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
Try reading Motorola AN211
RL
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