chapter 5: islamic university of gaza bjt ac analysis dr ... · 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

Common-Base Configuration • The input is applied to the

emitter.

• The output is taken from the

collector.

• Low input impedance.

• High output impedance.

• Very high voltage gain.

• No phase shift between input

and output.

2 Dr. Talal Skaik 2014




Calculations

eEi r||RZ

Co RZ

o

o

o C Cv

i e e

V ( )

V

V R RA

V r r

o C C C

e C

i ie C

e e

I R I R

I R

V VI R

r r

o

i

i

Assuming

IA 1

I

E e

e i

o e i

R r

I I

I I I

Input impedance:

Output impedance:

Voltage gain:

Current gain:


Av positive… Vi and Vo in phase.




Example 5.8


Determine re, Zi , Zo , Av , Ai




Common-Emitter Collector Feedback Configuration

• This is a variation of the common-emitter fixed-bias configuration

• Input is applied to the base

• Output is taken from the collector

• There is a 180 phase shift between input and output





Calculations

o C FZ R ||R

o Cv

i e

'

'

V RA

V r

o b

b o b

o o C b C

i ib o C

e e

I I I

For I I I I

V I R I R

V VI V R

r r

Output impedance:

Voltage gain:


Defining Zo




Calculations

F

C

ei

R

R1

rZ

Input impedance:


i

,

1' 1

( ') '

11

V 1 1

1 1

i ii o C

i e

o i o C i Ci ii

F F F e F F F e

i b e i e i e e

Ci i e e i

F e

e Ci e

F e

eii

i e C

F e

V VZ V R

I r

V V V R V RV VI V

R R R r R R R r

V I r I I r I r I r

RV I r rV

R r

r Ror I r

R r

rVZ

I r R

R r

1

1

C C ei

Ce e

F

R R rZ

Rr r

R




Determining the current gain using the voltage gain


L Li

Current Gain A , ,

.

A A

L

o oii i o

i i L

o

o oL ii

ii i L

i

iv

L

I VVI I

I Z R

V

I VR ZA

VI V R

Z

Z

R




L Li

Current Gain A ,

, 1.35 6.8

1.356.8 .6.8

1.35

1.35 ( 368.76) 73.2

6.8

1.35or A A ( 368.76) 73.2

6.8

L

oi

i

oii o

o

o oi

ii i

iv

L

I

I

VVI I

k k

V

I V kkAVI V k

k

k

k

Z k

R k

Determining the current gain using the voltage gain


From example 5.2

Zi=1.35 kΩ.

Av=-368.76




Effect of RL and RS


, , with

, with and

o ovNL vL L

i i

ovS L S

s

V VA A R

V V

VA R R

V




Effect of RL and RS


o C o C

e

o Cio C vL

e i e

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

V (R ||R )V (R ||R ) A

r V r

ib L b L b

LL

VI I I

V




O

i B e

o C

Input impedance: Z R || r

Output Impedance: Z R ||r

To find overall gain: ,

.

i s i ii

i s S i s

o o i i ivS vL vS vL

S i S i s i s

Z V V ZV

Z R V Z R

V V V Z ZA A A A

V V V Z R Z R

Effect of RL and RS





Darlington Connection

•The Darlington circuit provides a very high current gain—the

product of the individual current gains: D = 12

•A Darlington transistor connection provides a transistor having a

very large current gain, typically a few thousand.

•Darlington pairs are available as complete packages.

•A Darlington pair is sufficiently sensitive to respond to the small

current.





DC Bias of Darlington Circuits

BDBDE II)1(I

EEE RIV

EDB

BECCB

RR

VVI

Base current:

Emitter current:

Emitter voltage:

Base voltage:

BEEB VVV





Darlington Circuits


When light falls on the LDR,

its resistance reduces.

The bias voltage is supplied to

the transistor and this voltage is

enough to make the transistor

and relay work.

A variable resistor is also

connected on the base of

transistor to adjust the

sensitivity.

chapter 5: islamic university of gaza bjt ac analysis dr ... · robert l. boylestad and louis...

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