PURPOSE: To always maintain an operation at an optimum state against a variation of supply voltage of a power amplifier, and a variation of the load impedance, and to protect the power amplifier, by providing a voltage controlling circuit for controlling the supply voltage of a driving amplifier.

CONSTITUTION: A voltage controlling circuit 20 connected to an electric power supply terminal 6 is connected to a driving amplifier 3 connected to a high frequency signal input terminal 1. Also, to the respective circuit 20 and amplifier 3 are connected a positive electric power supply terminal 21 and a negative electric power supply terminal 22, and when voltage of the terminal 6 is raised by the circuit 20, an optimum operation of a power amplifier 5 which is operated by voltage of the terminal 6 is maintained by raising the supply voltage of the amplifier 3, and increasing an output of the amplifier 3. Also, when the impedance of a load 8 is lowered, an increase of a high frequency signal of the amplifier 5 is detected by a rectifying circuit 10 through a current transformer 9, the circuit 20 is controlled by an output of the circuit 10, supply voltage of the amplifier 3 is increased by the circuit 20, and an optimum operation of the amplifier 5 is maintained by an output of the amplifier 3.