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 Power Amplifier (DPA) for backhaul radio links. Both circuits, realized on the same 0.25μm GaN technology and in monolithic form, have fully been characterized with continuous wave (CW) and modulated signals. At 7 GHz the DPA shows 38dBm 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 32dBm 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.