Load Mismatch Sensitivity of Class-E Power Amplifiers

Abstract

Class-E RF Power Amplifiers (PAs) are very power efficient under nominal operating conditions. Due to incorporating two tuned tanks, the dependency on the load impedance is however relatively large, resulting in e.g. load dependent output power, power efficiency, peak voltages and peak (and average) currents which can lead to reliability issues. This paper presents load-pull analyses for class-E RF power amplifiers from a mathematical perspective, with analyses and discussions of the effects of the most common non-idealities of class-E PAs: the limited loaded quality factor (Q loaded) of the series filter, switch on-resistance, limited quality factor of the dc-feed inductor, loadmismatch dependent switch conduction loss and the limited negative voltage excursions (due to e.g. the reverse conduction of the switch transistor for negative voltage excursions). The theoretical findings are backed up by extensive circuit simulations and loadpull measurements of a class-E power amplifier implemented in 65 nm CMOS technology. The PA provides 18.1 dBm output power and 72% efficiency at 1.4 GHz under nominal operating condition employing an off-chip matching network.

Theory and Implementation of a Load-Mismatch Protective Class-E PA System

“…Load-pull contours for various amplifier metrics can be derived for a class-E PA designed for specific q, d and α and β [7]. To perform load-pull analyses, the PA load Z, is varied over a range of mismatch conditions.…”

“…For each load condition, PA metrics such as switch voltage, switch current, efficiency and output power can be derived. A full mathematical derivation of the loadpull analysis was presented in [7] and will not be repeated here. The load-pull contours show the peak switch voltage normalized to V DD , efficiency and the average switch current and output power normalized to those under nominal load and design conditions (Z = R nom , VSWR=1:1, q = q nom and d = d nom ).…”

“…Once the load-dependent switching parameters α and β are obtained, the maximum switch voltage normalized to V DD , average switch current normalized to that under nominal condition, output power normalized to that under nominal condition and efficiency follow. Using this theory, developed in [7], a theoretical load pull analysis is performed on a class-E PA with parameters q = 1.4, d = 1, m = 0.05, α = 0, β = 0. This design will henceforth be referred to as the nominal design.…”

“…destruction [7][8][9][10]. Antenna impedances are a strong function of their EM environment, with Voltage Standing Wave Ratios (VSWRs) reaching up to 10:1 [11][12][13].…”

“…In this work, we present the theoretical foundation for this technique, which is backed up by simulation results and more measurement results. It was shown in [7] that changing the relative resonance frequency of the switch tank and changing the switch duty cycle have approximately opposing effects on class-E PA behavior. The use of this duty cycle tuning as a complementary method to switch tank tuning is also described.…”