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1、UNIT2A:TheOperationalAmplifierOneproblemwithelectronicdevicescorrespondingtothegeneralizedamplifiersisthatthegains,AuorA~,dependuponinternalpropertiesofthetwo-portsystem(p,fl,R~,Ro,etc.)?~Thismakesdesigndifficultsincetheseparametersusuallyvaryfromdevicetodevic
2、e,aswellaswithtemperature.Theoperationalamplifier,orOp-Amp,isdesignedtominimizethisdependenceandtomaximizetheeaseofdesign.AnOp-Ampisanintegratedcircuitthathasmanycomponentpartsuchasresistorsandtransistorsbuiltintothedevice.Atthispointwewillmakenoattempttodescr
3、ibetheseinnerworkings.AtotallygeneralanalysisoftheOp-Ampisbeyondthescopeofsometexts.Wewillinsteadstudyoneexampleindetail,thenpresentthetwoOp-Amplawsandshowhowtheycanbeusedforanalysisinmanypracticalcircuitapplications.Thesetwoprinciplesallowonetodesignmanycircu
4、itswithoutadetailedunderstandingofthedevicephysics.Hence,Op-Ampsarequiteusefulforresearchersinavarietyoftechnicalfieldswhoneedtobuildsimpleamplifiersbutdonotwanttodesignatthetransistorlevel.Inthetextsofelectricalcircuitsandelectronicstheywillalsoshowhowtobuild
5、simplefiltercircuitsusingOp-Amps.Thetransistoramplifiers,whicharethebuildingblocksfromwhichOp-Ampintegratedcircuitsareconstructed,willbediscussed.ThesymbolusedforanidealOp-AmpisshowninFig.1-2A-1.Onlythreeconnectionsareshown:thepositiveandnegativeinputs,andtheo
6、utput.NotshownareotherconnectionsnecessarytoruntheOp-Ampsuchasitsattachmentstopowersuppliesandtogroundpotential.ThelatterconnectionsarenecessarytousetheOp-Ampinapracticalcircuitbutarenotnecessarywhenconsideringtheideal0p-Ampapplicationswestudyinthischapter.The
7、voltagesatthetwoinputsandtheoutputwillberepresentedbythesymbolsU+,U-,andUo.Eachismeasuredwithrespectt~groundpotential.Operationalamplifiersaredifferentialdevices.BythiswemeanthattheoutputvoltagewithrespecttogroundisgivenbytheexpressionUo=A(U+-U-)(1-2A-l)whereA
8、isthegainoftheOp-AmpandU+andU-thevoltagesatinputs.Inotherwords,theoutputvoltageisAtimesthedifferenceinpotentialbetweenthetwoinputs.Integratedcircuittechnologyallowsconstructionofma
