Amplifier system with AGC, as for an AM radio

DETAILED DESCRIPTION What is claimed is: 1.
A gain-controlled amplifier with tuned load comprising: first and second transistors of the same conductivity type having respectively small and respectively large area base-emitter junctions and having respective base, emitter and collector electrodes; an input signal terminal; a feed-forward capacitor connected between said input signal terminal and said tuned load means; means for connecting the collector electrode of said first transistor to said tuned load means and to an operating voltage; means connecting said second transistor for providing between its emitter and collector electrodes an electrically adjustable emitter degeneration resistance for said first transistor, controllable in response to the emitter-to-base potential of said second transistor, including a direct connection of the emitter electrode of said first transistor to the collector electrode of said second transistor, a connection of the emitter electrode of said second transistor to a reference voltage and signal ground, and means for applying a temperature-compensated quiescent potential between the emitter electrode of said second transistor and the base electrode of said first transistor that tends to maintain said second transistor in saturated conduction; means responsive to the amplitude of the response to said input signal appearing at said tuned load for developing an automatic gain control voltage; means for applying said automatic gain control voltage as the emitter-to-base potential of said second transistor; and means for selectively providing drive to the base electrode of said first transistor from said input signal terminal, including means for sensing when said automatic gain control voltage is of a level indicating that the amplitude of the response to said input signal appearing at said tuned load is sufficient, that the portion of that response attributable to input signal current fed forward through said feed-forward capacitor exceeds the portion of that response caused by the collector current of said first transistor, and including means responsive to that condition being sensed for interrupting drive otherwise provided to the base electrode of said first transistor from said input signal terminal