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1、UNIT2A:TheOperationalAmplifierOneproblemwithelectronicdevicescorrespondingtothegeneralizedamplifiersisthatthegains,AuorA~,dependuponinternalpropertiesofthetwo-portsystem(p,fl,R~,Ro,etc.)?~Thismakesdesigndifficultsincetheseparametersusuallyvaryfromdevicetodevice,aswe
2、llaswithtemperature.Theoperationalamplifier,orOp-Amp,isdesignedtominimizethisdependenceandtomaximizetheeaseofdesign.AnOp-Ampisanintegratedcircuitthathasmanycomponentpartsuchasresistorsandtransistorsbuiltintothedevice.Atthispointwewillmakenoattempttodescribetheseinne
3、rworkings.AtotallygeneralanalysisoftheOp-Ampisbeyondthescopeofsometexts.Wewillinsteadstudyoneexampleindetail,thenpresentthetwoOp-Amplawsandshowhowtheycanbeusedforanalysisinmanypracticalcircuitapplications.Thesetwoprinciplesallowonetodesignmanycircuitswithoutadetaile
4、dunderstandingofthedevicephysics.Hence,Op-Ampsarequiteusefulforresearchersinavarietyoftechnicalfieldswhoneedtobuildsimpleamplifiersbutdonotwanttodesignatthetransistorlevel.Inthetextsofelectricalcircuitsandelectronicstheywillalsoshowhowtobuildsimplefiltercircuitsusin
5、gOp-Amps.Thetransistoramplifiers,whicharethebuildingblocksfromwhichOp-Ampintegratedcircuitsareconstructed,willbediscussed.ThesymbolusedforanidealOp-AmpisshowninFig.1-2A-1.Onlythreeconnectionsareshown:thepositiveandnegativeinputs,andtheoutput.Notshownareotherconnecti
6、onsnecessarytoruntheOp-Ampsuchasitsattachmentstopowersuppliesandtogroundpotential.ThelatterconnectionsarenecessarytousetheOp-Ampinapracticalcircuitbutarenotnecessarywhenconsideringtheideal0p-Ampapplicationswestudyinthischapter.Thevoltagesatthetwoinputsandtheoutputwi
7、llberepresentedbythesymbolsU+,U-,andUo.Eachismeasuredwithrespectt~groundpotential.Operationalamplifiersaredifferentialdevices.BythiswemeanthattheoutputvoltagewithrespecttogroundisgivenbytheexpressionUo=A(U+-U-)(1-2A-l)whereAisthegainoftheOp-AmpandU+andU-thevoltagesa
8、tinputs.Inotherwords,theoutputvoltageisAtimesthedifferenceinpotentialbetweenthetwoinputs.Integratedcircuittechnologyallowsconstructionofma
