HP-200-IRE-Article 电路图.pdf

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1、topublishresultspromptlyandfullybytheaidoftheMonthlyBulletin,apublicationwhichcan beof summariesatleastmonthlyinperiodicals,radioconsiderablevaluetostudentsofradiowavepropa- broadcasts,andtheUrsigrams.gation.Healsohasthehonorofcongratulatingthe CONCLUSION numerousworkersonradiowave propagation for t

2、hemagnificentbodyofknowledgewhichtheyhave Thewritertakespleasureincomplimentingthebuiltupandwhichithasonlybeenpossibletoindi- GeneralSecretaryonhissuccessfulworkinissuingcateremotelyinthisReport. SomeApplicationsofNegativeFeedbackwithPar- ticularReferencetoLaboratoryEquipment* F.E.TERMANt,FELLOW,I.R

3、.E.,R.R.BUSSt,STUDENT,I.R.E., W.R.HEWLETTt,ASSOCIATE,I.R.E.,ANDF.C.CAHILLt,STUDENT,I.R.E. Summary-Theapplicationoffeedbacktoanentireamplifier ratherthanjusttothefinalstagemakesitpossibletorealizethechar- acteristicsofaperfectamplifieroverwidefrequencyranges.Theuse ofsuchamplifierstogivedirect-readin

4、gaudio-frequencyvoltmeters withpermanentcalibrationandanydesiredsensitivityisdescribed. Negativefeedbackcanbeusedtoreducethedistortionintheout- putoflaboratoryoscillatorsforallloadsfromopencircuittoshort circuitbytheexpedientofthrowingawayapartoftheoutputpower inaresistivenetwork. Meansaredescribedf

5、orapplyingfeedbacktotunedradio- frequencyamplifierssothattheamplificationdependsonlyuponthe constantsofthetunedcircuitandisindependentofthetubesand supplyvoltages. Theuseofnegativefeedbacktodevelopastabilizednegative resistancesubstantiallyindependentoftubesandsupplyvoltagesis considered,andvariousa

6、pplicationsdescribed. Highselectivitycanbeobtainedbyderivingthefeedbackvoltage fromtheneutralarmofabridge,onelegofwhichinvolvesaparallel resonantcircuit.It ispossiblebythismeanstoobtainaneffective circuitQofseveralthousand,usingordinarytunedcircuits,andthe selectivitycanbevariedwithoutaffectingtheam

7、plificationatreso- nance.Theuseofthesehighlyselectivecircuitsinwaveanalyzersis considered. Feedbackcanbeusedtogiveimprovedlaboratoryoscillators. Theseincluderesistance-stabilizedoscillators,inwhichtheampli- tude-limitingactionisalsoseparatedfromtheamplifieraction,and oscillatorsinwhichthefrequencyis

8、controlledbyaresistance- capacitancenetwork.Suchresistance-capacitanceoscillatorsrepre- sentasimpleandinexpensivesubstituteforbeat-frequencyoscil- lators,andhavecomparableperformance. Namplifierwithnegativefeedbackisanor- Adinaryamplifierinwhichavoltageisderived fromtheoutputandsuperimposeduponthe a

9、mplifierinputinsuchawayasundernormalcon- ditionstoopposetheappliedsignalvoltage.The presenceoffeedbackthenreducestheamplification andoutputdistortionbythefactor1/(1-AO), whereAistheamplificationintheabsenceoffeed- back,and1istheratioofvoltagesuperimposedon theamplifierinputtotheoutputvoltageofthe am

10、plifier.,2ThequantityA,Bdeterminesthemag- nitudeofthefeedbackeffect,andcanbeconven- ientlytermedthefeedbackfactor.Itwillbenotedthat whenA,Bislargecomparedwithunity,thatthe amplificationapproaches-1/A. * Decimalclassification:R363.Originalmanuscriptreceived bytheInstitute,November22,1938;abridgedmanu

11、scriptre- ceivedbytheInstitute,August1,1939.Presented,Thirteenth AnnualConvention,NewYork,N.Y.,June16,1938,andPacific CoastConvention,Portland,Ore.,August11,1938. tStanfordUniversity,California. 1H.S.Black,Stabilizedfeedbackamplifier,Elec.Eng., vol.53,pp.114-120;January,(1934). 2H.Nyquist,Regenerati

12、ontheory,BellSys.Tech.Jour., vol.11,pp.126-147;January,(1932). LABORATORYAUDIO-FREQUENCYAMPLIFIERS WITHNEGATIVEFEEDBACK Althoughfeedbackisusuallyemployedinaudio- frequencyamplifiersforthepurposeofreducingthe distortioninthepowerstage,thereismuchmoreto begainedinthecaseoflaboratoryamplifiersby applyi

