[Amps] Plate impedance

[Peter Chadwick]

Will,
those are the classic definitions.
Rp = dVa/dIa     gm = dIa/dVg   mu = dVa/dVg (Va being anode volts, Ia being anode current and Vg being grid volts) and of course, mu = ra.gm.
where d is the delta.
These days, though, its gm = dId/dVg    where dId is the change in drain current  for a dVg change in gate voltage. We don't use mu, but do use gm and the ac drain resistance.........and pretty low values of gm, too, with low power RF CMOS
Then the 'young' IC designers (those in their 40's and below) show surprise that we had tubes that could run to 15 or 20mA/V gm with plate currents of 15mA or less....and that Miller effect was found in tubes, and that induced grid noise is the same mechanism as induced gate noise and that drain-gate feedback capacity can lead to oscillation in tuned drain amplifiers unless neutralised or cascoded.
Parasitic oscillations are something else they don't know about, either........I met one who was sure that the distributed amplifier was an invention of the solid state era.
I'm getting old and grouchy.
73
Peter G3RZP