Identification of RF Power Amplifier Memory Effect Origins using Third-Order Intermodulation Distortion Amplitude and Phase Asymmetry

This paper describes a technique to determine the physical origins of memory effects in RF power amplifiers using amplitude and phase asymmetries of two-tone third order intermodulation distortion (IMD) products measured over a range of power levels and frequency spacing. Determining the origins of the memory effects is done by extracting the portion of the IMD product in each sideband that contributes to the asymmetry, and comparing these on a statistical basis to the responses of simple equivalent circuit models that represent thermal self-heating, and bias voltage feedback. A 90W, singled-ended GaN pHEMT PA stage is compared to a 650W push-pull combined Si LDMOS PA at 2.14 GHz. It is shown that the GaN PA memory effects are dominated by bias feedback (which is later improved), and that the LDMOS PA memory effects are dominated by thermal self-heating. These results are further quantified by comparing memory ratios