《電氣信息類專業(yè)英語》課件第2章

Part2AnalogandDigitalElectronicTechnology2.1TheOperationalAmplifiers2.2TheOperationalAmplifiers2.3DigitalElectronicTechnique2.4DataRegistersandCounters 2.5Application-specificIntegratedCircuitDesign

2.1.1語言現(xiàn)象——長句的翻譯

所謂長句，主要指語法結(jié)構(gòu)復(fù)雜，修飾成分頗多，內(nèi)容層次在兩個以上的復(fù)合句，亦可指帶有修飾性短語或附加成分的簡單句或并列句。修飾性短語包括介詞短語、不定式短語、分詞短語、動名詞短語和形容詞短語；附加成分主要指插入語、同位語和獨(dú)立成分。2.1TheOperationalAmplifiers在科技英語中，大量使用長句，以嚴(yán)密、準(zhǔn)確、客觀地表達(dá)多重密切相關(guān)的概念。但由于英漢兩種語言連接句子的手段不同，表述概念、敘述事情和論述事理的邏輯有別，在英漢互譯的過程中要注重了解兩者的差異，認(rèn)真進(jìn)行語法分析，吃透句子所要表達(dá)的深層意義，根據(jù)英漢表達(dá)的習(xí)慣，翻譯出符合譯入語習(xí)慣的句子?？萍加⒄Z中由于大量使用定語從句、狀語從句和各種短語，使句子常具有較長的復(fù)雜結(jié)構(gòu)。加之英漢兩種語言在詞法、句法、邏輯和思維等方面的差異，因而造成很多長句難懂、難譯。對待長句，要分清層次、突出重點(diǎn)，搞清各分句的內(nèi)容和結(jié)構(gòu)。譯文要邏輯嚴(yán)密、前后呼應(yīng)，借助語法關(guān)系和邏輯承接語，使譯文前后銜接、相互呼應(yīng)，把原文復(fù)雜的概念準(zhǔn)確通順地表達(dá)出來。長句的翻譯方法通常有順序法、變序法、分句法、并句法等典型的幾種，這些方法可根據(jù)不同情況單獨(dú)使用或結(jié)合使用。

1．順序法

當(dāng)英語原句所表達(dá)的思想與意義在句型結(jié)構(gòu)(如主謂結(jié)構(gòu)、主謂賓結(jié)構(gòu)、主系和部分主謂狀、主謂賓狀結(jié)構(gòu))、時間順序、句內(nèi)邏輯順序(如因果關(guān)系、對比轉(zhuǎn)折關(guān)系、動賓關(guān)系和條件假設(shè)關(guān)系)上基本與漢語句子的語序一致時，或句子同主多謂、同謂多賓時常采用順譯法，即大體按照英語句子的實(shí)際先后順序進(jìn)行翻譯。但不能拘泥于詞詞對照，經(jīng)常會有局部語序的變化。例如：

NosuchlimitationisplacedonanACmotor；heretheonlyrequirementisrelativemotion，andsinceastationaryarmatureandarotatingfieldsystemhavenumerousadvantages，thisarrangementisstandardpracticeforallsynchronousmotorratedaboveafewkilovolt-amperes．

交流電機(jī)不受這種限制，唯一的要求是相對運(yùn)動，而且由于固定電樞及旋轉(zhuǎn)磁場系統(tǒng)具有很多優(yōu)點(diǎn)，所以這種安排是所有容量在幾千伏安以上的同步電機(jī)的標(biāo)準(zhǔn)做法。

Thepoweramplifierhasnowayto“knowinadvance”whatkindofloudspeakeryouwilluse,sosimplyadoptstheconventionofassertingavoltageleveltoindicatetherequiredsignallevelateachfrequencyinthesignalandsupplyingwhatevercurrenttheloudspeakerthenrequires.

功率放大器不可能預(yù)先知道將會使用的揚(yáng)聲器類型，因此就簡單地按常規(guī)情況處理，提供一定大小的電壓，表示信號中任意頻率下所需信號的幅度，并提供揚(yáng)聲器所需要的電流。

Plasticsismadefromwaterwhichisanaturalresourceinexhaustibleandavailableeverywhere,coalwhichcanbeminedthroughautomaticandmechanicalprocessesatlesscostandlimewhichcanbeobtainedfromthecalcinationsoflimestonewidelypresentinnature.

