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1、DesignofOP-AMPS1OutlineI.GeneralConsiderationII.Two-StageCMOSOp-AmpIII.CascodeOp-AmpIV.Folded-CascodeOp-AmpV.Current-MirrorOp-AmpVI.FullyDifferentialOp-AmpVII.Common-ModeFeedbackCircuitVIII.Rail-to-RailOp-AmpIX.Conclusion2I.GeneralConsideration*Welooselydefinedanop-ampasa“hi
2、gh-gaindifferentialamplifier.”*Uptotwodecadesago,mostopampswereserveas“general-purpose”buildingblocks,satisfyingtherequirementsofmanydifferentapplications.Sucheffortssoughttocreatean“ideal”opamp,e.g.,withveryhighvoltagegain,highinputimpedance,andlowoutputimpedance,butattheco
3、stofmanyotheraspectsoftheperformance,e.g.,speed,outputvoltageswings,andpowerdissipation.*Bycontrast,today’sopampdesignproceedswiththerecognitionthatthetrade-offsbetweentheparameterseventuallyrequireamulti-dimensionalcompromiseintheoverallimplementation,makingitnecessarytokno
4、wtheadequatevaluethatmustbeachievedforeachparameter.3II.Two-StageOp-ampCcmpVinA1-A21VoutDifferentialSecondOutputinputstagegainstagebuffer(i)Thefirstgainstageisadifferential-inputsingle-endedoutputstage(ii)ThesecondgainstageisnormallyaAv⇑common-sourcegainstagethathasanactivel
5、oad.(iii)Thethirdstageisasourcefolloweroutswing⇑4Two-StageOp-ampMMMM101156M8MM12-+M16VVCinincVoutMM1412MM34M15M13MM97Rb5(a)Op-ampGain(i)Thefirststage(ii)Thesecondstage(iii)Thethirdstage+Vout2+rrds1ds2Vgs7Vgvgvgs8m1gs1m2gs2gm7vgs7r//rgm8vgs8-ds6ds7-Vout11Vout3gm3R=gm4vgs4rds4
6、R//r//rLds8ds9A=g(r//r)1(i)Thegainoffirststageis:v1m1ds2ds4=G(ii)Thesecondgainstageissimplyacommon-sourcegainstagewithaactiveload,M16:Av2=-gm7(rds6//rds7)(iii)Thethirdstageisabufferstage(sourcefollower)gm8A@v3G+g+g+gLm8ds8ds96(b)FrequencyResponseM5+VinCc-VinM1M2-A2A3VoutMMCe
7、q34(i)Millercapacitor:C=C1(+A)=CAeqc2c2(ii)Formid-bandfrequencies,theimpedanceofCeqdominates111Z=r//r//@@out1ds2ds4sCsCsCAeqeqc2g(iii)Ifweassumethat,thentheoverallgain:m1A3@1gAv(s)@w=m1sCc(iv)Findtheunity-gainfrequency:taCc7(c)SlewRateTheslewrateisthemaximumrateatwhichtheout
8、putchangeswheninputsignalsarelarge.SR=dVout=ID5QI=2I,C=gm1D5D1cwdtCtamaxc2I
