BJT Amplifier DC and AC Equivalent Circuits

bahmotherElectronics - Devices

Oct 7, 2013 (3 years and 11 months ago)

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Electronic Design Laboratory Spring 2006 jeg
1
BJT Amplifier DC and AC Equivalent Circuits
For all three circuits assume that I
C
= 4 mA, V
BE
= 0.7 V and V
CE
= 2.V. For the small-signal gain circuits
assume r
o
is infinite, r
π
= 0.025/I
B
. For Figure 2 (a), first calculate the value of R
1
and R
2
from the bias
conditions and then the voltage gain from the small-signal model. For your convenience the DC bias
circuit and small-signal gain equivalent circuit are given below.

DC Bias Circuit

R
2

12 V
R
1

12 V
+
V
BE

-
I
B
β
F

B

V
C

I







Small-Signal Gain Circuit



R
2
R
1

i
B
β
o

B

v
o

r
π

v
i



Electronic Design Laboratory Spring 2006 jeg
2
For Figure 2 (b), first calculate the value of R
1
and R
2
from the bias conditions and then the voltage gain
from the small-signal model. For your convenience the DC bias circuit and small-signal gain equivalent
circuit are given below.

DC Bias Circuit

R
2

12 V
R
1

+
V
BE

-
I
B
β
F

B

V
C

I







Small-Signal Gain Circuit


R
2
R
1

i
B
β
o

B

v
o

r
π

v
i



Electronic Design Laboratory Spring 2006 jeg
3
For Figure 2 (c), first calculate the value of R
1,
R
2
, R
3
and R
4
from the bias conditions and then the voltage
gain from the small-signal model. For your convenience the DC bias circuit and small-signal gain
equivalent circuit are given below. R
TH
is the parallel combination of R
3
and R
4
.

DC Bias Circuit

R
2
12 V
R
4

12 V
+
V
BE

-
I
B
β
F

B

V
C

I

R
3

R
1




Small-Signal Gain Circuit



R
2
R
TH

i
B
β
o

B

v
o

r
π

v
i

(
β
o
‫‱) R
1

β
o

β
o
‫ 1)


1