Lecture 4

Transistor

• A semiconductor device that can be modeled with dependent sources

• Transistor types:– Bipolar Junction Transistor (BJT)

– Field Effect Transistor (FET)

• A transistor has different operating modes with different i-v characteristics

2

INTRODUCTION

TRANSISTORSA bipolar transistor consists of a three-layer "sandwich" of doped

(extrinsic) semiconductor materials, either P-N-P or N-P-N. Each layer

forming the transistor has a specific name (Emitter - E, Base - B and Collector-C), and each layer is provided with a wire contact for

connection to a circuit

4

TERMINOLOGY AND SYMBOLS

•Both, PNP and NPN transistors can be thought of as two very closely spaced PN junctions.

•The base must be small to allow interaction between the two PN junctions.

5

•There are four regions of operation of a BJT transistor

•Since it has three leads, there are three possible amplifiers

6

Qualitative Description of Transistor Operation•Emitter doping is much larger than base doping

•Base doping larger than collector doping

•Current components: IE = IEp + IEn

IC = ICp + ICn

IB = IE - IC = IB1

+ IB2 + IB3

•IB2 = current due to electrons that replace the recombined electrons in the base,

•IB3 = collector current due to thermally-generated electrons in the collector that go in the base

•IB1 = current from electrons being back injected into the forward-biased emitter-base junction

7

Circuit DefinitionsBase Transport Factor aT :

T = ICp /IEp Ideally it would be equal to unity (recombination in the base reduces its value)

Emitter Injection Efficiency : = IEp /(ICp + IEp) = IEp /IE Approaches unity if emitter doping is much larger than base dopingAlpha-dc:

dc= IC /IE = (ICp+ ICn ) /(Iep + IEn) = ICp /(Iep + IEn ) = dc

Beta-dc:

dc = IC /IB = IC /(IE - IC) = dc /(1- dc) Current gain is large when dc approaches unity

Collector-reverse Saturation Current:

IBCo = ICn IC = ICp + ICn = dcIE + IBCo

8

Collector Current in Common-emitter Configuration:

IC =dc(IC + IB) +IBCo

IC ={dc /(1-dc)}* IB +IBCo /(1- dc) IC =dc +IECo

IECo =(1+ dc) IBCoLarge Current Gain Capability:

Small base current IB forces the E-B junction to be forward biased and inject large number of holes which travel through the base to the collector.

9

+ N P NEmitter

Base +

-

FB RB

Bipolar Transistor Biasing (NPN)

Collector

10

P N PEmitter Collector

Base

+

+

-

FB RB

Bipolar Transistor Biasing (PNP)

11

Bipolar Transistor Operation (PNP)•90% of the current carriers pass through the reverse biased base - collector PN junction and enter the collector of the transistor.

•10% of the current carriers exit transistor through the base.

•The opposite is true for a NPN transistor.

12

Transistor Characteristic Curve

IC

VCE

Q-Point

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA80 uA90 uA

Saturation

Cutoff

13

Bipolar Transistor Amplifiers

•Amplifier Classification–Amplifiers can be classified in three ways:

•Type (Construction / Connection)–Common Emitter–Common Base–Common Collector

•Bias (Amount of time during each half-cycle output is developed).

–Class A, Class B, Class AB, Class C

•Operation–Amplifier–Electronic Switch

14

Common Emitter Schematic

RB

RC

Q1

+

0

+VCC

Input Signal

+

0

Output Signal

Output Signal Flow Path

Input Signal Flow Path

15

• DC Kirchoff Voltage Law Equations and Paths

Kirchoff Voltage Law

RB

RC

Q1

+VCC

Base - Emitter Circuit

ICRC + VCE - VCC = 0

IBRB + VBE - VCC = 0

Collector - Emitter Circuit

16

Common Emitter Operation

Positive Going Signal

Negative Going Signal

Output Signal

Input Signal

+

+

0

0 Base becomes more (+) WRT Emitter FB IC VRC

VC VOUT ( Less + )

Base becomes less (+) WRT Emitter FB IC VRC

VC

VOUT ( More + )

