A High-Efficiency Low-Distortion GaN HEMT Doherty Power Amplifier With a Series-Connected Load

A distortion reduction method for a newly developed GaN HEMT Doherty amplifier (DA) with a series-connected load operating at 1.8 GHz is presented. Differing from conventional DAs with shunt-connected loads, the newly developed DA with a series-connected load has high input impedance, resulting in low-loss impedance matching and high-efficiency power combining. A distortion cancellation mechanism and its condition are derived, where odd-order nonlinear factors of transistors are considered, so as to retain inherent distortion cancellation between a peaking amplifier and a carrier amplifier. The validity of the design method is demonstrated using the developed 1.8-GHz GaN HEMT DA with a series-connected load. The third-order intermodulation distortion of the DA is improved by more than 15 dB at output powers from 5 to 20 dBm, compared to the case giving priority to power efficiency. The developed amplifier delivers a power-added efficiency (PAE) of 31% at an output power of 24 dBm, corresponding to 10-dB input backoff from a saturated output power of 31 dBm with a PAE of 58%. The proposed distortion reduction method can also be applied to shunt-connected-load-type amplifiers of other devices.