Bias optimized fet mixer for varactor tuner

Abstract From the Canadian patent

A field effect transistor is used as the mixer in a VHF varactor turner by the use of self-adjusting bias. Maximum conversion gain can be realized when the oscillator injection voltage is low, and low intermodulation and cross modulation products can be realized at higher oscillator injection voltages. Self-adjusting bias is achieved by use of the tuning voltage in a bias control circuit where FET, drain current is adjusted while FET bias voltage is maintained substantially constant. Keeping the bias voltage for the FET substantially constant ensures constancy of FET input and output capacitances, thus preventing the tuner's tuned circuits from being detuned and variable loaded.