Mosfet FM Tuner Design

Dual-gate MOSFETs in an FM tuner provide many advantages over bipolar, junction field-effect, and single-gate MOS field-effect transistors. RF amplification and mixing are aided by use of a low-feedback-capacitance transistor with low noise figure and large dynamic range. A second gate is available for either AGC or local oscillator injection. High stable RF and conversion gains are easily obtained with inexpensive, commercially available coils and without need for neutralization. This paper presents test data and design tips which can be used to design with the SFB8970* dual-gate MOSFET at 100 MHz. The SFB8970 is an N-channel, depletion mode, dual-gate, MOS transistor with integral back-to-back zener diodes between both gates and the source to eliminate the need for special precautionary handling procedures.