Dual Input Digitally Controlled Broadband Three-Stage Doherty Power Amplifier With Back-Off Reconfigurability

This article presents a design strategy to improve the bandwidth of the three-stage Doherty Power Amplifier (DPA) by controlling the phase difference between the dual RF inputs digitally. The theory also proposes multiple back-off reconfigurability without using tuning elements and extra circuitry. The proposed theory is validated by demonstration of a dual-input three-stage DPA which is designed and implemented using Wolfspeed's GaN HEMT transistors. The measured results show a drain efficiency of 50.4 - 58% and 46.1 - 65.4% at 12 dB and 6 dB back-off respectively, over the frequency range of 550-900 MHz. Over this 350 MHz band, the drain efficiency is between 53.97% and 71.5% at saturation, corresponding to 48.3% fractional bandwidth. The measurement results also show that both the back-offs are reconfigurable by changing the drain bias of main and first auxiliary power amplifier. The modulated measurements are carried with a 10 MHz LTE signal with a PAPR of 11.75 dB. The high average efficiency of 51.6%, 54.9%, and 52.6% is reported at 600, 700, and 800 MHz, respectively, at an average power of around 30 dBm. The measurement results validate the theory proposed making the presented DPA as a potential candidate for future wireless transmitters.