晶體管特性文獻翻譯

晶體管特性第一章中已經(jīng)指出，晶體管能夠放大電流。因此，晶體管在電子線路中應(yīng)用很廣，例如音頻放大器，助聽器，擴音機放大器，無線電接收機和電視接收機，測量儀表和工業(yè)控制。另外，晶體管也可以用作“電子開關(guān)”，它可使電流通路或者呈現(xiàn)高電阻或者呈現(xiàn)低電阻。這就是晶體管有可能在計算機電路和控制系統(tǒng)中獲得廣泛應(yīng)用。對每一項應(yīng)用都必須進行細致的電路設(shè)計，在能夠系統(tǒng)地進行設(shè)計工作之前，應(yīng)對晶體管這一電路元件的特性有個詳盡的了解，知道什么是最佳工作電壓和電流，對信號的阻抗有多大，晶體管的放大倍數(shù)有多大，什么是晶體管輸出端內(nèi)阻抗等等。這些特性資料可從各類晶體管有關(guān)數(shù)據(jù)中獲得，這些數(shù)據(jù)由制造廠商提供，印成“數(shù)據(jù)表”發(fā)行，使晶體管使用者能根據(jù)此進行初步設(shè)計，而不必自行量測。首要的問題是取得晶體管電壓電流關(guān)系曲線。通常要提供出兩組曲線；發(fā)射結(jié)正向電壓電流特性曲線，通常稱為發(fā)射極特性曲線或輸入特性曲線；以及集電結(jié)反向電壓電流曲線，通常稱為集電極特性曲線或輸出特性曲線。1基特性曲線::B77首先研究共基極電路，圖1所示為發(fā)射極基極正向特性曲線，它描述了發(fā)射極電流如何隨發(fā)射極基極電壓從零正向增高而增大。圖上所示的特性曲線是小型鍺管的典型曲線。由圖可看出，最初電流隨電壓增高而增加，但增加的幅度很小。在這期間，外加電壓逐漸克服pn結(jié)的勢壘。勢壘一旦被中和，電流就迅速增加。"0i0.2塩射極基極電壓圖1發(fā)射極基極正向特性曲線現(xiàn)在來研究發(fā)射極電流變化時集電極電路出現(xiàn)的情況。首先，發(fā)射極電流為零時集電極基極特性如圖2中IE=0的曲線所示。

它和前面圖所示的pn結(jié)反向特性曲線相似。圖中的小股電流稱為集電結(jié)的漏電流?，F(xiàn)在使發(fā)射結(jié)電流增加到1毫安（圖1中的A點），并使之維持不變。我們看到，幾乎全部發(fā)射極電流都傳送到集電極，通過集電極的電流量與集電極電壓的高低無關(guān)。這樣,集電極電壓電流特性曲線就成了圖2中Ie=2,3,4和5毫安時的集電極曲線B,C,D和E。集電極特性曲線幾乎處于水平位置這一性質(zhì)突出了高輸出電阻這一特性，因而集電極電壓發(fā)生大幅度變化時，電流變化很小。集電根電宦v-5-4-3-2-1o°Ie=2kF官if*Ie=2kF官if*-z'3-4-5圖2集電極基極特性由圖中可以看出，甚至在集電極電壓為零時仍然存在集電極電流。這是因為基極電流在通過基區(qū)電阻時在集電極基極回路中產(chǎn)生一小電勢差，從而在集電極兩端形成很小的反向偏壓。要使集電極電流減為零，就需要外加一很小的正向集電極電壓，如圖2所/示。圖2中集電極特性曲線畫在第三象限，以使人們注意到集電極反向偏置?，F(xiàn)在一般把它畫在第一象限，如圖3所示，這在某種程度上是由于熱離子管的輸出特性曲線本身就是這樣處理的。Ip=51<■EFf廠Ie=2f-Ie=1f°1020集電板電屋V5432?曲靂田斜niA圖3集電極特性曲線

