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Spring 2001
Some Practice Problems
1. The
gure shows the ac signal circuit of a cascode ampli
er. It is given that
β
=199
,
r
x
=20
,
V
T
=0
.
025
,
r
0
=
∞
,
I
E
1
=1
mA,
C
2
=100
pF,
R
B
=1
k
Ω
,
R
E
=20
Ω
,and
R
C
=10
k
Ω
.
Note that the answers below would be more complicated if
r
0
is not in
nity.
(a) What is the lowfrequency gain of the ampli
er? Calculate this with
C
1
an open circuit.
v
o
v
i
=
i
c
1
v
i
i
c
2
i
c
1
v
o
i
c
2
=
G
mb
1
α
2
(
−
R
C
)
where
R
tb
1
=
R
B
and
R
te
1
=
R
E
.
(b) What is
K
in applying the Miller theorem to
C
1
?
K
=
−
G
mb
1
(
−
r
ie
2
)
where
r
x
=0
,
R
tb
1
=0
,and
R
te
1
=
R
E
.
(c) What is the time constant for the base circuit of
Q
1
?
τ
b
1
=[
R
B
k
r
ib
](1+
K
)
C
1
(d) What is the time constant for the collector circuit of
Q
1
?
τ
c
1
=
r
ie
2
C
1
(e) What is the approximate upper cuto
f
frequency of the ampli
er?
f
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This note was uploaded on 05/26/2011 for the course ECE 3050 taught by Professor Hollis during the Summer '08 term at Georgia Institute of Technology.
 Summer '08
 HOLLIS

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