| 摘要 |
PURPOSE:To execute an automatic gain adjustment having good follow-up properties by connecting a filter capacitor through a resistance to an output part of an automatic gain adjusting amplifier, and providing a switch for discharging quickly this capacitor in accordance with an output of the amplifier. CONSTITUTION:When a detecting signal from an AM detector is supplied to an automatic gain adjusting amplifier 12, a voltage generated in a filter capacitor 16 connected to its output terminal through a parallel circuit of a resistance 20 and a switching circuit 22 is applied as an AGC voltage to a mixing circuit and an intermediate frequency amplifier. In this case, when an output of the automatic gain adjusting amplifier 12 drops in a state that the capacitor 16 has been charged with the AGC voltage, a base current of a transistor 26 of the switching circuit 22 flows and the transistor 26 becomes on, and a charge of the capacitor 16 is discharged quickly to the same level as an output of the amplifier 12. Accordingly, the voltage of the capacitor 16 is made to reach quickly the necessary AGC voltage, and a response of the gain adjustment can be improved.
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