized bias to the transistor.

7. Sudden application of voltage, in previously unchecked circuits, may damage a

transistor. When the circuit contains controls for varying the bias, set these

controls for minimum forward bias (emitter to base) and maximum reverse bias

(collector to base) before connecting the transistor. After the connection is made,

change the biases slowly to the required operating point.

8. Remember that a transistor is sensitive to heat. Do not put a transistor in hot

places, or next to hot circuit components. When soldering the transistor into a

circuit, use a heat shunt, or pliers with a wide grasping area, so as to conduct

heat out of the lead before it reaches the transistor body. When soldering con-

nections to transistors, do not keep the iron on the joint for long periods.

6.94 §4 : TRANSISTOR CHARACTERISTICS

Common Base Circuit Characteristics

Approximate Constant

Current Supply

CIRCUIT FOR PLOTTINGCOLLECTOR VOLTAGE

Against

COLLECTOR CURRENT CURVES -

COMMON- BASE CIRCUIT

You learnt in Part 2 of Basic Electronics that graphs of anode current plotted

against anode voltage and grid voltage could be used to illustrate the properties of a

valve. Similar curves can be drawn to illustrate the properties of a transistor.

These curves are in practice given you in the manufacturers' data books; but you

could equally well obtain them by using the circuit illustrated above.

Note that for plotting transistor characteristics, the current for both bias supplies

—

and particularly for the input circuit bias—must be kept as near constant as possible.

The reason is that the input and output circuits of a transistor are not isolated from

one another, so that a current change in any one of the elements will affect the current

in the other two.

In making characteristic curve measurements, two of the currents must be main-

tained constant, while the third is varied.

If no reliable constant current supply is available, the next best thing is to use a

high-voltage d.c. supply with the output connected in series with a large value of

fixed resistance. The resistance should be at least 100 times that of the transistor

circuit being supplied.

Thus, if you want to plot the collector current/collector voltage curve in the circuit

above, keep the current in the base-emitter circuit constant by connecting a 10-K

resistor in series with the transistor, and by supplying the bias voltage from a potentio-

meter across a 45-V battery.

§4 6.95

Common Base Circuit Characteristics {continued)

Set the 50-K potentiometer in the base-emitter circuit to give a certain value ofemitter current, say 5 mA, and then vary the collector current by means of the 10-Kpotentiometer in the base-collector circuit. Measure the collector voltage for eachvalue of collector current, and plot the results to give the characteristic curve. Thenplot similar curves for other values of emitter current, and you will get a family ofcurves as illustrated on the left below.

The curves illustrated are typical P-N-P junction transistor characteristics. Notethe resemblance between these curves and the IJVa curves for a pentode valve.

If you examine the curves, you will see that a change in emitter current produces aslightly smaller change in collector current. This indicates & current gain (a) of less

than one.

Further examination of the curves will show that the collector current is not zerowhen the collector voltage is zero. Indeed, to reduce the collector current to zero,you will have to reverse the polarity of the collector voltage.

You can also see that the collector current does not immediately drop to zerowhen the polarity of the collector voltage is reversed. This means that holes go onflowing from the emitter through the base and into the collector even though theremay be a small opposing bias across the base collector junction. The reason for thisis the high concentration of holes in the very thin base.

COLLECTOR VOLTAGE Against COLLECTORCURRENT CURVES FOR TYPICAL P-N-P

JUNCTION TRANSISTOR

COMMON-BASE CIRCUIT

-10 -20

collector voltage

The diagram above, right, shows the characteristics for a typical point-contacttransistor. Note that there is a current gain (a) of about 3, and that collector currentis zero when the collector voltage is zero.

6.96

Common-emitter Circuit Characteristics

B«

In the common-emitter circuit,

your main interest is in the effect

of changes in base current on

collector current. The diagram

below, left, shows the characteristic

curves of a typical P-N-P junction

transistor in a common-emitter

type of circuit. These curves show

variation of collector current

against collector voltage, for

different values of base current.

The circuit used to plot such

curves is shown on the right.

CIRCUIT FOR PLOTTINGCOLLECTOR VOLTAGE

Against

COLLECTOR CURRENT CURVES -

COMMON-EMITTER CIRCUIT

To plot these curves, maintain

the base current at one fixed value,

and vary the collector current;

note the collector voltage at

various values of collector current.

Repeat this for a variety of base

current values, and use the infor-

mation so gained to plot a family of

characteristic curves, as illustrated.

Examination of these curves will

show that very small changes in

base current produce large changes

in collector current. Indeed, it is

not unusual for a 25-jaA change in

base current to produce a 1-mAchange in collector current, giving

a j8 (or a) gain of 40.

§4] 6.97

Leakage Current

Another transistor characteristic usually given in manufacturers' data books is the

IJIe curve. Illustrated below, left, is such a curve for a typical P-N-P junction

transistor connected in a common-base type of circuit.

On the right is a magnified portion of the same characteristic as it approaches the

origin. You can see that there is a small flow of collector current even when the

emitter current is zero. This is known as the collector leakage current (Ico).

5 IO Ie(mA) O 50 KX> Ie(uA)

The value of Ico varies from transistor to transistor, and will also vary for any

individual transistor as its temperature changes. The manufacturer quotes the

value of Ic0 at a given temperature (usually 25°C). Ico may be from 0*5 fiA to 6 jiA.

There is a similar leakage effect in common-emitter-type circuits, as illustrated in

the IJIb curve shown below, left. In this case there is a flow of collector current

when the base current is zero. This leakage current is denoted by l'cm and may be

from 120 fzA to 400.'jxA. Again, I'c0 varies from transistor to transistor; and in

any individual transistor will increase as its temperature increases.

45V

O -IO Ib(uA)

The leakage current occurs because the collector attracts holes from the emitter

across the thin base. It is greater in the common-emitter type of circuit because of

the amplifying effect of the transistor when it is connected in such a way.

In the case of the common-base circuit, Ic0 is small enough to be neglected for most

practical purposes. i'cm however, is too large to be neglected; and it increases

rapidly with temperature. At 45°C, for instance, it may be six to eight times the

value of Veo at 25°C.

This leakage current is always present as part of the total collector current; and it

can interfere with the correct operation of transistor circuits. The effects of leakage

current, and the methods of countering these effects, are dealt with later on.

cos §5 : TRANSISTOR CIRCUITS

Introduction to Transistor Circuits

Now that you have understood some basic facts about transistors, you are readyto learn how transistors can be used in practical equipment.

You will learn about transistor circuits in a sequence similar to that which youfollowed when you were learning about valve circuits. You will begin by comparinga single-stage transistor audio amplifier with the equivalent single-stage valve audioamplifier. You will then learn the methods of coupling transistor stages together to

make up several types of complete audio amplifiers.

Following this, transistor r.f. circuits will be described, and then the completelytransistorized superheterodyne broadcast receiver.

Several times already, you have been invited to compare transistor and valvecircuits. In these comparisons, you learnt that the emitter, the base and the collector

of a transistor could often by usefully thought of as corresponding to the cathode,the grid and the anode of a valve.

Further comparisons of this type will still be helpful to you in bridging the gapbetween transistor and valve circuitry, but a point will fairly soon be reached at

which this comparison ceases to be useful. Better progress can be made at that timeby basing the explanation of more advanced transistor circuits on the knowledgeyou will then have gained about the more basic transistor circuits.

§5] 6.99

Basic Single-stage A.F. Amplifiers

You have already been told of two methods of obtaining the effects of amplification

from a transistor; and the circuits described can be converted easily into practical

audio amplifier stages. There is also a third method of obtaining signal gain from

a transistor, which you will be learning about very shortly.

The valve amplifier circuit with which you are most familiar is the one shown in

the diagram below. In this circuit, the input signal is applied across grid and earth,

and the output signal appears across anode and earth. Actually, because of the

cathode bypass capacitor, the input is effectively across grid and cathode; and the

output appears across anode and cathode. This amplifier is sometimes known as

the "grounded-cathode" circuit. More correctly it should be called the "common-

cathode" circuit, since both anode current and grid current (when present) flow

through the cathode circuit.

A brief review of this circuit will clarify the explanation of its transistor equivalent.

When the input signal makes the grid more positive (or less negative), the anode

current increases. The result is an increase in the voltage drop across the load

resistor, and the anode voltage decreases. When the input signal makes the grid

less positive (or more negative), the anode current decreases. The result is a decrease

in the voltage drop across the load resistor and the anode voltage increases.

