运算放大器电路固有噪声的分析与测量7new

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1、AnalysisandMeasurementofIntrinsicNoiseinOpAmpCircuitsPartVII:NoiseInsideTheAmplifierbyArtKay,SeniorApplicationsEngineer,TexasInstrumentsIncorporatedThisTechNotediscussesthefundamentalphysicalrelationshipsthatdeterminetheintrinsicnoiseofanoperationalamplifier(opamp).Board-and-systemleveldesigners

2、willgaininsightintoperformancetradeoffsmadebyintegratedcircuitdesignersbetweennoiseandotheropampparameters.Also,engineerswilllearnhowtoestimateworstcasenoisebasedontypicaldatasheetspecificationsatroomtemperatureandovertemperature.FiveRules-Of-ThumbForWorstCaseNoiseAnalysisAndDesignMostopampdatas

3、heetslistonlyatypicalvaluefornoise,withnoinformationregardingtemperaturedrift.Boardandsystemleveldesignerswouldliketohaveamethodforestimatingmaximumnoisebasedonthetypicalvalue.Furthermore,itwouldbeusefultoestimatenoisedriftwithtemperature.Therearesomefundamentalnoiserelationshipsfortransistorsth

4、atcanhelptomaketheseestimates.However,inordertomakeuseoftheserelationshipsprecisely,someknowledgeoftheinternaltopologyisrequired(egbiasingconfiguration,transistortype,etc).Nevertheless,itispossible,ifweconsidertheworstcaseconfiguration,tomakesomegeneralstatementsthatcoverthemajorityofconfigurati

5、ons.ThissectionoftheTechNotesummarizesfivebasicrules-of-thumbforworstcasenoiseanalysisanddesign.Thenextsectiongivesdetailedmathematicsbehindtheserules-of-thumb.Rule-Of-Thumb#1:Broadbandvoltagenoiseisveryinsensitivetosemiconductorprocesschanges.Thisisbecauseopampnoiseisgenerallyafunctionofbiascur

6、rentwhichistypicallyrelativelyconstantfromdevicetodevice.Alternatively,forsomedesigns,noisecouldbedominatedbythethermalnoiseoftheinputESDprotectionresistors.Withthisinmind,itisunlikelythatthebroadbandnoisewillchangemorethan10%fromthetypicalvalueandisusuallylessformostlow-noisedevices(seeFig.7.1)

7、.Fig.7.1:EstimateMaxRoomTemperatureBroadbandNoiseBasedonTypicalBroadbandcurrentnoiseismoresensitivethenvoltagenoise(forbipolarprocesses).Thisisbecausecurrentnoiseisrelatedtobasecurrent,whichissetbythetransistorscurrentgain(B