2共基極放大器的相位關(guān)系基極發(fā)射極回路是正向偏置。以pnp晶體管為例，它的發(fā)射極與基極相比為正。信號正向半周與vee串聯(lián)連接時，發(fā)射極比以前更正，使發(fā)射極基極電流增大。在晶體管中，發(fā)射極基極電流的增大使集電極電流相應(yīng)增加。由于Rl中的電流方向朝上，此電阻器上端與它的下端相比較就比以前更正了。因此，正向半周輸入信號引出正向半周輸出信號。這就是說共基極晶體管放大器沒有倒相問題。P-N-PP-N-P在許多類型的多級放大器、振蕩器和電視用視頻放大器中，相位關(guān)系是考慮的重要問題。對今后的應(yīng)用來說，重要的是要記住我們?nèi)绾闻袛嗟瓜嗯c否的方法。3單電源共基極電路U輸人RLU輸人RL圖5單電池共基極電路共基極電路的設(shè)計通常避免使用發(fā)射極電池vee。要做到這一點，只需加上一基極電阻（Rb）,并使此電阻的低端成為輸入和輸出回路兩者的公共端。此電路除了省去發(fā)射極電池外，還使一個輸入端和一個輸出端處于低電位。這樣，輸入和輸出兩個回路現(xiàn)在都有一個公共的參考電位（低電位）。流過Rb的小股電流Ico和地連接，它相對于基極是負的（這是npn晶體管）。因而不用電池就獲得了較小的正向偏壓。偏置電阻Rb可以旁路，這樣在可能出現(xiàn)交流信號時Rb的電壓降仍保持不變。4共發(fā)射極特性曲線共發(fā)射極連接時可得出與共基極連接時相類似的特性曲線。首先是輸入特性曲線，它表明基極電流隨發(fā)射極基極結(jié)兩端電壓正向升高而變化的情況，如圖6所示；其次是輸出特性曲線，它表明集電極電流隨不同基極電流下的集電極電壓變化已定的情況下，基極電流變化比發(fā)射極電流變化小。圖7的輸出特性曲線和共基極特性曲線很相似，只是電流曲線有明顯的坡度，即電流隨電壓而增大。這說明它的輸出電阻比基極電路低，但它仍然是相當高的。此外，集電極電壓為零時集電極電流也為零，這是因為基極電流所形成的電勢并沒有出現(xiàn)在集電極反射極回路。00.050.1□.1500.050.1□.150.20.25基極電壓V圖6共發(fā)射極特性曲線250200O50050圖7圖72N78輸出特性曲線電疾中矗田?5集電極曲線的應(yīng)用當你觀察集電極曲線時，首先映入眼中使你感興趣的是電流并不隨集電極電壓的變

化而急劇增大。它是一組十分近似于水平的直線，特別在基極電流很低的區(qū)域；即使在基極電流較高的區(qū)域，坡度也很平緩的。我們可以說：在制造廠商所推薦的區(qū)域范圍內(nèi),晶體管的集電極電流相對獨立于集電極電壓。Vc=5V/z//COICO15320030D760來屯根屯茴432圖8Ib—Ic曲線與之相反，在集電極電勢恒定的條件下，基極電流稍有變化就可使用集電極電流發(fā)生較大的變化?；鶚O電流作等值增長時，集電極電流是否相應(yīng)地等值上升呢？為了弄清這一點，我們選定一集電極電勢，譬如說5伏，然后沿這條5伏線上升。注意觀察基極電流每增加25毫安時集電極電流的變化情況。為幫助你估算集電極電流的變化量，我們繪制了集電極電勢為5伏的Ib—Ic曲線。雖然這條曲線并不完全是一條直線，但它確實很近似于直線，如果晶體管用于例如音頻放大電路，我們就可以說：如果Ib的變化局限在相當小的范圍內(nèi)，集電極電流就隨Ib作線性變化。這一提法可進一步解釋如下：如饋入基極的輸入電流是弱音頻電流，則集電極回路的電流變化比輸入電流的變化大，但其波形保持不變。這樣，我們得到一種度量我們對這類晶體管所能要求的保真度的方法，或者反過來說，度量晶體管固有畸變的方法。無畸變傳輸畸變輸入傳輸無畸變傳輸畸變輸入傳輸圖9晶體管畸變曲線只要Ib—Ic曲線是條直線，其固有畸變就為零。曲線的曲率越大，晶體管本身所造成的畸變越嚴重。

