A Reappraisal of Optimum Output Matching Conditions in Microwave Power Transistors

This paper presents a novel approach to the identification of output power and efficiency contours in microwave power transistors in compressed regime. The formulation is based on a polynomial representation of the drain-source voltage profile accounting for the knee region. Closed-form equations for the output power and efficiency as function of the fundamental load are demonstrated, enabling the plot of contours on a Smith Chart. From these, a further simplified drawing procedure for approximated contours is also derived, differentiating between two families of output characteristic. The first, with smooth knee, is usually experienced in GaN devices, while the second exhibits a steep knee which can be associated with GaAs devices' typical behavior. A 5-W GaN HEMT, a 2.5-W GaN HEMT, and a 0.7-W GaAs pHEMT are characterized with load-pull measurements. In all the three cases, the proposed method results in a very accurate contour construction, despite being based on an approximated output current/voltage profile and on a rough estimate of output equivalent capacitance.