PURPOSE: To obtain a good low frequency noise characteristics, by connecting an injection synchronous amplifier using an Si transistor between a GaAsFET limiter and a frequency discriminator.

CONSTITUTION: For the microwave FM, the AM noise is compressed by the limiter 1 using a GaAsFET. In this case, however, the low frequency noise is caused by the GaAsFET. Thus the low frequency noise is reduced by the transistor injection synchronous amplifier 8 using a microwave Si transistor having a good low frequency noise characteristics. Thereafter, a demodulation is carried out by the frequency discriminator 7. In such way, a microwave FM demodulator can be obtained with good low frequency noise characteristics.