电信 电流 外文翻译 外文文献恒流源.docx
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电信 电流 外文翻译 外文文献恒流源.docx
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电信电流外文翻译外文文献恒流源
CURRENTSOURCE
Acurrentsourceisanelectricalorelectronicdevicethatdeliversorabsorbselectriccurrent.Acurrentsourceisthedualofavoltagesource.Thetermconstant-currentsinkissometimesusedforsourcesfedfromanegativevoltagesupply.Figure1showsaschematicforanidealcurrentsourcedrivingaresistorload.
Figure1
Idealcurrentsources
Incircuittheory,anidealcurrentsourceisacircuitelementwherethecurrentthroughitisindependentofthevoltageacrossit.Itisamathematicalmodel,whichrealdevicescanonlyapproachinperformance.Ifthecurrentthroughanidealcurrentsourcecanbespecifiedindependentlyofanyothervariableinacircuit,itiscalledanindependentcurrentsource.Conversely,ifthecurrentthroughanidealcurrentsourceisdeterminedbysomeothervoltageorcurrentinacircuit,itiscalledadependentorcontrolledcurrentsource.SymbolsforthesesourcesareshowninFigure2.
Figure2
Anindependentcurrentsourcewithzerocurrentisidenticaltoanidealopencircuit.Forthisreason,theinternalresistanceofanidealcurrentsourceisinfinite.Thevoltageacrossanidealcurrentsourceiscompletelydeterminedbythecircuititisconnectedto.Whenconnectedtoashortcircuit,thereiszerovoltageandthuszeropowerdelivered.Whenconnectedtoaloadresistance,thevoltageacrossthesourceapproachesinfinityastheloadresistanceapproachesinfinity(anopencircuit).Thus,anidealcurrentsourcecouldsupplyunlimitedpowerforeverandsowouldrepresentanunlimitedsourceofenergy.Connectinganidealopencircuittoanidealnon-zerocurrentsourceisnotvalidincircuitanalysisasthecircuitequationwouldbeparadoxical,e.g.,5=0.
Norealcurrentsourceisideal(nounlimitedenergysourcesexist)andallhaveafiniteinternalresistance(nonecansupplyunlimitedvoltage).However,theinternalresistanceofaphysicalcurrentsourceiseffectivelymodeledincircuitanalysisbycombininganon-zeroresistanceinparallelwithanidealcurrentsource(theNortonequivalentcircuit).
Resistorcurrentsource
Thesimplestcurrentsourceconsistsofavoltagesourceinserieswitharesistor.Thecurrentavailablefromsuchasourceisgivenbytheratioofthevoltageacrossthevoltagesourcetotheresistanceoftheresistor.Foranearlyidealcurrentsource,thevalueofthisresistorshouldbeverylargebutthisimpliesthat,foraspecifiedcurrent,thevoltagesourcemustbeverylarge.Thus,efficiencyislow(duetopowerlossintheresistor)anditisusuallyimpracticaltoconstructa'good'currentsourcethisway.Nonetheless,itisoftenthecasethatsuchacircuitwillprovideadequateperformancewhenthespecifiedcurrentandloadresistancearesmall.Forexample,a5Vvoltagesourceinserieswitha4.7kohmsresistorwillprovideanapproximatelyconstantcurrentof1mA(±5%)toaloadresistanceintherangeof50to450ohms.
Activecurrentsources
Activecurrentsourceshavemanyimportantapplicationsinelectroniccircuits.Currentsources(current-stableresistors)areoftenusedinplaceofohmicresistorsinanalogintegratedcircuitstogenerateacurrentwithoutcausingattenuationatapointinthesignalpathtowhichthecurrentsourceisattached.Thecollectorofabipolartransistor,thedrainofafieldeffecttransistor,ortheplateofavacuumtubenaturallybehaveascurrentsources(orsinks)whenproperlyconnectedtoanexternalsourceofenergy(suchasapowersupply)becausetheoutputimpedanceofthesedevicesisnaturallyhighwhenusedinthecurrentsourceconfiguration.
JFETandN-FETcurrentsource
AJFETcanbemadetoactasacurrentsourcebytyingitsgatetoitssource.ThecurrentthenflowingistheIDSSoftheFET.Thesecanbepurchasedwiththisconnectionalreadymadeandinthiscasethedevicesarecalledcurrentregulatordiodesorconstantcurrentdiodesorcurrentlimitingdiodes(CLD).AnenhancementmodeNchannelMOSFETcanbeusedinthecircuitslistedbelow.
Simpletransistorcurrentsource
Figure3showsatypicalconstantcurrentsource(CCS).DZ1isazenerdiodewhich,whenreversebiased(asshowninthecircuit)hasaconstantvoltagedropacrossitirrespectiveofthecurrentflowingthroughit.Thus,aslongasthezenercurrent(IZ)isaboveacertainlevel(calledholdingcurrent),thevoltageacrossthezenerdiode(VZ)willbeconstant.ResistorR1suppliesthezenercurrentandthebasecurrent(IB)ofNPNtransistor(Q1).TheconstantzenervoltageisappliedacrossthebaseofQ1andemitterresistorR2.Theoperationofthecircuitisasfollows:
VoltageacrossR2(VR2)isgivenbyVZ-VBE,whereVBEisthebase-emitterdropofQ1.TheemittercurrentofQ1whichisalsothecurrentthroughR2isgivenby
.
