chapter 4 islamic university of gaza dc biasing...

Chapter 4

DC Biasing–BJTs

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

Voltage Divider Bias

This is a very stable bias circuit.

The currents and voltages (IC

and VCE) are nearly independent

of any variations in .

2 Dr. Talal Skaik 2014




Voltage Divider Bias Analysis


DC Circuit

Redrawing the input side of the network.






Determining RTH Determining ETH.

1 2||thR R R 22

1 2

CCth R

R VE V

R R






Inserting the Thévenin equivalent circuit.

0

( 1)

th B TH BE E E

E B

E I R V I R

I I

( 1)

th BEB

TH E

E VI

R R

( )CE CC C C EV V I R R




Example 4.8


Find ICQ , VCEQ




Example 4.10


Repeat example 4.8

with β=50. compare solutions for IC and VCE

β IC(mA) VCE (V)

100 0.84 mA 12.34 V

50 0.81 mA 12.69 V




Collector Feedback Configuration

Another way to

improve the stability

of a bias circuit is to

add a feedback path

from collector to base.

In this bias circuit

the Q-point is only

slightly dependent on

the transistor beta, .





Base-Emitter Loop

)R(RR

VVI

ECB

BECCB

From Kirchhoff’s voltage law:

0RI–V–RI–RI– V EEBEBBCCCC

Since IB << IC:

CI

BI

CI

CI'

Knowing IC = IB and IE IC, the

loop equation becomes:

0RIVRIRI– V EBBEBBCBCC

Solving for IB:


I IC B




Collector-Emitter Loop

Applying Kirchoff’s voltage law:

IE RE + VCE + I’CRC – VCC = 0

Since IC IC and IE IC :

IC(RC + RE) + VCE – VCC =0

Solving for VCE:

VCE = VCC – IC (RC + RE)





Example 4.12


Find ICQ , VCEQ




Example 4.13


Repeat example 4.12,

with β=135. (Find ICQ , VCEQ ).




Example 4.14


Determine the DC level of

IB and VC.




Emitter Follower (Common Collector)

Configuration


The output is taken

off the emitter terminal.




Emitter Follower (Common Collector) Configuration


0

using =( +1)

( +1)

0

B B BE E E EE

E B

B B B E EE BE

CE E E EE

I R V I R V

and I I

I R I R V V

V I R V

( +1)

EE BEB

B E

V VI

R R

CE EE E EV V I R




Example 4.16


Determine VCEQ and IEQ




Common Base Configuration


The applied signal is connected to the emitter terminal.




Common Base Configuration


Input DC equivalent

0EE E E BEV I R V

EE BEE

E

V VI

R

Determining VCE and VCB

0

( )

0

EE E E CE C C CC

E C CE EE CC E E C

CB C C CC

E C CB CC E C

V I R V I R V

I I V V V I R R

V I R V

I I V V I R




Example 4.17


Determine IE , IB , VCE

and VCB

chapter 4 islamic university of gaza dc biasing...

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