AUTOMATIC GAIN CONTROL CIRCUIT, E.G. FOR USE IN A HARD DISK DRIVE

An automatic gain control circuit is disclosed for amplifying an input signal so that it has a specific correspondence with a reference value (Vref). The circuit comprises an input terminal (2) for receiving the input signal, a variable gain amplifier unit (4) for amplifying said input signal in response to a gain control signal. A peak detector unit (14) is provided for detecting peaks in said amplified input signal. The peak detector unit (14) has an attack time constant (Ta) and a decay time constant (Td), so as to supply a peak detector output signal (Vp) which is representative of the amplitude of the peaks in said amplified input signal. Further, a comparator unit (20) is provided for comparing the peak detector output signal (Vp) with said reference value (Vref) so as to derive an error signal (e) therefrom. An integrator unit (30) is further present for integrating the error signal (e) so as to obtain said gain control signal. The peak detector circuit comprises means for controlling said decay time constant in response to said error signal, said means for controlling said decay time constant being adapted to decrease said decay time constant for increasing error signals, in a situation when the peak detector output signal (Vp) is larger than said reference value (Vref). Further, the following relationship is maintained between the attack time constant Td and the decay time constant T4 of the peak detector means: Td = c.Ta, where c is a positive constant and p is a constant larger than 1. p is preferably equal to 2 and for c holds 20