Professor Almudena Suárez
University of Cantabria, Spain

Power amplifiers often exhibit instabilities giving rise to frequency division by two or oscillations at non commensurate frequencies. These phenomena, observed from a certain level of input power, cannot be detected through a small-signal stability analysis of the circuit. Instead, a large-signal stability analysis must be performed. Other behaviors, like hysteresis and chaotic solutions, can also be obtained when the input power is varied. The qualitative changes in the output-power spectrum are due to bifurcations or qualitative stability changes in the circuit solution or in the number of solutions when the parameter is varied. The talk introduces the local and global stability concepts and the analysis techniques, based on harmonic balance. The first objective is to allow a good comprehension of the different phenomena. The second objective is to provide practical simulation tools for an efficient prediction and elimination of the undesired behavior. Different approaches for the local-stability analysis of nonlinear regimes will be presented, with emphasis on the pole-zero identification. Then, techniques will be shown for the detection of the most common types of bifurcations in power amplifiers. The final goal will be the stabilization of the circuit and the design corrections in order to suppress the undesired phenomena will also be presented. For illustration, the simulation tools will be applied to two different switching amplifiers developed at California Institute of Technology.

Almudena Suárez received the degree in electronic physics and the Ph.D. degree from the University of Cantabria, Santander, Spain, in 1987 and 1992, respectively. She received the Ph.D. degree in electronics from the University of Limoges in Limoges, France, in 1993. Since 1993, Dr. Suárez has been an associate professor (permanent since June 1995) at the University of Cantabria and a member of its Communications Engineering Department. She has co-authored the book Stability Analysis of Microwave Circuits (Artech House) and has contributed two articles to the Encyclopedia of RF and Microwave Engineering.