A Wideband and Highly Efficient Circulator Load Modulated Power Amplifier Architecture

A complete theoretical analysis is presented for a highly efficient power amplifier (PA) architecture based on a non-reciprocal combiner. The architecture consists of two amplifier branches and a microwave circulator acting as the output combiner, referred to as the circulator load-modulated amplifier (CLMA). The continuous-mode (CM) design technique has been further exploited for the CLMA to demonstrate the wideband capabilities of the architecture. The analysis reveals that the CLMA performs active load modulation to maintain high PA efficiency performance across a wide bandwidth and large output power dynamic range. As a proof of concept, a CLMA demonstrator circuit based on gallium nitride (GaN) transistors and a microwave circulator, is designed and characterized with continuous-wave and modulated-signal measurements. The implemented CLMA prototype circuit experimentally demonstrates Doherty-like efficiency enhancement over a large bandwidth covering 2.1 to 3.5 GHz. In measurements, the prototype exhibits a drain efficiency of 46.7-57.5% at peak output power and 35.6-50.6% at 7-dB output power back-off level, within the design bandwidth. When tested with a 60-MHz multi-carrier orthogonal frequency-division multiplexing (OFDM) signal having a 7-dB peak-to-average power ratio (PAPR), an average efficiency of 48.5% with better than −47.5-dBc adjacent channel leakage ratio (ACLR) is achieved after applying digital pre-distortion (DPD).