PURPOSE: To prevent leakage of an input signal without increasing the size of circuit by connecting a diode between a constant current load and one terminal of each one transistor(TR) at the constant current load.

CONSTITUTION: The cathode of a diode D1 is connected to a collector of a TR Q1, the anode is connected to a collector of a TR 3, and no effect is given onto the amplification of an active differential amplifier B by giving a high frequency signal to an input terminal 1 if a differential amplifier A is inactivated with a switch SW1 turned off. The cathode of a diode D2 is connected to a collector of a TR Q6 and the anode is connected to a base of the TR Q4, and no effect is given onto the amplification of the active differential amplifier A even by giving a high frequency signal to the input terminal 1 if a differential amplifier B is inactivated with a switch SW2 turned off. Thus, leakage of an input signal is prevented without increasing the size of the circuit.