2023年全國碩士研究生考試考研英語一試題真題（含答案詳解+作文范文）_第1頁
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1、CURRENTSOURCEAcurrentsourceisanelectricalelectronicdevicethatdeliversabsbselectriccurrent.Acurrentsourceisthedualofavoltagesource.Thetermconstantcurrentsinkissometimesusedfsourcesfedfromanegativevoltagesupply.Figure1show

2、saschematicfanidealcurrentsourcedrivingaresistload.Figure1IdealcurrentsourcesIncircuittheyanidealcurrentsourceisacircuitelementwherethecurrentthroughitisindependentofthevoltageacrossit.Itisamathematicalmodelwhichrealdevi

3、cescanonlyapproachinperfmance.Ifthecurrentthroughanidealcurrentsourcecanbespecifiedindependentlyofanyothervariableinacircuititiscalledanindependentcurrentsource.Converselyifthecurrentthroughanidealcurrentsourceisdetermin

4、edbysomeothervoltagecurrentinacircuititiscalledadependentcontrolledcurrentsource.SymbolsfthesesourcesareshowninFigure2.Figure2Anindependentcurrentsourcewithzerocurrentisidenticaltoanidealopencircuit.Fthisreasontheinterna

5、lresistanceofanidealcurrentsourceisinfinite.Thevoltageacrossanidealcurrentsourceiscompletelydeterminedbythecircuititisconnectedto.Whenconnectedtoashtcircuitthereiszerovoltagethuszeropowerdelivered.Whenconnectedtoaloadres

6、istancethevoltageacrossthesourceapproachesinfinityastheloadresistanceapproachesinfinity(anopencircuit).Thusanidealcurrentsourcecouldsupplyunlimitedpowerfeversowouldrepresentanunlimitedsourceofenergy.Connectinganidealopen

7、circuittoanidealnonzeroemitterresistR2.Theoperationofthecircuitisasfollows:VoltageacrossR2(VR2)isgivenbyVZVBEwhereVBEisthebaseemitterofQ1.TheemittercurrentofQ1whichisalsothecurrentthroughR2isgivenby.22)(22RVVRVIIBEERER??

8、??Figure3SinceVZisconstantVBEisalso(approximately)constantfagiventemperatureitfollowsthatVR2isconstanthenceIEisalsoconstant.DuetotransistactionemittercurrentIEisverynearlyequaltothecollectcurrentICofthetransist(whichintu

9、rnisthecurrentthroughtheload).Thustheloadcurrentisconstant(neglectingtheoutputresistanceofthetransistduetotheEarlyeffect)thecircuitoperatesasaconstantcurrentsource.Aslongasthetemperatureremainsconstant(doesntvarymuch)the

10、loadcurrentwillbeindependentofthesupplyvoltageR1thetransistsgain.R2allowstheloadcurrenttobesetatanydesirablevalueiscalculatedbysinceVBEis2R2RBEEIVV??265.02RZIVR??typically0.65Vfasilicondevice.(IR2isalsotheemittercurrenti

11、sassumedtobethesameasthecollectrequiredloadcurrentprovidedhFEissufficientlylarge).ResistanceR1atresistR1iscalculatedaswhereK=1.2toBZZSIKIVVR1???2(sothatR1islowenoughtoensureadequateIB)hFE(min)(min)2)(FERECBhIIII???isthel

12、owestacceptablecurrentgainftheparticulartransisttypebeingused.Amecommoncurrentsourceinintegratedcircuitsisthecurrentmirr.SimpletransistcurrentsourcewithdiodecompensationTemperaturechangeswillchangetheoutputcurrentdeliver

13、edbythecircuitofFigure3becauseVBEissensitivetotemperature.TemperaturedependencecanbecompensatedusingthecircuitofFigure4thatincludesastarddiodeD(ofthesamesemiconductmaterialasthetransist)inserieswiththeZenerdiodeasshownin

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