High-Efficiency RF Pulsewidth Modulation of Class-E Power Amplifiers

A new switch-mode power-amplifier (SMPA) topology particularly suitable for energy efficient amplification of RF pulsewidth modulation (RF-PWM) signals is derived. It is analytically shown that high efficiency can be maintained over a wide power dynamic range if the imaginary part of the Class-E load impedance is varied along with the duty cycle (pulsewidth). Using the theory developed, an explicit design procedure is presented that allows practical realization of the proposed topology from the circuit and component specifications. Following the design procedure, and using in-house (Chalmers University, Goteborg, Sweden) SiC varactor diodes to implement the tunable imaginary load impedance, a 2-GHz 10-W peak output power GaN HEMT circuit demonstrator is realized. RF-PWM input signals for characterization of the prototype power amplifier (PA) is generated with a dedicated 65-nm CMOS modulator. The measurements show that a drain efficiency > 70% can be obtained over an 6.5-dB dynamic range, which verifies the theory presented and demonstrates the feasibility of the proposed PA topology.