Design of microwave FET oscillator and modeling of FET

Abstract

First part of this dissertation work is devoted to the derivation of the equivalent circuit of GaAs FET. 8-element equivalent circuit for GaAs FET is employed to calculate their element values from the measured 8-data values of two port S-parameters exactly at each frequency. Three element values vary with frequency adn two additional small inductances are needed to account for the frequency dependence. 10-element equivalent circuit including these correction inductances closely predict the wafer measured S-parameter up to 18 GHz. Second part is about a noble design theory of GaAs FET oscillator. One may derive an analytic formulas to give the feedback and terminating reactances simultaneously which gives various values of negative resistances in the contour plots. The formulation includes common source, common gate, and common drain configuration and lossy feedback networks. From these formulations, one may derive the maximally loaded condition (the maximum negative resistance of the load) which is compared with the previously obtained results. The contours are proved to be the conic sections and the contours of the resistance seen from the load turned out to be in two classes, i.e. the bounded negative resistance and the unbounded negative resistance. Dispersion of the contours for different frequencies is also examined. The applications of the contour plot is tested by designing the fixed frequency oscillator and the voltage controlled oscillators. For the fixed frequency oscillators, the design procedure of a microstrip oscillator and a dielectric resonator oscillator is presented in detail. It is shown also that an undesired oscillation can easily be suppressed with this contour method. For voltage controlled oscillators, a design of YIG tuned oscillatro and that of varactor tuned oscillator is described. Existing design methodology for YIG tuned oscillatro (YTO) by S-parameter mapping or numerical cut and try is compared with this contour method. With thi...