Linearity Performance of Outphasing Power Amplifier Systems

As discussed in Chapter 1, in an outphasing power amplifier system, the AM of a bandpass waveform is transformed into PM of two signals prior to power amplification, and finally recovered by recombining the two amplified signals. As shown in Figure 1.16, with this approach, two highly efficient nonlinear power amplifiers can be used without distorting the original signal. This is the main advantage of the outphasing approach to linear power amplification. However, the bandlimited feature is no longer preserved— a wideband signal is added to and subtracted from the original source input to construct the two outphased component signals. Since the spectrum of the two additional signal components extends far into the adjacent channels, the system linearity performance relies on the precise cancellation of the wideband signals during power combining, and the tolerance on the matching between the two power amplifier branches becomes critical— a slight mismatch between the two paths can result in significant distortion in the adjacent and alternate frequency bands. The gain and phase characteristics of the two amplifier branches, which includes the digital baseband, analog, and RF portions, must be wellmatched for the acceptably small out-of-band rejection. A failure to achieve sufficient matching generally creates both out-of-band interference and inband distortion. The out-of-band interference is characterized by the ACPR, while the inband distortion may be described in terms of EVM. Various factors can contribute to path mismatch and the consequent linearity