
609Microwave Transistors
f g C L
c m s
= =/ 2 /4
gs
( )π υ π
(14.43)
where g
m
is the transconductance, L is the gate length,
υ
s
is the saturation
drift velocity and C
gs
is the gate–source capacitance. The C
gs
is given by
C
dQ
dV
V
gs
gs
constant
gd
=
=
(14.44)
From the charge carrier transit time (
τ
) discussed in connection with the
power frequency limitation of BJTs, the cutoff frequency is obtained to be
f L
c s
= =1/2 /2πτ υ π
(14.45)
This value is just half of the value obtained in Equation 14.43.
Maximum oscillation frequency (f
max
): The maximum frequency of oscilla-
tion, in a distributed circuit, depends on the transconductance and the drain
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