Feedback for High-Bandwidth Power Amplifier Linearization

Current RF power amplifier (PA) linearization techniques are presently being investigated to improve PA linearity and power efficiency. One such linearization technique, Cartesian feedback (CFB), makes use of analog feedback to linearize a nonlinear plant (Figure 1). The commanded baseband symbol, I and Q, are compared to the symbol being transmitted by the PA, the baseband symbol is predistorted to correct for PA nonlinearities. Due to significant delay through the PA and parasitic loop poles, the loop must be compensated (H(s) in Figure 1) to ensure an adequate phase margin. The loop dynamics inherently restrict the baseband symbol bandwidth. This limitation is juxtaposed with CFB's insensitivity to changes in PA characteristics over time, temperature, and load impedance. Given the difficulty in modeling these PA characteristics, CFB is attractive compared to other linearization methods requiring detailed PA models [1].