塑料是由水、煤和石灰石制成的，水是取之不盡的、到處可以獲得的天然資源；煤是用自動化和機(jī)械化的方法開采的，成本較低；石灰是由煅燒自然界中廣泛存在的石灰石得來的。

2．變序法

英語習(xí)慣前果后因和定、狀語后置，譯成漢語時可采用變序法，即先譯后部，再依次向前，改變原句的語序，按漢語習(xí)慣的語序譯出，從而便于表達(dá)和讀者理解。例如：

Theresistanceofanylengthofaconductingwireiseasilymeasuredbyfindingthepotentialdifferenceinvoltsbetweenitsendswhenaknowncurrentisfollowing．

已知導(dǎo)線中流過的電流，只要測出導(dǎo)線兩端電位差的伏特值，就能很容易得出任何長度導(dǎo)線的電阻值。

Itisveryinterestingtonotethedifferentlychosenoperatingmechanismbythedifferentmanufacturers,inspiteofthefactthattheoperatingmechanismhasamajorinfluenceonthereliabilityofthecircuit-breakers.

盡管操作機(jī)構(gòu)對斷路器的可靠性具有主要影響，但注意不同的制造廠按不同形式選擇操作機(jī)構(gòu)是非常有趣的。

Thiswillremaintruewhetherwearedealingwiththeapplicationofpsychologytoadvertisingandpoliticalpropaganda,orengineeringtothemassmediaofcommunication，orofmedicalsciencetotheproblemofover-populationoroldage.無論我們說的是把心理學(xué)應(yīng)用于廣告宣傳和政治宣傳，還是把工程學(xué)應(yīng)用于大眾傳播媒體，或是把醫(yī)學(xué)運(yùn)用于人口過剩問題和老年問題，這種情況總是如此。

3．拆分法

當(dāng)英語長句中的主句與從句、或介詞短語及分詞短語、副詞等所修飾的詞與詞之間的關(guān)系不是很密切時，為了使譯文結(jié)構(gòu)清楚，合乎表達(dá)習(xí)慣，有時可用拆譯法，即將原句分成幾個獨(dú)立的小句，順序基本不變，保持前后連貫，根據(jù)漢語一個小句表達(dá)一層意思的習(xí)慣，對原文的各個意思層進(jìn)行分解，把英語長句拆開成分句來譯。例如：

Thiskindoftwo-electrodestubeconsistsofatungstenfilament,whichgivesoffelectronswhenitisheated,andaplatetowardwhichtheelectronsmigratewhenthefieldisintherightdirection.這種二極管由一根鎢絲和一個極板組成。鎢絲受熱時放出電子，當(dāng)電場方向為正時，電子就移向極板。

Thesystemcanconceivablyeventuallyreplacetheyellowpages,newspaperandmagazineadvertisingandpersonalcontactfromsalespersonnel.

可預(yù)見，這一系統(tǒng)最終能取代電話黃頁、報刊廣告和銷售人員的個人聯(lián)系。

4．并句法

將原句中的某些部分合并翻譯，以使譯文簡潔通順。主要用于當(dāng)英漢長句不能按照上述三種方法進(jìn)行翻譯時，就要根據(jù)譯入語的習(xí)慣，適當(dāng)調(diào)整句子的順序，準(zhǔn)確反映句子的深層含義。例如：

Thereisagreatdealofdifferenceintheabilityofdifferentsubstancestoconductorinsulateandthatdecideswhichmaterialisthebesttouseforaparticularpurpose.

不同物質(zhì)的傳導(dǎo)和絕緣的能力有很大差別，這就決定了哪種材料最適用于某一特定的用途。

Itiscommonpracticethatelectricwiresaremadefromcopper．

電線通常是銅材制成的。

Myassistantwhohadcarefullyreadthroughtheinstructionsbeforedoinghisexperiment,couldnotobtainsatisfactoryresultsbecausehefollowedthemmechanically.

