Gain-controlled amplifier circuits

DETAILED DESCRIPTION What I claim is: 1.
A gain-controlled amplifier circuit which includes a pair of gain-controlled amplifiers, signal generating means operable to generate a pilot signal of constant amplitude during a repeated correction time interval, attenuator means operable to control the level at which said pilot signal is applied equally to the inputs of said two amplifiers, a first automatic gain control loop comprising means for subtracting the output of one amplifier from a reference gain control signal at a time other than during the correction time interval to derive a first error signal for application to the gain control input of said one amplifier, a second automatic gain control loop comprising means for subtracting the output of the other amplifier from the reference gain control signal to derive a second error signal for application to the gain control input of the said other amplifier only during the correction time interval, and a correction loop comprising means for subtracting the output of said one amplifier from the reference gain control signal to derive a correction signal for application to the attenuator means during the correction time interval to control the level of the pilot signal applied to the two amplifiers.
2.
A circuit as claimed in claim 1 which includes switching means operable to activate the signal generating means only during the correction time interval.
3.
A circuit as claimed in claim 2 including means for operating the switching means for derivation of said second error signal and said correction signal only during the period of operation of the signal generating means.
4.
A circuit as claimed in claim 1, said signal generating means comprising means for generating the pilot signal at a frequency within the pass-band of the two gain-controlled amplifiers.
5.
A circuit as claimed in claim 4, said signal generating means comprising means for generating the pilot signal at the normal center frequency of the two amplifiers.
6.
A circuit as claimed in claim 4 said signal generating means comprising means for generating a sinusoidal form