The wireless communications revolution has been driven by a confluence of technological advances, including improvements in communications theory, very large-scale integration (VLSI) technology, and radio frequency (RF) microelectronics. The microwave power amplifier represents one of the major factors in the low-cost and low-power implementation of these systems, since they are required to deliver tens of watts of power (in the case of a base station) with exacting standards of linearity and direct current (dc) power efficiency. The microwave power amplifier in the handset must deliver—at most—a few watts of power but dissipate very little dc power and sell for only a few dollars in large quantities. Clearly, these requirements represent an enormous challenge for the design and implementation of the power amplifier circuit.
This challenge has been met through a variety of creative approaches over the years, and the linearization and efficiency enhancement of microwave amplifiers remains an area of active research. Generally speaking, linearization techniques can be classified under the categories of either feedback or feed-forward approaches. Both of these approaches have wellknown advantages and disadvantages. Feedback approaches tend to exhibit stability problems, and feed-forward approaches suffer from matching limitations. Neither has achieved very widespread application in commercial communications systems.