MOSFET unity-gain freq

The unity gain frequency for a transistor is an important spec. because it gives us a good idea of where we can use it :)

For a FET, the transfer model (common source) is output current / input voltage (in the small signal sense) - the mutual transconductance, gm.

gm and wt (the unity gain frequency) are related roughly thus:

wt = gm / (Cgs+Cgd). As Cgd is _usually_ negligible with respect to Cgs, this is sometimes shortened to gm/Cgs. The longer version is wt = (Is/vgs) / (Cgs+Cgd) (all I did was substitute for gm)

Transposing and normalising, one gets:

vgs = Is/(Cgs+Cgd).

This is hardly surprising, as a FET is an electrostatic device - where Q = CV and therefore V = Q/C

Taking the differential, one gets

dV/dt = I/C

The similarity to the small signal analysis for FETs is not coincidental :)

Cheers

PeteS