雖然我的助手在做實(shí)驗前仔細(xì)閱讀過指導(dǎo)書，但是因為他機(jī)械地照搬，沒能取得滿意的結(jié)果。

以上幾種方法在不同情況下可以靈活掌握，結(jié)合使用。例如：

Thecomputerperformsasupervisoryfunctionintheliquid-levelcontrolsystembyanalyzingtheprocessconditionsagainstdesiredperformancecriteriaanddeterminingthechangesinprocessvariablestoachieveoptimumoperation．

在液位控制系統(tǒng)中，計算機(jī)執(zhí)行一種監(jiān)控功能。它根據(jù)給定的特性指標(biāo)來分析各種過程條件，并決定各過程變量的變化以獲得最佳操縱。2.1.2SpecifiedEnglishWords

ubiquitousadj.普遍存在的；無所不在的

pavevt.鋪設(shè)；安排；作鋪設(shè)之用

sandwichvt.夾入；擠進(jìn)；把……做成三明治n.三明治；夾心面包

bipolaradj.有兩極的，雙極的

fidelityn.保真度；忠誠；精確；盡責(zé)

diffuseadj.彌漫的；散開的vt.擴(kuò)散；傳播；漫射vi.傳播；四散

recombinevt.重組；再結(jié)合vi.重組；再結(jié)合

Shockleydiode肖克萊二極管

transconductancen.跨導(dǎo)

photonn.光子；輻射量子；見光度

photocurrentn.光電流(亦作photoelectriccurrent)

phototransistorn.光電晶體管

thresholdn.入口；門檻；開始；極限；臨界值

quadraticn.二次方程式adj.[數(shù)]二次的

depletionn.消耗；損耗；放血2.1.3Text

TransistorandItsApplication

Atransistorisasemiconductordeviceusedtoamplifyandswitchelectronicsignalsandpower.Itiscomposedofasemiconductormaterialwithatleastthreeterminalsforconnectiontoanexternalcircuit.Avoltageorcurrentappliedtoonepairofthetransistor’sterminalschangesthecurrentflowingthroughanotherpairofterminals.Becausethecontrolled(output)powercanbehigherthanthecontrolling(input)power,atransistorcanamplifyasignal.Today,sometransistorsarepackagedindividually,butmanymorearefoundembeddedinintegratedcircuits.

Thetransistoristhefundamentalbuildingblockofmodernelectronicdevices,andisubiquitousinmodernelectronicsystems.Followingitsdevelopmentintheearly1950sthetransistorrevolutionizedthefieldofelectronics,andpavedthewayforsmallerandcheaperradios,calculators,andcomputers,amongotherthings.

Theessentialusefulnessofatransistorcomesfromitsabilitytouseasmallsignalappliedbetweenonepairofitsterminalstocontrolamuchlargersignalatanotherpairofterminals.Thispropertyiscalledgain.Atransistorcancontrolitsoutputinproportiontotheinputsignal;thatis,itcanactasanamplifier.Alternatively,thetransistorcanbeusedtoturncurrentonoroffinacircuitasanelectricallycontrolledswitch,wheretheamountofcurrentisdeterminedbyothercircuitelements.

Therearetwotypesoftransistors,whichhaveslightdifferencesinhowtheyareusedinacircuit.Abipolartransistorhasterminalslabeledbase,collector,andemitter.Asmallcurrentatthebaseterminal(thatis,flowingfromthebasetotheemitter)cancontrolorswitchamuchlargercurrentbetweenthecollectorandemitterterminals.Forafield-effecttransistor,theterminalsarelabeledgate,source,anddrain,andavoltageatthegatecancontrolacurrentbetweensourceanddrain.

