Compact structure of automatic gain control circuit

摘要

An automatic gain control circuit for controlling the gain of an amplifier amplifying an ac signal is provided which includes a resistance circuit, a comparing circuit, a gain control determining circuit, and a gain controlling circuit. The resistance circuit includes a plurality of resistors connected in series through taps. The comparing circuit compares an output signal of the amplifier with reference upper and lower limit values to determine whether the output signal of the amplifier is within a desired amplitude range of the reference upper limit value to the reference lower limit value or not. The gain control determining circuit determines whether the output signal of the amplifier has exceeded the upper and lower limit values sequentially during a complete cycle thereof for determining whether the gain of the amplifier should be changed or not. The gain controlling circuit selectively short-circuits the taps on both sides of each of the resistors of the resistance circuit to change a total resistance of the resistance circuit for controlling the gain of the amplifier when the output signal of the amplifier has exceeded the upper and lower limit values sequentially.