A Reappraisal of Optimum Output Matching Conditions in Microwave Power Transistors

Abstract

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. IndexTerms-Field-effect transistors, high-efficiency amplifiers, power amplifiers (PAs).

Power and efficiency continuous modes in saturated GaN HEMT devices

“…However, when driving the device deeper into compression, some assumptions on the ideal device can no longer be sustained. For example, in devices with a smooth knee behaviour, such as GaN devices, the optimum loads for maximum power and efficiency are quite far apart, with the optimum for efficiency around twice the optimum for power [9], [8]. Moreover, while for linear operation it is enough to substitute the zerograzing condition with a knee-grazing condition, in saturation this approximation does not make sense anymore, since the knee can be explored down to 0 V, or lower [8].…”

“…Since a simplified knee voltage model has been successfully adopted to predict power and efficiency optimum loads in GaN devices [8], the same model is used here to explore how the optimum fundamental loads for power and efficiency change when the second harmonic is not at 0 Ω. This allows us to explore the continuity of power and efficiency terminations in saturation.…”

“…This formulation creates a dependence between voltage and current. In [8], the higher harmonics are all assumed to be short-circuited and the resulting simplified voltage allows for a general analytical solution. Setting the second harmonic to an arbitrary reactance makes the analysis very complicated, so in this work the model is implemented in a simulator to study the continuum.…”

“…In this paper, the continuity of power and efficiency optimum impedances in GaN HEMT devices driven into saturation is explored, for the first time, using a simplified kneemodel [8]. Source/Load-Pull characterization is then used to critically compare the results with experimental data.…”

Load Modulation of Harmonically Tuned Amplifiers and Application to Outphasing Systems

“…Output power is controlled, by reducing the fundamental component of the current waveform as a result of the clipping action while, the voltage waveform remains nearly constant, as shown in Fig. 3 [59]. In conventional outphasing systems, the level of gain compression of the transistors is directly proportional to the output power back-off as the envelope of the branch amplifier's input signal (V in,B1 , V in,B2 ) is not varied throughout the backoff range.…”