Bipolartransistorsaresonamedbecausetheyconductbyusingbothmajorityandminoritycarriers.Thebipolarjunctiontransistor(BJT),thefirsttypeoftransistortobemass-produced,isacombinationoftwojunctiondiodes,andisformedofeitherathinlayerofp-typesemiconductorsandwichedbetweentwon-typesemiconductors(ann-p-ntransistor),orathinlayerofn-typesemiconductorsandwichedbetweentwop-typesemiconductors(ap-n-ptransistor).Thisconstructionproducestwop-njunctions:abase–emitterjunctionandabase–collectorjunction,separatedbyathinregionofsemiconductorknownasthebaseregion(twojunctiondiodeswiredtogetherwithoutsharinganinterveningsemiconductingregionwillnotmakeatransistor).

TheBJThasthreeterminals,correspondingtothethreelayersofsemiconductor－anemitter,abase,andacollector.Itisusefulinamplifiersbecausethecurrentsattheemitterandcollectorarecontrollablebyarelativelysmallbasecurrent.InanNPNtransistoroperatingintheactiveregion,theemitter-basejunctionisforwardbiased(electronsandelectronholesrecombineatthejunction),andelectronsareinjectedintothebaseregion.Becausethebaseisnarrow,mostoftheseelectronswilldiffuseintothereverse-biased(electronsandholesareformedat,andmoveawayfromthejunction)base-collectorjunctionandbesweptintothecollector;perhapsone-hundredthoftheelectronswillrecombineinthebase,whichisthedominantmechanisminthebasecurrent.Bycontrollingthenumberofelectronsthatcanleavethebase,thenumberofelectronsenteringthecollectorcanbecontrolled.Collectorcurrentisapproximatelyβ(common-emittercurrentgain)timesthebasecurrent.Itistypicallygreaterthan100forsmall-signaltransistorsbutcanbesmallerintransistorsdesignedforhigh-powerapplications.

UnliketheFET,theBJTisalow-input-impedancedevice.Also,asthebase-emittervoltage(Vbe)isincreasedthebase-emittercurrentandhencethecollector-emittercurrent(Ice)increaseexponentiallyaccordingtotheShockleydiodemodelandtheEbers-Mollmodel.Becauseofthisexponentialrelationship,theBJThasahighertransconductancethantheFET.

Bipolartransistorscanbemadetoconductbyexposuretolight,sinceabsorptionofphotonsinthebaseregiongeneratesaphotocurrentthatactsasabasecurrent;thecollectorcurrentisapproximatelyβtimesthephotocurrent.Devicesdesignedforthispurposehaveatransparentwindowinthepackageandarecalledphototransistors.

Thefield-effecttransistor(FET),sometimescalledaunipolartransistor,useseitherelectrons(inN-channelFET)orholes(inP-channelFET)forconduction.ThefourterminalsoftheFETarenamedsource,gate,drain,andbody(substrate).OnmostFETs,thebodyisconnectedtothesourceinsidethepackage,andthiswillbeassumedforthefollowingdescription.

InaFET,thedrain-to-sourcecurrentflowsviaaconductingchannelthatconnectsthesourceregiontothedrainregion.Theconductivityisvariedbytheelectricfieldthatisproducedwhenavoltageisappliedbetweenthegateandsourceterminals;hencethecurrentflowingbetweenthedrainandsourceiscontrolledbythevoltageappliedbetweenthegateandsource.Asthegate-sourcevoltage(Vgs)isincreased,thedrain-sourcecurrent(Ids)increasesexponentiallyforVgsbelowthreshold,andthenataroughlyquadraticrate

(2-1-1)

whereVTisthethresholdvoltageatwhichdraincurrentbegins.Aquadraticbehaviorisnotobservedinmoderndevices,forexample,atthe65nmtechnologynode.

ForlownoiseatnarrowbandwidththehigherinputresistanceoftheFETisadvantageous.

FETsaredividedintotwofamilies:junctionFET(JFET)andinsulatedgateFET(IGFET).TheIGFETismorecommonlyknownasametal-oxide-semiconductorFET(MOSFET),reflectingitsoriginalconstructionfromlayersofmetal(thegate),oxide(theinsulation),andsemiconductor.UnlikeIGFETs,theJFETgateformsap-ndiodewiththechannelwhichliesbetweenthesourceanddrain.Functionally,thismakestheN-channelJFETthesolid-stateequivalentofthevacuumtubetriodewhich,similarly,formsadiodebetweenitsgridandcathode.Also,bothdevicesoperateinthedepletionmode,theybothhaveahighinputimpedance,andtheybothconductcurrentunderthecontrolofaninputvoltage.