Since the change in anode voltage is always in a direction opposite to that of the

grid voltage, there is a 180° phase reversal between the input and output signals.

BASIC VALVE

COMMON-CATHODEAMPLIFIER CIRCUIT

6.100 6*Basic Single-stage A.F. Amplifiers (continued)

The equivalent transistor stage is shown below. It can be seen that the emitter,

base and collector of the transistor correspond respectively to the cathode, grid andanode of the valve circuit. This circuit is known as the "grounded-emitter" circuit,

although it is more correct to call it the "common-emitter" circuit.

The operation of the circuit is the same whether a P-N-P or N-P-N transistor is

used, although the polarities of the bias voltages would need to be reversed (as is

shown in the twin diagram at the foot of the page).

First consider operation with an N-P-N transistor. When a positive-going input

signal drives the base more positive, the forward bias across the base-emitter junction

is increased. More electrons flow from the emitter into the base, and there is anincrease in collector current. This increase in current results in an increased voltage

drop across the load resistor, and the collector voltage decreases (becomes less

positive).

When the input signal becomes negative, some of the positive bias on the base is

cancelled; and the forward bias across the base-emitter junction is decreased. Fewerelectrons flow from the emitter into the base, and there is a decrease in collector

current. This decrease in current results in a decreased voltage drop across the loadresistor, and the collector voltage increases (becomes more positive).

Since the change in collector voltage is always in a direction opposite to that ofthe base voltage, there is a 180° phase reversal between the input and output signals.

There are similar results when a P-N-P transistor is used, but with all the bias

changes reversed. When a positive-going signal makes the base less negative, fewer

holes flow from the emitter into the base, the collector current decreases, and the

collector voltage increases (becomes more negative). A negative-going signal makesthe base more negative, more holes flow from the emitter into the base, the collector

current increases, and the collector voltage decreases (becomes less negative).

The outstanding advantage of this circuit is that it produces higher power ampli-

fication than do the other types you will be considering. Disadvantages are that

the circuit has the greatest tendency of the three to oscillate; and it also has im-

pedance-matching problems which will be considered later.

BASIC TRANSISTORCOMMON-EMITTERAMPLIFIER CIRCUITS

§5]

Basic Single-stage A.F. Amplifiers (continued)

6.101

The second basic amplifier circuit is known as the "grounded-grid," or "common-

grid," circuit in its valve form. In its transistor equivalent, the circuit is known as

the "grounded-base," or "common-base," circuit. This circuit resembles the one

you used to learn about alpha current amplification.

In the N-P-N transistor circuit, when the input signal becomes positive, part of the

forward bias across the emitter-base junction is cancelled. The result is a decrease

in emitter current and a decrease in collector current. The voltage drop across the

load resistor decreases, and the collector voltage becomes more positive.

When the input signal becomes negative, there is an increase in the forward bias

across the emitter-base junction. The result is an increase in both emitter and

collector current. The voltage drop across the load resistor increases, and the

collector voltage becomes less positive.

Since the collector voltage rises and falls with the input signal, there is no phase

reversal between the input and output signals.

The outstanding advantages of this circuit are that it produces the least noise and

has the least tendency to oscillate. The amplification produced is not as high as

that produced by the common-emitter circuit.

With the alterations to the circuit which you have now learnt to expect, the opera-

tion of the P-N-P transistor circuit produces similar results. It is suggested that you

check for yourself that this circuit also introduces no phase reversal between the

input and output signals.

6.102

Basic Single-stage A.F. Amplifiers (continued)

[§5

Basic N-P-NCommon -Collector

Amplifier Circuit

WhiInput

EQUIVALENT

P-N-PCIRCUIT

s$^!!!PM!|

The third basic circuit is known as the "cathode follower" circuit in its valve form.In its transistor equivalent, the arrangement is known as the "grounded-collector,"

or "common-collector," circuit. The collector is connected to earth, from an a.c.

point of view, by the capacitor across the battery.

Take, again, the N-P-N transistor circuit. When the input signal becomes positive,

the forward bias across the emitter-base junction is increased. The result is anincrease in emitter current and an increase in collector current. The voltage dropacross the load resistor (emitter to earth) increases, and the emitter voltage be-

comes more positive.

When the input signal becomes negative, part of the forward bias across the

emitter-base junction is cancelled. The result is a decrease in emitter current and adecrease in collector current. The voltage drop across the load resistor (emitter to

earth) decreases, and the emitter voltage becomes less positive (more negative).

Since the emitter voltage rises and falls with the input signal, there is no phasereversal between the input and output signals.

The outstanding characteristic of this circuit is that, as in the case of the cathodefollower, the voltage gain is always less than one, although power gains of over 30times may be produced.

Also, the circuit has input impedances in the order of 250,000 ohms, while the

output impedance is often below 1,000 ohms. This makes the circuit useful for

matching high impedance sources to low impedance input circuits.

Once again, you can check for yourself that the P-N-P transistor circuit, with oppo-site biases, will give similar effects.

§5] 6.103

Amplifier Circuit Variations

There exist quite a number of variations of the basic transistor amplifier circuits

which you havejust learnt. These variations often arise merely from different methods

of drawing the circuit; but they may, of course, arise also from actual differences in

the number and types of components used, and in the wiring.

The P-N-P transistor circuit diagrams below show how even identical circuits can

be redrawn so that at first glance they look different. But if you will take the

trouble to trace the connections carefully through, you will see that in each case

the diagram on the right represents simply a different layout of the circuit represented

also in the diagram on the left.

DBAWtNCr LAYOUT VACATIONS

COMMON-EMITTER

DRAWING LAYOUT VARIATIONS 3

II t \J£)tHh

J—COMMON-BASE

ii-l—®-_L||.

*T~^ 1"

COMMON-COLLECTOR ^

© ©

6.104 [§5

Amplifier Circuit Variations (continued)

The drawings on this page show five more variations of the common-emitter cir-

cuit shown on the previous page. Similar variations exist also for the common-baseand common-collector circuits.

In the case of diagram 3, although there have been some changes in drawinglayout, the main difference is that transformer coupling, rather than RC coupling,

has been used at the input and output.

In diagram 4, apart from differences in drawing layout, the major change is that onebias battery is used instead of two. In transistor applications this is often done.The two different biases which are required can easily be obtained from a single

battery, either by means of a voltage divider arrangement (as in diagram 4) or bymeans of individual series resistors from the battery to the collector and base (as in

diagram 5).

Diagram 6 shows how the primary winding of an output transformer can besubstituted for the battery-to-collector resistor. Also shown in that diagram is abypassed resistor in the emitter-to-earth circuit. This resistor generates a self-bias

voltage, similar to cathode bias, which maintains the desired operating character-

istics in spite of current variations in the other circuit components.Diagram 7 shows how self-bias can be generated by a resistor connected between

base and collector. The base bias resistor increases and decreases the base current

to the extent required to maintain stable operation.

ADDITIONAL VARIATIONS OF COMMON- EM] BiBBinBaii

TRANSFORMER § ©SINGLE BIASINPUT ANDOUTPUT

BATTERY WITHVOLTAGEDIVIDER

l-itT&

'SINGLE BIAS BATTERY|WITH SERIES RESISTORS|

SINGLE BIASBATTERY WITHSELF-BIASFOR BASE

tt

SINGLE BIAS BATTERYWITH TRANSFORMEROUTPUT

§5] 6.105

D.C. Stabilization

The variation in the common-emitter circuit shown in diagram 7 on the previous

page not only permits the use of one battery to supply both bias voltages. It also

makes it possible to counter the effects of leakage current.

Consider the effect which leakage current (see page 6.97) would have in the basic

common-emitter P-N-P transistor amplifier circuit shown below. The arrows

indicate the direction of electron flow in the circuit. Assume that the circuit is

operating at 25°C, and that the bias conditions have been chosen for correct operation

—that is, with forward bias emitter-to-base, and reverse bias collector-to-base. The

resistor in the base circuit would be about 10-KQ, and that in the collector circuit

about 3-KQ. The batteries would be 1-5 V and 4-5 V respectively.

HI lb

Ic to

INPUT

O

o

Ie OUTPUT

O

At the given temperature of 25°C, Ic in the steady state may be about 1 mA, and

Ib about 20 {iA. These currents flowing in the collector and base circuits respectively

cause voltage drops across the 3-K and 10-K resistors to provide a voltage of — 1-5 Vat the collector and of — 1*3 V at the base. There would thus be a reverse bias of

200 mV between collector and base.

