electronic devices and circuit theory 10th ed. boylestad - chapter 5

Chapter 5:

BJT AC Analysis

Copyright ©2009 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/e

Robert L. Boylestad and Louis Nashelsky

BJT Transistor Modeling

• A model is an equivalent circuit that represents the AC

characteristics of the transistor.

• A model uses circuit elements that approximate the

behavior of the transistor.

• There are two models commonly used in small signal AC

analysis of a transistor:

– re model

– Hybrid equivalent model





The re Transistor Model

• BJTs are basically current-controlled devices; therefore the re model

uses a diode and a current source to duplicate the behavior of the

transistor.

• One disadvantage to this model is its sensitivity to the DC level. This

model is designed for specific circuit conditions.





The re Transistor Model

Common-Emitter Configuration

• The equivalent circuit of Fig above will be used throughout the

analysis to follow for the common-emitter configuration.





Common-Emitter Fixed-Bias Configuration

• The input is applied to the base

• The output is from the collector

• High input impedance

• Low output impedance

• High voltage and current gain

• Phase shift between input and

output is 180





Common-Emitter Fixed-Bias Configuration

AC equivalent

re model





Common-Emitter Fixed-Bias Calculations

Co 10Rre

Cv

e

oC

i

ov

r

RA

r

)r||(R

V

VA

eBCo r10R ,10Rri

eBCo

oB

i

oi

A

)r)(RR(r

rR

I

IA

C

ivi

R

ZAA

Current gain from voltage gain:

Input impedance:

Output impedance:

Voltage gain: Current gain:

eE r10Rei

eBi

rZ

r||RZ

Co

O

R10rCo

Co

RZ

r||RZ





Common-Emitter Voltage-Divider Bias

re model requires you to determine , re, and ro.





Common-Emitter Voltage-Divider Bias Cal.


Input impedance:

Output impedance:

Voltage gain:

Current gain:

ei

21

r||RZ

R||RR

Co 10RrCo

oCo

RZ

r||RZ

Co 10Rre

C

i

ov

e

oC

i

ov

r

R

V

VA

r

r||R

V

VA

eCo

Co

r10R ,10Rri

oi

10Rrei

oi

eCo

o

i

oi

I

IA

rR

R

I

IA

)rR)(R(r

rR

I

IA

C

ivi

R

ZAA





Common-Emitter Emitter-Bias Config.





Impedance Calculations

Eb

Eeb

Eeb

bBi

RZ

)R(rZ

1)R(rZ

Z||RZ

Input impedance:

Output impedance: Co RZ





Gain Calculations


Voltage gain:

Current gain:

Eb

Eeb

RZE

C

i

ov

)R(rZEe

C

i

ov

b

C

i

ov

R

R

V

VA

Rr

R

V

VA

Z

R

V

VA

bB

B

i

oi

ZR

R

I

IA

C

ivi

R

ZAA





Feedback Pair

This is a two-transistor circuit that operates like a

Darlington pair, but it is not a Darlington pair.

It has similar characteristics:

• High current gain

• Voltage gain near unity

• Low output impedance

• High input impedance

The difference is that a Darlington

uses a pair of like transistors,

whereas the feedback-pair

configuration uses complementary

transistors.





Current Mirror Circuits

Current mirror circuits

provide constant current

in integrated circuits.

Example 4.26: Calculate the mirrored current I in the above circuit

given, Rx = 1.1 kΩ and +Vcc = 12 V.

electronic devices and circuit theory 10th ed. boylestad - chapter 5

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