PURPOSE: To remove output fluctuations in a steady state without impairing response characteristics in a transient period by making the time constant of a smoothing circuit network in the transient response period sufficiently small.

CONSTITUTION: A parallel connected circuit of diodes D3, D4 and resistances R5, R6 is connected in the base circuit of transistors Q1, Q2. If the voltage value of the input signal supplied between the input terminals 3, 4 is low, the diodes D3, D4 maintain a nonconducting state and the input signal passes the resistances R5, R6. If the voltage value of the input signal supplied between the input terminals 3, 4 is high, the diodes D3, D4 conduct and the greater part of the input signal passes through the diodes D3, D4.