PURPOSE: To obtain an AGC amplifier of a wide range, which generates no waveform distortion, by inserting one diode or transistor into a general conventional transistor AGC amplifier, by a theory of the variable gain by which a logarithmic transducer and an exponential transducer are cascade-connected.

CONSTITUTION: A logarithmic transducer 1 using one diode D1 and an expotential transducer 2 consisting of a transistor TR1 are cascade-connected. An input signal supplied through an input transformer T2 is supplied to the transistor TR1 of the exponential transducer 2 through the logarithmic transducer 1, and an output signal whose gain is adjusted is fetched from intermediate frequency transformer T1 inserted into a collecting circuit of this transistor TR1. A constant-current power source ID determines an operating point of the diode D1, and a constant-current power source Ic/β determines an operating point of the transistor. A linear amplifier of the variable gain can be constituted by changing each operating point.