RC

RB

Q1

17

Common Base Schematic

+

0

+

0+VCC

RBRCRE

Q1

CC

Input Signal Flow Path

Output Signal Flow Path

18

• DC Kirchoff Voltage Law Equations and Paths

Kirchoff Voltage Law

+VCC

RBRCRE

Q1

CC

Base - Emitter CircuitIBRB + VBE + IERE - VCC = 0

Collector - Emitter CircuitICRC + VCE + IERE - VCC = 0

19

Common Base Operation

Positive Going Signal

Negative Going Signal

+VCC

RBRCRE

Q1

CC

Base becomes more (+) WRT Emitter FB IC VRC

VC

VOUT ( More + )

Base becomes less (+) WRT Emitter FB IC VRC

VC VOUT ( Less + )Input

Signal

0Output Signal

+

0

20

Common Collector Schematic

RB

RE

Q1

+

0

+VCC

Input Signal +

0

Output Signal

Output Signal Flow Path

Input Signal Flow Path

21

• DC Kirchoff Voltage Law Equations and Paths

Kirchoff Voltage Law

RB

RE

Q1

+VCC

Base - Emitter CircuitIBRB + VBE + IERE - VCC = 0

Collector - Emitter CircuitICRC + VCE + IERE - VCC = 0

22

Common Collector Operation

Positive Going Signal

Negative Going Signal

RB

RE

Q1

+VCC

Base becomes more (+) WRT Emitter FB IE VRE

VE

VOUT ( More + )

Base becomes less (+) WRT Emitter FB IE VRE

VE VOUT ( Less + )Input

Signal

0 0

+ +

Output Signal

23

Transistor Bias Stabilization

Used to compensate for temperature effects which affects semiconductor operation. As temperature increases, free electrons gain energy and leave their lattice structures which causes current to increase.

24

Types of Bias Stabilization•Self Bias: A portion of the output is fed back to the input 180o out of phase. This negative feedback will reduce overall amplifier gain.•Fixed Bias: Uses resistor in parallel with Transistor emitter-base junction.•Combination Bias: This form of bias stabilization uses a combination of the emitter resistor form and a voltage divider. It is designed to compensate for both temperature effects as well as minor fluctuations in supply (bias) voltage.•Emitter Resister Bias: As temperature increases, current flow will increase. This will result in an increased voltage drop across the emitter resistor which opposes the potential on the emitter of the transistor.

25

Self Bias Schematic

RB

RC

Q1

+VCC

+

=

Initial Input

Self Bias Feedback

Resulting Input

+

+

+

+

o o

o

o

VOUT

26

Emitter Bias Schematic

RB

RC

Q1

+VCC

+

o

VOUT

RE

++

+

+

-

-Initial Input

+

o

CE

DC Component

AC Component

27

Combination Bias Schematic

RB1

RC

Q1

+VCC

+

o

VOUT

RE

++

+

+

-

-Initial Input

+

o

CE

DC Component

AC Component

RB2

28

Amplifier Frequency Response

•The range or band of input signal frequencies over which an amplifier operates with a constant gain.•Amplifier types and frequency response ranges.

•Audio Amplifier–15 Hz to 20 KHz

•Radio Frequency (RF) Amplifier–10 KHz to 100,000 MHz

•Video Amplifier (Wide Band Amplifier)–10 Hz to 6 MHz

29

Class ‘A’ Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA80 uA90 uA

Saturation

Cutoff

Q-Point

30

Class ‘B’ Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA

80 uA

90 uA

Saturation

Cutoff

Q-Point

31

Class ‘AB’ Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA

80 uA

90 uA

Saturation

Cutoff

Q-Point

Can be used for guitar distortion.

32

Class ‘C’ Amplifier Curve

IC

VCE

IB

0 uA

10 uA

20 uA

30 uA

40 uA

50 uA

60 uA

70 uA

80 uA

90 uA

Saturation

CutoffQ-Point

33

Transistor Fundamentals

Semiconductor devices that have-three or more elements are called transistors. The term transistor was derived from the words transfer and resistor. This term best describes the operation of the transistor.

