aaron_do
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I'm actually not that familiar with PA design, but anyway......
Harmonics arise as a result of the gain compression, but do not actually cause gain compression in the desired band unless they pass through a second nonlinear stage (then you get intermodulation).
For example if you input a sine wave and get out a square wave then you now have the fundamental frequency plus a bunch of harmonics. But the gain compression in the desired band is not due to the harmonics, they are a result of the nonlinearity.
As for the nonlinearity, the nonlinearity is basically a result of the gain compression due to the limited supply voltage. It isn't really a result of the transistor's intrinsic gm (where gm is taken assuming an unlimited supply voltage or zero output resistance). For example a MOSFET is ideally only second order nonlinear so there shouldn't be any gain compression in the fundamental frequency. But the gm of the transistor will change according to the limited supply voltage.
If you have an output y=a1x(t) + a2x^2(t) - a3x^3(t) and you input only a fundamental x(t)=A1cos(wt) then you will get
y(t) = a1A1cos(wt)-3/4a3A1^2cos(wt)
at the fundamental plus a whole bunch of harmonics. The -3/4 term is the gain compression.
So basically, 1)supply causes gain compression, 2)gain compression changes transistor gm, 3)harmonics are an effect not a cause
Sorry for taking so long...
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