13、ngfeedbacktotheentireamplifier.Bymaking thefeedbackfactorA/3muchlargerthanunity,and arrangingmatterssothefraction 3oftheoutput voltagethatissuperimposedupontheamplifier inputisindependentofthetubecharacteristics,the amplificationdependsprimarilyon:andissubstan- tiallyindependentoftubereplacements,el

14、ectrode voltages,agingoftubes,etc.Itisthenpossibleto engraveanaccuratecalibrationonthegaincontrol, sincethegaincalibrationbecomesaspermanentas thecharacteristicsofasmallmilliammeter.Further- more,ifAO3islargeandthefeedbackcircuitissuch astomakedindependentoffrequency,thenthe amplificationispractical

15、lyindependentoffrequency, thephaseshiftispracticallyzerooverthenormal frequencyrangeoftheamplifier,andtherangefor reasonablyflatresponseisgreatlyincreased. Theextentoftheimprovementsobtainableinthe performanceofanamplifiercanberealizedbycon- sideringTableI,whichcomparesperformanceswith andwithoutfee

16、dbackinahypotheticalcase. TABLEI COMPARISONOFRESISTANCE-COUPLEDAMPLIFIERSWITH ANDWITHOUTFEEDBACK NoFeedbackWithFeedback Voltagegain(middle-frequency range)2500 2500 Voltagegainwithtubeorsupply potentialchangethatincreases 31252520 A25percent(+1.94db)(+0.07db) Distortionwithfulloutput 2%0.04% Variati

17、onofgainoverrange15-50%+4% 30,000cycles (-6db)(+0.33db) Frequencyrangeforgainvaria-15to30,000 5to95,000 tionof50percentcyclescycles Phaseshiftoverrange15-30,000 cycles9004040 NOTE:Amplifierwithoutfeedbackistwo-stageresistance-coupled. Amplifierwithfeedbackistwosuchsectionsintandemwitheachsection hav

18、ingAS=-49inmid-frequencyrange. ProceedingsoftheI.R.E. 649 October,1939 RadioFans.CN 收音机爱 好者资料库 ProceedingsoftheI.R.E. Itisapparentthatwheneverflatnessofresponse, reproducibilityofgain,lowdistortion,orlowphase shiftareofimportance,anamplifiercannotbecon- sideredasbeingproperlydesignedunlessfulluseis

19、madeoffeedback.Thisisespeciallytrueinampli- fiersusedinmeasuringequipmentandforoscillo- graphpurposes. Feedbackcanalsobeusedtoimprovethebalance betweenthetwosidesofapush-pullclassAamplifier. TypicalcircuitsfordoingthisareshowninFig.1. (a)RESISTANCECOUPLING TWOSTAGES R5 R20RI4 hiTRANSFORMERCOUPUING O

20、NESTAGE XC)PHASEINVERTER thatnomatterhowlargeavoltagemayaccidentally beapplied,thethermcouplecannotbeburnedout.12 Theinstrumentwiththecircuitproportionsgiven inFig.2hasbeeninuseforseveralyearsandfound tobehighlysatisfactory.Ithasaninputresistance of1megohm,givesfull-scaleoutputwithaninput of3volts,a

21、ndcanbeusedasadirect-readinginstru- mentinthesamemannerasanordinarydirect-cur- rentvoltmeter.Thestabilityandflatnessofthefre- quencyresponseareindicatedbytheperformance testsreportedinTableII. I -HIGH:R2 RESIS- TANC 6J7 0.01 INPUTo. HF 71I NOTEFORASOVEFIGURES: RIPROOItCESFED-RACKTHAT CAUSESBALANCING

22、EFFECT R2ANDRpPROCUCEORDINARYFEED-RACKACTION R3ANDW3CONTROLAMOUNTOFFEED-BACK Fig.1-Useoffeedbacktomaintainbalancebetweenoutputs ofthetwosidesofapush-pullclassAamplifierorphasein- verter. Inthesearrangementsunbalanceproducesacurrent throughtheresistanceR1,resultinginthedevelop- mentofafeedbackvoltage

23、thatisappliedtothe tubesinsuchamannerastoreducethedifference intheoutputsofthetwosides.Theuseoffeedback inthiswaymakesitpossibletomaintainextremely accuratebalancewithoutthenecessityofusingcare- fullymatchedtubesineachpush-pullstage.Italso makespossiblealmostperfectbalancewhenphase invertersareused.

