| 摘要 |
PURPOSE:To operate a main PIF amplifier having an AGC function normally, by providing the AGC function for an intermediate frequency preamplifier, compressing the fluctuation of an input level and taking the maximum value of the input level of the main IF amplifier to a prescribed level or below. CONSTITUTION:The IF preamplifier 5' is provided between a reception mixer 4 and an IF BPF6, the amplifier 5' has a variable attenuation element 11 between amplifier elements 9 and 10, an output is obtained at the element 10 and a part of the output is sent to an output level detection circuit 12. In this case, the signal of the circuit 12 is applied to a control circuit 13, the output of which adjusts the bias current of PIN diode of the variable attenuation element 11 inserted in series between the amplifier element 9 and 10, for example, allowing to provide the AGC characteristics. As the range of the AGC, when the input level is higher than the standard level, the bias current of the PIN diode i s changed so that the output level is the same as that at the standard level. |
