S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Electrical and Computer Engineering (전기·정보공학부) Theses (Ph.D. / Sc.D._전기·정보공학부)
(A) study on multiband reconfigurable linear cmos power amplifier for mobile applications
이동통신 기기에 적합한 재구성이 가능한 다중대역 선형 CMOS 전력증폭기에 관한 연구
- 공과대학 전기·컴퓨터공학부
- Issue Date
- 서울대학교 대학원
- 학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2015. 2. 권영우.
- In this Dissertation, a study on multiband reconfigurable linear CMOS power amplifier (PA) is performed. Since a larger number of frequency bands is allocated for 3G/4G mobile communication standards nowadays, handset PAs are required to support the ever-increasing number of frequency bands. With the advent of high-speed wireless data transmission, handset PAs are also demanded to perform linear power amplification under the wide-band signal condition. Even though the CMOS technology has cost and size benefits, however, designing a watt-level linear CMOS PA is a challenging issue due to low breakdown voltage and nonlinear nature of the CMOS device.
To resolve the issues above, this study presents two methods suitable for multiband (MB) linear CMOS PA: a reconfigurable MB matching structure and a linearization technique. The proposed MB structure shares a PA core to reduce the cost and size, and contains the power- and frequency-reconfigurable matching networks as well as the output path-selection function. Thus, it can perform the MB operation requiring multiple frequency bands and target output powers. The reconfiguration mechanism is quantitatively analyzed and experimentally demonstrated. The fabricated tri-band reconfigurable 3G UMTS PA using an InGaP/GaAs heterojunction bipolar transistor (HBT) process for practical handset application showed minimal efficiency degradation of less than 2% by multi-banding, compared with a single-band reference PA.
For linearization of a CMOS PA, a phase-based linearization technique is presented. Since the PA nonlinearity is determined by the dynamic AM-AM and AM-PM, the two distortions should simultaneously be considered in linearization. Contrary to the previous works which have focused on the correction of AM-AM distortion by providing an envelope-dependent gate-bias, this work proposes an AM-PM linearizer using a varactor and an envelope-reshaping circuit. This linearizer helps the PA recover AM-AM distortion as well. To validate the usefulness of the proposed linearizer, 1.88 GHz and 0.9 GHz stacked-FET PAs using a 0.32-μm silicon-on-insulator (SOI) CMOS process were designed and fabricated. Measurement results showed that the fabricated 1.88 / 0.9 GHz linear CMOS PAs achieved linear efficiencies (meeting –39 dBc W-CDMA ACLR) of higher than 44 / 49%. Furthermore, a single-chain MB linear CMOS PA was implemented based on the proposed MB reconfiguration and linearization techniques. The fabricated MB PA, which has two outputs and covers five popular uplink UMTS/LTE bands (Band 1/2/4/5/8: 824 ~ 1980 MHz), showed minimal efficiency degradation (< 3.3%) compared to the single-band dedicated CMOS PA with W-CDMA efficiencies in excess of 40.7%.
Finally, the signal-bandwidth limiting effect of the envelope-based linear CMOS PA is discussed and a solution is proposed. Due to the time delay during envelope-detection and shaping, a timing mismatch between the incoming RF signal and envelope-reshaped signal occurs, thus resulting in no linearization effect under wide-band signal (LTE 20 MHz or more) conditions. To resolve the problem, a group delay circuit with a compact size is employed and thus the linearization effect of the proposed phase-based linearizer is maintained up to 40 MHz LTE bandwidth.