Efficiency Enhancement of GaN Doherty Power Amplifier at Large Power Back-Off with Virtual Short Stub Technique

We propose a new efficiency enhancement technique of Doherty power amplifiers (DPAs) using virtual short stub. In this technique, the peaking amplifier branch was utilized as an inductive short stub at output power combing point at low power level. Using this technique, it is possible to transform the low-power load impedance seen from the carrier amplifier into higher impedance than that of classical DPAs. This leads to efficiency enhancement at larger power back-off than conventional DPAs (>6dB). The relation of efficiency enhanced level and the circuit topology for DPAs with the virtual short stub are analytically derived. The proposed technique was experimentally verified by fabricating a 3.5 GHz 40 Watt Doherty power amplifier using GaN HEMTs. An average drain efficiency of 51% and adjacent power leakage ratio of -50 dBc was obtained using 10MHz LTE test signals with 8.5dB peak-to-average power ratio.