Stacked GaAs pHEMTs: Design of a K-band power amplifier and experimental characterization of mismatch effects

This paper presents the design and experimental characterization of a 3-stage stacked MMIC power amplifier in GaAs pHEMT technology. The amplifier targets an output power of 2W from 20 to 23 GHz through direct series power combination, and is fabricated in a super-compact stacked monolithic layout. The design, from basic-cell layout optimization to matching, is described. Despite the presence of early compression spots, CW measurements exhibit gain higher than 9.5 dB with output power larger than 31.8 dBm, and saturated efficiency in excess of 27%, in a 15% bandwidth around 21.5 GHz. To verify the overall stacked module in absence of high-frequency effects, the same basic cell has been manufactured for low frequency operations. Load pull characterization on this structure demonstrates the reliability of the solution, capable to deliver 2W on a 50 Ω load from 2 to 6 GHz, and permits to relate the K-band early compression spots, responsible of suboptimal performance, to the output mismatch.