前面所有討論的前提是，電路元件選擇恰當，才能使所用的晶體管產(chǎn)生正常的偏壓。不然，就可能出現(xiàn)與晶體管固有特性無關(guān)的畸變。癥田老tftf3mA.3.癥田老tftf3mA.3.5.2.^.二二二二二-二0.125mAZJc=0^6mA25mAsv集電極電壓

圖10Ic的改變影響Ib的改變我們可利用集電極特性曲線或者Ib—Ic曲線在幾秒鐘內(nèi)即可估算出晶體管的B值。晶體管集電極特性一般可在制造廠商的參數(shù)表中取得。讓我們利用它來校核晶體管2N78的B值，即常說的所謂的基極電流增益。晶體管2N78通常在5伏集電極電位下工作，我們先找出5伏線，然后沿此線選出基極電流變化的一般區(qū)域范圍內(nèi)，例如從100微安到125微安。利用我們已經(jīng)知道的等式3=AIC/AIb，取Ib從100微安到125微安的變化為AIb，它等于25微安。然后，我們可注意到在Ib的變換范圍內(nèi)Ic從2.9毫安變?yōu)?.5毫安。將這些值代入等式就得到：=24B=3.5-2.9二-0^=240.0250.0256單電池共發(fā)射極電路關(guān)于這個電路，讓我們先回顧第三節(jié)雙電池共基極放大器改成單電池電路的部分。你可能會發(fā)現(xiàn)，為了提供所需要的偏置電位，需要在基極回路上外加一電阻和電容。電阻Rb使基極相對于發(fā)射極（正向偏置）和集電極（反向偏置）都具有正確的極性。如果我們進一步分析雙電池共基極電路，顯然可知，需要外加偏壓元件的原因在于發(fā)發(fā)射極和集電極電流的流動方向。我們用pnp晶體管為例，可知Ie和IC在接向基極的公共引線中流動方向相反。由于用一個電池，不論放在電路的哪個部分，要在一個元件中產(chǎn)生兩股方向相反的電流是決不可能的，所以為了建立正常的偏置電位，借助于人為的輔助元件即偏置電阻是完全必要的。

