Bipolar AGC with RF transistor DC bias point stabilization

Claim 1

A double conversion television tuner comprising: an input for RF signals; a point of reference potential; a first amplifier including a first bipolar transistor arranged as a common emitter amplifier with a base coupled to said RF input, an emitter coupled to said point of reference potential, and a collector; a variable frequency local oscillator having an output; a doubly balanced mixer, including first and second baluns having respective unbalanced inputs coupled to respective ones of said collector of said first amplifier and said output of said variable local oscillator, respective balanced outputs, a mixing element coupled to said balanced outputs, an an output; a second amplifier having an input coupled to said output of said doubly balanced mixer, and an output; a fixed frequency local oscillator having an output; a second mixer having a first input coupled to said output of said second amplifier, a second input coupled to said output of said fixed local oscillator, and an output; AGC means, coupled to said output of said second mixer, for providing an automatic gain control voltage; and varying means DC coupled between the base and collector electrodes of said transistor for providing feedback therebetween and receiving said automatic gain control voltage for varying the base bias current in accordance with said automatic gain control voltage and thereby varying the gain of said amplifier, wherein said varying means includes a second bipolar transistor of opposite conductivity type with respect to said first bipolar transistor having an emitter DC coupled to the collector of said first bipolar transistor, a collector DC coupled to the base of said first bipolar transistor and a base coupled to said AGC means to receive said AGC voltage.