《功率放大器中的AMPM失真》

1、 chen.gaopengPA中的AM-PM效應分析陳高鵬在理想的線性PA中，輸入輸出之間的相位差應該是零或者常數(shù)，即輸出信號只是輸入信號經(jīng)過幅度放大和加入一定的延時。在實際PA中，由于其非線性的影響，會發(fā)生AM-AM失真和AM-PM失真。AM-AM失真是指輸出信號和輸入信號幅度上的失真，比如當輸入信號擺幅進入閾值電壓之下或者飽和電壓之上時，輸出電壓信號就會發(fā)生截斷或削頂，即為AM-AM失真。AM-PM失真是指，非線性PA輸入信號幅度上的變化，導致了輸出和輸入信號之間的相位差的變化。ThereisalsoagooddealofmysteryaboutexactlywhatcausesAM-PM

2、distortioninthefirstplace.Simpleclippingonsupplyrails,doesareasonablejobofexplaininggaincompression,butitisnotclearwherephasedistortioncomesfromevenwhenwearelookingatwell-clippedvoltageandcurrentwaveforms.下圖所示為一個1.9GHz1WPA的AM-PM測試結果：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失真的來源： 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.下圖所示為PA在變包絡信號激勵(上圖)下的輸入輸出相位差(下圖)：K(t)EvenwithnoattempttomodelthepreciseThephaseshiftswhichoccuratenvelopeaofthedoublesidebandmodulation,asindi(2叭ofthephasecharacteristicakeyfeatureemerges,demaximahavetwicethefun

7、damentalfequen4tiAuAM-PM效應的解析分析：設PA輸出信號(DSBAM)v(t)=coss(t)-cost+1+cos(2Ct)m2(1.1)t其中：為調制信號(即包絡信號)角頻率；為載波信號頻率()；mmA=.1+cos(2t)為AM-PM相位差(設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所以當v(t)包絡達到正、負峰值時，AM-PM相位差達到最大值。s chen.gaopeng公式(1.1)可以改寫為：(1.2)v(t)=cos(t)cos(t)cos中一sin(t)sinTsm其中：T=1+cos(2t

8、)。因為1，所以可以使用cosTa1,sinTT近似來簡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從公式(1.3)可以看到，AM-PM失真使得輸出信號中產(chǎn)生了與AM-AM失真相同頻率的6三階交調產(chǎn)物：土3，其幅度為?。m8AM-AM和AM-PM效應引起的三階

9、交調失真：統(tǒng)一考慮AM-AM和AM-PM失真時，PA輸出信號(DSBAM)為:(1.4)(1.5)(1.6)v(t)=cos(t)+卩cos(3t)+卩cos(5t)+.sm3m5m6cost+1+cos(2t)2m在這里，為了分析簡便，僅考慮三階AM-AM失真，即:v(t)=cos(t)+卩cos(3ot)m3m6cosot+1+cos(2t)2m將(1.5)式展開，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)可以得到輸出信號中的三階交調產(chǎn)物的幅度為:uv=3-cos(o3)t一+sin(o3)t(1.7)im32m28mThisshowstheprincipalcomponentsofthethirdorderIMproducts,withrepresentationsfrombothamplitudeandphasedistortionmechanisms.Notethatundertheongoingassumptionofsmall,thephasedistortioncontributionswillremainapproximate

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