There are many different types of transistors, but their basic theory of operation is all the same. The three elements of the two-junction transistor are:

(1)Emitter, which gives off, or emits," current carriers (electrons or holes); (2) Base, which controls the flow of current carriers; and

(3) Collector, which collects the current carriers.

Introduction to BJT Small Signal

Analysis

Bipolar Junction Transistor (BJT)

• BJTs are made up of alternating layers of n and p semiconductor materials joined metallurgically

• Two types of BJTs:– pnp-type: Principal conduction by positive

holes– npn-type:Principal conduction by negative

electrons

Bipolar Junction Transistor (BJT)

• BJT is a three terminal device: – Emitter (E)– Collector (C)– Base (B)

: Base-Emitter voltage

: Collector-Emitter voltage

: Collector Current

: Base Current

: Emitter Current

BE

CE

C

B

E

v

v

i

i

i

Bipolar Junction Transistor (BJT)

• Applying KCL to BJT:– Finding two currents can yield the third

one

• Three operating modes of the BJT transistors:– Active mode

– Cutoff mode

– Saturation mode

E C Bi i i

Active Mode• In active mode collector current is controlled by

base current and base-emitter voltage is constant:

voltage threshold:

gaincurrent forward :

V

Vv

ii

BE

BC

: forward current gain

: threshold voltage

C B

BE

i i

v V

V

39

RRee model model• Fails to account the output impedance Fails to account the output impedance

level of device and feedback effect from level of device and feedback effect from output to inputoutput to input

Hybrid equivalent modelHybrid equivalent model• Limited to specified operating condition Limited to specified operating condition

in order to obtain accurate resultin order to obtain accurate result

DisadvantageDisadvantage

40

• The transistor can be employed as an amplifying device. That is, the output sinusoidal signal is greater than the input signal or the ac input power is greater than ac input power.

• How the ac power output can be greater than the input ac power?

Amplification in the AC domain

41

Amplification in the AC domain

Conservation; output Conservation; output power of a system power of a system cannot be large than its cannot be large than its input and the efficiency input and the efficiency cannot be greater than 1cannot be greater than 1

The input dc plays the The input dc plays the important role for the important role for the amplification to amplification to contribute its level to the contribute its level to the ac domain where the ac domain where the conversion will become conversion will become as as ηη=P=Po(ac)o(ac)/P/Pi(dc)i(dc)

42

• The superposition theorem is applicable for the analysis and design of the dc & ac components of a BJT network, permitting the separation of the analysis of the dc & ac responses of the system.

• In other words, one can make a complete dc analysis of a system before considering the ac response.

• Once the dc analysis is complete, the ac response can be determined using a completely ac analysis.

Amplification in the AC Amplification in the AC domaindomain

43

BJT Transistor Model• Use equivalent circuit• Schematic symbol for the device can be replaced

by this equivalent circuits.• Basic methods of circuit analysis is applied.• DC levels were important to determine the Q-point• Once determined, the DC level can be ignored in

the AC analysis of the network.• Coupling capacitors & bypass capacitor were

chosen to have a very small reactance at the frequency of applications.

44

The AC equivalent of a network isobtained by:1. Setting all DC sources to zero & replacing them

by a short-circuit equivalent.2. Replacing all capacitors by a short-circuit

equivalent.3. Removing all elements bypassed by short-

circuit equivalent.4. Redrawing the network.

BJT Transistor ModelBJT Transistor Model

45

46

47

Example

48

Example

49

Example

50

The re transistor model

• Common Base PNP Configuration

51

Common Base PNP Configuration

• Transistor is replaced by a single diode between E & B, and control current source between B & C

• Collector current Ic is controlled by the level of emitter current Ie.

• For the ac response the diode can be replaced by its equivalent ac resistance.

52

Common Base PNP Configuration• The ac resistance of

a diode can be determined by the equation;

Where ID is the dc current through the diode at the Q-point.

EI

mVre

26

53

• Input impedance is relatively small and output impedance quite high.

