Chapter 5: Islamic University of Gaza BJT AC Analysis Dr ...site.iugaza.edu.ps/tskaik/files/chapter5_p1.pdf · Electronic Devices and Circuit Theory, ... Robert L. Boylestad and Louis

Chapter 5:

BJT AC Analysis

Islamic University of Gaza

Dr. Talal Skaik

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

2 Dr. Talal Skaik 2014






Removal of the dc supply and

insertion of the short-circuit

equivalent for the capacitors.

Capacitors chosen with very

small reactance at the frequency

of application → replaced by

low-resistance or short circuit.






Circuit redrawn for small-

signal ac analysis




The re Transistor Model


Common Emitter Configuration

1

11

beii

b b

be e e c b e b b e

b e

b ebei e e

b b

VVZ

I I

V I r I I r I I r

I r

I rVZ r r

I I




The re Transistor Model Common Emitter Configuration

26 e

E

mVr

I





The re Transistor Model

Common Emitter Configuration


0

0

1C

CE

CE

C

Islope

V r

Vr

I

The output resistance r is

typically in the range of

40 kΩ to 50 kΩ




Common-Base Configuration





Common-Base

Configuration


The output resistance

r0 is quite high.

typically extend into

the megaohm range.

Common Base re

equivalent circuit




Common Emitter Fixed Bias Configuration


Common-emitter fixed-bias

configuration.

Network after the removal of the effects

of VCC, C1 and C2.






Substituting the re model into the network.






o C

io C o o C o

e e

o C o Cv v r 10R

i e e

V (R ||r ) , , V (R ||r )r r

V (R ||r ) RA , A

V r r

ib b

V VI I

Input impedance:

Output impedance:

Voltage gain:

eE r10Rei

eBi

rZ

r||RZ

Co

O

R10rCo

Co

RZ

r||RZ






Demonstrating the 180°phase shift between input and

output waveforms.

o C ov

i e

V (R ||r )A

V r




Example 5.1


Determine re, Zi (with ro=∞), Zo (with ro=∞),

Av (with ro=∞).

Repeat with ro=50 kΩ.




Example 5.1 - Solution





Common-Emitter

Voltage-Divider Bias

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





Common-Emitter Voltage-Divider Bias

Input impedance:

Output impedance:

Voltage gain:

ei

21

r||RZ

R||RR

Co 10RrCo

oCo

RZ

r||RZ


o C

io C o o C o

e e

o C o Cv v r 10R

i e e

V (R ||r ) , , V (R ||r )r r

V (R ||r ) RA , A

V r r

ib b

V VI I




Example 5.2


Determine re, Zi , Zo (with ro=∞), Av (with

ro=∞). Repeat with ro=50 kΩ.




Example 5.2 - Solution





Common-Emitter

Emitter-Bias Configuration





Impedance Calculations

E

i B b

1

1

for R

Z R ||Z

i b e e E

i b e b E

ib e E

b

b e E e E

b E e

V I r I R

V I r I R

VZ r R

I

Z r R r R

Z R r

Input impedance:

Output impedance:

o CZ R





o

o

o Cv

i b

b e E

o Cv

i e E

b E

o Cv

i E

V

V

V RA

V Z

substituting Z (r R )

V RA

V r R

and for the approximation Z R

V RA

V R

o C b C

iC

b

I R I R

VR

Z

Gain Calculations

Voltage gain:





Example 5.3


Determine re, Zi , Zo , Av . ignore ro for ro ≥ 10(RC+RE)

Without CE (unbypassed):




Emitter-Follower Configuration

• This is also known as the common-collector configuration.

• The input is applied to the base and the output is taken from the emitter.

• There is no phase shift between input and output.






Input impedance:

i B b

b e E

b e E

b E E e

Z R ||Z

Z r ( 1)R

Z (r R )

Z R (for R >>r )






Output impedance:

E e

e b

o

o E e o e R r

, I =( +1)I

( +1)

( +1)

( +1)

sin ( +1)

To determine Z , is set to zero

Z R ||r , Z r

ib

b

i

b

ie

e E

ie

e E

i

VI

Z

V

Z

VI

r R

ce

VI

r R

V





Gain Calculations

Voltage gain:

E e E e E

Eo

E e

o Ev

i E e

ov R r , R r R

i

RV V

R r

V RA

V R r

VA 1

V

i





Example 5.7


Determine re, Zi , Zo , Av .




Example 5.7 - solution


Documents

Chapter 5: Islamic University of Gaza BJT AC Analysis Dr ...site.iugaza.edu.ps/tskaik/files/chapter5_p1.pdf · Electronic Devices and Circuit Theory, ... Robert L. Boylestad and Louis