自动化专业外文翻译运算放大器.docx

自动化专业外文翻译运算放大器.docx

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自动化专业外文翻译运算放大器

UNIT2

A:

TheOperationalAmplifier

Oneproblemwithelectronicdevicescorrespondingtothegeneralizedamplifiersisthatthegains,AuorA~,dependuponinternalpropertiesofthetwo-portsystem（p,fl,R~,Ro,etc.）?

~Thismakesdesigndifficultsincetheseparametersusuallyvaryfromdevicetodevice,aswellaswithtemperature.Theoperationalamplifier,orOp-Amp,isdesignedtominimizethisdependenceandtomaximizetheeaseofdesign.AnOp-Ampisanintegratedcircuitthathasmanycomponentpartsuchasresistorsandtransistorsbuiltintothedevice.Atthispointwewillmakenoattempttodescribetheseinnerworkings.

AtotallygeneralanalysisoftheOp-Ampisbeyondthescopeofsometexts.Wewillinsteadstudyoneexampleindetail,thenpresentthetwoOp-Amplawsandshowhowtheycanbeusedforanalysisinmanypracticalcircuitapplications.Thesetwoprinciplesallowonetodesignmanycircuitswithoutadetailedunderstandingofthedevicephysics.Hence,Op-Ampsarequiteusefulforresearchersinavarietyoftechnicalfieldswhoneedtobuildsimpleamplifiersbutdonotwanttodesignatthetransistorlevel.InthetextsofelectricalcircuitsandelectronicstheywillalsoshowhowtobuildsimplefiltercircuitsusingOp-Amps.Thetransistoramplifiers,whicharethebuildingblocksfromwhichOp-Ampintegratedcircuitsareconstructed,willbediscussed.

ThesymbolusedforanidealOp-AmpisshowninFig.1-2A-1.Onlythreeconnectionsareshown:

thepositiveandnegativeinputs,andtheoutput.NotshownareotherconnectionsnecessarytoruntheOp-Ampsuchasitsattachmentstopowersuppliesandtogroundpotential.ThelatterconnectionsarenecessarytousetheOp-Ampinapracticalcircuitbutarenotnecessarywhenconsideringtheideal0p-Ampapplicationswestudyinthischapter.Thevoltagesatthetwoinputsandtheoutputwillbe

representedbythesymbolsU+,U-,andUo.Eachismeasuredwithrespectt~groundpotential.Operationalamplifiersaredifferentialdevices.Bythiswemeanthattheoutputvoltagewithrespecttogroundisgivenbytheexpression

Uo=A（U+-U-）（1-2A-l）

whereAisthegainoftheOp-AmpandU+andU-thevoltagesatinputs.Inotherwords,theoutputvoltageisAtimesthedifferenceinpotentialbetweenthetwoinputs.

Integratedcircuittechnologyallowsconstructionofmanyamplifiercircuitsonasinglecomposite"chip"ofsemiconductormaterial.Onekeytothesuccessofanoperationalamplifieristhe"cascading"ofanumberoftransistoramplifierstocreateaverylargetotalgain.Thatis,thenumberAinEq.（1-2A-1）canbeontheorderof100,000ormore.（Forexample,cascadingoffivetransistoramplifiers,eachwithagainof10,wouldyieldthisvalueforA.）Asecondimportantfactoristhatthesecircuitscanbebuiltinsuchawaythatthecurrentflowintoeachoftheinputsisverysmall.Athirdimportantdesignfeatureisthattheoutputresistanceoftheoperationalamplifier（Ro）isverysmall.Thisinturnmeansthattheoutputofthedeviceactslikeanidealvoltagesource.

WenowcananalyzetheparticularamplifiercircuitgiveninFig.1-2A-2usingthesecharacteristics.First,wenotethatthevoltageatthepositiveinput,U+,isequaltothesourcevoltage,U+=Us.Variouscurrentsaredefinedinpartbofthefigure.ApplyingKVLaroundtheouterloopinFig.1-2A-2bandrememberingthattheoutputvoltage,Uo,ismeasuredwithrespecttoground,wehave

-I1R1-I2R2+U0=0（1-2A-2）

SincetheOp-Ampisconstructedinsuchawaythatnocurrentflowsintoeitherthepositiveornegativeinput,I-=0.KCLatthenegativeinputterminalthenyields

I1=I2

UsingEq.（1-2A-2）andsettingI1=I2=I,

U0=（R1+R2）I（1-2A-3）

WemayuseOhm'slawtofindthevoltageatthenegativeinput,U-,notingtheassumedcurrentdirectionandthefactthatgroundpotentialiszerovolts:

（U--0）/R1=I

So,U-=IR1

andfromEq.（1-2A-3）,U-=[R1/（R1+R2）]U0

SincewenowhaveexpressionsforU+andU-,Eq.（1-2A-l）maybeusedtocalculatetheoutputvoltage,

U0=A（U+-U-）=A[US-R1U0/（R1+R2）]

Gatheringterms,

U0=[1+AR1/（R1+R2）]=AUS（1-2A-4）

andfinally,

AU=U0/US=A（R1+R2）/（R1+R2+AR1）（1-2A-5a）

Thisisthegainfactorforthecircuit.IfAisaverylargenumber,largeenoughthatAR~>>（R1+R2）,thedenominatorofthisfractionisdominatedbytheAR~term.ThefactorA,whichisinboththenumeratoranddenominator,thencancelsoutandthegainisgivenbytheexpression

AU=（R1+R2）/R1（1-2A-5b）

ThisshowsthatifAisverylarge,thenthegainofthecircuitisindependentoftheexactvalueofAandcanbecontrolledbythechoiceofR1andR2.ThisisoneofthekeyfeaturesofOp-Ampdesigntheactionofthecircuitonsignalsdependsonlyupontheexternalelementswhichcanbe

easilyvariedbythedesignerandwhichdonotdependuponthedetailedcharacteroftheOp-Ampitself.NotethatifA=100000and（R1+R2）/R1=10,thepricewehavepaidforthisadvantageisthatwehaveusedadevicewithavoltagegainof100000toproduceanamplifierwithagainof10.Insomesense,byusinganOp-Ampwetradeoff"power"for"control."