Metal-semiconductorFETs(MESFETs)areJFETsinwhichthereversebiasedp-njunctionisreplacedbyametal-semiconductorjunction.These,andtheHEMTs(highelectronmobilitytransistors,orHFETs),inwhichatwo-dimensionalelectrongaswithveryhighcarriermobilityisusedforchargetransport,areespeciallysuitableforuseatveryhighfrequencies(microwavefrequencies;severalGHz).

Unlikebipolartransistors,FETsdonotinherentlyamplifyaphotocurrent.Nevertheless,therearewaystousethem,especiallyJFETs,aslight-sensitivedevices,byexploitingthephotocurrentsinchannel-gateorchannel-bodyjunctions.

FETsarefurtherdividedintodepletion-modeandenhancement-modetypes,dependingonwhetherthechannelisturnedonoroffwithzerogate-to-sourcevoltage.Forenhancementmode,thechannelisoffatzerobias,andagatepotentialcan“enhance”theconduction.Fordepletionmode,thechannelisonatzerobias,andagatepotential(oftheoppositepolarity)can“deplete”thechannel,reducingconduction.Foreithermode,amorepositivegatevoltagecorrespondstoahighercurrentforN-channeldevicesandalowercurrentforP-channeldevices.NearlyallJFETsaredepletion-modeasthediodejunctionswouldforwardbiasandconductiftheywereenhancementmodedevices;mostIGFETsareenhancement-modetypes.

Thebipolarjunctiontransistor(BJT)wasthemostcommonlyusedtransistorinthe1960sand70s.EvenafterMOSFETsbecamewidelyavailable,theBJTremainedthetransistorofchoiceformanyanalogcircuitssuchasamplifiersbecauseoftheirgreaterlinearityandeaseofmanufacture.Inintegratedcircuits,thedesirablepropertiesofMOSFETsallowedthemtocapturenearlyallmarketsharefordigitalcircuits.DiscreteMOSFETscanbeappliedintransistorapplications,includinganalogcircuits,voltageregulators,amplifiers,powertransmittersandmotordrivers.

TheimageinFig.2.1representsatypicalbipolartransistorinacircuit.Chargewillflowbetweenemitterandcollectorterminalsdependingonthecurrentinthebase.Sinceinternallythebaseandemitterconnectionsbehavelikeasemiconductordiode,avoltagedropdevelopsbetweenbaseandemitterwhilethebasecurrentexists.Theamountofthisvoltagedependsonthematerialthetransistorismadefrom,andisreferredtoasVBE.

Transistorsarecommonlyusedaselectronicswitches,bothforhigh-powerapplicationssuchasswitched-modepowersuppliesandforlow-powerapplicationssuchaslogicgates.

Inagrounded-emittertransistorcircuit,suchasthelight-switchcircuitshowninFig.2.2,asthebasevoltagerises,thebaseandcollectorcurrentriseexponentially.Thecollectorvoltagedropsbecauseofthecollectorloadresistance(inthisexample,theresistanceofthelightbulb).Ifthecollectorvoltagewerezero,thecollectorcurrentwouldbelimitedonlybythelightbulbresistanceandthesupplyvoltage.Thetransistoristhensaidtobesaturated－itwillhaveaverysmallvoltagefromcollectortoemitter.Providingsufficientbasedrivecurrentisakeyproblemintheuseofbipolartransistorsasswitches.Thetransistorprovidescurrentgain,allowingarelativelylargecurrentinthecollectortobeswitchedbyamuchsmallercurrentintothebaseterminal.Theratioofthesecurrentsvariesdependingonthetypeoftransistor,andevenforaparticulartype,variesdependingonthecollectorcurrent.Intheexamplelight-switchcircuitshown,theresistorischosentoprovideenoughbasecurrenttoensurethetransistorwillbesaturated.

