The term "gm doubling" as I understand it refers to the increase in gain that occurs from when one half of the output stage is conducting, to when both halves are conducting. The idea is that there's "double the transistors", so there's "double the transconductance". The static gain is of course no where near double because of the local feedback intrinsic in the output stage.
"Gm doubling" is the main problem with a Class AB amplifier, and manifests itself as a sharp nonlinearity at the point of transition between Class A and Class B operation. This can easily be seen when looking at the distortion output on a 'scope - as the signal is increased into a resistive load, the nonlinearity on the distortion residual slides, in alignment, from the tips of the waveform toward the center, as the amplifier transitions from Class A to Class B. And you're correct in the assertion that increasing bias doesn't improve the problem, it just increases the signal level at which it occurs.
Class A amplifiers don't have this problem, because "gm doubling" occurs all the time - since both halves are always conducting, there's no change in gain to the point where they're not. Class B amplifiers must walk a thin line between cutoff (underbiased) and gm-doubling (overbiased) - this is why bias tracking is so critical on class B designs.
"Gm doubling" is the main problem with a Class AB amplifier, and manifests itself as a sharp nonlinearity at the point of transition between Class A and Class B operation. This can easily be seen when looking at the distortion output on a 'scope - as the signal is increased into a resistive load, the nonlinearity on the distortion residual slides, in alignment, from the tips of the waveform toward the center, as the amplifier transitions from Class A to Class B. And you're correct in the assertion that increasing bias doesn't improve the problem, it just increases the signal level at which it occurs.
Class A amplifiers don't have this problem, because "gm doubling" occurs all the time - since both halves are always conducting, there's no change in gain to the point where they're not. Class B amplifiers must walk a thin line between cutoff (underbiased) and gm-doubling (overbiased) - this is why bias tracking is so critical on class B designs.