FETS in Communication Circuit Applications

The diffused-gate field-effect transistor has improved non-linearity characteristics over the conventional (bipolar) transistor and the vacuum-tube as well. This improvement is contributed to the nearly square-law transfer characteristic. Some comparative non-linearity performance figures, related to signal handling problems, (cross-modulation and inter-modulation distortions) are given. The improvements clearly demonstrate the desireability of obtaining good gain-stability and noise performance with FETs for use at very high frequencies. A "cascode" configuration, using two FET\´s are interconnected to reduce the reverse feed-back capacitance inherent in these devices. Improved gain-stability relationship is thus achieved at frequencies to 250 megacycles. The FET cascode development is detailed relating the improved performance obtainable through this circuit configuration. A 200 megacycle circuit is designed using an FET cascode and evaluated for gain-stability, noise figure and cross-modulation performance.