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le13.4,13.5,13.6
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
http://www.4p8.com/eric.brasseur/vtranen.html
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