You saw on page 6.97 that Ic includes the leakage current I'co, and that this leakage

current increases rapidly with temperature. Suppose now that the temperature

rises. The value ofFco will increase, and will cause Ic to be increased by, say, 50 (iA.

This will increase the drop across the 3-K resistor in the collector circuit, and will

change the collector voltage to — 1-35 V.

The reverse bias voltage between collector and base will now be reduced to 50 mVand increased base current will flow. This increased base current will in turn cause

a further increase in collector current, and so will further heat the junction.

The initial increase in temperature, and hence of leakage current, will therefore

upset the bias conditions in the circuit; and will start a train of events which could

result in excessive collector current, and possibly even in destruction of the transistor

junction.

6.106 £§5

D.C. Stabilization (continued)

Now consider again the common-emitter circuit (diagram 7 on page 6.104, which

is for convenience reproduced below).

;60K

HF U

INPUT

4-5V

I

OUTPUT

In this circuit, Ic will be of the order of 3 mA, and the collector voltage about 4 V.

The base current flowing through the 60-K resistor gives the reverse bias collector-

to-base.

The amount of base current flowing is proportional to the voltage on the col-

lector. If the temperature increases, and there is an increase in leakage (and hence

of collector current), the voltage drop across the resistor in the collector circuit will

be greater and the collector voltage will fall—i.e., become less negative.

This will reduce the base current, and so counter the effect of the temperature rise.

The d.c. operating conditions of the circuit are therefore stabilized.

This circuit, however, has two main disadvantages:

(a) initially, high collector currents are inevitable; and(b) a feed-back path from output to input is introduced.

The circuit below provides bias from one battery, and also gives d.c. stabilization

without the disadvantages mentioned.

The bias voltage for the base is provided by the resistance divider R1-R2 in con-

junction with the resistorRe in the emitter circuit. Re is decoupled at audio frequencies.

D.C. stabilization is effected as follows. An increase in collector current (caused

by increased leakage) means that the emitter current has increased. This causes

increased drop across Re ; and the voltage at the emitter becomes more negative, so

reducing the forward bias base-emitter. This reduces the base current; and this,

in turn, restores the collector current to its original value.

§5]

Multiple-stage Transistor Amplifiers

6.107

VALVECOUPLINGMETHODS

-*~HT+

Direct Coupling

Although large amounts of power, voltage and current amplification can be

produced by a single transistor stage, the requirements of practical equipment gen-

erally demand even greater amplification between the input and output. Youhave learnt various methods of coupling together, or "cascading," a number of

valve amplifier stages. The same general methods are used in multiple-stage tran-

sistor amplifiers.

As in valve amplifiers, the input stage must be impedance-matched to the signal

input source—the microphone, the aerial or the signal generator, for example; andthe output stage must be impedance-matched to the output load—the loudspeaker,

earphones, transmission line or meter, etc.

Methods which can be used for coupling the amplifier stages to the input source

and output load, or for coupling amplifier stages to each other, include transformer

coupling and resistance-capacitance coupling. The diagram on the left, above,

illustrates these two methods of coupling valves.

Another method of coupling, direct coupling, makes use of a direct connection be-

tween the anode of one stage and the grid of the next stage. In such circuits,

the bias requirements of the second valve are met by maintaining its cathode at anappropriately high d.c. potential. The diagram on the right, above, is of a direct-

coupled amplifier circuit.

6.108

Multiple-stage Transistor Amplifiers (continued)

[§5

In multiple-stage transistor amplifiers, the problem of coupling the various stages

together is complicated by the characteristics of the different types of individual

amplifier stages.

Both the grounded-base and the grounded-emitter types have input resistances of

1,000 ohms or lower. In the grounded-base type, indeed, input resistance may be

as low as 25 ohms. On the other hand, the output resistances of these stages may be

as high as 50,000 ohms for the grounded-emitter type and 500,000 ohms for the

grounded-base type.

If resistance-capacitance coupling were to be used in cascading amplifier stages of

these types, the very large difference between the output resistance of one stage and

the input resistance of the next stage would result in a very large power loss in the

coupling network between the stages. Such losses result in significantly reduced

gain through the overall amplifier.

To make up for these losses, it is sometimes necessary to add one or more stages

to the amplifier.

§5] 6.109

Multiple-stage Transistor Amplifiers (continued)

Although the use of resistance-capacitance coupling results in a large loss in

amplification, it is simpler and less expensive than are the alternatives. For this

reason, several methods have been devised for retaining this type of coupling while

partially overcoming the important problem of mismatch.

Matching the first amplifier stage to the signal source can be accomplished by

selecting a type of amplifier circuit which has an input resistance approximately

equal to the source. When the input signal source has a resistance between 25 and

300 ohms (as is the case with many transmission lines, low-impedance microphones

and gramophone pick-ups) a common-base type of input stage is Used. If the

output impedance of the signal source is in the range between 300 and 1,000 ohms,

either a common-base or a common-emitter type of input stage may be used. But if

the signal source has an impedance in the order of several hundred thousand ohms, a

common-collector type of input stage is used.

A similar technique is used to match the final amplifier stage to the output load.

For load resistances below 5,000 ohms, a common-collector type amplifier is used as

the output stage. For loads between 5,000 and 100,000 ohms, a common-emitter

type output stage is used. For load resistances above 100,000 ohms, the common-base type of output amplifier stage is used.

There exists, unfortunately, no very efficient direct method of transmitting high

levels of signal power to a load of less than 500 ohms. In such cases a transformer

is generally required.

In power amplifiers, a common-collector stage, with its high input impedance

(100,000 to 350,000 ohms) and low output impedance (700 to 25,000 ohms), can be used

to present a fairly good impedance match between stages of the other types. Al-

though the voltage gain is less than one, there is good power gain (about 30 times),

and the mismatch losses are effectively eliminated.

6.110 [§5

Multiple-stage Transistor Amplifiers {continued)

Transformers offer not only the most efficient method of matching transistor

stages to their inputs and outputs, but also a fairly simple one. The diagram below

shows a three-stage, transformer-coupled P-N-P transistor amplifier.

The reason for the efficiency of transformers is that they can be manufactured to

have almost any desired combination of input and output impedances, so that almost

perfect matches can be achieved. Fewer stages, too, are required in a transformer-

coupled amplifier than in a resistance-capacitance amplifier of equivalent gain.

But the use of transformers brings about the disadvantages of greater cost, and

added space and weight; so that, despite the use of mass-production techniques,

and although miniaturized models are widely available, it is a rare transistor amplifier

which uses transformer-coupling exclusively.

INPUT

Two methods of directly coupling transistor amplifier stages are shown at the foot

of .the page. They are used in special amplifiers where the phase shift introduced

by other coupling methods cannot be tolerated. Direct coupling is also used in the

amplification of signals at frequencies too low to be coupled efficiently by the methods

previously described.

Output devices such as meters, relays, earphones and other units can be direct-

coupled by connecting them in series with the output element of the final amplifier

stage. If the device has a high impedance, it can be used in conjunction with the

common-base and common-emitter types; if it has an impedance below 5,000 ohms,

it can be connected in series with the output of a common-collector stage.

Note that in the direct-coupled amplifier illustrated on the left, below, P-N-P andN-P-N transistors are used in conjunction, this resulting in a simplified biasing

arrangement without impedance-matching problems. This method is known as the

"complementary symmetry" circuit arrangement, and further examples of it will be

met in the description of transistorized push-pull audio amplifier circuits.

OUTPUT

LIRECT-rOl'PLLI)

AMPLIFIER

INPUT

§5]

Push-pull Audio Amplifiers

6.111

INPUT

In your study of valve amplifiers, you learnt that the power output needed is

sometimes more than can be produced by a single output stage. When this is so,

the power output can be increased by using two valves in the output stage, the push-

pull arrangement producing the highest power output with the least distortion.

A basic push-pull amplifier of the valve type is shown in the diagram above.

In this circuit, the output signal of the previous stage is applied to the primary of a

transformer. The secondary of the transformer is centre-tapped, and applies equal

and opposite voltages to the grids of the push-pull valves. Because opposite ends

of the transformer secondary are applied to the two grids, the signal voltage on one

grid is becoming more positive while the voltage on the other grid is becoming morenegative. Thus the anode current in one valve is increasing while that in the other

is decreasing.

