Operation of this power amplifier
D1 is forwarded biased by R4 so the voltage is 0.7V above the voltage at A. You have to pay attention to the signs. The NPN transistor emitter is about 0.7V lower, so almost equal to the voltage at A. 

The same is true of the PNP transistor, with signs reversed, so the outout is at about the same voltage as A.