Automatic gain control system radio receivers and the like

Claim 1

In an apparatus for receiving and detecting a modulated carrier wave high frequency signal, an automatic gain control system comprising: a pair of transistors whose emitters are commonly connected to each other to form a differential-type amplifier; an additional transistor whose collector is connected with the common emitters of said pair of transistors to directly control said pair of transistors; means for applying a received high frequency signal between the base and emitter of said additional transistor; a semiconductor detection device; means for supplying a forward bias current through said semiconductor detection device to produce an offset voltage thereacross; means for applying a potential difference, defined by said offset voltage across said semiconductor detection device, between the respective bases of said pair of transistors to set one of these transistors in cutoff state thereof when no high frequency signal is received; means connecting the output from the other one of said pair of transistors to said semiconductor detection device for applying an amplified high frequency signal thereto; and means for applying a carrier level signal derived from the output of said semiconductor detection device to the base of said one of said pair of transistors to vary the potential difference between the respective bases of said pair of transistors, thereby to control the effective gain of the said other transistor when in the conductive state in accordance with the carrier level signal.