EE 438
Homework #18
1.
The small-signal equivalent circuit of the common-emitter amplifier shown in Fig. 1 on page 4
is given below.
_
+
V
i
R
i
C
µ
C
π
g
m
V
π
V
π
C
1
R
B
C
R
′
V
o
+
_
c
b
e
The elements in the equivalent circuit have the following values.
.
fF
20
and
pF,
10
,
k
2
||
,
mS
40
F,
2
,
k
4
||
||
,
1
1
2
1
=
=
Ω
=
=
′
=
=
Ω
=
=
Ω
=
µ
π
π
µ
C
C
r
R
R
g
C
r
R
R
R
k
R
o
C
C
m
B
B
B
i
(a)
Use the low frequency approximation (
C
π
and
C
µ
open-circuited) to determine
A
vl
(
s
) in the
(
s
+1/
T
) form with all constants, including
A
mid
, expressed as numerical values.
(b)
Using the high frequency approximation (
C
1
shorted) we find that
A
vh
(
s
) is given by
( )
(
)(
)
[
]
{
}
(
)
2
||
1
||
1
1
s
C
C
R
R
R
s
C
C
R
g
R
R
C
R
s
g
C
A
s
A
C
i
B
C
m
i
B
C
m
mid
vh
π
µ
π
µ
µ
µ
′
+
+
′
+
+
′
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
=
.
Solve the denominator quadratic to express
A
vh
(
s
) in the
⎟
⎠
⎞
⎜
⎝
⎛
+
T
s
/
1
1
form with all constants,
including
A
mid
, expressed as numerical values.
(c)
Combine
A
vl
(
s
) and
A
vh
(
s
) to form
A
v
(
s
) with all constants evaluated numerically.
2.
Draw a small signal equivalent circuit of the common-drain amplifier shown in Fig. 2 on
page 4. Include the two device parasitic capacitors
C
gs
and
C
gd
(do not include
C
gb
and
C
db
) in
your circuit. Indicate the locations of
V
i
and
V
o
and the transistor terminals.
Do not analyze the
circuit
.
3.
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