Fig.2.1Simplecircuitabipolartransistor

Fig.2.2Electronicswitch

Thecommon-emitteramplifierisdesignedsothatasmallchangeinvoltage(Uin)changesthesmallcurrentthroughthebaseofthetransistor;thetransistor’scurrentamplificationcombinedwiththepropertiesofthecircuitmeanthatsmallswingsinUinproducelargechangesinUout.

Variousconfigurationsofsingletransistoramplifierarepossible,withsomeprovidingcurrentgain,somevoltagegain,andsomeboth.

Frommobilephonestotelevisions,vastnumbersofproductsincludeamplifiersforsoundreproduction,radiotransmission,andsignalprocessing.Thefirstdiscretetransistoraudioamplifiersbarelysuppliedafewhundredmilliwatts,butpowerandaudiofidelitygraduallyincreasedasbettertransistorsbecameavailableandamplifierarchitectureevolved.

DarlingtonpairisasshowninFig.2.3,thisistwotransistorsconnectedtogethersothatthecurrentamplifiedbythefirstisamplifiedfurtherbythesecondtransistor.Theoverallcurrentgainisequaltothetwoindividualgainsmultipliedtogether:

Darlingtonpaircurrentgainβ12=β1×β2(β1andβ2arethegainsoftheindividualtransistors).

ThisgivestheDarlingtonpairaveryhighcurrentgain,suchas10,000,sothatonlyatinybasecurrentisrequiredtomakethepairswitchon.

ADarlingtonpairbehaveslikeasingletransistorwithaveryhighcurrentgain.Ithasthreeleads(B,

C

and

E)whichareequivalenttotheleadsofastandardindividualtransistor.Toturnontheremustbe0.7

Vacrossboththebase-emitterjunctionswhichareconnectedinseriesinsidetheDarlingtonpair,thereforeitrequires1.4

Vtoturnon.

Darlingtonpairsareavailableascompletepackagesbutyoucanmakeupyourownfromtwotransistors;TR1canbealowpowertype,butnormallyTR2willneedtobehighpower.ThemaximumcollectorcurrentIc(max)forthepairisthesameasIc(max)forTR2.

ADarlingtonpairissufficientlysensitivetorespondtothesmallcurrentpassedbyyourskinanditcanbeusedtomakeatouch-switchasshowninFig.2.4shown.ForthiscircuitwhichjustlightsanLEDthetwotransistorscanbeanygeneralpurposelowpowertransistors.The100kΩresistorprotectsthetransistorsifthecontactsarelinkedwithapieceofwire.

Fig.2.3darlingtonpair

Fig.2.4touchswitchcircuit

Notes

1．Sometransistorsarepackagedindividually,butmanymorearefoundembeddedinintegratedcircuits.

一些晶體管單獨(dú)封裝，但更多的是嵌入在集成電路中。

2．Bipolartransistorscanbemadetoconductbyexposuretolight,sinceabsorptionofphotonsinthebaseregiongeneratesaphotocurrentthatactsasabasecurrent。

雙極晶體管暴露于光線中可以導(dǎo)電，因為在基區(qū)吸收的光子產(chǎn)生光電流，即得到基極電流。

3．Darlingtonpairsareavailableascompletepackagesbutyoucanmakeupyourownfromtwotransistors。

達(dá)林頓對管常常作為一體來使用，你也可以使用兩個晶體管做這個對管。2.1.4ReadingMaterials

Op-ampsCharacteristics(1)

Anidealop-ampisusuallyconsideredtohavethefollowingproperties,andtheyareconsideredtoholdforallinputvoltages:

Infiniteopen-loopgain(whendoingtheoreticalanalysis,alimitmaybetakenasopenloopgainAOLgoestoinfinity).

Infinitevoltagerangeavailableattheoutput(Uo)(inpracticethevoltagesavailablefromtheoutputarelimitedbythesupplyvoltagesVS+andVS-).Thepowersupplysourcesarecalledrails.

Infinitebandwidth(i.e.,thefrequencymagnituderesponseisconsideredtobeflateverywherewithzerophaseshift).

Infiniteinputimpedance(so,inthediagram,Rin=∞,andzerocurrentflowsfromU+toU-).

