What is a complementary symmetry amplifier?

Answer 1

A complimentary-symmetry amplifier is a power output stage using two transistors. One is an NPN and the other is PNP, and normally the designer will use transistors that are compliments of one another (TIP31C for NPN and TIP32C for PNP, for example).

The transistors are hooked in series in a sort of push-pull arrangement. The NPN's collector connects to the positive voltage rail and the PNP's collector connects to either A) ground or B) a negative but equal voltage rail. The emitters are connected together and are at either A) 1/2 the positive voltage supply or B) zero volts. In A) a large coupling capacitor must be used to connect to the speaker while in B) there is no DC difference and the speaker can be directly connected.

This configuration is similar to a "self-inverting push-pull" circuit in tube-speak. A positive-going input pulse will cause the NPN to conduct more and the PNP to conduct less. A negative-going pulse will cause the PNP to conduct more and the NPN less. It's useful for a small, transformerless output stage but there are more efficient and clearer designs. I can't tell you much more as I'm just now getting into transistors and am still experimenting with this circuit.

That was probably more info than you were wanting, however. Good luck.