| 摘要 |
PURPOSE:To attain easily and stably strong input gain adjustment by detecting surely a control voltage changed in response to a true strong AGC input from a constant circuit section of a frequency mixer circuit. CONSTITUTION:A reception signal is impressed to a mixer circuit 4 from an antenna 2 via a tuning circuit 18 and also an oscillation output is fed from a local oscillating circuit 20, and a mixer output is outputted to an intermediate frequency amplifier via an intermediate frequency tuning circuit 42. When the AGC input 12 is strong, the gain applied with an AGC current corresponding to an emitter AGC voltage of a transistor (TR) 56 giving the operating current of the mixer circuit 4 is controlled via a voltage/current converting circuit 72. Then the base-emitter voltage is decreased exponentially so as to control a strong input gain control circuit 50 via a TR54, the AGC input is converted into an exponential function change and controlled. Thus, the stable AGC operation is obtained and a proper AGC point is set through the selection of resistors 58, 60.
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