Abstract: The invention relates to an automatic gain control circuit for restoring an input signal whose variable level is to be regulated throughout the whole frequency band, in particular to restore the input signal to a constant level. A gain control loop uses n phase shifters (3,4) each phase shifting at the lower limit of the frequency band of the input signal by 2.pi./(n+1), being connected in cascade to the output of the circuit and followed by a rectifier and summing circuit (5) acting on the output signals of the circuit (S) and of each of the phase shifters (3 and 4) to generate a single polarity signal which, filtered by a cell (7) with a short time constant, constitutes the gain control signal. The circuit can be used in a modulator-demodulator to transmit data, in particular facsimile signals, over a telephone line.