• range from a few Ω to max 50 Ω

• Typical values are in the M Ω

CBi reZ

CBZo

Common Base PNP Configuration

54

The common-base characteristics

55

Voltage Gain

re

RA

re

R

rI

RI

V

VA

rI

ZI

ZIV

RI

RI

RIV

LV

L

ee

Le

i

OV

ee

ie

iii

Le

LC

Loo

:gain voltage

: ageinput volt

)(

: tageoutput vol

56

Current Gain

1

i

e

e

e

C

i

oi

A

I

I

I

I

I

IA

• The fact that the polarity of the Vo as determined by the current IC is the same as defined by figure below.

• It reveals that Vo and Vi are in phase for the common-base configuration.

57

Common Base PNP Configuration

Approximate model for a common-base npn transistor configuration

58

Example 1: For a common-base configuration in figurebelow with IE=4mA, =0.98 and AC signal of 2mV isapplied between the base and emitter terminal:a) Determine the Zi b) Calculate Av if RL=0.56kc) Find Zo and Ai

e

b b

c

ec I αI

IcIe

common-base re equivalent cct

re

59

Solution:

5.6m4

m26

I

26mr Za)

Eei

43.845.6

)k56.0(98.0

r

RA b)

e

Lv

98.0I

IA

Ω Zc)

i

oi

o

60

Example 2: For a common-base configuration in previous example with Ie=0.5mA, =0.98 and AC signal of 10mV is applied, determine:a) Zi b) Vo if RL=1.2k c) Av d)Ai e) Ib

20m5.0

m10

I

V Za)

:Solution

e

ii

88mV5(1.2k)0.98(0.5m)

RIRIV b) LeLco

8.58m10

m588

V

VA c)

i

ov

98.0A d) i

A10

)98.01(m5.0

)1(m5.0

I-I

I-II e)

ee

ceb

Common Emitter NPN Configuration

• Base and emitter are input terminal

• Collector and emitter are output terminals

61

Common Emitter NPN Configuration

• Substitute re

equivalent circuit

• Current through diode

62

bc II

bbe

bbbce

III

IIIII

)1(

63

• Input impedance

ei

ei

b

ebi

eb

eei

b

be

i

ii

rZ

rZ

I

rIZ

rI

rIV

I

V

I

VZ

; 1an greater thusually

)1(

)1( that so

)1(

:ageinput volt

:impedanceinput

64

The output graph

65

bI

c

e

bIi=Ib

re model for the C-E transistor configuration

rero

e

0AbI

c

e

bIi=Ib

rero

e

Vs=0V

= 0A

oZ

impedance)high cct,(open ΩZ

the thusignored is r if

rZ

o

o

oo

Output impedance Zo

66e

LV

eb

Lb

i

oV

eb

iii

Lb

Lco

Loo

r

RA

rI

RI

V

VA

rI

ZIV

RI

RIV

RIV

that so

:ageinput volt

: tageoutput vol

i

b

b

b

C

i

oi

A

I

I

I

I

I

IA

Voltage GainVoltage Gain Current GainCurrent Gain

re model for common-emitter

67

68

Example 3: Given =120 and IE(dc)=3.2mA for a common-emitter configuration with ro= , determine:

a) Zi b)Av if a load of 2 k is applied c) Ai with the 2 k load

975)125.8(120rZ

125.8m2.3

m26

I

26mr a)

ei

Ee

:Solution

15.246125.8

k2

r

Rb)A

e

Lv

120I

IA c)

i

oi

69

Example 4: Using the npn common-emitter configuration, determine the following if =80, IE(dc)=2 mA and ro=40 k

a) Zi b) Ai if RL =1.2k c) Av if RL=1.2k

k04.1)13(80rZ

13m2

m26

I

26mr a)

ei

Ee

:Solution

bI

cbIi=Ib

re model for the C-E transistor configuration

rero

e

RL

Io

70

67.77

)80(k2.1k40

k40

Rr

r

IRr

)I(r

A

Rr

)I(rI

I

I

I

IiAb)

(cont)Solution

Lo

o

b

Lo

bo

i

Lo

boL

b

L

i

o

6.8913

k40k2.1

r

rRvAc)

e

oL

71

Common Collector Configuration

• For the CC configuration, the model defined for the common-emitter configuration is normally applied rather than defining a model for the common-collector configuration.

END

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