| 摘要 |
PURPOSE:To perform ALC operation smoothly to variation in input level by composing a comparing circuit which compares the output voltage of a low frequency amplifying circuit with a reference voltage by using a differential amplifier whose emitter connects with a diode. CONSTITUTION:The output voltages of plural low frequency amplifying circuits A are inputted to the base of the transistor (TR) Q27 of the comparing circuit C. The reference voltage from a reference voltage source B is inputted to the base of the TRQ26 of the circuit C. Then, TRs Q26 and Q27 constitute the differential amplifier, whose emitter is connected to the collector of a constant current source TRQ28 through diodes D7 and D8 respectively. The collector output of the TRQ27 is amplified by a driving circuit D, whose output is used to control a variable impedance element E, performing ALC operation for the circuits A. Variation in the collector current of the constant current source TR with the base voltage difference of the differential amplifier of the circuit C is made slow through the diodes connected to the emitter to perform the smooth ALC operation accompanying an increase in input level to the amplifier A. |
