PURPOSE: To stabilize a gain, and to reduce an influence of a linear distortion of a diode by connecting the diode and a series resistance as a load, to a differential amplifier whose voltage gain is set by a value of a constant-current supplied from a variable constant-current source.

CONSTITUTION: A differential amplifier 34 is constituted of transistors TR 30, 32. A diode 38 and a resistance 40 are connected in series as a load of the differential amplifier 34, between the collector of the TR30 and a current supply terminal 36, and a diode 42 and a resistance 44 are connected in series between the collector of the TR32 and the power source supply terminal 36, too, in the same way. Input terminals 46, 48 are formed between the bases of the TRs 30, 32, and output terminals 50, 52 are formed between its collectors. The emitters of the TRs 30, 32 are connected in common through resistances 54, 56, and a variable constant-current source 60 whose constant-current output can be varied is connected between a connecting point of the resistances 54, 56 and a reference potential terminal 58.