c
°
Copyright 2008. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of
Electrical and Computer Engineering.
FET CurrentMirror Examples
CommonSource Ampli
fi
er
Figure 1 shows a commonsource ampli
fi
er. The active device is
M
1
. Its load consists of a current
mirror active load consisting of
M
2
and
M
3
. The current source
I
REF
sets the drain current in
M
3
which is mirrored into the drain of
M
2
. Because the sourcetodrain voltage of
M
2
is larger than
that of
M
3
, the Early e
ff
ect causes the dc drain current in
M
2
to be slightly larger than
I
REF
.
Figure 1: Commonsource ampli
fi
er.
The voltage
V
G
is the dc component of the gate input to
M
1
. It is a dc bias voltage which sets
the dc drain current in
M
1
. This current must be equal to the drain current in
M
2
in order for the
dc output voltage to be stable. In applications,
V
G
would usually be set by feedback. It will be
assumed that
r
01
=
∞
in calculating
i
o
(
sc
)
but not in calculating
r
out
. Note that the body e
ff
ect
must be accounted for in
M
1
. Assume that
I
D
1
=
I
D
2
=
I
D
3
=
I
REF
.
(a) Use the source equivalent circuit to calculate the shortcircuit output current. Assume that
r
01
=
∞
.
i
o
(
sc
)
=
−
i
0
d
1
=
−
i
0
s
1
=
−
v
i
(1 +
χ
1
)
r
is
1
+
R
S
r
is
1
=
r
s
1
1 +
χ
1
=
1
g
m
1
(1 +
χ
1
)
where
g
m
1
= 2
p
K
1
I
REF
(b) Calculate the output resistance. Assume that
r
01
<
∞
.
r
out
=
r
id
1
k
r
02
where
r
id
1
=
r
01
μ
1 +
R
S
r
is
1
¶
+
R
S
=
r
01
[1 + (1 +
χ
1
)
g
m
1
R
S
] +
R
S
1
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r
01
=
λ
−
1
+
V
DS
1
I
REF
r
02
=
λ
−
1
+
V
DS
2
I
REF
(c) Calculate the output voltage and the voltage gain.
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 Summer '08
 HOLLIS
 output voltage, Thévenin's theorem, Voltage source, Norton's theorem, Current Source

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