The automatic gain control unit for a radiofrequency receiver includes a first current source for charging a storage capacitor by a first current, four comparators for comparing on the lower side an in-phase positive intermediate signal, an in-phase negative intermediate signal, a quarter-phase positive intermediate signal, a quarter-phase intermediate signal with a first reference threshold to a high amplitude level of the intermediate signals. Each comparator controls a MOS transistor connected in series between a second current source and the storage capacitor in order to discharge the storage capacitor when each transistor is in a conductive state. A bi-stable trigger element is connected to the storage capacitor and to provide an AGC signal depending on the voltage level on the storage capacitor for attenuating or not the gain of the low noise amplifier or of a mixer unit amplifier of the receiver.