PURPOSE: To obtain a high output while securing protection against a surge voltage developed, specially, at a power source, by taking a countermeasure to the surge voltage through a single end class "B" push-pull circuit.

CONSTITUTION: Between the base and emitter of a transistor TR13, a TR23 and a TR21 for controlling the TR23 through a resistance 22 are added. If a high surge voltage is developed at a power source, TRs 3 and 6 turn on by receiving the output of an over-voltage detecting circuit 100 connected between the power source Vcc and a ground point GND to form short circuits between the bases and emitters of TRs 12 and 11, performing protection against collector-emitter dielectric strength BVCES. Then, TRs 21 and 23 turn on respectively to form a short circuit between th base and emitter of the TR13, performing protection against collector-emitter dielectric strength BVCES.