Because the d.c. anode currents in the output transformer primary flow in opposite

directions, the transformer core is not easily saturated; and high operating efficiency

can be obtained from the transformer.

In audio amplifiers, the power output which can be obtained from a single output

stage employing ordinary transistors is usually limited to about 150 milliwatts.

Such a low output is quite adequate for driving earphones, but is normally considered

insufficient for use with a loudspeaker. By using a push-pull power transistor

arrangement in the final amplifier stage, however, an output of 500 milliwatts can be

readily, obtained with Class B operation. This is quite adequate for driving the

loudspeaker of a portable radio.

With the aid of high-power transistors, outputs of up to 20 watts can be obtained

from a push-pull circuit.

A transistorized push-pull amplifier circuit is also shown in the diagram. It can

be seen that the two circuits are very similar, and the explanation given applies equally

well to both of them.

Note that the operating characteristics of the two transistors must be carefully

matched if efficient operation and low distortion are to be obtained.

6.112 [§5

Push-pull Audio Amplifiers (continued)

Very efficient push-pull operation can be achieved by the use of a circuit with

complementary symmetry. Such circuits are possible because P-N-P and N-P-Ntransistors can be made which have equivalent operating characteristics while de-

manding oppositely polarized bias sources.

Circuits having complementary symmetry can often do without several of the

coupling components usually required. The circuit illustrated below, for instance,

works well without either input or output transformers.

Both transistor bases are driven without phase reversal by the output signal of the

previous stage. The balanced voltage divider arrangement shown at the left of the

circuit keeps the two bases equally and oppositely biased with respect to one another.

The resistors are low in value, so that there is little loss in signal amplitude; andequal signals appear on both bases.

The transistor circuits are of the common-collector type. Since one transistor

is of the P-N-P type while the other is of the N-P-N type, push-pull operation is

obtained even though the bases are driven in phase with one another. When there is

no signal applied, the currents through the load resistor are equal and opposite, andso cancel out. The biasing selected usually cuts off the collector current when zero-

signal is applied, giving Class B operation for maximum power output.

When the input signal becomes positive, the upper transistor does not conduct.

The lower transistor does conduct, however, and current flows from the emitter

through the load resistor and into the battery centre tap.

When the input signal becomes negative, only the upper transistor conducts; andcurrent flows from the battery centre tap through the load resistor to the emitter.

Thus push-pull operation is achieved.

Since a common-collector circuit is employed, a low impedance output is achieved

without an output transformer. The negative feedback obtained through the

common load resistor balances out small differences in transistor operating character-

istics, and precise matching of the two transistors is not required.

§ 6 : THE TRANSISTORIZED SUPERHETERODYNE 6.1,3

RECEIVERIntroduction

RFAmplifier

AFAmplifier

LocalOscillator

SUPERHETERODYNE RECEIVERBLOCK DIAGRAM

Now that you know something about transistor audio amplifiers, you are ready to

go on to transistor superheterodyne receiver circuits. Since you are already familiar

with the valve versions of these circuits, only a brief review of their workings will benecessary.

You will remember that the superheterodyne receiver aerial picks up transmitted

signals and delivers them to the input transformer of the r.f. amplifier. The r.f.

amplifier steps up the amplitude of the signal to which it is tuned, but does notamplify signals at other frequencies. Thus the purpose of the r.f. amplifier is to

add selectivity and sensitivity to the receiver. The r.f. amplifier is often eliminated

in domestic receivers, and the aerial signal is fed directly to the frequency changer

or mixer.

In the mixer, the incoming signal and the output of a local oscillator are mixedtogether. The anode current of the mixer varies according to both of these signals,

which are at different frequencies. A beat, or difference, signal appears at the

output; and this signal is fed to the i.f. amplifier, which is tuned to this difference

frequency.

The i.f. signal has the same modulation as the r.f. carrier, and the" only change is alowering of the frequency of the carrier signal. When the r.f. amplifier and mixer

are tuned to the incoming signal, the oscillator is also tuned. The tuning is so

arranged that the difference signal is always at the same frequency, and the i.f. ampli-

fier can be fixed-tuned.

The i.f. amplifier is permanently tuned to the fixed-frequency signal coming fromthe mixer stage. Since no variable tuning is required, the i.f. amplifier is designed

for maximum amplification and high selectivity. From one to three i.f. amplifiers

are normally included in a superheterodyne receiver, and the intermediate frequency

normally used in broadcast receivers is in the range 450-475 kilocycles.

The detector stage receives the output of the i.f. amplifier, removes the i.f. carrier

signal and leaves only the audio signal. This audio signal is stepped up in ampli-

tude by the audio amplifier, the output ofwhich drives the earphones or loudspeaker.

6.114

The I.F. and R.F. Amplifier Stages

B*

(COMMON-EMITTER CIRCUIT)

The designs of the transistor i.f. amplifier and of the transistor r.f. amplifier are

basically similar—just as is the case with their valve-type counterparts. Once again,

too, the major design problem in both stages is how to achieve adequate gain at high

frequencies. In both cases, this is normally done by using tuned circuits.

The diagram shows a valve-type i.f. amplifier, together with two transistor-type

equivalents.

The first of these transistor circuits is a common-base arrangement. Between it

and its valve-type equivalent, only two significant differences can be seen. First are

the differences arising from the change in bias requirements. And second is the

fact that the emitter circuit is connected to a tap on the secondary winding, rather

than to the end of that winding. The purpose of this is to provide a good impedance

match between the two stages; the high output impedance of the collector circuit is

effectively matched to the low input impedance of the emitter which follows, while

maintaining the normal tuning requirements of an i.f. transformer.

In the second transistor-type i.f. amplifier circuit shown in the diagram above, a

common-emitter arrangement is used. The major difference here is that the im-

pedance match is accomplished in the transformer primary. Instead of being con-

nected to the end of the primary winding, the collector is connected to a tap on the

primary. Also, the secondary winding is untuned, as is sometimes done in valve

circuits.

Transistor-type r.f. amplifiers closely resemble the transistor i.f. amplifiers shown in

the diagram, except that their frequency range is both higher and wider.

Just as is the case with valves, some transistors are designed for use at r.f. and

others for use at a.f.

§6]

The Oscillator and Mixer Stages

Transistor oscillators operate by means of a

positive feedback network, just as do valve oscil-

lators. This arrangement takes a portion of the

output signal and feeds it back to the input circuit.

The phase relationship is such that the input

signal is reinforced, and oscillation takes place at a

frequency determined by the tuned circuit. The

basic principles involved are identical to those

described in Part 3 of Basic Electronics.

Just as there is a wide variety of valve oscillators,

so there are many kinds of transistor oscillators.

There are, for instance, transistorized versions of

the Armstrong and Hartley oscillators, which are

shown in the diagrams opposite, top. Theoperating principles are the same in both cases;

and the only significant differences are those

necessary for maintaining the proper bias on the

various elements.

There are, however, also a number of oscillator

circuits peculiar to transistors. These can always

be recognized in a schematic diagram by the fact

that they all contain a series- or parallel-tuned

circuit with some form of coupling between the

input and output circuits.

Also shown on this page (third diagram) is

the schematic diagram of a typical mixer circuit.

The emitter is frequently biased by means of a

bypassed resistor to earth, and the local oscillator

signal is injected into it by means of transformer

coupling. The modulated r.f. signal from the

aerial or r.f. amplifier is coupled to the base.

The transistor version of a frequency changer

stage is shown in the bottom diagram opposite.

6.115

6.116

Transistor Receiver Operation

K6

The diagram above shows the circuit of a complete transistor-type superhet re-

ceiver—incorporating a frequency changer (OC 44), two i.f. amplifier stages

(2xOC 45), a germanium diode detector (OA 70), an audio-frequency amplifier

(OC 71), and a Class B push-pull output stage (2 x OC 72). The receiver operates

from a 9-V battery, and tunes over the medium- and long-wave bands.

The two OC 72 transistors in the output stage have their bases biased by the two

resistors Ri6 and Rn across the battery supply. Bias is applied to the emitters by

the voltage developed across i?18 . It is important that the two OC 72 transistors

be "matched"—that is to say, that their characteristics be very similar. Drive for

the push-pull output stage is provided by an OC 71 working as an audio-frequency

amplifier transformer-coupled to the push-pull output stage.

Bias is applied to the base of the OC 71 by a practical divider comprising R10

and Ri 3 . Emitter current flowing through R14 and R15 provides the emitter bias.

