Automatic gain control of a single gate GaAs FET amplifier

Claim 1

In a radio receiver operating in the GHz radio frequency (RF) range, and adapted to receive a modulated RF input signal, an automatic gain control (AGC) circuit comprising; a controllable power source having a control input and including means for supplying direct current (DC) operating voltage between a supply line and a ground plane, said DC operating voltage having a magnitude; an RF preamplifier which includes a single GaAs FET having gate, drain and source electrodes, and biasing means connected to said DC operating voltage, said gate electrode being adapted to receive said RF input signal, said drain electrode being AC isolated and DC connected to said ground plane, and said source electrode being connected to said biasing means, said FET providing an amplified RF signal at the drain electrode, the RF preamplified gain varying in response to variations in the magnitude of the DC operating voltage supplied by said controllable power source; circuit means having an input adapted to receive said amplified RF signal, said circuit means including a fixed gain amplifier, and providing an amplified IF signal at an output; a level control circuit having an input adapted for connection to the output of said circuit means, said level control circuit being responsive to the amplitude of said amplified IF signal so as to provide a variable DC control signal to the control input of said controllable power source, whereby the gain of the RF preamplifier is varied inversely to the amplitude of said amplified IF signal, so as to substantially maintain the amplitude of said amplified IF signal constant over a predetermined range of RF input signal amplitudes.