Chapter 6 BJT Amplifiers - ::: الموقع الرسمي لجامعة ميسان ::: · · 2014-02-03Transistor AC Model The five resistanceparameters (r-parameters) can be used

Chapter 6

Basic Electronic Devices and Circuits

EE 111

Electrical Engineering

Majmaah University

2nd Semester 1432/1433 H

© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.

Electronic Devices, 9th edition

Thomas L. Floyd1

BJT Amplifiers

Introduction

• The things you learned about biasing a transistor in Chapter 5 are now applied in this chapter where BJT circuits are used as small-signal

amplifiers.

• The term small-signal refers to the use of signals that take up a



Thomas L. Floyd2

relatively small percentage of an amplifier’s operational range.

•Additionally, you will learn how to reduce an amplifier to an equivalentdc and ac circuit for easier analysis.

AC Quantities

AC quantities are indicated with a lowercase italic

subscript;

rms values are assumed unless otherwise stated.

Vce

VceVce

rmsavg

V

rms: 1/√2 ≈ 0.707avg: 2/π ≈ 0.6366



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unless otherwise stated.ce

Vce

vce

VCE

00

t

Vce

The figure shows an example of a specific waveform for the collector-emitter voltage. Notice the DC component is VCE and the ac component is Vce.

Resistance is also identified with a lowercase subscript when analyzed from an ac standpoint.

Linear Amplifier

A linear amplifier produces a replica of the input signal at the output.

+VCC

Vb

Ic

ICQ



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For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape.

RCR1

RER2 RL

C2

b

IbRs

ICQ

Vce

VCEQ

Vs

C1IBQ

VBQ

AC Load Line

Operation of the linearamplifier can be illustrated using an ac load line.

0 VCE

I b

IC

Ic

Q

Vce

ICQ

I BQ

VCC/(RC||RL +RE)



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The ac load line is differentthan the dc load line because a capacitor looks open to dcbut effectively acts as a short to ac.

Thus the collector resistor RC appears to be in parallelwith the load resistor RL.

VCEQ

ac short; ZC = 1/( j ω C )

ac ground

Solution:

Projections on the graph of Figure 6–4 show the collector current Ic varying from 6 mA to 4 mA for a peak-to-peak



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c

to 4 mA for a peak-to-peak value of 6 – 4 = 2 mAand the collector-to-emitter voltage Vce varying from 1 V to 2 V for a peak-to-peak value of 2 – 1 = 1 V.

Transistor AC Model

very small

very large



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Thomas L. Floyd8

Transistor AC Model

The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit.

For most analysis work, the simplified r-parameters give good results.

The simplified r-parameters are shown in relation to the transistor



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βacIb

C

B

E

βacIb

C

B

E

Ib ′re

′re

shown in relation to the transistor model.An important r-parameter is re'. It appears as a small ac resistance between the base and emitter.

'

E

25 mVer

I=

assuming an abrupt junction between the nand p regions, and an ambient temperature of 20°C

QuizQuizQuiz

Q. The equation for finding the ac emitter resistance of a BJT is

a. '

B

25 mVer

I=



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b.

c.

d.

B

'

E

'

B

'

E

25 mV

0.7 V

0.7 V

e

e

e

rI

rI

rI

=

=

=



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IE = 25 mV / 8 Ω = 3.125 mA

Comparison of βac to βDC

Curve of IC versus IB is nonlinear!



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slope slope

h (hybrid) parameters



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Relationships of h Parameters and r Parameters

forward current gain, common-base

forward current gain, common-emitter

voltage feedback ratio, common-emitter

output admittance (conductance), common-emitter



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input impedance (resistance), common-emitter

Documents

Chapter 6 BJT Amplifiers - ::: الموقع الرسمي لجامعة ميسان ::: · · 2014-02-03Transistor AC Model The five resistanceparameters (r-parameters) can be used