Investigation of load modulated inverse Class-F power amplifier with extended conduction angle

Abstract

A novel design space of load modulated (LM) inverse Class-F power amplifiers (PAs) with extended conduction angle is proposed. The effects of the driven level factor β and the biasing operation factor ρ on the third-harmonic generation are discussed. The harmonic generation mechanism shows that the knee voltage effects are the source of third-harmonics for LM inverse Class-F PAs. The definition of the inverse Class-F mode is consistently valid under the circumstance of load modulation in a limited output power back-off (OPBO) range. Meanwhile, the conduction angle θ0 can be extended from π/2 to a limited value (<110°) to keep the standard waveforms of inverse Class-F mode and maintain high efficiency. After introducing the “continuous concept” and the second harmonic manipulation method, the mathematical design space of inverse Class-F PAs with extended conduction angle is derived. Calculation shows that the purely conductive load modulation can enable high back-off efficiency operation for LM inverse Class-F PAs. As proof of concept, a demonstrator amplifier is fabricated and measured. The experimental results show that the power added efficiency (PAE) with optimum Vctrl is improved by 5% over an OPBO range of 6 dB compared with the same PA with fixed Vctrl.