Switch-mode class-E power amplifiers: a contribution toward high performance and reliability

Ali Ghahremani

    Research output: ThesisPhD Thesis - Research UT, graduation UT

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    Abstract

    The increasing demands for high data rate necessitate the use of complex modulation schemes that require highly linear transmitters to optimize both the signal quality and the bandwidth usage. These two metrics are usually expressed in terms of error vector magnitude (EVM) and adjacent channel leakage ratio (ACLR). The power amplifiers (PAs), being the last active building block in a transmitter chain, greatly affect the signal quality. Also, typically, PAs are the most power hungry block that have a significant impact on the overall efficiency of a transmitter. Switch-mode class-E PAs have shown great potential for power efficient amplifi-cation of RF signals. Under certain conditions for the transistor voltage and cur-rent waveforms, this class of PAs provide (ideally) 100% efficiency. Also, due to the switched-mode operation of the transistors, class-E PAs are CMOS-friendly and show only a weak dependency on process variations. However, due to incorporating two tuned tanks, the dependency on the load impedance is 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. Load-mismatch can be due to (unintended) changes in the antenna environment or can be due to (intended) load modulation as with e.g. outphasing systems. This thesis work aims at high performance and reliable class-E PA. The first part of this thesis presents load pull analyses for class-E RF power amplifiers from a math-ematical perspective, with analyses and discussions of the effects of the most common non-idealities of class-E PAs. This includes the limited loaded quality factor (Qloaded) of the series filter, switch on-resistance, limited quality factor of the DC-feed induc-tor, load mismatch 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 simu-lations and load pull measurements of a class-E power amplifier implemented in 65nm CMOS technology. Due to switch-mode operation, a single class-E with constant supply only allows phase modulation or On-Off Keying (OOK) modulation. One may use load mod-ulation through outphasing to also enable amplitude modulation. The second part of this thesis presents an analysis of outphasing class-E Power Amplifiers (OEPAs), using load-pull analyses of single class-E PAs. This analysis led to an approach that allows to rotate and shift power contours and rotate the efficiency contours to im-prove the efficiency of OEPAs at deep power back-off, to improve the Output Power Dynamic Range (OPDR) and to reduce switch voltage stress. The theory was vali-dated using a 65nm CMOS demonstration that includes a pcb transmission-line based power combiner. OEPAs using isolating power combiners and an inverse cosine signal component separator are inherently linear but suffer from low efficiency at power back-off. For high efficiency both at maximum output power and at power back-off, non-isolating power combiners are required. In the third part of this thesis the linearity of OEPAs using non-isolating power combiners is studied theoretically and validated by mea-surement of an OEPA implemented in a standard 65nm CMOS technology using an off-chip transmission-line based combiner. The developed theoretical model for the linearity is then employed to define digital pre-distortion (DPD) parameters for the implemented OEPA. Using this theory-based DPD and without any AM/AM and AM/PM characterizations, the implemented OEPA provides a competitive linearity performance compared to the state of the art OEPAs. -31dB RMS EVM level and below -30dB ACLR were measured for a 13.1dBm 6.25MHz 30Mbit/s 7dB PAPR 64QAM signal with 41.8% drain efficiency and 33.6% power added efficiency. Finally, this thesis introduces a technique to self-protect/self-heal Class-E PAs against the effects of load variations, with only a minor impact on output power and efficiency. To validate the proposed technique, load-pull measurements are conducted on a class-E PA implemented in a standard 65nm CMOS technology, employing an off-chip matching network, augmented with a fully automated self-protective/self-healing control loop. It is shown that the proposed self-protective PA can reduce its peak switch voltage from 5.4×VDD to below 3.8×VDD for all load mismatch conditions with VSWR up to 19:1 while output power and efficiency are not considerably affected. This allows to reduce the class-E PA’s design margins significantly and to choose a higher VDD (to have a higher output power) compared to the case that the self-protective control loop is disabled. The designed self-protective class-E PA provides 17.5dBm measured output power from a 1.2V supply under nominal load conditions (when all the losses of the matching network are included) and the switch voltage is always below the value allowed by the technology for all load mismatch conditions with VSWR up to 19:1. Overall, this thesis contributes to design of high performance and reliable switch-mode class-E PAs.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • University of Twente
    Supervisors/Advisors
    • Nauta, Bram, Supervisor
    • Annema, Anne J., Co-Supervisor
    Thesis sponsors
    Award date28 Oct 2020
    Place of PublicationEnschede
    Publisher
    Print ISBNs978-90-365-5039-0
    DOIs
    Publication statusPublished - 28 Oct 2020

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