You will recognize that these components provide the conditions necessary for d.c.

stabilization.

Negative feedback is connected from the secondary of the output transformer,

through Ri9 and C18 , to the emitter of the OC 71. The purpose of this negative

feedback is both to reduce any distortion that may be present, and to flatten the

audio-frequency response curve. The 680-pF capacitor will allow more feedback

at the higher audio frequencies and less feedback at the lower audio frequencies,

thereby compensating for the rising characteristic of the frequency response.

By reason of the low input impedance of the OC 71 when connected in the

common-emitter condition, the value of the coupling capacitor 8 fxF is higher than

that which would be used to couple valve circuits.

RVi serves a dual purpose. It acts as a volume control, as well as providing the

detector load.

§6]

Transistor Receiver Operation (continued)

6.117

The detector consists simply of a germanium diode (OA 70) connected in a half-

wave rectifier circuit, its load being RVx. The decoupling capacitor C12 bypasses

the i.f. component to earth.

The lead from the junction of MRU RVt and C12 which connects to R5 supplies

AGC voltage to the i.f. amplifier. Since P-N-P transistors are used in this stage, the

action of the AGC is to feed back a positive d.c. voltage as bias in order to reduce

the gain of the i.f. stage.

Audio-frequency components present on theAGC line are decoupled to earth by C8.

The detector is fed from the output of the i.f. amplifier, the intermediate frequency

being 470 kc/s. The i.f. amplifier consists of two stages using OC 45 transistors.

Both these transistors have bias voltages applied to base and to emitter, the collectors

being connected to tapped points on the primaries of the i.f. transformers.

The reason for this latter connection is that the output impedance of a transistor

connected in a common-emitter condition is not as high as is the impedance of the

primary tuned circuit of the i.f. transformer at the i.f. frequency. The collector

is therefore connected to an impedance matching point on the primary.

The secondaries of the i.f. transformers are untuned, since this allows a better

impedance match to the input of the transistors. Sufficient gain is obtained from the

i.f. stages, despite their untuned secondaries, because the coils of the transformers are

specially wound on "ferrite cores," and so have high values of "Q."

An RC network, C19-i?2i-Qo-^20» is connected from the secondary of T3 to the

secondaries of Tx and T2 . These are neutralizing components, providing negative

feedback between output and input in order to prevent oscillations in the i.f. ampli-

fier. Since rather high capacitances exist in the transistor (i.e. collector-to-base

and base-to-emitter), neutralization is necessary, even at 470 kc/s, in order to prevent

positive feedback which would cause the amplifiers to oscillate.

The combined gain of the two i.f. amplifier stages is about 60 dB.

6.118

Transistor Receiver Operation (continued)

[§6

The frequency changer uses an OC 44 transistor; bias voltages are applied to bothbase and emitter. A coupling winding, consisting of a few turns in the collector

circuit, couples a portion of the output back to the emitter circuit to maintain

oscillations.

The frequency of the oscillator is controlled by a tuned circuit consisting of acoil, a 163-pF variable capacitor (C7), and a trimming capacitor. To reduce the

frequency of oscillations for long-wave-band operation, an additional 310-pFcapacitor (C5) is switched in parallel with the coil.

The output of the collector is applied to the primary ofTlt where the i.f-of 470 kc/s

is selected and passed for amplification.

The aerial input circuit consists of tuned windings wound on a "ferrite rod," andloosely coupled by low impedance windings to the base of the OC 44. Switching

facilities are provided, to allow reception on either medium- or long-wave bands.

The primary of the long-wave coil is earthed when the stage is switched to medium-band working, in order to prevent any resonance from the long-wave winding fromaffecting the medium-wave winding,

§ 7 : FAULT-FINDING ON A TRANSISTOR 6.119

SUPERHET

The Basic Procedure

The basic procedure for fault-finding on a transistor superhet is no different from

that which you learnt for a valve superhet in Part 5 of Basic Electronics. Once more,

here it is:

1. COLLATE THE SYMPTOMS.2. DECIDE WHICH STAGE IS FAULTY.3. INSPECT THE EQUIPMENT.4. CONFIRM YOUR DEDUCTION BY SIGNAL INJECTION AND

TRACING.5. FIND THE FAULTY COMPONENT BY VOLTAGE, CURRENT

AND RESISTANCE MEASUREMENTS WITHIN THE FAULTYSTAGE.

Steps 1, 2 and 3 are exactly the same as those you applied to the valve receiver, so

they will not be repeated. But you will have to learn how to apply steps 4 and 5 to

the transistor superhet.

Signal Injection and Signal Tracing

First, it would be usual to check both audio stages together, by injecting a signal

from an audio-frequency generator between base and earth of the first audio amplifier

transistor (OC 71). You should connect a d.c. blocking capacitor in series with

the a.f. generator output, in case a d.c. path should affect the bias voltage present

between base and earth of the transistor.

If the audio stages are operating correctly, you will hear a tone from the loud-

speaker; and an output meter connected across the secondary winding of the output

transformer will give an indication.

You can check for distortion by connecting an oscilloscope across the output and

observing the waveform. Some of the ways in which a sine wave appears distorted

are shown in the illustration below.

6.120 K 7

Signal Injection and Signal Tracing (continued)

Next, you can check the operation of the detector stage by connecting an r.f.

signal generator across the secondary winding of the i.f. transformer T3, having first

set the signal generator output to 470 kc/s modulated. If the stage is working, anaudio signal will be heard from the loudspeaker, or an indication noted on an out-put meter.

To test the i.f. amplifiers, keep the signal generator set to 470 kc/s amplitude modu-lated, and connect it across base and earth of each i.f. amplifier transistor in turn.

Either an audio signal from the loudspeaker or an indication on an output meterwill prove that the i.f. amplifiers are working.

The frequency changer can be checked by loosely coupling the signal generator tothe medium- or long-wave band coils on the ferrite rod. "Loose coupling," in this

case, means that you construct a simple loop of wire and place it near one of thewindings on the ferrite rod; you then tune the receiver until the signal generatoroutput is heard. This method of injecting the signal is necessary because directconnection of the signal generator to the tuned aerial coil would connect the lowimpedance of the signal generator across the tuned circuit, thereby damping it andmaking the circuit less sensitive and less selective.

A good overall check on the working of the receiver as a whole can now be made,by tuning the receiver to signals on both medium- and long-wave bands. If nosignals are heard when the signal generator is coupled to the aerial (and provided, ofcourse, that the i.f. and audio stages have been proved to be in working order) youshould next check the oscillator section of the frequency changer.Two ways of doing this are:—

1. Use a valve voltmeter to see whether the oscillator is oscillating; or2. Measure the emitter voltage or current, and note whether the reading changes

when the oscillator circuit is shorted out.

§*] 6.121

Using the Multi-meter in Transistor Circuits

Tests of voltage and resistance will normally be made with a multi-range meter-similar to the one you have used for similar measurements in valve circuits. A fewspecial points about the use of a multi-range meter in transistor circuits should,however, be noted.

1. In P-N-P transistor circuits, the positive battery terminal is connectedto "earth," and the H.T. line is negative. In N-P-N transistor circuits, thenegative battery terminal is connected to "earth," and the H.T. line is positive.

2. The shunting effect of the meter must be taken into account when measuringvoltages; for example, when measuring base-to-earth voltages in common-emitter circuits.

3. When you are measuring emitter voltages in common-emitter circuits, take careto avoid shorting the emitter to earth (chassis). Such a short would cause alarge current to flow in the emitter, and hence in the collector; it might evencause "thermal runaway." Thermal runaway is the term used to describe whathappens when the junctions are heated by excessive current to a temperaturewhich automatically increases the current still higher, and so on until thejunctions themselves are in danger of being broken down.

4. When resistances are being measured between two points connected by parallel

paths, one of which contains a transistor, it may be necessary to disconnect oneof the relevant transistor leads. Failure to do so may give rise to misleadingresistance readings; forthe reason that there are two resistances across a transistor

junction, a "forward resistance" and a "back resistance." Which of these tworesistances is actually being measured at any one time depends on the polarity ofthe multi-range meter leads when the measurement is being made.

5. Transistor circuits nearly always include low working-voltage electrolytic

capacitors. When such components are being checked with an ohm-meter,or with the ohms range of a multi-meter, it is essential that the voltage andpolarity of the meter itself be first checked over the appropriate range. Neverconnect either too high a working voltage, or a voltage of incorrect polarity,to electrolytic capacitors of this kind.

