| 摘要 |
PURPOSE:To adjust easily and stably the gain at a strong input by detecting and outputting a control voltage changed in response to a true strong input of an AGC input from a constant current section of a frequency mixer circuit. CONSTITUTION:A reception signal is applied to the mixer circuit 4 from an antenna 2 via a tuning circuit and also an oscillation output is applied from a local oscillator 20. When the AGC input 12 is strong, the AGC current corresponding to the AGC voltage is fed to an emitter of a transistor (TR) 56 giving the operating current to the mixer circuit 4 via a voltage/current converting circuit 72 and the gain is controlled. Further, a base-emitter voltage and a voltage of a diode 58 are decreased in a way of exponential function and a strong input gain control circuit 50 is controlled via a TR54. Thus, the operation at strong AGC input is made stable and a weight is given to the slope of the AGC voltage by changing the number of the diodes 58 thereby increasing the control capability.
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