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

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.