Chapter5 BJT AC Analysis

8/3/2019 Chapter5 BJT AC Analysis

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Chapter 5:

BJT AC Analysis


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Copyright 2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert 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 ACanalysis of a transistor:

re model

Hybrid equivalent model


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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. Thismodel is designed for specific circuit conditions.


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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.


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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


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Common-Emitter Fixed-Bias Configuration

AC equivalent

re model


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Common-Emitter Fixed-Bias Calculations

Co 10Rr

e

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

iviR

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


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Common-Emitter Voltage-Divider Bias

re

model requires you to determine , re, and r

o.


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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

V

A

eCo

Co

r10R,10Rri

oi

10Rrei

oi

eCo

o

i

oi

I

IA

rR

R

I

IA

)rR)(R(r

rR

I

IA

C

iviR

ZAA


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Common-Emitter Emitter-Bias Config.


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Impedance Calculations

Eb

Eeb

Eeb

bBi

RZ

)R(rZ

1)R(rZ

Z||RZ

Input impedance:

Output impedance:Co RZ


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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

bBB

ioi

ZR

R

I

IA

Civi

R

ZAA


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Feedback Pair

This is a two-transistor circuit that operates like aDarlington pair, but it is not a Darlington pair.

It has similar characteristics:

High current gain

Voltage gain near unity Low output impedance

High input impedanceThe difference is that a Darlington

uses a pair of like transistors,whereas the feedback-pair

configuration uses complementary

transistors.


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Current Mirror Circuits

Current mirror circuitsprovide constant current

in integrated circuits.

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

given, Rx = 1.1 k and +Vcc = 12 V.

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Chapter5 BJT AC Analysis