When the maximum efficiency of a linear amplifier occurs?
You are talking about instantaneous efficiency rather than the efficiency over one cycle of the input waveform which is what is usually considered. The latter efficiency is the ratio of the RMS power delivered to the load divided by the DC input power. For a sine wave and a linear class A amplifier the maximum efficiency is 50% which occurs when the output voltage reaches the supply voltage and is only possible in an ideal amplifier for which the output transistors have no internal voltage drop at maximum output.As the output voltage drops, the efficiency will drop also because the difference between the output voltage and the DC power supply now appears across the transistor and, when multiplied by the output current, this represents a power loss.