Analysis of an RF power amplifier for envelope tracking based on load-pull data

Recently, due to the development of high efficiency supply modulators, the envelope tracking (ET) technique has grown in interest. ET PAs show a promising potential when it comes to improve efficiency when amplifying signal with peak-to-average ratio of several dBs. In this work several practical experiments are performed to find the potential of a test PA when envelope tracking is implemented. A 16 QAM signal with peak-to-average ratio of 6.4 dB is used in the analysis. The experiments include load-pull measurements, power sweeps at different drain voltages and collection of baseband waveforms from a modulated signal to estimate phase compression. With this approach an accurate model of the transistor is not necessary since the PA design are based on measurements. From the collected data, the PA performance can be estimated using any type of digitally modulated signal. The collected data also give information about the load that maximizes efficiency at different power levels which can be used in load modulation analysis. The analysis of the collected data shows that an overall efficiency of 59.5% is attainable with envelope tracking, assuming a supply modulator with 80% efficiency and an ACPR level of -35.5 dB, compared to 37.7% for a constant Vd of 12 V.