GB748500A - Improvements in or relating to devices for the amplification of electrical signals and having automatic gain control - Google Patents

Abstract

748,500. Automatic gain control. TELEFONAKTIEBOLAGET L. M. ERICSSON. April 10, 1953 [April 10, 1952], No. 9819/53. Class 40 (5). An amplifier has an automatic gain control circuit which has a larger gain for noise than for signal so that the signal output is held at one level and in the absence of a signal the noise is held at a given lower level. In Fig. 1 the amplifier 1 is provided an A.G.C. path through a rectifier 6 responsive only to the signal and an A.G.C. path, through a rectifier 5, responsive only to noise, filters being included in either or both paths to select signal and noise respectively and the noise path including an amplifier 3 so that the output noise level of the amplifier 1 is reduced to a lower value than the signal. To avoid the disadvantage that the noise A.G.C. voltage is reduced when the signal A.G.C. becomes effective the control voltages derived from the amplifier output 2 are applied to a circuit 4 (Fig. 2) which passes only noise to a rectifier circuit 5, 9 producing a control voltage at 10. The signal is fed through an attenuator 11, 12 to a rectifier 6, so that when the output from the attenuator exceeds the noise level, rectifier 6 produces a sufficient bias to render rectifier 5 ineffective and alone provides the control voltage. The noise may be selected on a frequency basis from outside the signal band, from an unused space within the band, or, when the signal is of pulse form, from a region where the envelope of the pulse spectrum is zero. When the signal is of pulse form the noise may alternatively be selected on a time basis. Thus, a device sensitive to the pulse repetition frequency or to synchronizing pulses may block a noise amplifier or attenuator during the pulses or for the duration of the signal (Fig. 3, not shown). When the pulses are short compared to their spacing the noise detector may be of the averaging type and the signal detector of the peak type and no separation of noise from signal is necessary (Fig. 4, not shown). A single gain control path could be used if it is made more sensitive to noise than signals by desensitizing the control path by means of a voltage separated on a time or frequency basis from the signal (Fig. 6, not shown). The circuit of Fig. 1 could be modified so that a delayed A.G.C. voltage from the signal path desensitizes amplifier 3 (Fig. 5, not shown)..