.Working

6.122[§7

A Typical Fault-finding Sequence

Let us now go through a typical fault-finding sequence on a transistor superhet.

Let us say that the symptoms are that the receiver is dead, no signals at all being

heard on either wave-band. . .

The first step is to check the battery voltage. Say it measures 8-5 V d.c. Satis-

factory.

Next, injection of an a.f. signal between the base and earth of TR4 (see diagram

on page 6.116) produces an audio output at the loudspeaker. So the audio stages

are working.

A modulated i.f. signal of 470 kc/s is then applied across the secondary of T3

(see below). An audible output indicates that the detector stage is in order. At this

point you should check the action of the volume control, and confirm that the loud-

ness of the output signal varies with the setting of the control.

Now move the modulated i.f. signal to the base of TRy Once again an audio out-

put indicates that the stage is operating.

But application of an i.f. signal between base and earth of TR2 gives no audio

output. So the fault is in the first i.f. stage, comprising TR2 and its associated com-

ponents.

Had this been a valve circuit, you would next have tried replacing the valve. But

transistors are less likely to be faulty than are valves; and the task of replacing one

of them also takes longer. So your next step should be to check the transistor

voltages.

The collector voltage of TR2 measures 6-2 V d.c, which is satisfactory; but voltage

checks at emitter and base show no voltage at either. No voltage at the emitter

means that there is either no emitter current, or else a short circuit from emitter to

earth. A resistance check from emitter to earth measures 680 ohms; so the emitter

of TR2 is not short-circuited.

Next, therefore, you check the TR2 base circuit. No voltage. Why? Well,

R4 could be an open circuit; or there could be a short circuit to earth from the base of

TR2 .

A resistance check from the base of TR2 to earth shows zero ohms. The 8-(xF

AGC decoupling capacitor C8 is suspect; and removal and checking of this com-

ponent shows it to be faulty.

A replacement capacitor is put in, and the receiver is back in working condition.

§7] 6.123

Transistor Connections

There will come a time, when you are fault-finding on transistorized equipments,

when you will suspect a transistor itself of being faulty; and you will want to test it.

Just as valve manufacturers give data of pin connections for valves, so do transistor

manufacturers give information enabling you to identify transistor element con-

nections. One widely used method of arranging the collector, emitter and base

leads of a transistor so that its various connecting leads can be easily identified is

pictured in the top diagram.

In this diagram of a typical transistor, the

collector lead is the one nearest to the red or

white spot painted on the transistor body.

The emitter is connected to the lead farthest

from the collector. And the base connecting

lead is the one between the emitter and the

collector.

Notice also that the spacing between the three

connecting leads is different.

Some transistors are fitted with metal shields,

or "heat sinks," which slide over the outer

casing of the transistor. When the transistor

is fitted into an equipment, the heat sink is fixed

to a point on a metal chassis; and helps to

conduct heat away from the transistor body.

In larger transistors, the collector is normally

connected to the outer casing; and the emitter

and base connecting leads are identified by

coloured sleeving, as shown in the lower

illustration.

Note that, although these two methods of,

marking the leads of a transistor are widely

used, they are not universal. When the leads

of a transistor cannot be certainly identified,

you should at once consult the manufacturer's

data book for this vitally important piece of

information.

6.124 [§7

Transistor Testing

The complete testing of a transistor is a big job, requiring special testing equipment

which is both more complex and more expensive than are most valve testers. There

are, however, two important tests which can be carried out on low-power transistors

with comparatively simple equipment; and they are the testing of its current gain

and measurement of leakage current. Transistors are connected in a grounded-

emitter condition for these tests.

You know that the current gain of a transistor in a grounded-emitter condition

s symbolized as j3, or a ; and that

=the change in collector current

the change in base current

lOOK

A/WW

Your first step is to note the collector current meter-reading (1^) when the con-

nection to the base of the transistor is open circuited; this reading represents the

collector leakage current (Ico) multiplied by the current gain of the transistor.

The value of this leakage current is normally of the order 0-1 to 0-3 mA. If there

is either no leakage current at all, or else a current well in excess of the given range,

it is an indication that the transistor is unserviceable.

The current gain is found by supplying current to the base, adjusting the

1-megohm variable resistor for a collector current of 1 mA, and noting the reading

of base current. Re-adjust the 1-megohm resistor for a collector current of 2 mA,and note the new value of base current. The ratio of change-of-collector-current to

change-of-base-current will give you j8.

Assume, as an example, that the base current needed to obtain 1-mA collector

current was 20 micro-amps, and that the base current to obtain 2 mA was 40 micro-

amps. Then you get the following simple equation:

j8 (or a') =2 mA— 1 mA _ 1 mA40(xA-20(xA ~ 20 fxA

= 50

In other words, the current gain of the transistor under test, when it is in the

grounded-emitter condition, is 50.

§8 : GENERAL REVIEW OF TRANSISTORS 6.125

Junction Transistor, A junction

transistor consists of semi-conduc-

tor materials in an N-P-N or a

P-N-P sequence. The junctions

may be formed either during a

crystal-growing process (grown

junctions), or by a dissolving and

re-crystallization process (alloy

junctions), or by an etching and

plating process.

P N P

mv * ' K FEmitter Base Collector

Point-contact Transistor, Apoint-contact transistor consists of

a block of N-type semi-conductor

material in contact with two

closely spaced pointed metal wires.

The construction can be considered

as the equivalent of a P-N-Pjunction arrangement.

PROTECTIVE CASE

Transistor Amplification. Am-plification is achieved because the

transistor transfers current from a

low-resistance circuit to a high-

resistance circuit. This is whythey are called "transfer resistors,"

or transistors.

Common-emitter Amplifier.

This type of circuit operates in the

same general manner as does the

common-cathode valve amplifier.

The circuit has the highest power

amplification, but the greatest ten-

dency to oscillate. Input resis-

tance is 1,000 ohms or lower ; output

resistance is as high as 50,000

ohms.

6.126

REVIEW of Transistors (continued)

Common-base Amplifier, This

circuit operates in the same general

manner as does the grounded-grid

valve amplifier. It produces lower

power amplification than does the

common-emitter circuit, but has

the least tendency to oscillate.

Input resistance may be as low as

25 ohms; output resistance is as

high as 500,000 ohms.

[§8

Common-collector Amplifier.

This circuit operates in the same

general manner as does the cathode-

follower. The voltage gain is less

than one; but power gains of over

30 times can be produced. Input

resistance is in the order of

25,000 ohms; output resistance is

as low as 1,000 ohms.

Multiple-stage Amplifiers. Thecoupling of transistor amplifier

stages together brings important

impedance-matching problems.

These are solved by transformer-

coupling, or by careful selection of

the types of stages to be used in

sequence. The same coupling

methods are used as in valve

amplifiers.

laptduc* Matching la Transistor Am pllfisrs

25-300 COMMON-BASE

CIRCUIT

COMMON -

BASECIRCUIT

100K ohmohm input or higher load

300-1000COMMON-BASE-OR COMMON-

EMlTTEflCIRCUIT

COMMON-EMITTERCIRCUIT

100K ohmohm input

""

or lower load

100K-500K mCOMMON-COLLECTORCIRCUIT

COMMON-COLLECTORCIRCUIT

5000 ohm ^ohm input or tower load

""

Transistor Applications. Tran-

sistors can be used in many of the

electronic circuits for which valves

have hitherto been needed. Tran-

sistors have, however, frequency

and power limitations which pre-

vent their completely replacing

valves at the present stage of then-

development.

§9 : INTRODUCTION TO SYNCHROS

AND SERVOMECHANISMS6.127

You have now progressed far enough in your study of Electronics to be able to con-sider in more detail some of the practical applications of the circuits and componentsyou have learnt.

The great technical advances made in military equipment during the SecondWorld War are now producing momentous consequences in industrial applications.

Automatic process control—more popularly known as "automation"—is being

installed in efficient plants in all industrially advanced countries of the world—in

oil refineries; in steel, textile and paper mills; in chemical plants; in the manufacture

and assembly of automobile and electronic assemblies; and in the rapid "processing"

of unbelievable quantities of detailed information in scientific and technical labora-

tories, and in offices tackling problems of stock-control, sales-tabulations, the proper

phasing of production with anticipated demand, the calculation and checking of

wage-packets and the like.

