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It is positive, or in-phase, with the fundamental, and from a frequency domain viewpoint, the action of the partially cutoff transistor is to generate a substantial amount of second harmonic,
which reduces the dips of the fundamental sinewave and sharpens the peaks, resulting
in a lower mean level for approximately the same overall peak-to-peak amplitude.
We can note, in passing, that using the cutoff characteristic of a transistor is
not the only possible method for generating such a desirable second harmonic component.
It may be possible, for example, to tailor the frequency response of the input
matching network such that the drive voltage has some appropriate in-phase second
harmonic enhancement, assuming it is coming from another, possibly nonlinear,
amplifier driver stage.