Transistor Circuit Analysis

Bipolar Junction


Transistor Circuit Analysis

EE314

Chapter 13: Bipolar
Junction Transistors




1.
Large signal DC analysis

2.
Small signal equivalent

3.
Amplifiers

BJT Transistor Circuit Analysis

Circuit with BJTs

Our

approach
:

Operating

point

-

dc

operating

point



Analysis

of

the

signals

-

the

signals

to

be

amplified

Circuit

is

divided

into
:

model

for

large
-
signal

dc

analysis

of

BJT

circuit




bias

circuits

for

BJT

amplifier




small
-
signal

models

used

to

analyze

circuits

for





signals

being

amplified


Remember !

Large
-
Signal dc Analysis:

Active
-
Region Model

Important
:

a

current
-
controlled

current

source

models

the


dependence

of

the

collector

current

on

the

base

current

The

constrains

for

I
B

and

V
CE

must

be

satisfy

to

keep

BJT

in

the

active
-
mode

V
BE

forward bias

V
CB

reverse bias

?

?

Large
-
Signal dc Analysis:

Saturation
-
Region Model

V
BE

forward bias

V
CB

forward bias

?

?

Large
-
Signal dc Analysis:

Cutoff
-
Region Model

V
CB

reverse bias

V
BE

reverse bias

?

?

If

small

forward
-
bias

voltage

of

up

to

about

0
.
5

V

are

applied,

the

currents

are

often

negligible

and

we

use

the

cutoff
-
region

model
.

Large
-
Signal dc Analysis:

characteristics of an npn BJT

Large
-
Signal dc Analysis

Procedure
:

(
1
)

select

the

operation

mode

of

the

BJT



(
2
)

use

selected

model

for

the

device

to

solve

the

circuit



and

determine

I
C
,

I
B
,

V
BE
,

and

V
CE




(
3
)

check

to

see

if

the

solution

satisfies

the

constrains

for



the

region,

if

so

the

analysis

is

done


(
4
)

if

not,

assume

operation

in

a

different

region

and



repeat

until

a

valid

solution

is

found


This

procedure

is

very

important

in

the

analysis

and

design


of

the

bias

circuit

for

BJT

amplifier
.


The

objective

of

the

bias

circuit

is

to

place

the

operating

point

in

the

active

region
.



Bias

point

–

it

is

important

to

select

I
C
,

I
B
,

V
BE
,

and

V
CE


independent

of

the

b

慮d

ope牡rion

te浰e牡r畲u
.

b硡浰le13.4,13.5,13.6

Large
-
Signal dc Analysis: Bias Circuit

From Example 13.6

Remember
:

that

the

Q

point

should

be

independent

of

the

b



(
sta扩lity

iss略)


V
BB

&

V
CC

provide

this

stability,

however

this

impractical

solution

Other

approach

is

necessary

to

solve

this

problem
-
resistor

network

V
BB
acts as a short

circuit for ac signals

Large
-
Signal dc Analysis: Four
-
Resistor Bias Circuit

1

2

3

4

Thevenin

equivalent

Equivalent
circuit for
active
-
region
model

Solution

of

the

bias

problem
:

Input

Output

Small
-
Signal Equivalent Circuit

Thevenin

equivalent

Small

signal

equivalent

circuit

for

BJT
:

and

Common Emitter Amplifier

Find voltage gain:

First perform DC analysis to find
small
-
signal equivalent
parameters at the operating point.

Find input impedance:

Common Emitter Amplifier

Find power gain:

Find current gain

Find output impedance:

Problem 13.13:


Suppose that a certain
npn

transistor has V
BE

= 0.7V for I
E

=10mA.
Compute V
BE

for I
E

= 1mA.


Repeat for I
E

= 1µA. Assume that V
T

= 26mV.

Problem 13.14:


Consider the circuit shown in Figure P13.14. Transistors Q
1

and Q
2

are
identical, both having
I
ES
= 10
-
14
A
and
β = 100
. Calculate V
BE

and I
C2
.
Assume that
V
T

= 26mV
for both transistors.


Hint:

Both transistors are operating in the active region.

Because the transistors are identical and have identical values of V
BE
,
their collector currents are equal.

Problem 13.50:

The transistors shown in Figure P13.50 operate in active region and
have
β = 100, V
BE
=0.7V
. Determine I
C

and V
CE

for each transistor.

I
1

I
E2

V
BE

I

I
1

I
E

Problem 13.52:

Analyze the circuit of Figure P13.52 to determine I
C

and V
CE
.

I
B

I
C

Problem 13.45:

Analyze the circuits shown in Figure P13.45 to determine I and V. For
all transistors, assume that
β = 100
and
|V
BE
| = 0.7V
in both the active
and saturation regions. Repeat for
β = 300
.

Problem 13.45: Contd.

I
1

Problem 13.67:

Consider the emitter
-
follower amplifier of Figure P13.67 . Draw the dc
circuit and find I
CQ
. Next, determine the value of r
π
. Then, calculate
midband values for A
v
, A
voc
, Z
in
, A
i
, G and Z
0
.

npn

BJTs
–

Practical Aspects

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