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1
Introduction to Embedded System DesignProfessor Dhananjay V. Gadre (NSUT) & Badri SubudhiIndian Institute of Technology, JammuLecture 10Power Supply for Embedded Systems ContinuedHello,welcometoanewsessionforthisonlinecourseonIntroductiontoEmbeddedSystemDesign.(Refer Slide Time: 0:45)So,inthisconfiguration,ifwehavethetransistor,youwantthetransistortohandlemoreamountofcurrent,thenyoucanusewhatiscalledasaDarlingtonpair,whereyouusetwotransistortomultiplythebetaandthisisthetypicalconfigurationofaDarlingtonpairhere.Thisbecomesyourcollector,thisbecomesyourbaseandthisbecomesyouremitter.HereistheDarlingtonpairwe used two transistors and the effective gain, the current gain of this transistor.Ifthisisthegain,beta1isthegainofthefirsttransistorandthisisthecurrentgainonthesecondtransistor.Theeffectivegainbecomesbeta1intobeta2.Andsothisconfigurationcanhandlemuchlargercurrent.However,pleasenotethatnowthebaseemittervoltageisnolongerVBEitisVBE1forthefirsttransistor,plusVBE2forthesecondtransistor,andtherefore,ifyouare
goingtousetheDarlingtonpairinthatpowersupplyconfiguration,theVouthastoovercomethis.Andsotheoutputvoltageoftheopampwillbesuchthatitovercomesthisrequirement.AndsoifIamgoingtousethispowersupplyconfigurationandIwantmorecurrent,whichsingletransistor may not provide me the required current gain and I could use this Darlington pair.(Refer Slide Time: 2:38)However,still,thispowersupplysuffersfromaninabilitytoprovideagainstanyshortcircuitingattemptattheoutput.Andsowemustbuildinsomemechanismsothatshortattheoutputdoesnotendupdestroyingthepowersupplyandthewayitisachievedisthroughthiscircuithere,throughtheintroductionofthistransistorT2,thisensuresthatthecurrentisnevergoingtoexceed a certain amount.Inthiscase,themaximumcurrentthatthispowersupplywillprovideisequaltotheVBEoftransistorT2dividedbythevalueofR2.So,ifthisvoltageissay0.7voltsandifyouchooseR2tobe1ohmthatmeansthispowersupplycanprovide700milliamperesofcurrent.Whathappensifyoushortit?IfIshortthispowersupply,yes,thecurrentwilltrytoexceed700milliamperes,atwhichpointitwillenableT2itwillturnT2onanditwillprovideanalternativepathforthecurrentfromtheopampinsteadofgoingintoT1andgettingmagnifiedbyafactorof