Steve says:
>In my experience, the formula holds at 0.5-2dB gain compression, but only
if
>the amplifier if truely linear up to when it starts to compress. If the
>formula is mathematically derived, it has to assume linearity up to V/2,
>then look at compression at full output.
Yes, that's what it does.
> It can't differentiaite between hard and soft compression either (although
the effect in practice wasn't
>that great in my tests).
>It is constructive to draw a graph of gain vs output power - ideal amps
have
>a constant line until you approach full output, when saturation starts and
>the gain falls. The formula seems to work well for such amps.
Unfortunately,
>very few seem to be like this. Many power mosfets have gain which drops
>continuously with power (Po/Pin is banana shaped) latest LDMOS has an 'S'
>shaped Po/Pi characteristic at most bias levels. Most bipolar amps are
>dreadful because the particular transistor isn't suited to linear operation
>and/or the biassing arrangements are rubbish.
I'm told that if you do the maths for the exponential transfer
characteristic of a bipolar transistor, the 1dB gain compression comes out
at 10dB below third order intercept. This does, however, take no notice of
the effects of Re (as opposed to re) producing negative feedback, which
especially at high currents, can't be neglected. Or the effects of hfe sag
at high collector currents.
73
Peter G3RZP
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