Gary Schafer wrote:
> I am guessing that the 3:1 ratio for IM3 happens in pure class A?
> Also from what I have read recently that the IM5 is a 5:1 ratio.
Yes - although IMD5 (or higher order) is rarely significant with class A
amps running low IMD3.
> The data that I cited earlier about the IM products being lower at
> higher power levels was for AB1 operation. Apparently IM performance is
> entirely different in AB1 and A.
Very! It's a whole new world where driver stages are much bigger than
you think they need to be. Say your output stage will deliver the wanted
power at -50dBc IMD3 - if the driver also runs at -50, the overall
result will be -44dBc. The driver has to run at -56 or better to affect
the overall IMD by 1dB or less. You churn the linearity numbers, and
it's a bigger device than you might think at first. With older
semiconductors where each stage has a gain of 8-10dB, a typical lineup
would have 1 driving 2 driving 4 identical devices, all running the same
class A dissipation - massive overkill by class AB standards (imagine
4CX250 driving 3-500 driving 2x3-500).
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