PURPOSE: To prevent the occurrence of impedance mismatching in the output terminal by supplying the supply voltage, which is changed to linearly power- amplify an input signal, to a power amplifier.

CONSTITUTION: A CPU 20 reads out data of a DC voltage VCC corresponding to the level of an inputted audio signal and data of a control bias voltage Vb from a RAM 22 and outputs them to a data buffer memory 26 and a D/A converter 27. Data of the DC voltage VCC is temporarily stored in the memory 26 and is outputted to a programmable voltage source 5a, and the voltage source 5a outputs the DC voltage VCC corresponding to data to a power amplifier 4. Meanwhile, the converter 27 subjects data to D/A conversion to generate the control bias voltage Vb and outputs this voltage to a variable attenuator 8. Thus, the input signal is linearly amplified by the power amplifier 4, and consequently, the impedance mismatching state does not occur in the output terminal of the amplifier 4.