电信 电流 外文翻译 外文文献恒流源.docx

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