Optimal fundamental load modulation for harmonically tuned switch mode power amplifier

This paper presents a mathematical model, along with simulation and experimental validation for the optimal fundamental load modulation design space for harmonically tuned switch mode power amplifiers. Starting from conventional class-F−1 PA with a conventional optimal load modulation on the real impedances axis, it is shown mathematically that by moving one of the harmonic terminations along the edge of the smith chart (purely reactive impedance), the optimum fundamental load trajectory is moved away from the conventional real impedances axis to a complex impedance trajectory that maintains constant high power efficiency for different output power back-off values. A 10 W GaN transistor is measured in a harmonic load pull system at 2.14 GHz in order to validate the theory.