Design Strategies for Efficient and Linear Rf Power Amplifiers

In this contribution, research activities and trends in the field of RF and microwave power amplifiers design are discussed. Design strategies for high efficiency and linearity amplifiers are focused and the results attained in three years of TARGET NoE activities are presented, showing interesting and state-of-the-art power amplifier design, employing GaAs or GaN technologies. Introduction The power amplifier (PA) is a key element in transmitter systems, whose main task is to increase the power level of signals at its input up to a predefined level. PA’s requirements are mainly related to the absolute achievable output power levels, in conjunction with highest efficiency and linearity performances. Nowadays, the increasing demand for portable apparatuses, whose main characteristic is battery duration and overall size, naturally translates into a low power electronic system. Since the main power supply consumer is the PA, the low power feature is directly translated to PA specifications. Moreover, due to the widespread diffusion of communication applications, the PA designer has usually to trade-off among the contrasting goals of high transmitted power, low power consumption and highly linear operation. The relative weight of the above goals and the resulting compromise may vary depending on the radio link to be established and overall system specifications [1]. The resulting challenge has however heavily influenced, in the last decade, industrial, technical and research directions in the PA field. From the PA designer point of view, both the selection of the active devices composing the PA and especially the exploration of their non-linear operating regions, to fully exploit the output power capabilities become critical [2]. Dedicated and non linear design methodologies to attain the highest available performance become therefore crucial for successful results [2]. In this contribution, moving from an overview of the available technologies for PA realisation and related performances, PA design strategies for high efficiency and high linearity performances are revisited and established through several experimental results carried out in the first three years of TARGET NoE activities. Design Approaches PA design strongly depends on operating frequency and application, as well as on the available device technology. Available approaches are not therefore not unique. Moreover, the design is further complicated by active device non linear behaviour, even if simplified approaches are often adopted to infer preliminary device figures [3]. For high frequency applications however, two broad PA design methodologies classes are available: SwitchingMode (SM) amplifiers [4] and transconductance-based amplifiers with Harmonic Tuned terminations (HT) [5]. Switching-Mode Power Amplifiers This class of amplifiers is featured by the assumption of an ideal switching-mode behaviour of the selected active devices, which are therefore driven by a large-signal input to act as a switch, rather than a current source, as in HT amplifiers. For this reason, the resulting SM amplifier is often considered as a dc/rf converter rather than a RF amplifier. Starting from the early Class E configuration proposed by Sokal in 1975 [6], several approaches and design solutions were proposed later [7, 8]. The approach is widely adopted, especially in RF wireless systems, due to its relatively simple scheme and availability of closed form expressions for its circuit components [7]. The basic amplifier schematic is depicted in Fig. 1, even if slightly different topologies have been proposed [8, 9].