Doherty power amplifiers are widely used below 6 GHz to improve Power Added Efficiency (PAE) for communications applications. Designing Doherty PAs incurs many challenges, which increase as the frequency of operation moves towards mmWave. LDMOS, which is commonly used below 6GHz, cannot be used, and more integrated approaches need to be utilised to minimise parasitic inductances and capacitances. Recently, short gate-length GaN-on-SiC MMIC processes have become commercially available, opening up the possibility of realising medium power, high-efficiency Doherty power amplifiers at mmWave frequencies.
The design of a Doherty MMIC for the 28GHz 5G frequency band will be described. First pass design success was achieved using an asymmetrical topology designed on the commercially available 0.15µm G28v5 GaN-on-SiC process from Wolfspeed. The MMIC was packaged in a cost-effective, compact 4x4mm QFN package. Details of the design, simulation, layout, and packaging will be described.