Zeroinputcurrent(i.e.,thereisassumedtobenoleakageorbiascurrentintothedevice).

Zeroinputoffsetvoltage(i.e.,whentheinputterminalsareshortedsothatU+=U-,theoutputisavirtualgroundorUo=0).

Infiniteslewrate(i.e.,therateofchangeoftheoutputvoltageisunbounded)andpowerbandwidth(fulloutputvoltageandcurrentavailableatallfrequencies).

Zerooutputimpedance(i.e.,Ro=0,sothatoutputvoltagedoesnotvarywithoutputcurrent).

Zeronoise.

InfiniteCommon-moderejectionratio(CMRR).

InfinitePowersupplyrejectionratioforbothpowersupplyrails.

Theseidealscanbesummarizedbythetwo“goldenrules”:

1．Theoutputattemptstodowhateverisnecessarytomakethevoltagedifferencebetweentheinputszero.

2．Theinputsdrawnocurrent.

Thefirstruleonlyappliesintheusualcasewheretheop-ampisusedinaclosed-loopdesign(negativefeedback,wherethereisasignalpathofsomesortfeedingbackfromtheoutputtotheinvertinginput).Theserulesarecommonlyusedasagoodfirstapproximationforanalyzingordesigningop-ampcircuits.

Inpractice,noneoftheseidealscanbeperfectlyrealized,andvariousshortcomingsandcompromiseshavetobeaccepted.Dependingontheparametersofinterest,arealop-ampmaybemodeledtotakeaccountofsomeofthenon-infiniteornon-zeroparametersusingequivalentresistorsandcapacitorsintheop-ampmodel.Thedesignercanthenincludetheeffectsoftheseundesirable,butreal,effectsintotheoverallperformanceofthefinalcircuit.Someparametersmayturnouttohavenegligibleeffectonthefinaldesignwhileothersrepresentactuallimitationsofthefinalperformance,thatmustbeevaluated.

Realoperationalamplifierssufferfromseveralnon-idealeffects:

Finitegain.Open-loopgainisinfiniteintheidealoperationalamplifierbutfiniteinrealoperationalamplifiers.Typicaldevicesexhibitopen-loopDCgainrangingfrom100,000toover1million.Solongastheloopgain(i.e.,theproductofopen-loopandfeedbackgains)isverylarge,thecircuitgainwillbedeterminedentirelybytheamountofnegativefeedback(i.e.,itwillbeindependentofopen-loopgain).Incaseswhereclosed-loopgainmustbeveryhigh,thefeedbackgainwillbeverylow,andthelowfeedbackgaincauseslowloopgain;inthesecases,theoperationalamplifierwillceasetobehaveideally.

Finiteinputimpedances.Thedifferentialinputimpedanceoftheoperationalamplifierisdefinedastheimpedancebetweenitstwoinputs;thecommon-modeinputimpedanceistheimpedancefromeachinputtoground.MOSFET-inputoperationalamplifiersoftenhaveprotectioncircuitsthateffectivelyshortcircuitanyinputdifferencesgreaterthanasmallthreshold,sotheinputimpedancecanappeartobeverylowinsometests.However,aslongastheseoperationalamplifiersareusedinatypicalhigh-gainnegativefeedbackapplication,theseprotectioncircuitswillbeinactive.Theinputbiasandleakagecurrentsdescribedbelowareamoreimportantdesignparameterfortypicaloperationalamplifierapplications.

Non-zerooutputimpedance.Lowoutputimpedanceisimportantforlow-impedanceloads;fortheseloads,thevoltagedropacrosstheoutputimpedanceoftheamplifierwillbesignificant.Hence,theoutputimpedanceoftheamplifierlimitsthemaximumpowerthatcanbeprovided.Inconfigurationswithavoltage-sensingnegativefeedback,theoutputimpedanceoftheamplifieriseffectivelylowered;thus,inlinearapplications,op-ampsusuallyexhibitaverylowoutputimpedanceindeed.Negativefeedbackcannot,however,reducethelimitationsthatRloadinconjunctionwithRoutplaceonthemaximumandminimumpossibleoutputvoltages;itcanonlyreduceoutputerrorswithinthatrange.

