Digital Signal Processing Reference
In-Depth Information
accordingly. On the other hand, if the bias current needs to be kept fixed, a
reduction of the overdrive voltage across the gate-source terminals results in
an inverse proportional increase of the transconductance. Remark that the di-
mensions of the transistor should be scaled appropriately to maintain a fixed
bias current.
At first glance, one would prefer a reduction of the overdrive voltage, while
the bias current remains unaltered. After all, this method seems to increase
the dc-gain of the transistor without raising the power consumption. Unfortu-
nately, there is no such thing as a free lunch. In order to clarify this universal
misfortune, let us first introduce some important performance parameters of
the mos transistor.
Cut-off frequency of a mos transistor
The
cut-off frequency
(
f
T
) of a single mos transistor is defined as
the frequency at which the small-signal gate current becomes equal
to the small-signal drain current (with drain shorted to ac-ground). It
is the transition frequency (hence the abbreviation
f
T
) beyond which
the transistor does not provide any useful current gain. The cut-off fre-
quency is a measure for the intrinsic speed (excluding junction and
wiring impedance) of a transistor and is given by (A.4):
g
m
2
πC
gs
f
T
=
[Hz] ,
(A.4)
where
C
gs
is a rough estimation of the parasitic gate-source capaci-
tance. In the strong inversion region, this capacitance is dominated by
the oxide capacitance between the gate and the inversion layer at the
source node implant of the mos transistor [Eyn89, Raz94].
Maximum frequency of oscillation
The
maximum frequency of oscillation
(
f
max
)ofamos device is de-
fined as the maximum frequency at which the unilateral power gain
1
of
a transistor drops below 0 dB [Dro55]. The
f
max
limit can be reached
if the impedance of parasitic capacitances at gate and drain are com-
pensated using appropriately sized inductors.
2
1
Unilateral power gain: the power gain of a transistor when feedback has been used to neutralize S
12
.Also,
both the in- and the output reflection coefficients (
S
11
and
S
22
) have been matched to zero.
2
Such as is done in an oscillator, hence the term
maximum frequency of oscillation
.
