Tim,
IMD can be caused by two things, saturation at high power and
cross-over distortion near idle conditions.
You missed one.? Anytime the output is not a uniform multiple of the input.
Or what is know ans non linear behavior.
Good example the HFE of 3904 at 10ma is 100, at 100ma its 40.? So is the?
waveform causes it to swing from 10 to 100ma the gain supplied is varying.
The base current required to get 10ma and 100 ma is not ten times higher
it is more like 25 times higher and the change is varying for intermediate
currents.??
For power MOSfets this is pronounced at low gate voltages as first we have
threshold where below that there is no conduction. as we go up from that point
the curve is markedly curved then straightens and at some high current bends
again.
For information look at the Ids VS Vds for Vgs curves.? From that graph we can get a
load line that reflects input voltage.? Old tube hands likely the curves looking like?
many power tubes.? ?It is on the page upper right corner.? Do not let pulsed be a bother
as in some cases those curves represent power well above what the device can do
sustained.? Needless to say its hard to run the drain to zero volts and remain linear.?
It also indicates a Gm of about 1S? IE 1Vgs change causes a 1A Drain current change.
Allison
