Switching transients in class-D RF power amplifiers

Class-D RF-power amplification offers both high efficiency and increased power output. When used as part of a Kahn-technique transmitter, it also offers excellent linearity. Class-D RF power amplifiers (PAs) for the HF and VHF bands generally use the transformer coupled topology because it is well-suited to the available n-channel RF-power MOSFETs. Output transformers with two or even three decades of bandwidth are readily implemented. Modern l-GHz RF-power MOSFETs makes possible class-D power amplifiers with subnanosecond switching speeds. Changing the drain capacitance through the inductance of the RF-output transformer produces a drain-voltage transient that can necessitate operating the PA at a reduced supply voltage and output power. Driving the MOSFETs with a sine-wave of moderate amplitude increases the switching time and eliminates the transients, allowing the PA to be operated at full power. This paper therefore examines the mechanism for generating switching transients and their control with sine-wave drive.