Simplified All-in-One Load Network of the Broadband Doherty Power Amplifier

Abstract

A simplified all-in-one load network of Doherty power amplifier (DPA) was proposed for extended back-off range and broadband operation in this study. Although the proposed load network was simplified to just have a postmatching network (PMN) and compact L-section matching networks for both carriers and peaking amplifiers, it could fully incorporate previously known back-off range extending methods, such as asymmetric structure, virtual stub (VS), out-phased current combining (OCC) method, and complex combining load (CCL). Accurate closed-form expressions for the elements and impedances of the load network based on the back-off range extending methods were derived and used to design the structure of the load network. Among available configurations of synthesized load networks, a structure suited for broadband operation was derived. To verify the proposed load network and its closed-form expression, an asymmetric DPA based on GaN-HEMT with an extended back-off range of 8 dB was designed and implemented for a frequency band of 2.9–4.0 GHz, which includes the N78 5G frequency band. Lumped inductors for the initial configuration were transformed using transmission lines for a quasi-lumped structure which was optimized to have a frequency response similar to the original inductor. Implemented DPA exhibited a drain efficiency (DE) of 50%–56% under an output power back-off (OBO) of 8.0 dB from a saturated output power of about 43 dBm, while it showed a DE of 62%–76% over a frequency range of 2.9–4.0 GHz using a continuous-wave (CW) signal. An efficiency of as high as 50.6%–58.0% and an adjacent channel leakage power ratio (ACLR) of under −46.0 dBc after a digital predistortion (DPD) based on a memory polynomial were achieved at an average output power of 35 dBm using a 5G new radio (NR) signal with a signal bandwidth of 100 MHz and a peak-to-average power ratio (PAPR) of 7.8 dB.