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A Highly Efficient SiGe Differential Power Amplifier Using. An Envelope-Tracking Technique for 3GPP LTE Applications. Yan Li, Jerry Lopez, Donald Y.C. Lie, ...
A Highly Efficient SiGe Differential Power Amplifier Using An Envelope-Tracking Technique for 3GPP LTE Applications Yan Li, Jerry Lopez, Donald Y.C. Lie, Kevin Chen*, Stanley Wu*, and Tzu-Yin Yang* Electrical and Computer Engineering Dept., Texas Tech University, Lubbock, TX, USA *Information & Communications Research Laboratories, Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan Abstract — This paper presents a highly-efficient polar transmitter (TX) system that adopts the envelopetacking (ET) technique with a differential SiGe power amplifier (PA) for 3GPP Long Term Evolution (LTE) applications. The differential PA was designed using a cascode topology, reaching power-added efficiency (PAE) of 50% at output power of 22dBm in continuous wave (CW) mode. The experimental data also shows that the proposed ET-based polar TX system with the cascode PA delivers 21dBm average output power with 33.6% PAE at 1.42 GHz, while also meeting the LTE 16QAM linearity specs for both error vector magnitude (EVM) and TX emission mask without the need of PA predistortion.

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Fig. 1 An open-loop polar TX system using the ET technique with a 3GPP LTE baseband for this work

Vector Magnitude (EVM) is 17.5% for QPSK modulation and 12.5% for 16QAM modulation. Regarding Adjacent Channel Leakage Ratio (ACLR) the value higher than 33 dB has to be achieved by the transmitter. Therefore, in the presence of any LTE signal excitation, the transmitted signal has to exhibit good linearity performances in order to maintain quality data transmission and avoid adjacent channel interference. Moreover, it is critical to maximize the power-added-efficiency (PAE) for most batteryoperated RF TX systems, especially for portable wireless applications. Nonlinear switch-mode or saturated PAs are more efficient than linear PAs, and a very attractive TX architecture for efficiency enhancement utilizes polar modulations with nonlinear PAs (i.e., “polar transmitters”). Polar TXs have demonstrated excellent system efficiency and linearity recently for GSM, EDGE, WLAN, WiMAX, etc. [2]-[7], and it is interesting to investigate their performance for 3GPP LTE as well, which is the focus of this study. For Si-based PAs, using a differential topology can reduce the parasitic ground inductance at RF that can hurt the gain and PAE of the PA significantly; it also helps to de-sensitize the PA performance against the bondwires variation during packaging. Furthermore, it can also reduce the second harmonics at the output. Recent research results have shown the potential of using differential cascode PAs with EnvelopeElimination-and-Restoration (EER)-based polar TXs [5]-[7]. In this paper, a differential cascode PA is

Index Terms — Differential Cascode Power Amplifier (PA), envelope-tracking (ET), envelope amplifier, SiGe PA, linearity, power-added efficiency (PAE)

I. INTRODUCTION 3GPP LTE is one of several evolving 3G wireless standards loosely referred to as 3.9G. The idea is that 3GPP LTE will enable much higher speeds (~100Mbps downlink) for cellular communication, along with much lower packet latency (20dB at the frequency of 1.42 GHz while achieving PAE of 50% at 22 dBm output power in CW mode. However, its inherent nonlinearity (especially the high out-of-band emission even at 6dB back-off) makes it unsuitable for LTE applications. Our experimental data indicates that an ET technique can be utilized to enhance the linearity behaviors of this 124