fl錨1>>?fl錨1>>?圖11雙電池共發(fā)射極電路現(xiàn)在我們來分析雙電池共發(fā)射極電路。V使電子流向通過Ri（或通過信號源本身，ee假設(shè)信號源是連續(xù)直流的）和基極到發(fā)射極，再向下經(jīng)過公共導線回到電池的正端，如深色箭頭所示。Vee使電子流向通過R和集電極到發(fā)射極，再向下經(jīng)過公共連接線回到L電池的正端，如淺色箭頭所示。由此可知，從發(fā)射極到兩組電池正極結(jié)點的公共導線中電流的流向相同。這就很容易用一組電池來完成原來兩組電池的工作。不管我們把電池接到那里，一定要記住，pnp晶體管的基極相對于發(fā)射極應(yīng)是負的，但負的程度不如集電極。恃輸恃輸PUP圖12圖12發(fā)射極和集電極電流的流動同向圖13單電池基極發(fā)射極放大電路實際上去掉第二組電池很容易，這不能不使我們感到奇怪，為什么在共發(fā)射極電路中曾經(jīng)使用過兩組電池。我們只注意到Vee和Vcc相對于發(fā)射極都是負的。全面檢查線路的連接，我們很快就可看出，集電極電流的通路與過去完全相同；從電池的負端通過集電極，離開發(fā)射極，又回到正端。與此同時，同一電池按深色箭頭指示的方向把電流（雖然可能很?。┧腿牖鶚O回路。這股電流的流動和采用兩組電池時的情況完全相同。剩下要作的只是選擇適當?shù)碾娮柚担够鶚O電壓大小合適。由于Ri阻值大小在很大程度上取決于晶體管類型、Vcc的電勢和環(huán)境溫度，所以我們無法提供某一定值。晶體管2N78在標準中頻電路室溫下的Ri典型阻值為10,000歐姆左右。來自《晶體管原理》附：英文原文TransistorCharacteristicsIthasbeenshowninthepreviouschapterthatthetransistoriscableofamplifyingelectriccurrents.Asaresult,itcanbeusedformanyapplicationsinelectroniccircuits,suchasaudioamplifiers,hearingaids,pubicaddressamplifiers,radioandtelevisionreceivers,instrumentationandindustrialcontrol.Also,thetransistorcanbeusedasan"electronicswitch",thatisitcanpresenteitherahighoralowresistancetothepassageofcurrent.Thisopensupthepossibilityofwideuseincomputercircuitsandcontrolsystems.Foreachapplicationcarefulcircuitdesignworkmustbecarriedout.Beforethiscanbedonesystematically,itisnecessarytohavedetailedknowledgeofthecharacteristicsofthecharacteristicsofthetransistorasacircuitelement,thatis,toknowwhatisthebestoperatingvoltageandcurrent,whatimpedanceispresentedtothesignal,whatamplificationthetransistorwillgive,whatistheinternalimpedanceofthetransistorattheoutput,andsoon.Datafromwhichinformationofthisnaturecanbeobtainedispreparedbythemanufactureroneachtypeoftransistorandpublishedas“datasheets”sothattheusercancarryouthispreliminarydesignwithouthavingtomakemeasurementshimself.First,itisimportanttoderivegroupsofvoltage-currentrelationshipsforthetransistor.Twosetsofcurvesarenormallypresented,theforwardvoltage-currentcharacteristicsoftheemitterjunction,referredtoastheemittercharacteristicscurvesofthecollectorjunction,calledthecollectorcharacteristicsortheoutputcharacteristics.CommonBaseCharacteristicsConsideringfirstthecommonbasearrangement,Fig.1showstheemittertobaseforwardcharacteristic,thatis,howtheemittercurrentincreaserastheemittertobasevoltageisincreasedpositivelyfromzero.Thecharacteristicshownistypicalofasmallgermaniumtransistor.Itwillbeseenthatatfirstthecurrentincreasesonlyveryslightlyasthevoltageisincreased.Duringthisregiontheappliedvoltageisovercomingthepotentialbarrierofthejunction.Oncethebarrierhasbeenneutralized,thecurrentincreasesrapidly.

a□.1aEMITTER-BASEVOLTAG-E-,VOLTSFis1Inputcharacteristicvft-asccircuit.Ie=dAHJ中CJD丿IE=4EJ1Ie=dAHJ中CJD丿IE=4EJ1IE=5COLLECTORVOLTAGEVOLTS-5—4-3-2-1□-5Fig2Collectorcharateristic?coinmonbasecircuit.Similarly,astheemittercurrentisincreasedinfurtherstepscollectorcurvesB,C,DandEareobtainedforemittercurrentIE=2,3,4and5mA.Thealmosthorizontalnatureofthecollectorcharacteristicsemphasizesthehighoutputresistance,alargechangeofcollectorvoltageproducingonlyaverysmallchangeofcurrent.Itwillbeseenthatthecollectorcurrentismaintainedevenatzerocollectorvoltage.This

