Intermediate-frequency amplifier with wide-range continuously variable bandwidth selection

Claim 1

A varactor-controlled adjustable bandwidth fixed-frequency amplifier comprising a plurality of cascaded amplifier stages and a double-tuned filter connected between each amplifier stage and the succeeding amplifier stage, a. each such filter including a T-network comprising first and second varactors in the respective series legs of the network and a third varactor in the shunt leg thereof, b. a source of adjustable control voltage applied to the junction point of said T-network, c. said first and second varactors being poled relative to said junction point to vary their effective capacitances in one sense relative to changes in said control voltage, and said third varactor being poled relative to said junction point to vary its effective capacitance in the opposite sense relative to changes in said control voltage; d. said amplifier including an inductance in series with said third varactor, forming therewith a series-resonant circuit having a resonant frequency higher than the design center frequency of said amplifier, to present an amplified value of effective capacitance change in said shunt leg of the Tnetwork; e. and each such filter including a pair of tuned circuits coupled by said T-netwoRk, and fourth and fifth varactors respectively connected in shunting relation to said tuned circuits; and voltage-shaping circuit means connecting said fourth and fifth varactors to said source of control voltage, to hold constant the center frequency of said amplifier for all useful values of said control voltage.