Harmonic Tuning of Stacked SiGe Power Amplifiers Using Active Load Pull

Stacked power amplifiers (PAs) have gained a lot of attention recently because of increased output power through equal distribution of output voltage swing among the stacked transistors. However, commonly employed PA design techniques, such as waveform engineering or harmonic tuning, often suffer in stacked configuration, and the enhancement in power-added efficiency (PAE) remains poor. This letter analyzes the feasibility of harmonic tuning techniques applied to stacked amplifiers. An analysis is presented to show how harmonic tuning at the output affects harmonic optimization of stacked transistors and how this depends on the base termination impedance. Three different device configurations (cascode, tri-stacked, and quad-stacked) have been characterized at $X$ -band. Measured results show harmonic PAE enhancement degrades as the number of stacked transistors increases, which is in agreement with the analysis presented.