To attain a stable operation of the circuit and to reduce the circuit scale even when multi-stage connection of a TR circuit and a composite circuit are configured by providing a bias means and an input frequency selection means to the bias circuit deciding the operating point of TRs.

The bias circuit provides a bias current to a base and a collector of a TR 1 to decide the operating point of the TR 1 and a frequency selection means is provided to the base. The frequency selection means is made up of a resonance circuit 13 consisting of a resistor 10, a distributed constant line 11 and a capacitor 12 and passes a signal within a prescribed band and attenuates the frequency signal at the outside of the prescribed band. Thus, in the case of connecting amplifier circuits in multi-stage or configuring a composite circuit having amplifier circuits and a mixer circuit inbetween, they are stably operated without having an effect the noise characteristic and the bias circuit is realized with a small scale without addition of a complicated circuit.