| 摘要 |
PURPOSE:To prevent deterioration in S/N by performing AGC control so that when a reception signal increases in input level, the gain of an intermediate frequency amplifier is decreased and then the gain of a high frequency amplifier is decreased. CONSTITUTION:When the reception signal increases in electric field intensity, an AGC voltage VAGC from a detecting circuit 5 rises and currents of transistors TR24 and 27 of differential amplifier circuits 100 and 300 increase. The circuits 100 and 200 receive constant currents from constant current sources 25 and 28, so the collector currents IC1 and IC2 of TRs 23 and 26 decrease and the high frequency amplifier circuit 2 and intermediate frequency amplifier circuit 4 both decrease in gain. In this case, the fixed bias voltage VB2 of the TR26 is lower than the fixed bias voltage VB1 of the TR23, so the current IC2 decreases before the current IC1 decreases. Therefore, the circuit 4 decreases in gain earlier than the circuit 2 and a noise level generated by the circuit 2 instantaneously is decreased and sent to the circuit 5. |
