PURPOSE: To make an amplifier sufficient in dynamic range corresponding to an input signal and high in efficiency by using a four-terminal FET added with a threshold voltage controlled electrode as an amplifier and impressing a voltage at a certain level corresponding to the amplitude of this input signal to this electrode.

CONSTITUTION: An amplitude detection circuit 2 detects a peak level in the amplitude of the input signal with the passage of time and supplies a level signal expressing the size of this amplitude to a bias control circuit 7, and the threshold control voltage is provided in the circuit 7 and impressed to a threshold voltage controlled electrode 3a of a four-terminal FET 3. Corresponding to the amplitude of the envelope of the input signal, the bias control circuit 7 sets threshold control voltage to be impressed to the electrode 3a of the FET 3. At such a time, the threshold control voltage to the FET 3 corresponding to the maximum value of the amplitude is decided so as to perform the A-class operation when the envelope amplitude is maximum. When the envelope amplitude is reduced, the threshold voltage of the FET 3 is changed to the positive side by changing the voltage to be impressed to the electrode 3a from the threshold control voltage to the negative direction.