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1、SimulationandMeasurementofOpAmps11/05/2004D.DeVenuto1Simulationandmeasurementtechniques•Thecategoriesofopampmeasurementsandsimulationsdiscussedinclude:open-loopgain,open-loopfrequencyresponse(includingthephasemargin),input-offsetvoltage,common-modegain,power-supplyr
2、ejectionratio,common-modeinput-andoutput-voltageranges,open-loopoutputresistance,andtransientresponseincludingslew-rate.•Simulatingormeasuringtheopampinopenloopconfigurationisoneofthemostdifficultstepstoperformsuccessfully.Thereasonisthehighdifferentialgainoftheopam
3、p.11/05/2004D.DeVenuto2Open-loopmodewithoffsetcompensation1)method:Vosisanexternalvoltagewhosevalueisadjustedtokeepthedcvalueofvoutbetweenthepower-supplylimits.WithoutVostheopampwillbedriventothepositiveornegativepowersupplyforeitherthemeasurementorsimulationcases.T
4、hemethodworksinsimulationbutnotinmeasurements11/05/2004D.DeVenuto3Amethodofmeasuringtheopen-loopcharacteristicswithdcbiasstability2)Amethodmoresuitableforbothmeasuringandsimulatingtheopen-loopgainis:InthiscircuititisnecessarytoselectthereciprocalRCtimeconstantafacto
5、rofAv(0)timeslessthantheanticipateddominantpoleoftheopamp.Undertheseconditions,theopamphastotaldcfeedback,whichstabilizesthebias.11/05/2004D.DeVenuto4•Simulationormeasurementoftheopen-loopgainoftheopampwillcharacterizetheopen-looptransfercurve,theopen-loopoutput-swi
6、nglimits,thephasemargin,thedominantpole,theunity-gainbandwidth,andotheropenloopcharacteristics.•Thedesignershouldconnecttheanticipatedloadingattheoutputinordertogetmeaningfulresults.11/05/2004D.DeVenuto5Configurationforsimulatingormeasuringtheopen-loopfrequencyrespo
7、nseformoderate-gainopamp•Insomecases,wheretheopen-loopgainisnottoolarge,theopen-loopgaincanbemeasuredbyapplyingvandmeasuringinvandv.InthisconfigurationRislargeenoughtonotcauseaoutIdccurrentloadontheoutput11/05/2004D.DeVenuto6DCinputoffset•Ifthedcinputoffsetvoltageis
8、toosmall,itcanbeamplifiedbyusingaresistordividerinthenegativefeedbackpath.OnemustrememberthatVoswillvarywithtimeandtemperatureandisverydif