isbecausethebasecurrent,inflowingoutthroughtheresistanceofthebaseregion,setsupasmallpotentialwhichappearsinthecollector-basecircuit,andconstitutesasmallreversebiasacrossthecollectorjunction.ToreducethecollectorcurrenttozeroitisnecessarytoapplyasmallforwardcollectorvoltageasshowninFig.2.21rIe=jr21rIe=jrTr專1f01!□COLLECTORVOLATAGE,VOLTSFig?Collctorcharateristic,commonbasecircuitInFig.2thecollectorcharacteristicshavebeenshowninthethirdquadrantasareminderthatthecollectorjunctionisbiasedinthereversedirection.ItisnowcustomarytopresenttheminthefirstquadrantasshowninFig.3,tosomeextentbecausetheoutputcharacteristicsofthermionicvalveswerealwaysdrawninthisway.HaseRelationsinaCommon-BaseAmplifieracommciti-basetratnsistoramplifierThebase-emittercircuitisforward-biased.Inthecaseofthepnptransistorusedasanexample,thismeansthattheemitterismorepositivethanthebase.Whenapositive-goinghalf-cycleisnowinsertedinserieswithV,theemitterbecomesmorepositivethanbefore,increasingtheemitter-basecurrent.Inatransistor,anincreaseofemitter-basecurrentproducesacorrespondingincreaseofcollectorcurrent.SincethedirectionofcurrentflowisupwardinRL,thetopterminalofthisresistormustbecomemorepositivethanitwasbeforewithrespecttothebottomterminal.Henceapositive-goinginputhalf-cyclegivesrisetoapositive-goingoutputhalf-cycle.Thismeansthatthereisnophaseinversioninthecommonbasetransistoramplifier.Phaserelationsareimportantconsiderationsinmanytypesofmultistageamplifiers.Inoscillators,andinvideoamplifiersfortelevision.Itisimportanttorememberhowwedeterminewhetherphaseinversiondoesordoesnotoccurforfutureuse.Single-batteryCommon-baseCircuitVCEFig5ThecircuitAcommon-basecircuitisnormallydesignedtodoawaywiththeneedforanemitterbatteryVee.Todothisweneedmerelyaddabaseresistor(Rb)andmakethelowerterminalofthisresistorcommontobothinputandoutputcircuits.Inadditiontodoingawaywiththeemitterbattery,thiscircuitmakesitpossibletomaintainoneinputandoneoutputterminalatgroundpotential.Thereisnowacommonreferencepotential(ground)forbothinputandoutput.ThesmallleakagecurrentICOflowingthroughRbplacesthebaseataslightlyhigherpositivepotentialthanground.SincetheemitterisconnectedtogroundthroughRe,thiselementmustbenegativewithrespecttothebase(thisisannpntransistor).Thus,thesmallamountoffor-ward-biasisobtainedwithouttheneedforabattery.ThebiasresistorRbmaybebypassedtomaintainthevoltagedropacrossitconstant,despitethepossiblepresenceofalternatingsignalcurrents.CommonEmitterCharacteristicsWiththecommonemitterconnection,similarcharacteristicscanbeprepared.First,theinputcharacteristic,whichshowshowthebasecurrentvariesasthevoltageacrossthe

emitter-basejunctionisincreasedintheforwarddirection,asshowninFig.7.ItwfromFig.6thattheinputresistancishigherthanforthecommonbasearrangement,thechangeofbasecurrentbeingsmallerthanthechangeofemittercurrentforagivencemittertobasevoltage.TheoutputcharacteristicsinFig.7aresimilartothecommcharacteristfexceptthatthereisnowanoticeablelopeonthecurrentlines,hecurrentincreasingwithvoltage.Thisindicatesthattheoutputresistanceislowerthanintbasearrangement;butneverthelessitisstillhigh.Also,thecollectorcurrentisnzerocollectorvoltagesincethepotentialproducedbythebasecurrentdoesnotappecollectortoemittercircuit.D■口5□.1□.15n.2□.25COLLEJCTORVOLATAGE,VOLTS卜些7D■口5□.1□.15n.2□.25COLLEJCTORVOLATAGE,VOLTS卜些7Collctorcharateiisticncontm.011emittercircuitBASEVOLATAGE,VOLTSFig6IputcharacteriuticofcctniRonemittercircuitUsingtheCollectorCurvesOneofthefirstinterestingthingsthatstrikesyouasyoulookatthecollectorthatthecurrentdoesnotriseveryrapidlywithchangesofcollectoroltageNoticehow

