A Design Approach to Maximize the Efficiency vs. Linearity Trade-Off in Fixed and Modulated Load GaN Power Amplifiers

This paper proposes a design method to minimize the phase distortion (AM/PM) in gallium nitride (GaN) power amplifiers (PAs) without significantly worsening other key features such as efficiency, amplitude distortion, and gain. The design strategy consists in the adoption of a smart two-stage architecture, in which the driver stage is devised to act as a sort of “analogue AM/PM predistorter”for the final one. Accounting for the actual trend in PA solutions, the proposed approach has been fine-tuned to implement a two-way class AB PA and a Doherty PA (DPA) for backhaul radio links. Both circuits, realized on the same 0.25-μm GaN technology and in monolithic form, have fully been characterized with continuous wave and modulated signals. At 7 GHz, the DPA shows 38 dBm of saturated output power and less than 3° of phase distortion with a power-added efficiency (PAE) higher than 41% in 6 dB of power back off. The class AB PA achieves almost the same level of saturated output power and maximum PAE, with an inevitable lower efficiency in back-off operation, whereas the registered AM/PM is lower than 1.5°. Moreover, when tested with modulated signals, at 32 dBm of average output power and without any digital predistortion, the DPA shows a spectral regrowth of around 36 dBc and a PAE of 40%, whereas the class AB PA achieves 40 dBc and 30%, respectively.