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 , a 5 Vpowersupplymonolithicamplifierintendedf oruseovera voltagegainrangeof 1 to +6,theCLC404offersa DCto175 MHzfrequencyrangewhiledelivering12dBmpow erintoa 50 loadwhiledissipatingonly110m ,aswellashighersupplyvoltage(andhencehig herpoweroutput)

2 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).Foradescription ofthecurrentfeedbackopamptopologyandtran sferfunction,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 , DCvoltagegenerallyhaslittlemeaningin (AC)

3 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. 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.

4 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. GettingZo= 50 simplyrequiresa series50 resistorin theoutput, ,the(+)inputis groundreferenced,whilethesignalchannelin putimpedancebecomestheparallelcombinatio nofRgandRM.

5 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. 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.

6 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(+)inputbiasis setbythetwoRb s thebiaspath,butnotsolargeastogenerateexc essiveDCerrorsduetotheamplifier s ,Rg, is ,the(+) bothcircuitstolimittheDCcurrentthatwould berequiredif a ,Non-InvertingOpAmpOperationForthesingle supplyinvertingamplifierofFigure8, westillrequirethemidpointreferencetobebr oughtin onthe(+)

7 De-couplingcapacitoronthatnodeis againACcoupledtoyielda , aswellasRTin Figure7, , ,alongwithimpedancesin , TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, , ,InvertingOpAmpOperation9 SNOA390B September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, (reference[1]). Theseare:ScatteringParametersRFAmplifier SpecificationS11 InputreflectionInputVSWRS22 OutputreflectionOutputVSWRS21 ForwardtransmissionAmplifiergainandbandw idthS12 ReversetransmissionReverseisolationThese frequencydependentspecificationsaremeasu redusinga ,usedforthemeasurementsreportedhere,inco rporatesfull12termerrorcorrectionin thecablingandtesthardware(reference[2]).

8 Figure9 andFigure10showthetwoconfigurationsforth eCLC404usedin ,theS-parametertestsetplacesthedeviceint oa 50 voltagegainof6 totheoutputpinand3 tothe50 gainof20 Iog(3)= RFapplicationsis widerangeofgainscouldhavebeenselectedfor thetestcircuitsofFigure9. andFigure10. 6 wasselectedtoalloweasycomparisonstotheCL C404 s datasheetspecifications,whicharealldefin edata gainof+ ,RMalongwithRgsetstheinputimpedanceto50 . AnRTof50 isretainedonthenon-invertinginputtolimit thepossibilityofself-oscillationin thenon-invertinginputtransistors(seeOA-1 5 FrequentFauxPasin ApplyingWidebandCurrentFeedbackAmplifier sApplicationReport(SNOA367). TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, (VSWR)is a measureofhowwelltheinputandoutputimpedan cesarematchedtothesourceimpedance.)

9 (Itis assumedthroughoutthatthetransmissionline characteristicimpedanceis alsoequaltothesourceimpedanceofbothports -50 in thiscase).It is desirablethattheinputandoutputimpedances beascloselymatchedaspossibletothesourcef ormaximumpowertransferandminimumreflecti ons.(1)Ideal,VSWR= 1 Typically,VSWR= ,forRF-ampsovertheiroperatingfrequencyra ngeMeasuringtheinputVSWRis simplya ofFigure9 andFigure10(S11). A (S22).Asdescribedearlier,anopamp s ,I/OVSWRis nevershownonanopamp s ,nevertheless,beachievedusingthecomponen tsshownin Figure4 September1993 RevisedApril2013OA-11A TutorialonApplyingOpAmpstoRFApplications SubmitDocumentationFeedbackCopyright 1993 2013, s , ,however,theopamp s outputimpedancewillbegintoincreaseasthel oopgainrollsoff(reference[3], page237).Thisinductivecharacteristiccanb epartiallycompensatedbya smallshuntcapacitanceacrossRO.

10 Figure11showsthis,foreithergainpolarity, ,tosomeextent,onthegainsetting,andwasdet erminedempiricallyforthistestbyusinga smalladjustablecap(5-20pF) :1whenCTis (S21) lightlyandwillbeleftin is principallysetbyRT. Asthefrequencyincreases, sonegligibleovertheexpectedoperatingfreq uencyrange,however,thatnotuningis ,atlowfrequencies,setbytheparallelcombin ationofRgandRM. Thisholdsverywellaslongastheamplifier s invertinginputactslikea ,theinvertinginputis actuallya driven, s impedancewill,however, voltagefeedbackamplifier ,theincreasein invertinginputimpedancewillbeseenata lowerfrequencythanfora andFigure10. Inthiscase, TutorialonApplyingOpAmpstoRFApplications SNOA390B September1993 RevisedApril2013 SubmitDocumentationFeedbackCopyright 1993 2013, :1at200 MHz,whiletheinverting,thoughhigher, fixedgain,asdefinedin Figure1, witha , currentfeedbackopamp,a s specifiedvoltagegainrangeof 1 to 10,andincludingtheadditional6dBlossfromt heoutputtotheload, ,oranyothercurrentfeedbackamplifier,with somesacrificein bandwidth(seeOA-13 CurrentFeedbackLoopGainAnalysisandPerfor manceEnhancementApplicationReport(SNOA36 6)).