运算放大器的单电源供电

Oneofthemostcommonapplicationsquestionsonopera-tionalamplifiersconcernsoperationfromasinglesupplyvoltage.“CanthemodelOPAxyzbeoperatedfromasinglesupply?”Theanswerisalmostalwaysyes.Operationofopampsfromsinglesupplyvoltagesisusefulwhennegativesupplyvoltagesarenotavailable.Furthermore,certainap-plicationsusinghighvoltageandhighcurrentopampscanderiveimportantbenefitsfromsinglesupplyoperation.ConsiderthebasicopampconnectionshowninFigurela.Itispoweredfromadualsupply(alsocalledabalancedorsplitsupply).Notethatthereisnogroundconnectiontotheopamp.Infact,itcouldbesaidthattheopampdoesn’tknowwheregroundpotentialis.Groundpotentialissome-wherebetweenthepositiveandnegativepowersupplyvoltages,buttheopamphasnoelectricalconnectiontotellitexactlywhere.VINVOUT=VING=+1+VS=15V–VS=15VVINVOUT=VING=1+VS=30V(a)(b)FIGURE1.Asimpleunity-gainbufferconnectionofanopampillustratesthesimilarityofsplit-supplyop-eration(a)tosingle-supplyoperationin(b).Thecircuitshownisconnectedasavoltagefollower,sotheoutputvoltageisequaltotheinputvoltage.Ofcourse,therearelimitstotheabilityoftheoutputtofollowtheinput.Astheinputvoltageswingspositively,theoutputatsomepointnearthepositivepowersupplywillbeunabletofollowtheinput.Similarlythenegativeoutputswingwillbelimitedtosomewherecloseto–VS.Atypicalopampmightallowoutputtoswingwithin2Vofthepowersupply,makingitpossibletooutput–13Vto+13Vwith±15Vsupplies.Figure1bshowsthesameunity-gainfolloweroperatedfromasingle30Vpowersupply.Theopampstillhasatotalof30Vacrossthepowersupplyterminals,butinthiscaseitcomesfromasinglepositivesupply.Operationisotherwiseunchanged.Theoutputiscapableoffollowingtheinputaslongastheinputcomesnocloserthan2Vfromeithersupplyterminaloftheopamp.Theusablerangeofthecircuitshownwouldbefrom+2Vto+28V.Anyopampwouldbecapableofthistypeofsingle-supplyoperation(withsomewhatdifferentswinglimits).Whythenaresomeopampsspecificallytoutedforsinglesupplyapplications?Sometimes,thelimitonoutputswingnearground(the“negative”powersupplytotheopamp)posesasignificantlimitation.Figure1bshowsanapplicationwheretheinputsignalisreferencedtoground.Inthiscase,inputsignalsoflessthan2Vwillnotbeaccuratelyhandledbytheopamp.A“single-supplyopamp”wouldhandlethisparticularapplicationmoresuccessfully.Thereare,however,manywaystouseastandardopampinsingle-supplyapplicationswhichmayleadtobetteroverallperformance.Thekeytotheseapplicationsisinunderstandingthelimitationsofopampswhenhandlingvoltagesneartheirpowersupplies.TherearetwopossiblecausesfortheinabilityofastandardopamptofunctionneargroundinFigure1b.Theyare(1)limitedcommon-moderangeand(2)outputvoltageswingcapability.TheseperformancecharacteristicsareeasilyvisualizedwiththegraphicalrepresentationshowninFigure2.Therangeoverwhichagivenopampproperlyfunctionsisshowninrelationshiptothepowersupplyvoltage.Thecommon-moderange,forinstance,issometimesshownplottedwithrespecttoanotherparametersuchastemperature.A±15Vsupplyisassumedinthepreparationofthisplot,butitiseasytoimaginethenegativesupplyasbeingground.InFigure2a,noticethattheopamphasacommon-moderangeof–13Vto+13.5V.Forvoltagesontheinputtermi-nalsoftheopampofmorenegativethan–13Vormorepositivethan+13.5V,thedifferentialinputstageceasestoproperlyfunction.Similarly,theoutputstagesoftheopampwillhavelimitsonoutputswingclosetothesupplyvoltage.Thiswillbeload-dependentandperhapstemperature-dependentalso.Figure2bshowsoutputswingabilityofanopampplottedwithrespecttoloadcurrent.Itshowsanoutputswingcapabilityof–13.8Vto+12.8Vforal0kΩload(approximately±1mA)at25°C.®©1986Burr-BrownCorporationAB-067PrintedinU.S.A.March,1986SINGLE-SUPPLYOPERATIONOFOPERATIONALAMPLIFIERSSBOA059Oneofthemostcommonapplicationsquestionsonopera-tionalamplifiersconcernsoperationfromasinglesupplyvoltage.“CanthemodelOPAxyzbeoperatedfromasinglesupply?”Theanswerisalmostalwaysyes.Operationofopampsfromsinglesupplyvoltagesisusefulwhennegativesupplyvoltagesarenotavailable.Furthermore,certainap-plicationsusinghighvoltageandhighcurrentopampscanderiveimportantbenefitsfromsinglesupplyoperation.ConsiderthebasicopampconnectionshowninFigurela.Itispoweredfromadualsupply(alsocalledabalancedorsplitsupply).Notethatthereisnogroundconnectiontotheopamp.Infact,itcouldbesaidthattheopampdoesn’tknowwheregroundpotentialis.Groundpotentialissome-wherebetweenthepositiveandnegativepowersupplyvoltages,buttheopamphasnoelectricalconnectiontotellitexactlywhere.VINVOUT=VING=+1+VS=15V–VS=15VVINVOUT=VING=1+VS=30V(a)(b)FIGURE1.Asimpleunity-gainbufferconnectionofanopampillustratesthesimilarityofsplit-supplyop-eration(a)tosingle-supplyoperationin(b).Thecircuitshownisconnectedasavoltagefollower,sotheoutputvoltageisequaltotheinputvoltage.Ofcourse,therearelimitstotheabilityoftheoutputtofollowtheinput.Astheinputvoltageswingspositively,theoutputatsomepointnearthepositivepowersupplywillbeunabletofollowtheinput.Similarlythenegativeoutputswingwillbelimitedtosomewherecloseto–VS.Atypicalopampmightallowoutputtoswingwithin2Vofthepowersupply,makingitpossibletooutput–13Vto+13Vwith±15Vsupplies.Figure1bshowsthesameunity-gainfolloweroperatedfromasingle30