To prevent the collector current of fifth and sixth transistors from largely changing by the change of a power source voltage.

The first and the third transistors Q1 and Q3 and the second and fourth transistors Q2 and Q4 are connected by inverted Darlington, the emitters of the third and fourth ones are connected to the bases of the fifth and the sixth transistors Q5 and Q6 and a bias circuit V3 is arranged between the bases of the first and the second ones, the collector of the seventh transistor Q7 is connected to the intermediate connection point of the first and the third ones, a first diode D1 is arranged between the base of the seventh one and a positive power source V1, the collector of the eighth transistor Q8 is connected to the intermediate connection point of the second and the fourth ones, the second diode D2 is arranged between the base of the eighth one and a negative power source V2, the first and the second Zener diodes D3 and D4 are arranged between the bases of the seventh and eighth ones and the emitters of the first to eighth ones are connected to the positive power source V1 or the negative power source V2.