NONLINEAR TRANSISTOR MODELS AND DESIGN TECHNIQUES FOR HIGH-EFFICIENCY MICROWAVE POWER AMPLIFIERS

In recent years, electronic technologies oriented to communications went through a continuous and pressing development due to several factors. On one hand, the devel-opment of the internet network and its related information systems caused an increasing interest of the people in using devices capable of ensuring a constant connection to those services. This aspect greatly improved the wide diffusion of mobile devices and new generation technologies, such as 3G and 4G/LTE, were developed to satisfy more and more demanding requirements. On the other hand, other systems such as geolocation services (e.g., GPS and GLONASS), initially built for military purposes, are now diffused and commonly adopted by an increasing number of people. While the consumer market has given a significant boost to communication tech-nologies, other sectors have seen a tremendous development. As an example, satellite systems for Earth observation (such as the COSMO-SkyMed system for the observa-tion of the Mediterranean basin) plays today a fundamental role in the prevention and the management of natural phenomena. The aforementioned examples of communication systems exploit microwave fre-quency technologies for the transmission of large amounts of data, thanks to the availability of larger bandwidths. This necessarily implies use of high-power and high-efficiency technologies in line with the requirements of the systems where they are exploited. When these aspects are taken into account, the attention focuses on the basic ele-ment which mainly determines the performance of an electronic circuit: the transistor. New technologies based on particular semiconductors such as Gallium Arsenide (GaAs) and Gallium Nitride (GaN) are revealing themselves as great solutions for the realization of transistors with excellent performance at micro- and mm-wave frequencies. Because of their relative immaturity compared to well-assessed technologies, such as Silicon, they are of great interest in the research field, in order to identify their limitations and margins of improvement. The research activities carried out during my PhD program lie in this framework. In particular, the attention has been focused on the nonlinear modeling of transistors for microwave applications and on the study, as well as the application, of design techniques to optimize the performance of power amplifiers. In Chapter 1 nonlinear transistor modeling techniques will be briefly reviewed. Then, the attention will be focused on the problem of the low-frequency dispersion affecting new generations of electron devices, which strongly influences their dynam-ic behavior and, therefore, their performance at high frequency. To this end, a low-frequency measurement setup oriented to the analysis of this phenomenon will be described since it has been widely used throughout the research activity. Successively, two different modeling approaches, namely the compact and the behavioral ones, will be considered. Two techniques based on the setup described in Chapter 1 have been analyzed and developed in the PhD activity and will be present-ed in Chapters 2 and 3 respectively. Finally, Chapter 4 will be devoted to the design of microwave power amplifiers. In particular, the problem of identifying the optimal operating condition for an active device will be analyzed, with particular interest in the maximization of the efficiency. In this context, a recently proposed design technique, based on large-signal low-frequency measurements will be applied to obtain accurate information on the tran-sistor behavior. This technique will be also compared with conventional approaches (e.g., load pull) and validated with the realization of a prototype of a microwave power amplifier