Digital Signal Processing Reference
In-Depth Information
Fig. 3.1
Schematic view
and small-signal equivalent
scheme of a single-ended
amplifier.
r
L
is the output
resistance of a non-ideal
current source
I
L
V
DD
V
in
M
1
v
in
v
out
V
out
g
m
r
ds
r
L
I
L
V
SS
(a)
(b)
amplifier for which p-type solutions are provided, follows. After a behavioral com-
parison also simulation results are presented that compare differential amplifiers,
all built with a different load topology. Finally the applicability of a set of standard
circuit techniques, such as cascoding and gain boosting, is discussed.
3.2.1 Single-Ended Amplifier
The p-type single-ended amplifier is presented in Fig.
3.1
. This amplifier is built up
with one p-type transistor
M
1
and a current source. The DC voltage of the output node
is determined by the component equations of both the transistor
M
1
and the current
source. The component equation of
M
1
biased in saturation is given in Eq. (
3.1
):
2
1
W
L
(
V
SD
V
E
·
K
p
·
I
SD
=
V
SG
−
V
T
)
+
(3.1)
L
where
K
p
is a technology-defined constant and
V
E
is the Early voltage of the tran-
sistor. The output voltage of the amplifier is obtained by rewriting this equation into
Eq. (
3.2
):
1
V
E
·
I
L
V
out
=
V
DD
−
V
SD
=
V
DD
−
2
−
L
(3.2)
W
K
p
·
L
(
V
SG
−
V
T
)
An acceptable value for the DC level of
V
out
is
V
DD
/
2. Then the equation is
slightly simplified to Eq. (
3.3
):
