PURPOSE: To output a signal giving a change of amplification factor proportional or inversely proportional to a control input by applying logarithmic expansion to a signal voltage under the control of a control output voltage.

CONSTITUTION: The titled circuit consists of operational amplifiers A1∼A4, a subtraction circuit A5, log diodes D1∼D3, resistors R1∼R7, a constant current circuit I and a transistor (TR). A signal voltage Vs applying logarithmic compression to a signal input by the diodes, a control voltage Vc applying logarithmic compression to a control input by the diodes and a reference voltage Vr applying logarithmic compression to a constant current by the diodes, a control output voltage Vcr being the result of input of the control voltage Vc and the reference voltage Vr to the subtraction circuit As are formed, and a signal output amplified variably is generated by applying logarithmic expansion to the signal voltage Vs under the control of the control output voltage Vcr. Thus, the amplification factor is changed proportional or inversely proportional to the control input with simple constitution.