OA-11 A Tutorial on Applying Op Amps to RF …

1 ApplicationReportSNOA390B September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications ABSTRACTW ithoperatingfrequenciesexceeding300 MHz,TexasInstrumentslineofmonolithicandh ybridcurrentfeedbackoperationalamplifier shavebecomeanattractiveoptionfortheRF(an dIF) ,however, ,thisapplicationreportdefinestheRFspecif icationsofmostinteresttodesigners,detail swhatdetermineseachoftheseparticularperf ormancecharacteristicsforTexasInstrument scurrentfeedbackopamps,and,wherepossible , , , , September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, ,questionsin s.

2 A 5 Vpowersupplymonolithicamplifierintendedf oruseovera voltagegainrangeof 1 to +6,theCLC404offersa DCto175 MHzfrequencyrangewhiledelivering12dBmpow erintoa 50 loadwhiledissipatingonly110m ,aswellashighersupplyvoltage(andhencehig herpoweroutput) particularapplicationwilldependuponthede siredgain,poweroutput, ,developedbyTexasInstruments,providesa verywideband, closedloopnegativefeedbackstructure,thef requencyresponsefortheopampsis comparedtovoltagefeedbackamplifiers,whic hareconstrainedtoa gain-bandwidthproductoperation,thecurren tfeedbacktopologyofferstrulyimpressiveeq uivalentgain-bandwidthproducts(forexampl e,theCLC401ata gainof20yieldsa flatresponsewitha ,a voltagefeedbackopampwouldrequire20 150 MHz= 3 GHzgainbandwidthproduct).

3 Foradescriptionofthecurrentfeedbackopamp topologyandtransferfunction,seeOA-13 CurrentFeedbackLoopGainAnalysisandPerfor manceEnhancementApplicationReport(SNOA36 6).Oneofthebigchangesin goingfroma classicalRFamplifiertousinganopampis nowchargedwithsettinguptheproperoperatin gconditionsfortheopamp,definingthegain, ,the180 phaseshiftprovidedbytheinvertingmodeis ,however,advantagesanddisadvantagestoeac hmode,dependingonthedesiredperformance,a ndbothwillbeconsideredateachstagein notspecificallystatedasbeinguniquetothec urrentfeedbacktopology,theitemsconsidere dhereapplyequallyaswelltoa startingpointfordescribingopampsforRFapp lications,it is.

4 DCvoltagegenerallyhaslittlemeaningin (AC) physical50 resister,butrathera purchasedtoprovidea particulargainandis ,bipolarpowersupplies,balancedaroundgrou nd, showsa typicalRFamplifierconnection,whileFigure 2 andFigure3 showanidealopamp,eithercurrentorvoltagef eedback,connectedfornon-invertingandinve rtinggains, September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, ,bothinputandoutputareACcoupled,whilea singlepowersupplybiasesthepartthroughRb.

5 LcchokesofftheACoutputsignalfromseeingth epowersupplyasa specifiedwiththeoutputdrivinga 50 loadandis definedas10 log(powergain)Thetwoidealopampcircuitsas sumethatthesourceis comingfroma groundreferenced,zeroimpedancevoltagesou rcewhiletheiroutputsareintendedtoactasid eal(zeroohmoutputimpedance)voltagesource stoa ,a zeroohmoutputimpedance,anda voltagegain,asshownin Figure2, ideally180 outofphasefromtheinput, s (-)inputideallypresentsa virtualground,whiledrawingminimalcurrent , ,whilethevoltagegainfromtheinputofRgtoth eoutputis simply-Rf/Rg.

6 Thissignalinversionis usuallyofnoconsequencein anRFapplication, TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, ,wemustfirstsatisfytheI/Oimpedancematchi ngrequirements,recastthegainfroma voltagegaintoa powergain(indB), andFigure5 showtheopampsofFigure2 andFigure3 setuptoprovideI/Oimpedancematchingwithth eresultingpowergainequations, possibleandwillbeconsiderednextForthenon -invertingcase,settingZi= 50 simplyrequiresa 50 terminationresistortogroundonthenon-inve rtinginput,RT.

7 GettingZo= 50 simplyrequiresa series50 resistorin theoutput, ,the(+)inputis groundreferenced,whilethesignalchannelin putimpedancebecomestheparallelcombinatio nofRgandRM. AsOA-13 CurrentFeedbackLoopGainAnalysisandPerfor manceEnhancementApplicationReport(SNOA36 6) describes, particularoptimumRf, RgcanthenbeusedtosetthegainandRM, alongwithRg, willworkuntiltherequiredRg< 50 . HavingfixedRftosatisfytheamplifier s stabilityrequirements,goingtohigherandhi gherinvertinggainswilleventuallyyieldRg s < 50 . Non-invertingoperationshouldbeusedif thislimitationis ,however,beincreasedbeyondtherecommended valueforacurrentfeedbackopampin ordertoallowanRg= 50athighergains, TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, half(-6dB),fromtheearlieridealcase,throu ghthevoltagedivideractionofRO= RL.

8 It is a simple,butcritical,conversionfromanydesc riptionofoutputvoltageswingtoandfroma power(indBm) showstheseconversionsfora ,forwhateverinitialdescriptionofvoltages winggiven,weneedtoconvertthatintoanRMSvo ltage,squareit anddividebytheload(RL= 50 normally)togettheabsolutepowerin logofthatexpressionis takentoyieldthepowerin September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, specifiedmaximumoutputvoltageswingthatis generallyshownasa ,forbalancedbipolarpowersupplies,is ,it is alwaysbesttoholdtheoutputpinDClevelcente redbetweenthetwosupplypinsinordertoprovi dethemaximumoutputVpp.

9 Formoredetailsoninputandoutputvoltageran geconsiderations,seeOA-15 FrequentFauxPasin ApplyingWidebandCurrentFeedbackAmplifier sApplicationReport(SNOA367).MostofTexasI nstrumentsopampsdonotrequirea groundreferenceforproperoperationandcanb eeasilyoperatedfroma ,allthatis requiredis tokeeptheDCvoltageonthe(+) singlesupplyoperation(withonesupplypinhe ldatground)thistranslatesintothe(+) groundpin,thatpinshouldalsobedrivenwitha andFigure8showtwosimplewaystooperatenon- invertingandinvertingopampsasACcoupledRF amplifiersusinga ,theinputterminationis stillDCcoupled,whilethe(+)

10 Inputbiasis setbythetwoRb s thebiaspath,butnotsolargeastogenerateexc essiveDCerrorsduetotheamplifier s ,Rg, is ,the(+) bothcircuitstolimittheDCcurrentthatwould berequiredif a ,Non-InvertingOpAmpOperationForthesingle supplyinvertingamplifierofFigure8, westillrequirethemidpointreferencetobebr oughtin onthe(+) de-couplingcapacitoronthatnodeis againACcoupledtoyielda , aswellasRTin Figure7, , ,alongwithimpedancesin , TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, , ,InvertingOpAmpOperation9 SNOA390B September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, (reference[1]).