Study andminimization oftheOut-phasing amplifiers nonlinearity (2006)

Thispaperdeals withtheminimization ofsource ofnonlinearity inout-phasing amplifiers, whichwaswidely reported in previous publications. The loadmodulation, produced bythelossless combiner, leads toahighsensitivity to thereactive partofthetwotransistors' outputimpedance attributed totheextrinsic parasitics. Theseparasitics introduce anunbalance between thetwobranches andcontribute tothe nonlinear behavior. Theminimization ofsuchparasitics allowed asignificant improvement inthelinearity oftheamplifier. IndexTerms- out-phasing power amplifier, power efficiency, linearity, transistor parasitics. I.INTRODUCTION Theconstraints imposed bythenewwireless generation standards arebecoming moreandmorestringent andmaking thedesign ofthecorresponding Radio Frequency (RF)front- endsmoreandmorechallenging. Infact, inthetransmitter side, thehigh-power amplifier isthemostpower-consuming andcostly (building) block. Accordingly, high-efficiency linear amplifiers havedrawnconsiderable research attention inthepastdecade, particularly inthecontext ofamodulated RFcarrier withanon-constant envelope engendered bythe spectrally efficient modulation schemes. Inorder toobtain higher Power-Added Efficiency (PAE), amplifiers ofClass AB,B,C,orswitched modeClass E,D orFareoften used. Therefore, highefficiency andhighlinearity areinconsistent, andadvanced techniques aresuggested intheliterature to enhance thelinearity versusefficiency trade-off. Among others, linearization techniques suchasfeedback, feed- forward andpre-distortion arelargely investigated. In addition, advancedload modulation amplification techniques, suchasDoherty andLinear amplification with Nonlinear Components (LINC)(1)-(8), wereproposed to overcomethelimitations ofprevious approaches. The enhancement ofthepowerefficiency forlarge back-off came atthecostofahigher complexity, caused bytheir double branchbasedtopologies. However,theystill face implementation difficulties, whicheither keepsuch approaches intheconceptual phase orlimit their outcomes. LINCbasedamplification scheme exploits anout-phasing technique inorderto amplify signals withvariable amplitudes using twosaturated andhighefficient power amplifiers fedwithconstant envelope andphase modulated signals. After combining thetwoamplifiers output signals, anamplified version oftheoriginal signal isrecovered. For that, theoutput powercombiner could beeither matched and isolated (Wilkinson combiner) orlossless andnon-isolated (out-phasing combiner). Despite therelatively good linearity, theLINCamplifier using aWilkinson combiner leads toanuninteresting powerefficiency (5)whichis attributed tothepooraverage efficiency ofthecombiner overthewholeinput signal dynamic range. Incontrary, the lossless andnon-isolated combiner permits a significant improvement oftheaverage powerefficiency. Indeed, the powerattwo-combining ports oftheout-phasing combiner varyasfunction oftheout-phase 0asaresult oftheload modulation introduced by thetwobranches cross-talk. Hence, forloworiginal signal powerlevels, thecombiner input powerdecreases andthepowercombining efficiency is consequently improved comparing tothatobtained with Wilkinson combiner forthesamevalue of0.Theseveral attempts forimplementing theout-phasing amplifiers with discrete transistors, reported intheopenliterature, were morefocused ontheefficiency benefits (1)-(4). However, a significant, disadvantageous nonlinearity wasreported. This problem isstill notwellunderstood andthesources that stand behind itarenotwellidentified. Theremainder ofthis paper isorganized asfollows. The nextsection willgiveanoverview oftheout-phasing amplifier inthecaseoftwoideal voltage sources. Then, the nonlinear effects ofthetwotransistors' output reactance are pointed out.Thethird section will explain howtominimize thenonlinearity oftheout-phasing amplifier.