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Design optimization and DPD linearization of GaN-based unsymmetrical Doherty

IEEE TRANSACTIONS ON MICROWA VE THEORY AND TECHNIQUES, VOL. 57, NO. 9, SEPTEMBER 20092105

Design Optimization and DPD Linearization of GaN-Based Unsymmetrical Doherty Power Amplifiersfor 3G Multicarrier Applications

Sung-Chan Jung, Oualid Hammi , Member, IEEE , and Fadhel M. Ghannouchi , Fellow, IEEE

Abstract—In this paper, a design optimization approach for gal-lium–nitride(GaN)-basedDoherty power amplifiers(PAs)is pro-posed to enhance linearizability and maximize the power-added efficiency(PAE)for multicarrier wideband code division multiple access (WCDMA)applications. This is based on the use of an input offset line at the peaking amplifierpath to compensate, at a given backoff level, for the bias and power-dependant phase variation through the carrier and peaking paths. At a 6-dB output power backoff (OPBO),measurement results of the unsymmetrical un-linearized Doherty amplifierusing a four-carrier WCDMA signal achieved results of close to 50%for the PAE and about 30dBc for the adjacent channel leakage ratio. Linearization of the de-signed Doherty PA using a baseband digital predistortion tech-nique led to quasi-perfect cancellation of spectrum regrowth. At an OPBO equal to the input signal’speak-to-average power ratio, 63-and 53-dBc adjacent channel power ratios were obtained when the Doherty amplifierwas driven by single-and four-carrier WCDMA signals, respectively. To the best of the authors’knowl-edge, this represents the best reported results for PAE and linearity for GaN-based Doherty PAs linearized over 20MHz of instanta-neous bandwidth.

Index Terms—Digital predistorter, Doherty amplifier,gallium–nitride (GaN),linearization, power amplifier(PA),power effi-ciency, wideband code division multiple access (WCDMA).

I. I NTRODUCTION

P

OWER amplifiers(PAs),which are used for modern wireless communication systems, require high system efficiencyin both low-and high-power regions, due to the use of modulated signals with very high peak-to-average power ratios (PAPRs),such as wideband code division multiple access

Manuscript received April 01, 2009, revised June 05, 2009. First published August 18, 2009; current version published September 04, 2009. This work was supported by the Alberta Informatics Circle of Research Excellence (iCORE),the Natural Sciences and Engineering Research Council of Canada (NSERC),the Canadian Space Agency, Focus Microwaves, and Nanowave Technologies, and under the Canada Research Chairs (CRC)Program. The work of S.-C. Jung was supported by the Korean Government (MOEHRD)under Korea Research Foundation Grant KRF-2007-357-D00177.

Design optimization and DPD linearization of GaN-based unsymmetrical Doherty

Design optimization and DPD linearization of GaN-based unsymmetrical Doherty

Design optimization and DPD linearization of GaN-based unsymmetrical Doherty

S.-C. Jung was with the Intelligent RF Radio Laboratory (iRadioLabora-tory), Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4. He is now with the Semiconductor Material Research and Development Center Samsung Techwin, Seoul 135-980, Korea (e-mail:sungchan.jung@http://www.sodocs.net/doc/2acab99a52ea551811a6876c.html ).

O. Hammi and F. M. Ghannouchi are with the Intelligent RF Radio Labora-tory (iRadioLaboratory), Department of Electrical and Computer Engineering, Schulich School of Engineering, University of Calgary, Calgary, AB, Canada T2N 1N4(e-mail:ohammi@ucalgary.ca;fadhel.ghannouchi@ucalgary.ca).Color versions of one or more of the figuresin this paper are available online at http://www.sodocs.net/doc/2acab99a52ea551811a6876c.html .

Digital Object Identifier10.1109/TMTT.2009.2027076

(WCDMA)and orthogonal frequency division multiplexing (OFDM)signals. The efficiencyof a conventional linear PA is drastically decreased since it must operate far from its satura-tion region. In addition, the linearity of the PA is important in maintaining good quality of the signal. However, the linearity and efficiencyof conventional PAs normally show opposite tendencies with the drive level. Therefore, to simultaneously achieve high linearity and high efficiency,many techniques have been proposed [1]–[15].

The Doherty amplifieris one of the most useful techniques to improve efficiency.It has several advantages compared with other efficiencyenhancement techniques in terms of circuit complexity, cost effectiveness, and fabrication [1]–[11].Some of the efficiencyenhancement techniques, such as envelope elimination and restoration (EER)[12],envelope tracking (ET)[13],dynamic bias switching (DBS)[14],and linear amplifica-tion with nonlinear components (LINC)[15]require additional control and/orpreprocessing circuits. This contrasts with the Doherty amplifier,which, in principle, does not need additional control circuitry to improve efficiency[1]–[11].However, in most current wireless applications, linearization techniques have to be used to compensate for the nonlinear distortions of the amplifiers[16]–[19].In addition to linearization techniques, many useful techniques have been proposed to improve the lin-earity and efficiencyof the Doherty amplifier.These techniques can be brieflysummarized as:1) unequal input power splitter [3];2) drain bias control [6];3) harmonic termination circuit [7];4) input phase delay line [9];and 5) optimized impedance matching network for each amplifier[10].

The carrier amplifieris usually operated as class AB in order to satisfy linearity specificationsin the low-power region. The peaking amplifieris usually biased in class-B or class-C to maximize the efficiencyin the high-power region. However, the linearity of the Doherty amplifiercan be made to be higher than that of the balanced class-AB amplifierif the third-order

transconductance,

, of the peaking amplifiercancels out its counterpart of the carrier amplifier.This can be achieved by proper selection of the bias conditions of both amplifiers[8]or, as proposed in this paper, by aligning the phase variation between the Doherty amplifier’sbranches at a given average operating power when multicarrier signals are used.

In this research, a class-AB balanced amplifierwas ini-tially designed as a reference design. A symmetrical Doherty amplifierwith an appropriate offset length to optimize the load–pullingand efficiencywas then designed and built. A third amplifier,the unsymmetrical design, was optimized for

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