Peak Class F and Inverse Class F Drain Efficiencies Using Si LDMOS in a Limited Bandwidth Design

This paper compares two popular high power, high efficiency modes of operation, class F and inverse class F, and assesses the peak obtainable drain efficiencies when using Si LDMOS devices in a limited bandwidth design. Optimum class F and inverse class F conditions are presented using active harmonic load-pull measurements, and it was found that a higher drain efficiency was achieved in the class F configuration. This result is due to the limitations imposed by the soft voltage breakdown occurring due to the extended voltage swings inherent to inverse class F, as a consequence generating unwanted current content during the off cycle. This significantly reduces the peak measured efficiency using Si LDMOS devices when implementing an inverse class F design with a drain bias of 28 V. By reducing the drain bias to 18 V, to accommodate the voltage extension of inverse class F, it became possible to achieve peak measured efficiencies much closer to what theory predicted.