UNIVERSITY OF CALIFORNIA AT BERKELEY
College of Engineering
Department of Electrical Engineering and Computer Sciences
Homework #7 Solutions
Due Tuesday, October 16, 2007
EE 105
Fall 2007
Prof. Liu
Problem 1. DC Biasing of Cascode Stage
a)
V
b
2
=
V
T
ln
parenleftbigg
I
C
I
S
+ 1
parenrightbigg
=
792 mV
b) To obtain the appropriate current, we need
V
BE
1
=
V
BE
2
(assuming
I
C
1
≈
I
C
2
). We know
that
V
E
1
=
V
C
2
=
V
b
2
−
300 mV, since the basecollector junction is forward biased by
300 mV. Since
V
b
1
=
V
C
2
+ 792 mV, we have
V
b
1
= 1
.
284 V
.
If we want to be more precise, we need:
I
E
1
=
1 +
β
β
I
S
parenleftBig
e
V
BE
1
/V
T
−
1
parenrightBig
=
I
C
1
V
BE
1
=
V
T
ln
parenleftbigg
βI
C
1
(1 +
β
)
I
S
+ 1
parenrightbigg
= 792 mV
As you can see, our approximation that
I
C
1
=
I
C
2
is a good approximation, since we obtain
the same value for
V
BE
1
.
Problem 2. ShortCircuit Transconductance
We know
R
out
from our analysis of a cascode previously (when we short the input to ground,
we obtain the same circuit).
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 Fall '07
 KingLiu
 Electrical Engineering, Smallsignal model, Early effect

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