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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,SEPTEMBER20092105 Design Optimization and DPD Linearization

of GaN-Based Unsymmetrical Doherty Power

Amplifiers for3G 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)-based Doherty 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 amplifier path to compensate,at a given backoff level,for the bias and power-dependant phase variation through the carrier and peaking paths.At a6-dB output power backoff(OPBO),measurement results of the unsymmetrical un-linearized Doherty amplifier using a four-carrier WCDMA signal achieved results of close to50%for the PAE and about30dBc 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’s peak-to-average power ratio, 63-and53-dBc adjacent channel power ratios were obtained when the Doherty amplifier was 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 over20MHz 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 efficiency in 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 April01,2009,revised June05,2009.First published August18,2009;current version published September04,2009.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.

S.-C.Jung was with the Intelligent RF Radio Laboratory(iRadio Labora-tory),Department of Electrical and Computer Engineering,Schulich School of Engineering,University of Calgary,Calgary,AB,Canada T2N1N4.He is now with the Semiconductor Material Research and Development Center Samsung Techwin,Seoul135-980,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(iRadio Laboratory),Department of Electrical and Computer Engineering, Schulich School of Engineering,University of Calgary,Calgary,AB,Canada T2N1N4(e-mail:ohammi@ucalgary.ca;fadhel.ghannouchi@ucalgary.ca). Color versions of one or more of thefigures in 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 efficiency of 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 efficiency of 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 amplifier is one of the most useful techniques to improve efficiency.It has several advantages compared with other efficiency enhancement techniques in terms of circuit complexity,cost effectiveness,and fabrication[1]–[11].Some of the efficiency enhancement 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/or preprocessing 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 efficiency of the Doherty amplifier.These techniques can be briefly summarized as:1)unequal input power splitter [3];2)drain bias control[6];3)harmonic termination circuit [7];4)input phase delay line[9];and5)optimized impedance matching network for each amplifier[10].

The carrier amplifier is usually operated as class AB in order to satisfy linearity specifications in the low-power region.The peaking amplifier is usually biased in class-B or class-C to maximize the efficiency in the high-power region.However, the linearity of the Doherty amplifier can be made to be higher than that of the balanced class-AB amplifier if the third-order

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

transconductance,,of the peaking amplifier cancels 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’s branches at a given average operating power when multicarrier signals are used.

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

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