Optimal Fundamental Load Modulation for Class-X Harmonically Tuned Power Amplifier

This paper presents a mathematical model of the optimal fundamental load modulation design space for Class-X harmonically tuned power amplifiers (PAs). Starting from Class-X PA with maximally flat waveform, the optimal fundamental load trajectory that maintains constant high efficiency for different output power back-off level is obtained. Optimal fundamental load trajectory with different sets of second and third harmonic impedances are studied and verified with harmonic load-pull measurement. By carefully choosing the second and third harmonic load impedance, the optimal fundamental load trajectory can cross the real axis, which provides more flexibility in amplifier design. A 10W Cree GaN transistor, CG2H40010F, is measured at 800 MHz. The measured load-pull contours locate on the calculated optimal fundamental trajectory which validates the theory proposed in this work.