On the Double-Inflection Characteristic of the Continuous-Wave AM/AM in Class-F−1 Power Amplifiers

• Published in: IEEE microwave and wireless components letters Vol. 28; no. 12; pp. 1131 - 1133
• Main Authors: Sharma, Tushar, Roberts, Jeffery, Holmes, Damon G., Darraji, Ramzi, Jones, Jeffery K., Ghannouchi, Fadhel M.
• Format: Journal Article
• Language: English
• Published: IEEE 01.12.2018
• Subjects: Active load–pull; AM/AM inflection; amplitude-to-amplitude modulation (AM/AM); class-F; Gallium nitride; gallium nitride (GaN); Harmonic analysis; Impedance; linearity; power amplifier (PA); Power amplifiers
• ISSN: 1531-1309; 1558-1764
• DOI: 10.1109/LMWC.2018.2874156

This letter investigates the impact of open second-harmonic impedance on the static amplitude-to-amplitude modulation (AM/AM) characteristic of class-F −1 power amplifiers (PAs). The phenomenon of double gain compression in harmonically tuned PAs is analyzed for the first time. The study starts with presenting a theoretical postulation explaining the occurrence of inflection points in AM/AM profiles of class-F −1 PAs. Thereafter, a simplified mathematical model providing closed-form expressions for predicting the locations of output power levels of the first and second AM/AM inflection points is presented. For practical validation, an active load-pull system is used to synthesize class-F −1 load conditions using a 0.7-mm NXP GaN die at a frequency of 2.6 GHz. The gain curve profile varies quite significantly as a function of second harmonic load reactance and it has been measured to capture the AM/AM inflection phenomenon at low power levels. Under class-F −1 harmonic loading conditions at 28 and 48 V, the active load-pull measurements demonstrate the phenomenon of AM/AM inflection and agree well with the mathematical analysis presented in this letter.