运算放大器驱动容性负载
- 格式：pdf
- 大小：308.34 KB
- 文档页数：9
OPAMPSDRIVINGCAPACITIVELOADS
Q.WhywouldIwanttodriveacapacitiveload?
A.It'susuallynotamatterofchoice.Inmostcases,theloadcapacitanceisnotfroma
capacitoryou'veaddedintentionally;mostoftenit'sanunwantedparasitic,suchasthe
capacitanceofalengthofcoaxialcable.However,situationsdoarisewhereit's
desirabletodecoupleadcvoltageattheoutputofanopamp-forexample,whenanop
ampisusedtoinvertareferencevoltageanddriveadynamicload.Inthiscase,you
mightwanttoplacebypasscapacitorsdirectlyontheoutputofanopamp.Eitherway,
acapacitiveloadaffectstheopamp'sperformance.
Q.Howdoescapacitiveloadingaffectopampperformance?
A.Toputitsimply,itcanturnyouramplifierintoanoscillator.Here'show:
Opampshaveaninherentoutputresistance,Ro,which,inconjunctionwitha
capacitiveload,formsanadditionalpoleintheamplifier'stransferfunction.Asthe
Bodeplotshows,ateachpoletheamplitudeslopebecomesmorenegativeby20dB/
decade.Noticehoweachpoleaddsasmuchas-90°ofphaseshift.Wecanview
instabilityfromeitheroftwoperspectives.Lookingatamplituderesponseonthelog
plot,circuitinstabilityoccurswhenthesumofopen-loopgainandfeedback
attenuationisgreaterthanunity.Similarly,lookingatphaseresponse,anopampwill
tendtooscillateatafrequencywhereloopphaseshiftexceeds-180°,ifthisfrequency
isbelowtheclosed-loopbandwidth.Theclosed-loopbandwidthofavoltage-feedback
opampcircuitisequaltotheopamp'sbandwidthproduct(GBP,orunity-gain
frequency),dividedbythecircuit'sclosedloopgain
(ACL
).
Phasemarginofanopampcircuitcanbethoughtofastheamountofadditionalphase
shiftattheclosedloopbandwidthrequiredtomakethecircuitunstable(i.e.,phase
shift+phasemargin=-180°).Asphasemarginapproacheszero,theloopphaseshift
approaches-180°andtheopampcircuitapproachesinstability.Typically,valuesof
phasemarginmuchlessthan45°cancauseproblemssuchas"peaking"infrequency
response,andovershootor"ringing"instepresponse.Inordertomaintain
conservativephasemargin,thepolegeneratedbycapacitiveloadingshouldbeatleast
adecadeabovethecircuit'sclosedloopbandwidth.Whenitisnot,considerthe
possibilityofinstability.
Q.SohowdoIdealwithacapacitiveload?
A.Firstofallyoushoulddeterminewhethertheopampcansafelydrivetheloadon
itsown.Manyopampdatasheetsspecifya"capacitiveloaddrivecapability".Others
providetypicaldataon"small-signalovershootvs.capacitiveload".Inlookingat
thesefigures,you'llseethattheovershootincreasesexponentiallywithaddedload
capacitance.Asitapproaches100%,theopampapproachesinstability.Ifpossible,
keepitwellawayfromthislimit.Alsonoticethatthisgraphisforaspecifiedgain.
Foravoltagefeedbackopamp,capacitiveloaddrivecapabilityincreasesproportionallywithgain.SoaVFopampthatcansafelydrivea100-pFcapacitanceat
unitygainshouldbeabletodrivea1000-pFcapacitanceatagainof10.
Afewopampdatasheetsspecifytheopenloopoutputresistance(Ro),fromwhich
youcancalculatethefrequencyofgain-theaddedpoleasdescribedabove.Thecircuit
willbestableifthefrequencyoftheaddedpole(fP)ismorethanadecadeabovethe
circuit'sbandwidth.
Iftheopamp'sdatasheetdoesn'tspecifycapacitiveloaddriveoropenloopoutput
resistance,andhasnographofovershootversuscapacitiveload,thentoassure
stabilityyoumustassumethatanyloadcapacitancewillrequiresomesortof
compensa-tiontechnique.Therearemanyapproachestostabilizingstandardopamp
circuitstodrivecapacitiveloads.Hereareafew:
Noise-gainmanipulation:Apowerfulwaytomaintainstabilityinlow-frequency
applications-oftenoverlookedbydesigners-involvesincreasingthecircuit's
closed-loopgain(a/k/a"noisegain")withoutchangingsignalgain,thusreducingthe
frequencyatwhichtheproductofopen-loopgainandfeedbackattenuationgoesto
unity.Somecircuitstoachievethis,byconnectingRDbetweentheopampinputs,are
shownbelow.The"noisegain"ofthesecircuitscanbearrivedatbythegiven
equation.
Sincestabilityisgovernedbynoisegainratherthanbysignalgain,theabovecircuits
allowincreasedstabilitywithoutaffectingsignalgain.Simplykeepthe"noisebandwidth"(GBP/ANOISE)atleastadecadebelowtheloadgeneratedpoletoguarantee
stability.
Onedisadvantageofthismethodofstabilizationistheadditionaloutputnoiseand
offsetvoltagecausedbyincreasedamplificationofinput-referredvoltagenoiseand
inputoffsetvoltage.TheaddeddcoffsetcanbeeliminatedbyincludingCDinseries
withRD,buttheaddednoiseisinherentwiththistechnique.Theeffectivenoisegain
ofthesecircuitswithandwithoutCDareshowninthefigure.
CD,whenused,shouldbeaslargeasfeasible;itsminimumvalueshouldbe10
ANOISE/(2pRDGBP)tokeepthe"noisepole"atleastadecadebelowthe"noise
bandwidth".
Out-of-loopcompensation:Anotherwaytostabilizeanopampforcapacitiveload
driveisbyaddingaresistor,RX,betweentheopamp'soutputterminalandtheload
capacitance,asshownbelow.Thoughapparentlyoutsidethefeedbackloop,itacts
withtheloadcapacitortointroduceazerointothetransferfunctionofthefeedback
network,therebyreducingtheloopphaseshiftathighfrequencies.