AsimilarmathematicalanalysiscanbemadeonanyOp-Ampcircuit,butthisiscumbersomeandtherearesomeveryusefulshortcutsthatinvolveapplicationofthetwolawsofOp-Ampswhichwenowpresent.

1）ThefirstlawstatesthatinnormalOp-Ampcircuitswemayassumethatthevoltagedifferencebetweentheinputterminalsiszero,thatis,

U+=U-

2）ThesecondlawstatesthatinnormalOp-Ampcircuitsbothoftheinputcurrentsmaybeassumedtobezero:

I+=I-=0

ThefirstlawisduetothelargevalueoftheintrinsicgainA.Forexample,iftheoutputofanOp-AmpisIVandA=100000,then（U+-U-）=10-SV.Thisissuchasmallnumberthatitcanoftenbeignored,andwesetU+=U-.ThesecondlawcomesfromtheconstructionofthecircuitryinsidetheOp-Ampwhichissuchthatalmostnocurrentflowsintoeitherofthetwoinputs.

B:

Transistors

Putverysimplyasemiconductormaterialisonewhichcanbe'doped'toproduceapredominanceofelectronsormobilenegativecharges（N-type）;or'holes'orpositivecharges（P-type）.AsinglecrystalofgermaniumorsilicontreatedwithbothN-typedopeandP-typedopeformsasemiconductordiode,withtheworkingcharacteristicsdescribed.Transistorsareformedinasimilarwaybutliketwodiodesback-to-backwithacommonmiddlelayerdopedintheoppositewaytothetwoendlayers,thusthemiddlelayerismuchthinnerthanthetwoendlayersorzones.

Twoconfigurationsareobviouslypossible,PNPorNPN（Fig.1-2B-l）.Thesedescriptionsareusedtodescribethetwobasictypesoftransistors.Becauseatransistorcontainselementswithtwodifferentpolarities（i.e.,'P'and'N'zones）,itisreferredtoasabipolardevice,orbipolartransistor.

Atransistorthushasthreeelementswiththreeleadsconnectingtotheseelements.Tooperateinaworkingcircuititisconnectedwithtwoexternalvoltageorpolarities.Oneexternalvoltageisworkingeffectivelyasadiode.Atransistorwill,infact,workasadiodebyusingjustthisconnectionandforgettingaboutthetophalf.Anexampleisthesubstitutionofatransistorforadiodeasthedetectorinasimpleradio.Itwillworkjustaswellasadiodeasitisworkingasadiodeinthiscase.

Thediodecircuitcanbegivenforwardorreversebias.Connectedwithforwardbias,asinFig.l-2B-2,drawnforaPNPtransistor,currentwillflowfromPtothebottomN.Ifasecondvoltageisappliedtothetopandbottomsectionsofthetransistor,withthesamepolarityappliedtothebottom,theelectronsalreadyflowingthroughthebottomNsectionwillpromotea

flowofcurrentthroughthetransistorbottom-to-top.

Bycontrollingthedegreeofdopinginthedifferentlayersofthetransistorduringmanufacture,thisabilitytoconductcurrentthroughthesecondcircuitthrougharesistorcanbeverymarked.Effectively,whenthebottomhalfisforwardbiased,thebottomsectionactsasageneroussourceoffreeelectrons（andbecauseitemitselectronsitiscalledtheemitter）.Thesearecollectedreadilybythetophalf,whichisconsequentlycalledthecollector,buttheactualamountofcurrentwhichflowsthroughthisparticularcircuitiscontrolledbythebiasappliedatthecenterlayer,whichiscalledthebase.

Effectively,therefore,therearetwoseparate'working'circuitswhenatransistorisworkingwithcorrectlyconnectedpolarities（Fig.1-2B-3）.Oneistheloopformedbythebiasvoltagesupplyencompassingtheemitterandbase.Thisiscalledthebasecircuitorinputcircuit.Thesecondisthecircuitformedbythecollectorvoltagesupplyandallthreeelementsofthetransistor.Thisiscalledthecollectorcircuitoroutputcircuit.（Note:

thisdescriptionappliesonlywhentheemitterconnectioniscommontobothcircuits~knownascommonemitterconfiguration.）Thisisthemostwidelyusedwayofconnectingtransistors,butthereare,ofcourse,twootheralternativeconfigurations--commonbaseandcommonemitter.But,thesameprinciplesapplyintheworkingofthetransistorineachcase.

Theparticularadvantageofferedbythiscircuitisthatarelativelysmallbasecurrentcancontrolandinstigateaverymuchlargercollectorcurrent（or,morecorrectly,asmallinputpoweriscapableofproducingamuchlargeroutputpower）.Inotherwords,thetransistorworksasanamplifier.

Withthismodeofworkingthebase-emittercircuitistheinputside;andtheemitterthroughbasetocollectorcircuittheoutputside.Althoughthesehaveacommonpaththroughbaseandemitter,thetwocircuitsareeffectivelysepara