Theory and Design of Quadrature-Balanced GaN Power Amplifier as Magnetic-Less Simultaneous Transmit and Receive (STAR) Front-End

This article presents the design and implementation of a magnetic-less high-power front-end that can be exploited for long-range full-duplex (FD) communications, based on the quadrature-balanced power amplifier (QB-PA). A unique reflective loadline theory is proposed, which leverages the design space of the continuous modes of PAs for achieving spontaneous high transmitter efficiency and low receiver loss. A corresponding design flow with practical devices is developed, which is further proven to be agnostic to the GaN processes and device sizes. Based on the proposed theory and design methodology, a functional simultaneous transmit and receive (STAR) front-end is designed using commercially available GaN devices. Experimentally, the developed prototype achieves up to 80% PA efficiency at a peak power of 42.5 dBm at 1.6 GHz together with a low reception loss of $\approx 1.5$ dB across varying RX frequencies from 1.64 to 1.72 GHz. STAR operation is evaluated with concurrent stimuli of TX and RX with 4G long-term evolution (LTE) signals at slightly deviated center frequencies, which is due to the fact that self-interference cancellation (SIC) is not fully applied in this study. A low RX error vector magnitude (EVM) is measured across a variation of average TX power and bandwidth, and meanwhile, a high average TX efficiency (>30%) and linearity (EVM <4%) are experimentally demonstrated.