24、Thegreatpracticalvalueofthe resultinlaboratorypush-pullamplifiersrequiring accuratebalanceisobvious. USEOFFEEDBACKAMPLIFIERSIN VOLTAGEMEASUREMENTS Thestabilityofamplificationthatresultswhena largeamountoffeedbackisemployedinanaudio- frequencyamplifieriscomparablewiththestability ofthesmalldArsonvalm

25、eterscommonlyusedin laboratorywork.This,coupledwiththeveryuni- formresponsethatcanbeobtainedoverawide frequencyrangeopensupmanypossibilitiesinmeas- uringequipment. TheinstrumentsshowninFigs.2and3areindica- tionsofwhatcanbedone.Thefirstoftheseconsists ofatwo-stageamplifierwithalargeamountoffeed- back

26、,deliveringitsoutputtoavacuumthermo- couple.Thisgivestheequivalentofasquare-law vacuum-tubevoltmeterbuthasapermanentcali- brationandrequiresnozeroadjustment.Further- morebyproperdesignthefinaltubecanbemadeto overloadatslightlyabovefull-scaledeflection,so I 28DM00W r 6K74.0 0.25 C g -*1- METER L OOH

27、Oo Cyo2 n5A1TMTHERMO- COUPLE + 4250 NOTEI:FEEDSACKADJUSTMENTMADEmYSCREWDRIVERTO GIVEREQUIREDSENSITIVITY. NOTE2:LiANDRIREQUIREDTOPREVENTRUSHOFCHARGING CURRENTTOC,THATWOULDBURNOUTTHERMO-COUPLE. Fig.2-Circuitdiagramofafeedbackvoltmeterdesigned foraudio-frequencyservice. 6J76K7o,1 oHt INPUTtvs2.02IOINPU

28、T 0- GM5OUTPUT |ADJtUSTMENT l.l_ 4250 NOTESGAINOFEXACTLYDOOSTAIEDOSYSETTINGFEED-BACK ADJUSTMENTWITHASCREWDRIVER. R-ADJUSTEDT04000OHMSFROMPOINT STO GROUND. Fig.3-Circuitdiagramofstandard-gainamplifierforincreasing thesensitivityofvacuum-tubeandfeedbackvoltmetersat audiofrequencies.Themaximumgainis50,

29、andtheoutput istappedasshownsothatgainsof20,10,5,and2arelikewise available. TABLEII CHARACTERISTICSOFFEEDBACKMEASURINGINSTRUMENTS FeedbackVolt-Standard-Gain meterofFig.2AmplifierofFig.3 Dropinresponseat40cyclesunder-1%under-1% Dropinresponseat20000cyclesunder-1%under-1% Changeingainwhenplate-supply

30、voltageisvariedfrom250voltsto 140voltsunder-1under-0.5% 400voltsunder+1under+0.5% Changeingainwhenheatervolt- agevariedfrom6voltsto 7.5voltsunder0.5Wunder1% 5.3voltsunder0.5%under1% TheinstrumentofFig.3isanamplifierforex- tendingtherangeofthefeedbackvoltmeterofFig.2 andofordinaryvacuum-tubevoltmeter

31、s.Thiscon- sistsofanamplifiermadestableandgivenagood frequencyresponsebyintroducingalargeamountof feedback.Thisfeedbackissetbymeansofascrew- driveradjustmenttogiveagainofexactly50,and 650 October t . tl RadioFans.CN 收音机爱 好者资料库 Terman,Buss,Hewlett,andCahill:ApplicationsofNegativeFeedback theoutputres

32、istanceisthentappedasindicatedso thatoutputvoltagesthatare2,5,10,20,or50times theinputvoltagecanbeobtainedacrossa1-megohm loadaccordingtotheswitchposition.Thepropor- tionsaresuchthatanoutputofapproximately3 voltseffectivecanbeobtainedonanyrangewithout overload.ThedesignindicatedinFig.3wasintended fo

33、raudio-frequencyservice,andasseenfromTable IIhasexcellentstabilityandpracticallyanideal frequencyresponse. ApplicationofFeedbacktotheOutputAmplifierof LaboratoryOscillators Inawell-designedlaboratoryoscillatorthemajor partofthedistortionoccurringintheoutputresults fromharmonicsgeneratedintheoutputam

34、plifier. Theuseofnegativefeedbacktoreducethisdistor- tioniscomplicatedbythefactthattheloadimped- ancetowhichtheoscillatoroutputisdeliveredmay varyfromshortcircuittoopencircuitunderdiffer- entconditionsofuse.Thesituationcan,however, behandledbythrowingawayafractionoftheoutput inaresistivenetworkassho

35、wninFig.4.HereR2 preventstheoutputofthepowertubefromever beingshort-circuited,whilethecombinationR1+R2 introducesfeedbackthatmakesthevoltageEoa substantiallydistortionlessreproductionoftheinput signalEs. AquantitativeanalysisofFig.4(a)showsthatfor maximumoutputpowerdeliveredtotheload,the loadshouldb

