Seminar on Industrial Problems

Links Between the Nonlinear Behavior of Transistors and the Spectral Characteristics of Portable Wireless Power Amplifiers

April 24, 1998

Presented by: Peter A. Blakey, Motorola

---

Cell phone technology is moving from analog systems to digital systems. Analog systems use simple modulation schemes that do not place stringent demands on the linearity of the transistor used as the final power amplifier (PA) in the transmitter. This makes it possible for circuit designers to achieve DC-to-RF conversion efficiencies that exceed 70%. Digital systems use more complicated modulation schemes that place very stringent demands on the linearity of the PA. As a result, the DC-to-RF conversion efficiencies of digital phones are quite low, typically around 40%.

The linearity of a PA is determined by several factors. These include the RF drive level, the nonlinear characteristics of the output transistor, and topology and element values associated with the surrounding circuit. Figures of merit that characterize the linearity of analog PA's, and links between these figures of merit and device nonlinearities, are well established. The figures of merit that characterize the linearity of PA's used in digital systems are quite different, and the relationships between these figures of merit and transistor nonlinearities are not well understood. However, it is known from experiments that there is little correlation between the figures of merit that are applicable to analog PA's and the figures of merit that are applicable to digital PA's.

The first part of this presentation will present an overview of analog modulation schemes and figures of merit that are used to characterize the linearity of analog PA's. The second part will cover the nonlinear characteristics of transistors and some approaches (such as Volterra series) that have been used to establish links between transistor nonlinearities and the figures of merit for analog PA linearity.. The third part of the presentation will describe digital modulation schemes and the linearity figures of merit applicable to digital PA's. The presentation ends with the question: Can we establish useful theoretical links between transistor nonlinearities and the spectral characteristics of digital power amplifiers?

A solution to this problem would assist both technology discrimination (determining what type of transistor is best suited for use in digital PA's) and device optimization (for a given type of transistor, determining what structural parameters lead to optimum performance).