Impact of Nonideal Auxiliary Current Profile on Linearity of Microwave Doherty Amplifiers: Theory and Experiments

A new nonlinear effect of Doherty power amplifier (DPA) caused by the nonideal auxiliary current is disclosed. In contrary to traditional assumption, the auxiliary current profile is found to be a nonlinear function of input drive. Although perfect load modulation conditions can be enforced at the transition and saturation power levels (i.e., two efficiency peaks), inside the Doherty region, the nonlinear auxiliary current incurs insufficient load modulation, leading to voltage clipping effect. Theoretical analysis reveals that the conventional design (DPA-I) can suffer from degraded linearity performance with distorted waveforms, increased harmonic regrowth, and poor phase distortion. For further investigation, a modified design (DPA-II) is contrived with remedied clipping effect. Simulation studies show that the linearity of DPA-II is largely improved with minimal reduction in efficiency, which indicates this non-ideal current profile is one of the fundamental limiting factors of DPA linearity and cannot be overlooked. For verification, two 42-dBm 2-GHz symmetrical DPAs based on GaN HEMT are designed, fabricated, and measured. Experimental results show that DPA-II provides 11-dB suppression in third-order intermodulation distortion (IMD3) and 8-dB enhancement in adjacent channel leakage power ratio (ACLR), compared to DPA-I. Meanwhile, excellent efficiency performance was attained with drain efficiency of 54%/71.5% at back-off/saturation.