Linearity considerations for a high power Doherty amplifier
This paper proposes the calculation of memory residual (MR) and adjacent channel power ratio (ACPR) as figures-of-merit (FOM) for the analysis of linearizability of a high power RF Doherty amplifier circuit. Two-tone and wideband code division multiple access (WCDMA) signals of increasing bandwidths and varying power levels are used for the calculation of the MR and ACPR performance respectively. It is found that MR and ACPR performances are not only dependent on the input signal power to the high power amplifier (HPA) but these metrics are also dependent on the bandwidth of the signal used. Agreement is found in MR and ACPR performance trends for the increasing input power and signal bandwidths. For the investigation of linearizability and ACPR measurements of the Doherty HPA, various predistorter (Memory Polynomial and Volterra-series based) schemes are used. The Volterra-series based predistorter scheme using dynamic deviation reduction (Volterra DDR) is found to give the best ACPR performance in the presence of significant memory effects. For the validation, simulation and hardware measurements of MR and ACPR performances are presented for high power Doherty power amplifier circuits operating at 2.14GHz.