To provide an automatic gain feedback loop circuit without the use of a full-wave rectifier by deciding the operating time of a charge pump, based on the results obtained through the comparison performed between the differential outputs of both differential input and output variable gain amplifiers and the paired outputs, obtained by shifting the levels of those differential outputs respectively.

A variable gain amplifier 200 inputs a signal of 100 MHz, for example, which is read out of a disk drive and amplifies the input signal at a degree of amplification that is decided by the gain control voltage outputted from a gain control circuit 290. Then the amplifier 200 outputs the differential output signal of a desired level, via an LPF 210 and an amplifier 220. Program level shifters 230 and 240 are the emitter follower circuits which add the programmed DC voltages together. Compactors 250 and 260 compare the differential output signals with the paired signals, obtained by shifting the levels of both differential output signals respectively, detect the peaks in half cycles respectively to decide the pulse width, and fix the operating cycle of a charge pump circuit 280 to generate the gain control voltage.