A recursive house-of-cards digital power amplifier employing a λ/4-less Doherty power combiner in 65nm CMOS

This paper presents a DC-RF power inverter that efficiently synthesizes high-voltage RF waveforms directly from a battery voltage using thin-oxide CMOS switches. Instead of stacking transistors or employing large inductive transformation ratios, high output power is generated by switching individual class-D power amplifier (PA) cells in a 2-phase house-of-cards (HoC) topology to provide voltage addition of the cells outputs without exceeding device voltage ratings, effectively resulting in a solid-state RF impedance transformer. High-efficiency at backoff is then achieved by capacitively combining the output of two HoC networks nominally set to generate different amplitudes, enabling voltage-mode Doherty-like backoff without a bulky transmission line. The PA is implemented in 65nm bulk LP CMOS, operates from 4.8V, and provides a battery-to-RF efficiency above 40% at both 23dBm and 6dB backoff at 720MHz.