horizontalthegraphlinesare,especiallywhenthebasecurrentislow.Evenforhigherbasecurrents,theslopesareveryshallow.Wecansayitthisway:overtherangerecommendedbythemanufacturer,thecollectorcurrentofatransistorisrelativelyindependentofthecollectorvoltage.Ontheotherhand,asmallchangeofbasecurrentalwaysproducesarelativelylargechangeincollectorcurrent,訐thecollectorpotentialismaintainedconstant.Doesthecollectorcurrentriseinequalstepsforequalincrementsofbasecurrent?Toseethis,chooseacertaincollectorpotential,say5volts,thenfollowthe5-voltlineupwardandnotehowthecurrentchanges,wehavedrawnIb一Iccurveforacollectorpotentialof5volts.Ve-J/r///Ve-J/r///7541口50inn15D2DD25D目口口BASECUTTENTFig￡Ib-Icctktvefor2N7Strarisistorwith7cheldat5volts.Althoughthiscurveisnotaperfectstraightline,itcertainlydoesapproachitclosely.Ofthetransistorisusedinanaudioamplifiercircuit,forexample,wemightthensay:IftherangeofvariationofIbisheldwithinreasonablysmalllimits,thecollectorcurrentvarieslinearlywithit.Thismaybeinterpretedasfollows:iftheinputcurrenttothebaseisaweakaudiocurrent,thecurrentvariationsinthecollectorcircuitwillbelargebutwillhavethesamewaveform.Thus,wearriveatameasureofthefidelityor,conversely,theinherentdistortionthatwemayexpectfromatransistorofthistype.AslongastheIb一Iccurveisastraightline,theinherentdistortionwillbezero.Thegreaterthecurvatureofthegraph,thegreaterwillbethedistortionthatcanbeattributedtothetransistoritself.

NODISTORTIONINPUTDISTORTIONtfithSistOftramsistorINPUTOUTPUT/NODISTORTIONINPUTDISTORTIONtfithSistOftramsistorINPUTOUTPUT/~~OUTPUTFig?Acuivedttamsistafcharateristic

mayintroduesdistrotionAlloftheforegoingpresupposesthatthecircuitcomponentshavebeenselectedtoproducetheproperbiasforthespecifictransistorused.Ifthisisnotthecase,distortionmayoccurthathasnoconnectionwiththeinherentcharacteristicsofthetransistor.COLLECTORVOLATAGE,VOLTSFig10Asmallpartofth已COLLECTORVOLATAGE,VOLTSFig10Asmallpartofth已CollctorcharatetisticHNHHa口u?ID.L337nooEitherthecollectorcharacteristiccurvesortheIb一Iccurvemaybeusedtoestimatethebetaofatransistorinjustafewseconds.Sincethecollectorcharacteristicsarethosegenerallyfoundinmanufacturers'ratingsheets,supposeweusethesetocheckthebeta,orbasecurrentgainasitisoftencalled,ofthe2N78.Sincethistransistornormallyoperatesatacollectorpotentialof5volts,wefirstlocatethe5-voltline.Thenweselectanaverageregionofbasecurrentchangealongthisline,sayfrom100microamperesto125microamperes.Usingourknowledgethatbeta=AIc/AIb,wecantakethechangeofIbfrom100口Ato125口AforatotalofAIb=25口A.WethennotethatIcgoesfrom2.9mato3.5maoverthisrangeofIb.Substitutingthesevaluesintheequationwehave:B=3.5-2.9B=3.5-2.90.0250.025=24TheSingle-BatteryCommon-EmitterCircuitAtthispoint,letusreferbacktowhereweconvertedthetwo-batterycommon-baseamplifierintoasingle-batterycircuit.Youwillfindthatitwasnecessarytoaddaspecialresistanceandcapacitorinthebasecircuittoprovidetherequiredbiasingpotentials.Thatis,Rbpermitsthebasetohavethecorrectpolaritywithrespecttotheemitter(forwardbias)andwithrespecttothecollector(reversebias).Ifweanalyzethetwo-batterycommon-basecircuitfurther,itisapparentthatthereasonfortheneedofanextrabiasingcomponentliesinthedirectionsofflowoftheemitterandcollectorcurrents.Usingapnptransistorasanexample(thenpnisanalyzedthesamewayexceptthatallcurrentdirectionsarereversed),weseethatIeandIcflowinoppositedirectionsinthecommonleadgoingtothebase.Sinceonebattery,placedanywhereatallinthecircuit,couldneverproducetwooppositely-directedcurrentsinacommonelement,itisnecessarytocreatethecorrectbiaspotentialsbymeansofanartificialaid——thebiasresistor.Nowletusanalyzethetwo-batterycommon-emittercircuit.VeeforcescurrentupthroughRi(orthroughthesignalsourceitself訐thesourcehasd-ccontinuity),throughthebasetotheemitter,thendownwardthroughthecommonleadbacktothepositiveterminalofthebatteryasshownbythedarkarrows.Vdrivescu