At the heart of the complex, electronically-controlled mechanisms which makesuch operations possible, lies a group of devices known as Servomechanisms. It is

to a basic study of the Servomechanism "family"—and of its close relations, the

Synchros—that the next two Parts of this Illustrated Course of Elementary

Technician Training are devoted.

.vxv"!^

This way to

THE COURSE>ttth BASIC SYNCHROS AH9 SCRVOMECHAHISMS

INDEX TO PART 6(Note: A cumulative index covering all six

index

AFC, 6,14

AGC, 6.44

Audio correction, 6.19, 6.26

Base, 6.81

Bias

forward, 6.70, 6.80

reverse, 6.71, 6.80

Collector, 6.81

Common-base circuit, 6.94, 6.126

Common-collector circuit, 6.102, 6.126

Common-emitter circuit, 6.96, 6.125

D.C. stabilization, 6V105

De-emphasis, 6.45

Diodejunction, 6.67, 6.79

point contact, 6.69, 6.80

semi-conductor, 6.72

Discriminator, 6.32, 6.50

basic, 6.37

Foster-Seeley, 6.39

ratio detector, 6.41, 6.50

Emitter, 6.81

Fault-finding on transistor circuits, 6.119

Ferrite reactor, 6.24, 6.25

FM, 6.25

advantages of, 6.4

definition of, 6.2

disadvantage of, 6.5

receiver, 6.27

transmitter, 6.6, 6.12, 6.22, 6.25

tuner, 6.46

Foster-Seeley discriminator, 6.39

I.F. amplifier

FM, 6.28, 6.49

transistor, 6.114

Junction diode, 6.67

Junction transistor, 6.82, 6.125

Leakage current, 6.97

Limiter, 6.32

Mixer, transistor, 6.115

Modulator FMferrite reactor, 6.24, 6.25

link phase, 6.20

reactance valve, 6.8, 6.25

Multi-stage transistor a.f. amplifier, 6.107,

6.126

N-type material, 6.65

Oscillator, transistor, 6.115

P-type material, 6.66

Phase modulation, 6.16, 6.26

P-N junction, 6.70

Point-contact diode, 6.69, 6.80

Parts will be found immediately following this

to Part 6)

Point-contact transistor, 6.81, 6.125

Pre-emphasis, 6.15

Push-pull transistor a.f. amplifier, 6.111

Ratio detector, 6.41, 6.50

Reactance valve, 6.8, 6.25

Receiver FM, 6.27, 6.49

AGC, 6.44

de-emphasis, 6.45

discriminator, 6.32, 6.50

i.f. amplifier, 6.28, 6.49

limiter, 6.32, 6.50

Receiver, transistor, 6.113

circuit, 6.116

operation, 6.116

R.F. transistor amplifier, 6.114

Semi-conductor, 6.56

materials, 6.64, 6.79

N-type, 6.65

P-type, 6.66

Semi-conductor diodes

applications of, 6.76, 6.80

basic construction of, 6.67, 6.69

characteristics of, 6.72

history of, 6.56

junction, 6.67, 6.79

point-contact, 6.69, 6.80

Single-stage transistor a.f. amplifier, 6.99

Superhet, transistor, 6.116

fault-finding on, 6.119

Transistor

advantages of, 6.62

care and handling of, 6.93

characteristics, 6.94

connections, 6.123

current amplification, 6.91

junction, 6.82, 6.125

leakage current, 6.97

N-P-N, 6.83

P-N-P, 6.90

point-contact, 6.81, 6.125

superhet, 6.113

testing, 6.124

voltage and power gains, 6.88

Transmitter FM, 6.6

AFC, 6.14

audio correction, 6.19, 6.26

block diagram, 6.12

complete, 6.22

pre-emphasis, 6.15

Tuner, FM, complete, 6.46, 6.51

Zener effect, 6.74

CUMULATIVE INDEX (PARTS 1-6)

