| 摘要 |
PURPOSE:To switch easily a noise reduction circuit without providing any switch, by switching the gain of a variable gain controlling circuit between a fixed gain state and a gain control state corresponding to an input signal level. CONSTITUTION:A switching circuit 34 turns on a transistor (TR) Q1 when an input terinal 40 is held at a high level, and a TRQ2 is turned off by the output of an inverter 41; and the output controlling current of a fixed control signal generating circuit 35 is applied to control terminals of variable gain controlling circuits 31 and 32 through a diode D2 to fix the gains of the circuits 31 and 32 to one time forcibly. When a noise reduction switching signal goes down to a low level, the TRQ1 turns off and the TRQ2 turns on; and a diode D1 is conductive and the diode D2 is cut off to convert a voltage corresponding to the amplitude of the input signal into a current through a voltage-current converting circuit 30D, controlling the gains of the variable gain control circuits 31 and 32 through the diode D1. |
