Graduate Thesis Or Dissertation
 

Switch-capacitor Power Amplifier with Enhanced Power Back-off Efficiency and Linearity

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/q811ks48c

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  • Nowadays, wireless communication systems use high-order quadrature amplitude modulation (QAM) together with orthogonal frequency-division multiplexing (OFDM) to increase the link capacity and robustness. These signals always have large peak-to-average power ratio (PAPR) and require power amplifiers (PAs) achieve both high efficiency and linearity simultaneously. For the efficiency, the complementary metal oxide semiconductor (CMOS) switched-capacitor power amplifier (SCPA) architecture proposed in 2011 enabled the design of fully-integrated radio frequency (RF) transmitters with much higher power-added efficiency (PAE) than previous approaches. Due to the flexibility of highly digital behavior of SCPA, new techniques can still be used to improve the efficiency more at power back-off (PBO). Also, nonlinearity of the CMOS switch in SCPA should be minimized to reach the strict requirements of error vector magnitude (EVM) and spectrum emission musk. This dissertation demonstrates projects on SCPA with various design techniques to improve the PBO efficiency and linearity. The main contributions of this dissertation are as follow. The first project is to further improve the PBO efficiency and hence the overall efficiency of the transmitter, especially when transmitting high-PAPR signals with transmission power control (TPC). An eight-core Class-G polar SCPA is proposed with an eight-way digitally-scalable transformer (DST) and a new pseudo-differential Class-G switch. Eight seamless efficiency peaks are introduced at the 0 dB, 2.5 dB, 6 dB, 8.5 dB, 12 dB, 14.5 dB, 18 dB and 24 dB PBO levels. The prototype chip was designed and fabricated with a 65nm CMOS technology. This 10-bit eight-core Class-G SCPA can generate a maximum output power of 27.2 dBm at the carrier frequency of 2.42 GHz. Compared to the normalized Class-B PA, the measured drain efficiency is improved by at most 3.3x which means up to 70% power saving. A lookup-table based static digital-predistortion (DPD) is used to linearize the SCPA. For a single-carrier 64 QAM signal with 1 MHz bandwidth, the measured EVM is -28.6 dB, and the adjacent channel leakage ratio (ACLR) is -36.4 dBc after DPD. The second project is for linearity. Low error vector magnitude (EVM) and adjacent channel power ratio (ACPR) are required for high-speed wireless communication system. With carefully design, conventional SCPA can achieve good EVM without any DPD. However, the undesired spurs still need to be reduced especially the third-order counter inter-modulation products (C-IM3) when signal concentrates in a narrow band. Based on the analysis of the nonlinearity in I/Q-sharing SCPA, a baseband harmonic-rejection (BBHR) technique is used to minimize the C-IM3. Also, using a Wilkinson combiner, the cross-coupling between two sub-SCPAs is isolated to enable a linear harmonic-rejection summation. With the proposed topology, the simulated linearity is improved significantly compared to a conventional I/Q-sharing SCPA. More specifically, the C-IM3 is reduced by ~25 dB with a single-tone input and by ~22 dB with modulated signal.
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  • Pending Publication
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  • 2021-12-01 to 2024-01-11

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