Dual threshold wide band/narrow band AGC

Claim 1

A dual threshold AGC circuit for a radio receiver that responds to variable strength signals with a first narrow band threshold and a second wide band threshold, both of which thresholds must be exceeded for said AGC to become functional, said receiver including a tuner comprising a variable gain radio frequency amplifier to produce an intermediate frequency signal, an intermediate frequency amplifier having an input coupled to said tuner, and means coupled to said intermediate frequency amplifier for developing a tuning meter output voltage which has a magnitude proportional to the in band intermediate frequency signal amplitude, said AGC circuit comprising: a differential amplifier having first and second inputs, a single ended output, and a tail current supply; means for modulating said tail current supply with said tuning meter voltage; means coupled to said tail current supply for introducing a threshold in said tail current response to said tuning meter voltage whereby said tuning meter voltage must exceed said narrow band threshold to modulate said tail current; a wide band control amplifier coupled to receive the input to said intermediate frequency amplifier and to drive said differential amplifier first and second inputs; means for offsetting said first and second inputs of said differential amplifier to produce said wide band threshold; and means responsive to said single ended output of said differential amplifier for developing an automatic gain control current for controlling the gain of said radio frequency amplifier.