A.C./D.C. half-wave rectifier, 1.91, 1.103

Aerials, 4.59, 4.71

dipole, 4.60

radiation pattern, 4.66

radiation resistance, 4.63

receiver, 5.13, 5.20

selecting and installing, 5.16

tuning, 4.65

types of, 5.14

A.F. amplifier, 5.41

in the superhet receiver, 5.57, 5.67

in the TRF receiver, 5.38, 5.42

tone control, 5.39

volume control, 5.40

AFC, 6.14

AGC, 6.44

Alignment, 5.68, 5.71, 5.72

Amplification, 2.1

classes of, 2.29

factor, 2.19

Amplifiers, 2.10, 4.9, 4.16

audio power, 2.70

beam tetrode, 2.51, 2.53

Class C amplifier, 4.10

classes of operation, 4.9

frequency response, 2.61, 2.64, 2.65, 2.88

high-frequency response, 3.5

low-frequency response, 3.4

pentode, 2.47, 2.53

push-pull, 2.77, 2.83, 2.85

r.f., 3.13, 3.31, 3.39

single-stage, 2.54

two-stage RC coupled, 2.57, 2.65

two-stage transformer coupled, 2.58, 2.66

video, 3.2, 3.11

Amplitude modulation, 4.77, 4.91

modulation patterns, 4.84

percentage modulation, 4.89

Anode modulation, 4.81

resistance, 2.20

Anode-bend detector, 5.35

Armstrong oscillator, 3.54

frequency of oscillations, 3.58

frequency stability of, 3.60

how oscillations are maintained, 3.56

Audio correction, 6.19, 6.26

Automatic gain control (AGC), 5.58

Band switching in receivers, 5.24

Bandwidth, 3.34

Base, 6.81

Beam tetrode, 2.51, 2.53

Bias, 2.23

battery, 2.31

cathode, 2.34

forward, 6.70, 6.80

from power supply, 2.32

grid-leak, 3.55, 3.59

reverse, 6.71, 6.80

Bleeder resistors, 1.72

Bridge rectifier circuit, 1.46

Capacitors

ganged, 5.25

padder, 5.65, 5.73

trimmer, 5.26, 5.72

tuning, 3.23

Cathode

bypass capacitor, 2.37

follower, 5.96

Cavity resonators, 3.87

Class C amplifiers, 4.10, 4.16

bias arrangements, 4.12

Coils, tuning, 3.24

Collector, 6.81

Colpitts oscillator, 3.63

Common base circuit, 6.94, 6.126

Common collector circuit, 6.102, 6.126

Common emitter circuit, 6.96, 6.125

Compensating networks

for high-frequency compensation, 3.6

for low-frequency compensation, 3.7

Coupling

RC, 2.57, 2.65

transformer, 2.58, 2.66

Coupling circuits, 4.41

tuned, 4.42

Crystal detector, 5.30

Crystal oscillator, 3.68, 3.70, 3.73

tuning the, 3.71

Crystal receiver, 5.10

Current flow

in full-wave rectifier circuit, 1.43

in half-wave rectifier circuit, 1.30

CW transmission, 4.72, 4.92

keying circuits, 4.73

D.C. stabilization, 6.105

Decibels, 2.86, 3.36

Decoupling filter, 2.55

De-emphasis, 6.45

Detector, 5.37

anode-bend, 5.35

Cumulative Index

crystal, 5.30

diode, 5.32

grid-leak, 5.33

in the superhet receiver, 5.57, 5.67

in the superhet receiver for CW working,

5.64

in the TRF receiver, 5.29

Diode

junction, 6.67, 6.79

point-contact, 6.69, 6.80

semi-conductor, 6.72

Diode detector, 5.32

Diode valve, 1.20, 1.35

cold-cathode type, 1.80

current flow in, 1.25, 1.107

gas-filled, 1.32

Dipole, 4.60

Discriminator, 6.32, 6.50

basic, 6.37

Foster-Seeley, 6.39

ratio detector, 6.41, 6.50

Earphones, 2.89, 2.95, 2.97

Electron-coupled oscillator, 3.74

Emitter, 6.81

Fault-finding, 5.76, 5.91

in the superhet receiver, 5.84

procedure, 5.77

signal injection, 5.82

signal tracing, 5.81

testing within stages, 5.83

Fault-finding on transistor circuits, 6.119

Ferrite reactor, 6.24, 6.25

Fidelity in a receiver, 5.9

Filter capacitors, 1.54, 1.57, 1.71, 1.75

dry electrolytic, 1.59

paper, 1.57

wet electrolytic, 1.59

Filter chokes, 1.66

Filters, 1.49, 1.75

capacitor input, 1 .69

choke input, 1.67

RC, 1.61, 1.95

resonant, 1.70

FM, 6.25

advantages of, 6.4

definition of, 6.2

disadvantage of, 6.5

receiver, 6.27

transmitter, 6.6, 6.12, 6.22, 6.25

tuner, 6.46

Foster-Seeley discriminator, 6.39

Frequency, 4.44

Frequency changing valves, 5.54

Frequency multipliers, 4.35

doubling, 4.37

tripling, 4.38

tuning indicators, 4.39

Frequency response, 2.61, 2.65

curves, 2.64, 2.88

methods of improving of amplifier, 3.6

Frequency spectrum, 4.70

Frequency stability of oscillators, 3.53

factors affecting, 3.60

Gain, 2.57

Grid-leak bias, 3.55, 3.59

Grid-leak detector, 5.33

Grid modulation, 4.82

Hartley oscillator, 3.61

how oscillation is maintained, 3.62

Heterodyne principle, 5.63

I.F. amplifier, 5.56, 5.66

FM, 6.28, 6.49

transistor, 6.114

I.F. transformer, 5.55

Impedance matching, 2.75

Junction diode, 6.67

Junction transistor, 6.82, 6.125

Keying, 4.73

Klystron valve, 3.88

Leakage current, 6.97

Limiter, 6.32

Local oscillator, 5.49, 5.65

Loudspeakers, 2.89, 2.96, 2.97

Mains

filter, 1.40

transformer, 1.6

Metal rectifiers, 1.14, 1.19

Microphones, 2.89, 2.90, 2.97

Mixer, transistor, 6.115

Mixing, 5.52

mixer stage in the superhet receiver,

5.53, 5.66

mixer valves, 5.54

Modulation (AM), 4.7, 4.91

anode, 4.81

grid, 4.82

other methods of, 4.83

Modulator, FMferrite reactor, 6.24, 6.25

link phase, 6.20

reactance value, 6.8, 6.25

Motor generators, 1.101, 1.105

Cumulative Index

Multi-stage transistor a.f. amplifier, 6.107,

6.126

Multi-vibrator, 3.83

Mutual conductance, 2.21

N-type material, 6.65

Negative feedback, 3.8

Neutralization, 4.28, 4.34

Oscillations, 3.44

Oscillator, beat frequency, 5.64

transistor, 6.115

Oscillator stage in the superhet receiver,

5.65

Oscillators, 3.42, 3.64, 3.73, 3.89

Armstrong, 3.54

Colpitts, 3.63

crystal, 3.68

electron-coupled, 3.74

Hartley, 3.61

high-frequency, 3.85

RC, 3.82

relaxation, 3.83

TATG, 3.65

variations in circuit, 3.78

Output transformer, 2.73

impedance matching, 2.75

P-type material, 6.66

Pentode, 2.47, 2.53

Pentodes

variable /*, 3.29

why used in r.f. amplifiers, 3.26

Phase inverters, 2.80

Phase modulation, 6.16, 6.26

Phase splitter, 2.82

P-N junction, 6.70

Point-contact diode, 6.69, 6.80

Point-contact transistor, 6.81, 6.125

Power supplies, 1.4, 1.103

for d.c. sources, 1.97

metal rectifiers in, 1.14, 1.19

transformer type, 1.36, 1.41

transformer-less type, 1.91

voltage doubler, 1.93

Pre-emphasis, 6.15

Push-pull amplifier, 2.77, 2.83, 2.85

advantages of, 2.84

Push-pull transistor a.f. amplifier, 6.111

Q, 3.22

Radio telephony, 4.8

Ratio detector, 6.41, 6.50

RC oscillator, 3.82

Reactance valve, 6.8, 6.25

Receiver, 5.92

crystal, 5.10

fidelity, 5.9

introduction to, 5.1

selectivity, 5.8, 5.46

sensitivity, 5.7

superhet, 5.12, 5.43, 5.61, 5.74

the jobs performed by, 5.5

TRF, 5.11,5.21

Receiver FM, 6.27, 6.49

AGC, 6.44

de-emphasis, 6.45

discriminator, 6.32, 6.50

i.f. amplifier, 6.28, 6.49

limiter, 6.32, 6.50

Receiver, transistor, 6.113

circuit, 6.116

operation, 6.116

Rectifiers, 1.7

copper oxide, 1.15

gas-filled diode, 1.32

in bridge circuit, 1.46

in full-wave circuit, 1.42, 1.48

in half-wave circuit, 1.38, 1.41

metal-type, 1.14, 1.19

selenium, 1.15

valve-type, 1.20

R.F. amplifier, 3.13, 5.41

coupling between stages, 3.37

in the superhet receiver, 5.47

in the TRF receiver, 5.22, 5.28

single-stage, 3.26, 3.31

two-stage, 3.32, 3.39

R.F. transformer, 5.23

R.F. transistor amplifier, 6.114

Relaxation oscillator, 3.83

Resistors, bleeder, 1.72

Rotary converters, 1.102

Rotary transformers, 1.101

Selectivity, 3.22, 3.34

Selectivity in a receiver, 5.8, 5.46

Semi-conductor, 6.56

materials, 6.64, 6.79

N-type, 6.65

P-type, 6.66

Semi-conductor diodes

applications of, 6.76, 6.80

basic construction of, 6.67, 6.69

characteristics of, 6.72

history of, 6.56

junction, 6.67, 6.79

Cumulative Index

point contact, 6.69, 6.80

Sensitivity in a receiver, 5.7

measurements, 5.69

Series and shunt feed in oscillators, 3.78

Sidebands, 4.78, 4.94

Single-stage transistor a.f. amplifier, 6.99

Sound, 2.89

Superhet receiver, 5.12

complete circuit of, 5.61

fault-finding in, 5.84

Superhet, transistor, 6.116

fault-finding on, 6.119

TATG oscillator, 3.65, 3.73

feedback in, 3.66

Test instruments, 5.86

Tetrode, 2.42

secondary emission, 2.43

eliminating effects of, 2.46

Three-stage transmitter, 4.17, 4.34, 4.93

complete diagram for, 4.21

FPA, 4.20

IPA, 4.19

oscillator, 4.18

tuning methods, 4.24

valve filament circuit, 4.22

Time-base generator, 5.97

Tone control, 5.39

Transformers, mains, 1.6

Transistor Tft

advantages of, 6.62

care and handling of, 6.93

characteristics, 6.94

connections, 6.123

current amplification, 6.91

junction, 6.82, 6.125

leakage current, 6.97

N-P-N, 6.83

P-N-P,6.90

point-contact, 6.81, 6.125

superhet, 6.113

testing, 6.124

voltage and power gains, 6.88

Transmission

AM, 4.8

FM, 4.8

keyed, 4.7

voice, 4.8

Transmitter, 4.3

three-stage, 4.17, 4.34

Transmitter FM, 6.6

AFC, 6.14

audio correction, 6.19, 6.26

block diagram, 6.12

complete, 6.22

pre-emphasis, 6.15

Transmitter lines, 4.40, 4.94

applications, 4.55

characteristics impedance, 4.47

non-resonant and resonant lines, 4.49

open-circuited line, 4.51, 4.54

short-circuited line, 4.51, 4.53

TRF receiver, 5.11, 5.21

Triode, 2.11, 2.22

amplifier, 2.16, 2.23, 2.40

characteristics, 2.19, 2.20, 2.21

Tuned circuits, 3.17, 3.25

as oscillators, 3.47

LC parallel, 3.19

LC series, 3.18

Tuned lines, 3.86

Tuner, FM complete, 6.46, 6.51

Valve,

pentode, 2.47, 2.53

tetrode, 2.42

triode, 2.11, 2.22

Valve testing, 5.87

Valves, 1.20

bases, 1.28, 1.31

discovery of diode, 1.21

jobs of, 1.111

method of representation in diagrams,

1.27, 1.31

rectifiers, 1.20

stabilizer, 1.80, 1.88

thermionic emission in, 1.23

Vibrators, non-synchronous, 1.97, 1.104

synchronous, 1.100, 1.104

Video amplifier, 3.2, 3.11

Voltage stablization, 1.78, 1.88

circuit for, 1.82

valve, 1.80

Voltage stabilizer circuit, 5.99

Volume control

automatic (AGC), 5.58

manually operated, 5.27, 5.40

Wavelength, 4.44

Wave propagation, 4.67

ground wave, 4.68

space wave, 4.68, 4.69

Zener effect, 6.74

WIGANCENTRALLIBRARY