Unbalanced Power Amplifier: An Architecture for Broadband Back-Off Efficiency Enhancement

In this article, we present a new broadband power amplifier (PA) architecture with a back-off efficiency enhancement that supports very wide modulation bandwidths. The unbalanced PA is composed of two cooperating sub-PAs using the Lange couplers as input power splitter and output power combiner. The PA operation is controlled by the transistors' width ratio and coupling coefficients of the Lange couplers. The output power back-off (OPBO) level is given by the transistors' width ratio and coupling coefficient of the output coupler, while the maximum efficiency is achieved at the back-off point. These features provide more design flexibility compared with the conventional Doherty PA, where the OPBO can be set only by the transistors' width ratio, and the maximum efficiency is achieved at the peak power. Using broadband harmonic matching networks, the main and auxiliary sub-PAs operate in the continuous mode to improve efficiency over a broad bandwidth. A fully integrated unbalanced PA, implemented in a 250-nm GaN-on-SiC process, achieves 32.2-34.3-dBm output power, 27%-37% efficiency at peak power, and 27%-40% at 5-6-dB back-off, across 4.5-6.5 GHz. The PA provides 3.7/4.5% (-28.6/-26.9 dB) rms error vector magnitude (EVMrms) and 30% average efficiency for a 256-QAM signal with 100-/200-MHz bandwidth, 7.2-dB PAPR, and 25.5-dBm average output power, without using any predistortion.