PURPOSE: To improve an S/N in a small signal and to expand a dynamic range by providing a control circuit which controls a wave distribution supply source and a bias current in proportion to the rectifying output level of an output signal.
CONSTITUTION: Transistors 3, 4 comprise a first differential amplifier circuit, and transistors 10, 9 comprise a second differential amplifier circuit. A voltage- current conversion circuit 21 converts output VOUT to a current, and supplies the current representing an output signal level to an adder circuit 11 via a full-wave rectifier circuit 20. Also, the adder circuit 11 supplies the current in which the currents from the full-wave rectifier circuits 13, 20 are added on the current from a coastant current source 12 to a current distribution circuit 8, and the current distribution circuit 8 supplies the bias currents I6, I7 of the differential amplifier circuits that flow on a common emitter connection point with a prescribed ratio. ln such a way, it follows that the bias current can be controlled in propertion to the size of a gain, which improves the S/N in the gain of the small signal and expands the dynamic range.