Figure3
SinceVZisconstantandVBEisalso(approximately)constantforagiventemperature,itfollowsthatVR2isconstantandhenceIEisalsoconstant.Duetotransistoraction,emittercurrentIEisverynearlyequaltothecollectorcurrentICofthetransistor(whichinturn,isthecurrentthroughtheload).Thus,theloadcurrentisconstant(neglectingtheoutputresistanceofthetransistorduetotheEarlyeffect)andthecircuitoperatesasaconstantcurrentsource.Aslongasthetemperatureremainsconstant(ordoesn'tvarymuch),theloadcurrentwillbeindependentofthesupplyvoltage,R1andthetransistor'sgain.R2allowstheloadcurrenttobesetatanydesirablevalueandiscalculatedby
or
sinceVBEistypically0.65Vforasilicondevice.(IR2isalsotheemittercurrentandisassumedtobethesameasthecollectororrequiredloadcurrent,providedhFEissufficientlylarge).ResistanceR1atresistorR1iscalculatedas
where,K=1.2to2(sothatR1islowenoughtoensureadequateIB),
andhFE(min)isthelowestacceptablecurrentgainfortheparticulartransistortypebeingused.
Amorecommoncurrentsourceinintegratedcircuitsisthecurrentmirror.
Simpletransistorcurrentsourcewithdiodecompensation
TemperaturechangeswillchangetheoutputcurrentdeliveredbythecircuitofFigure3becauseVBEissensitivetotemperature.TemperaturedependencecanbecompensatedusingthecircuitofFigure4thatincludesastandarddiodeD(ofthesamesemiconductormaterialasthetransistor)inserieswiththeZenerdiodeasshownintheimageontheleft.Thediodedrop(VD)trackstheVBEchangesduetotemperatureandthussignificantlycounteractstemperaturedependenceoftheCCS.
ResistanceR2isnowcalculatedas
SinceVD=VBE=0.65V,
Therefore,
.
Figure4
ThismethodismosteffectiveforZenerdiodesratedat5.6Vormore.Forbreakdowndiodesoflessthan5.6V,thecompensatingdiodeisusuallynotrequiredbecausethebreakdownmechanismisnotastemperaturedependentasitisinbreakdowndiodesabovethisvoltage.
SimpletransistorcurrentsourcewithLED
AnothermethodistoreplacetheZenerdiodewithalight-emittingdiodeLED1asshowninFigure5.TheLEDvoltagedrop(VD)isnowusedtoderivetheconstantvoltageandalsohastheadditionaladvantageoftracking(compensating)VBEchangesduetotemperature.R2iscalculatedas
andR1as
whereIDistheLEDcurrent.
Figure5Figure6
AnothercommonmethodistousefeedbacktosetthecurrentandremovethedependenceontheVbeofthetransistor.Figure6showsaverycommonapproachusinganopampwiththenon-invertinginputconnectedtoavoltagesource(suchastheZenerinanaboveexample)andtheinvertinginputconnectedtothesamenodeastheresistorandemitterofthetransistor.Thiswaythegeneratedvoltageisacrosstheresistor,ratherthanboththeresistorandtransistor.(Fordetails,seethearticleontheidealopamp-thenullor.)Thearticleoncurrentmirrordiscussesanotherexampleoftheseso-calledgain-boostedcurrentmirrors.
Otherpracticalsources
Inthecaseofopampcircuitssometimesitisdesiredtoinjectapreciselyknowncurrenttotheinvertinginput(asanoffsetofsignalinputforinstance)andaresistorconnectedbetweenthesourcevoltageandtheinvertinginputwillapproximateanidealcurrentsourcewithvalueV/R.
Inductortypecurrentsource
Amongstotherapplications,thecircuitofFigure7usingtheLM317voltageregulatorisusedtopresentasourceofconstantcurrentinClassE(switching)electronicamplifiers.
Figure7
Currentandvoltagesourcecomparison
Mostsourcesofelectricalenergy(mainselectricity,abattery,...)arebestmodeledasvoltagesources.Suchsourcesprovideconstantvoltage,whichmeansthataslongastheamountofcurrentdrawnfromthesourceiswithinthesource'scapabilities,itsoutputvoltagestaysconstant.Anidealvoltagesourceprovidesnoenergywhenitisloadedbyanopencircuit(i.e.aninfiniteimpedance),butapproachesinfinitepowerandcurrentwhentheloadresistanceapproacheszero(ashortcircuit).Suchatheoreticaldevicewouldhaveazeroohmoutputimpedanceinserieswiththesource.Areal-worldvoltagesourcehasaverylow,butnon-zerooutputimpedance:
oftenmuchlessthan1ohm.
Conversely,acurrentsourceprovidesaconstantcurrent,aslongastheloadconnectedtothesourceterminalshassufficientlylowimpedance.Anidealcurrentsourcewouldprovidenoenergytoashortcircuitandapproachinfiniteenergyandvoltageastheloadresistanceapproachesinfinity(anopencircuit).Anidealcurrentsourcehasaninfiniteoutputimpedanceinparallelwiththesource.Areal-worldcurrentsourcehasaveryhigh,butfiniteoutputimpedance.Inthecaseoftransistorcurrentsources,impedancesofafewmegohms(atDC)aretypical.
Anidealcurrentsourcecannotbecon
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