PURPOSE: To realize an automatic level control circuit with a MOS integrated circuit by converting the variance of an input level to the change of the sampling frequency of a first switched capacitor and moving the frequency characteristic curve of a filter in parallel to compensate the variance of the input level.

CONSTITUTION: All of used operational amplifiers consist of MOSFETs. An input level vi, an output vRE of a rectifier 1, an oscillation frequency fs of a voltage controlled oscillator(VCO) 2, and an output v4 of a switched capacitor filter(SCF) 4 in the reference state are vio, vREO, fso, and v40 respectively, and the output vRE of the rectifier 1 is changed to aVREO when the input level vi is changed to avio. As the result, the oscillation frequency fs of the VCO 2 is changed to afso. That is, when the sampling frequency is raised by (a)-number of times in SCFs 4 and 5, the frequency characteristic is moved in parallel in the direction of the frequency base because the cut-off frequency is raised by (a)-number of times also. Thus, the automatic level control circuit is realized with the MOS integrated circuit.