Data sheets for BJTs usually report the maximum frequency they can be
used to amplify a signal, while this data is omitted in (MOS)FET data
sheets. How do I determine if a field effect transistor is suitable to
work in, say, an UHF preamplifier?
Thanks!
The transition frequency Ft of a bjt is not the 'maximum frequency'of
use, it is the frequency where the current transfer ratio beta can be
expected to drop to 1.4 (3dB). As the voltage or power gain of a
circuit is only partially dependant on beta, useful frequency ranges
are application dependant - the requirements to produce an input
current and the effect of the resulting output current are
device(parasitic or stray) and circuit (source and load) dependant.
High frequency characteristics are better illustrated by measured
typical spot frequency parameters.
Fets, in general, are specified for tranconductance gm or forward
transfer admittance yfs. As with bipolars, the requirements to produce
an input voltage and the effect of the resulting output current are
both device(parasitic Ciss Crss Coss or stray) and application
dependent, over frequency. If the parts aren't characterized for the
frequency range of interest (a value-added design feature), the work
of doing so is left to the end-user without any assurance of
consistency between fabs.
RL
