| 摘要 |
PURPOSE:To ensure gain adjustment at strong input by providing a switching circuit conductive at the strong input to an input section of a frequency mixer circuit so as to decrease the level of signal input. CONSTITUTION:A reception signal is applied to the mixer circuit 4 from an antenna 2 via a tuning circuit. Further, an oscillation output is fed from a mixer oscillator 48 and a mixer output is outputted to an intermediate frequency amplifier via an intermediate frequency tuning circuit 52. When an AGC input 12 is a strong input in excess of a threshold value, a base current of a transistor (TR) 34 flows via a voltage/current converting circuit 72 in response to the strong input, a TR34 is conductive, the impedance between bases of TRs 14, 16 is decreased and a common mode input is given to a differential input of the TRs 14, 16. Thus, the AGC effect for suppressing the effect of the common mode amplification gain of the differential amplifier is obtained at the strong input and the gain is adjusted without decreasing remarkably the input impedance of the mixer circuit.
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