Overview

From APPLIED MICROWAVE & WIRELESS, VOL. 12, NO. 1, JANUARY 2000

The use of feedforward linearization in power amplifiers has become widespread in the wireless arena, because it offers the ultimate performance in terms of intermods and spectral regrowth. It also provides the ultimate in distortion correction. It is often used with some form of predistortion. Feedforward can be used for amplifiers from under a watt to several hundred watts and beyond, and with almost any modulation technique. In the early 1990s it was used in wireless applications primarily with FM multicarrier modulation. Now the feedforward technique is being used for multiple code-division multiple access channels (CDMA) through one amplifier, global system for mobile communication (GSM), time-division multiple access (TDMA), and has potential for use with wideband CDMA (third generation) systems.

Although the architecture of the standard feedforward amplifier is fairly straightforward on paper, significant challenges must be overcome in practical amplifiers. Controlling phase to less than a degree over a band of frequencies is just one problem if distortion terms are to be 60 or 70 dB down from the main carriers. Considering that a typical regrowth spec of ?60 dBc means that the distortion energy is on the order of 0.1 percent of the signal energy, in voltage units (if the distortion bandwidth and the signal bandwidth were the same), this distortion performance is equivalent to some of the best...