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Active Load Modulation for High Efficiency RF Transmitters

  • Author(s): Vorapipat, Voravit
  • Advisor(s): Asbeck, Peter M
  • et al.
Abstract

This dissertation focuses on the development of active load-modulation techniques to improve back-off efficiency of power amplifiers (PAs) while maintaining wide bandwidth and high linearity suitable for modern wireless handsets. High back-off efficiency is required because of the use of high spectral efficiency modulation with high peak-to-average power ratio (PAPR), which causes the PA to operate in a back-off condition most of the time, typically reducing the efficiency. The Doherty architecture is a widespread active load modulation technique. A well-known issue for Doherty PAs, however, is limited bandwidth due to the use of frequency-sensitive quarter-wave transmission line or its equivalent as impedance inverter. In this work, the impedance inverter is eliminated by using voltage-source PA building blocks, rather than current-source PAs used in conventional Doherty PAs. Such idea was shown in Doherty’s original paper (1936), however it was long forgotten due to the unavailability of voltage-source PAs. Mainstream Doherty amplifiers are implemented using two current source PAs and an impedance inverter. Recently, advances in CMOS devices and PA architecture have led to a high efficiency, high linearity voltage source PA, a switched capacitor power amplifier (SCPA). In this dissertation, a Doherty PA without impedance inverter, the Voltage-Mode Doherty (VMD), is realized by using SCPA building blocks. The VMD is fabricated in 65nm CMOS and achieves 24dBm saturated power (Psat) with power-added efficiency (PAE) of 45%/34% at 0dB/5.6dB back-off, comparable to the state of the art however with much wider 1dB fractional bandwidth from 750MHz to 1050MHz. With memoryless linearization, the VMD transmits 40MHz 256-QAM 802.11ac modulation with -34.8dB EVM and 22% PAE. Compared to the state of the art, the VMD achieves much better modulation accuracy with similar PAE. The wide bandwidth, high back-off efficiency, and excellent linearity of VMD architecture enable high efficiency RF transmitters for future wideband and high spectral efficiency modulation.

Another issue of commonly used active load modulation techniques is a rather “shallow” back-off efficiency enhancement of about 6dB. Since modern modulation signals have PAPR of about 10dB, achieving deeper back-off enhancement can further improve the overall efficiency. Several hybrid architectures have been demonstrated to have efficiency enhancement beyond 6dB back-off. However, they come at the cost of a mode-switching glitch which occurs when the PA changes configuration. The mode-switching glitch causes increased distortion in the transmitted signal proportional to the modulation bandwidth, thus preventing these PAs to be used with future wideband modulation. The second part of the dissertation combined the class-G SCPA and VMD used in the first part of the dissertation to implement a PA with efficiency enhancement up to 12dB back-off without mode-switching glitch. The CG-VMD PA is fabricated in 45nm CMOS SOI with integrated balun. At 3.5GHz, it achieved 25.3dBm Psat and 30.4%/25.3%/17.4% PAE at 0/6/12 dB back-off, comparable to the state of the art with much wider bandwidth of 1GHz. With memoryless linearization, the PA achieved 19.2% PAE with −35.8dB EVM while transmitting a 40MHz 256QAM 10.1dB PAPR 802.11ac modulation. The excellent achieved EVM is superior to what is achieved in current state of the art hybrid architecture due to the absence of mode-switching glitch.

The third part of the dissertation investigates the distortion caused by the mode-switching glitch in dynamic reconfiguration PAs, and compares it to the glitch-free technique developed in this dissertation. The PA described above provides sufficient flexibility that with software control it can be used to create a mode-switching discontinuity in the same way as the power-combined transformer reconfiguration architecture used in several recent high efficiency PA reports. The measured results show a significant glitch in the signal envelope when the mode-switching occurs, causing both in-band and out-of-band distortion. A correction technique was developed to partially correct the distortion caused by the glitch. It is shown, however, that the in-band distortion can be corrected at the cost of increased out-of-band distortion. With optimal operation of the PA demonstrated in the dissertation, no glitch is observed in the signal envelope, and in-band and out-of-band distortion are much improved. This result quantitatively demonstrates the benefits of the VMD and CG-VMD glitch-free architectures for future wideband high spectral efficiency modulation.

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