PURPOSE: To make operation stable by the consitution using a differential amplifier circuit and a common emittr-type amplifier circuit.

CONSTITUTION: Differential amplifier circuit 10 constitutes a differential input- stage element by transsistors Q1 and Q2, and emitters are connected commonly, and the common connection end is ground through resistance R2. The collector of Q1 is connected to power supply V1 through the collector and the emitter of Q3 which are load elements, and diode D is connected between the base and the emitter of Q3. The output end of amplifier circuit 10 is connected to the base of emitter follower Q4, and the collector of Q4 is connected to power supply V1, and the emitter is grounded through resistance R3 and bleeder resistance R4, thereby constituting buffer circuit 13. The connection end of R3 and R4 is connected to the base of Q5 of common emitter-type amplifier circuit 14, and the collector of Q5 is connected to power supply V1 through R5. The collector of Q5 is also connected to the base of Q2 through R7, and the connection end of this base and R7 is grounded through by-pass capacitor C3. The collector of Q5 is connected to the base of Q6 of detector circuit 15, and the collector of Q6 is connected to power supply V1, and the emitter is grounded through R6.