Low-impedanceoutputstypicallyrequirehighquiescent(i.e.,idle)currentintheoutputstageandwilldissipatemorepower,solow-powerdesignsmaypurposelysacrificelowoutputimpedance.

Inputcurrent.Duetobiasingrequirementsorleakage,asmallamountofcurrent(typically～10nanoamperesforbipolarop-amps,tensofpicoamperesforJFETinputstages,andonlyafewpAforMOSFETinputstages)flowsintotheinputs.Whenlargeresistorsorsourceswithhighoutputimpedancesareusedinthecircuit,thesesmallcurrentscanproducelargeunmodeledvoltagedrops.Iftheinputcurrentsarematched,andtheimpedancelookingoutofbothinputsarematched,thenthevoltagesproducedateachinputwillbeequal.Becausetheoperationalamplifieroperatesonthedifferencebetweenitsinputs,thesematchedvoltageswillhavenoeffect(unlesstheoperationalamplifierhaspoorCMRR,whichisdescribedbelow).Itismorecommonfortheinputcurrents(ortheimpedanceslookingoutofeachinput)tobeslightlymismatched,andsoasmalloffsetvoltage(differentfromtheinputoffsetvoltagebelow)canbeproduced.Thisoffsetvoltagecancreateoffsetsordriftingintheoperationalamplifier.Itcanoftenbenulledexternally;however,manyoperationalamplifiersincludeoffsetnullorbalancepinsandsomeprocedureforusingthemtoremovethisoffset.Someoperationalamplifiersattempttonullifythisoffsetautomatically.

Inputoffsetvoltage.Thisvoltage,whichiswhatisrequiredacrosstheop-amp’sinputterminalstodrivetheoutputvoltagetozero,isrelatedtothemismatchesininputbiascurrent.Intheperfectamplifier,therewouldbenoinputoffsetvoltage.However,itexistsinactualop-ampsbecauseofimperfectionsinthedifferentialamplifierthatconstitutestheinputstageofthevastmajorityofthesedevices.Inputoffsetvoltagecreatestwoproblems:First,duetotheamplifier’shighvoltagegain,itvirtuallyassuresthattheamplifieroutputwillgointosaturationifitisoperatedwithoutnegativefeedback,evenwhentheinputterminalsarewiredtogether.Second,inaclosedloop,negativefeedbackconfiguration,theinputoffsetvoltageisamplifiedalongwiththesignalandthismayposeaproblemifhighprecisionDCamplificationisrequiredoriftheinputsignalisverysmall.

Common-modegain.Aperfectoperationalamplifieramplifiesonlythevoltagedifferencebetweenitstwoinputs,completelyrejectingallvoltagesthatarecommontoboth.However,thedifferentialinputstageofanoperationalamplifierisneverperfect,leadingtotheamplificationoftheseidenticalvoltagestosomedegree.Thestandardmeasureofthisdefectiscalledthecommon-moderejectionratio(denotedCMRR).Minimizationofcommonmodegainisusuallyimportantinnon-invertingamplifiers(describedbelow)thatoperateathighamplification.

Outputsinkcurrent.Theoutputsinkcurrentismaximumcurrentallowedtosinkintotheoutputstage.Somemanufacturersshowtheoutputvoltagevs.theoutputsinkcurrentplot,whichgivesanideaoftheoutputvoltagewhenitissinkingcurrentfromanothersourceintotheoutputpin.

Temperatureeffects.Allparameterschangewithtemperature.Temperaturedriftoftheinputoffsetvoltageisespeciallyimportant.

Power-supplyrejection.Theoutputofaperfectoperationalamplifierwillbecompletelyindependentfromripplesthatarriveonitspowersupplyinputs.Everyrealoperationalamplifierhasaspecifiedpowersupplyrejectionratio(PSRR)thatreflectshowwelltheop-ampcanrejectchangesinitssupplyvoltage.CopioususeofbypasscapacitorscanimprovethePSRRofmanydevices,includingtheoperationalamplifier.

Drift.Realop-ampparametersaresubjecttoslowchangeovertimeandwithchangesinte