《功率放大器中的AMPM失真》

1、 chen.gaopengPA中的AM-PM效应分析陈高鹏在理想的线性PA中，输入输出之间的相位差应该是零或者常数，即输出信号只是输入信号经过幅度放大和加入一定的延时。在实际PA中，由于其非线性的影响，会发生AM-AM失真和AM-PM失真。AM-AM失真是指输出信号和输入信号幅度上的失真，比如当输入信号摆幅进入阈值电压之下或者饱和电压之上时，输出电压信号就会发生截断或削顶，即为AM-AM失真。AM-PM失真是指，非线性PA输入信号幅度上的变化，导致了输出和输入信号之间的相位差的变化。ThereisalsoagooddealofmysteryaboutexactlywhatcausesAM-PM

2、distortioninthefirstplace.Simpleclippingonsupplyrails,doesareasonablejobofexplaininggaincompression,butitisnotclearwherephasedistortioncomesfromevenwhenwearelookingatwell-clippedvoltageandcurrentwaveforms.下图所示为一个1.9GHz1WPA的AM-PM测试结果：driveleveluptothecompressionpoint(circleindicated),whereitstartsamo

3、rerapidreversal.SohereisfinaltwistinthereducedconductionanglePAstory,itseemsthatdeepABoperationmaycausesubstantialAM-PMproblemsintheprecompressionzone.尤其是在深ClassAB工作模式下(“AB”)：showsanalmostlinearphasechangeversus.dB2PA中AM-PM失真的来源： chen.gaopeng chen.gaopengIngeneral,AM-PMeffectscanbetracedtothesignal-

4、leveldependenceofseveralkeytransistormodelelements.ForFETstheinputcapacitanceandboththedepletionandjunctionresistanceofthegate-sourcediodecanbeprimaryculprits.Itshouldbenotedthatnonlinearresistance,inthepresenceoflinearreactance,cancauseAM-PMeffects,justasmuchasnonlinearreactance.ForBJTs,thenonlinea

5、rbase-collectorcapacitanceaddsanimportantadditionalsourceofdrive-dependentphaseshift.Alloftheseeffectsaredetailed,interactive,andhighlycomplexinthemselves,andposegreatchallengesforphysicalmodeling,suchasthatwhichismoresuccessfullyemployedforcompressionandclippingeffects.SotheonlywaythatAM-PMcanbetre

6、atedinaconcisemanneristoresorttoempiricaldescribingfunctionsfittedtophysicalmeasurements.下图所示为PA在变包络信号激励(上图)下的输入输出相位差(下图)：K(t)EvenwithnoattempttomodelthepreciseThephaseshiftswhichoccuratenvelopeaofthedoublesidebandmodulation,asindi(2叭ofthephasecharacteristicakeyfeatureemerges,demaximahavetwicethefun

7、damentalfequen4tiAuAM-PM效应的解析分析：设PA输出信号(DSBAM)v(t)=coss(t)-cost+1+cos(2Ct)m2(1.1)t其中：为调制信号(即包络信号)角频率；为载波信号频率()；mmA=.1+cos(2t)为AM-PM相位差(设1)。2m令AM-PM相位差人二怎.1+cos(2-1)=0，可解得：t=n兀(n=0,l,2.)。2mm所以当v(t)包络达到正、负峰值时，AM-PM相位差达到最大值。s chen.gaopeng公式(1.1)可以改写为：(1.2)v(t)=cos(t)cos(t)cos中一sin(t)sinTsm其中：T=1+cos(2t

8、)。因为1，所以可以使用cosTa1,sinTT近似来简2m化公式(1.2)：6v(t)=cos(t)cost一1+cos(2t)sin(t)st2m66=cos(t)cos(t一)+cos(2t)sin(t)m22m666(1.3)=cos(t)cos(t一)+sin(+2)t+sin(+2)tm24m4m1616=cos(+)t+cos(一)t2m22m26+_sin(+)t+sin(+3)t+sin()t+sin(3)t8mmmm从公式(1.3)可以看到，AM-PM失真使得输出信号中产生了与AM-AM失真相同频率的6三阶交调产物：土3，其幅度为?。m8AM-AM和AM-PM效应引起的三阶

9、交调失真：统一考虑AM-AM和AM-PM失真时，PA输出信号(DSBAM)为:(1.4)(1.5)(1.6)v(t)=cos(t)+卩cos(3t)+卩cos(5t)+.sm3m5m6cost+1+cos(2t)2m在这里，为了分析简便，仅考虑三阶AM-AM失真，即:v(t)=cos(t)+卩cos(3ot)m3m6cosot+1+cos(2t)2m将(1.5)式展开，得到：1616v(t)=cos(+)t一+cos()t一2m22m26+sin(+)t+sin(+3)t)+sin()t+sin(3)t8mmmm+3cos(+3)t一+3cos(3)t一2m22m2+3sin(3)t+sin(

10、+5)t+sin(3)t+sin(5)t8由式(1.6)可以得到输出信号中的三阶交调产物的幅度为:uv=3-cos(o3)t一+sin(o3)t(1.7)im32m28mThisshowstheprincipalcomponentsofthethirdorderIMproducts,withrepresentationsfrombothamplitudeandphasedistortionmechanisms.Notethatundertheongoingassumptionofsmall,thephasedistortioncontributionswillremainapproximate

11、lyinquadraturewiththeamplitudeones.Sothereisnopossibilityofsignificantcancellationoccurringbetweenthecomponentsgeneratedbythetwodistortionmechanisms.TheAM-AMandAM-PMcomponentswillalwayscombinetoproduceIM3sidebandswhichhaveahigheramplitudethaneitheroftheseindividualparts.ExperimentalmeasurementoftherelativecontributionsofAM-AMandAM-PMcontributionsrequi