36、earesistanceequaltoR3,whilethe resistanceformedbyR1+R2inparallelwithR3 shouldequaltheplate-loadresistancewhichgives maximumpoweroutputfromthetubeoperatedas R2 (a) +tE, e(b)+EB Fig.4-Circuitarrangementsinwhichfeedbackisarrangedto beeffectiveinreducingthedistortionoftheoutputamplifier ofalaboratoryosc

37、illatorirrespectiveoftheloadimpedance. anordinaryamplifier.Undertheseconditionsthe maximumpowerthatcanbedeliveredtotheload is(P0/4)X(R1+R2)/(R1+R2+R3),wherePoisthe maximumundistortedoutputwhichthetubeis capableofdeveloping.Intheusual casewhere (R1+R2)R3,theoutputobtainable henceap- proachesP0/4. TUN

38、EDAMPLIFIERSEMPLOYING NEGATIVEFEEDBACK Theamplificationofatunedamplifiercanbe madesubstantiallyindependentofthetubeandthe supplyvoltagesbymeansofthecircuitshownin Fig.5.Herethecurrentthatthetubedeliverstothe tunedoutputcircuitalsoflowsthrougharesistance R,acrosswhichisdevelopedafeedbackvoltagethat i

39、sproportionaltothecurrentpassedthroughthe tunedcircuitandisindependentoffrequency.When thefeedbackfactorobtainedinthiswayislarge,the voltagedevelopedacrosstheresistanceR1,andhence thecurrentthroughthetunedcircuit,isstabilized. Theamplificationisthendeterminedsolelybythe IILI? FEEDBACKVOLTAGE (A) DEV

40、ELOPEDHEREE9 BROADLYTUNEDSTAGE LL RESISTANCECOUPLE3 (b) +E8 Fig.5-Circuitdiagramsoftunedamplifiersinwhichfeedback isusedtomakethegainindependentoftubeconditions.These circuitscanbeeasilymodifiedforband-passaction. tunedcircuit,andbecomesindependentofthetube orelectrodevoltages.ThetwocircuitsshowninF

41、ig.5 accomplishthesameresult,butthearrangementat (b)isbyfarthebetterbecauseitgivesappreciable gainevenwhenthefeedbackfactorAOiislarge. ArrangementsofthetypeshowninFig.5canbe usedtoadvantageintheintermediate-frequency andradio-frequencystagesoffield-strength-measur- ingequipment.Itispossibleinthisway

42、toavoidthe necessityoffrequentcalibration,andinfactitisen- tirelyfeasibletomakeacalibrationofthefieldstrength intermsofgain-controlsetting,withtheassurancethat theonlyfactorsthatwillaffectthecalibrationappre- ciablyaretemperatureeffectsandmisalignment. HIGHSELECTIVITYBYMEANSOF NEGATIVEFEEDBACK Negat

43、ivefeedbackprovidessomeremarkablepos- sibilitiesforobtainingtheequivalentofahigh-Q tunedcircuit.Onemethodofdoingthisistouse feedbacktoprovideastabilizednegativeresistance thatcanbeusedforregeneration.Anothermethodof approachistoprovideafeedbackamplifierinwhich thefeedbacknetworkisacircuithavingatran

44、smis- sioncharacteristicthatdependsuponfrequency. StabilizedNegativeResistance ThecircuitofFig.6givesanegativeresistance acrosstheterminalsaathatissubstantiallyinde- pendentofthetubesandsupplyvoltages,andwhich canbemadeconstantover awiderangeoffrequen- cies.Thisarrangementcanbeanalyzedbyassuming asi

45、gnalvoltageEBisappliedtotheinput,andthen 1939 651 RadioFans.CN 收音机爱 好者资料库 ProceedingsoftheI.R.E. evaluatingtheratioE,/18,where18isthecurrent thatflowsintotheinputterminalsaa.Assumingthat thegridofthefirsttubeisnotallowedtogopositive, andreferringtoFig.6,onecanwrite Es-EoEs-AEsE_(, RR R/(1-A) whereEo

46、istheamplifiedvoltage,andAistheratio EolE8.IftheamplifiedvoltageEohasthesamephase asE8,thentheresistancewhichtheterminalsaa +ES Fig.6-Circuitfordevelopingastabilizednegativeresistanceby usinganegative-feedbackamplifiertogivestabilizedregen- eration,togetherwithseveralapplications. offertothevoltageE

47、8isobviouslyanegativere- sistancehavinganabsolutemagnitudeR/(A-1)I. Byusingalargeamountofnegativefeedbackthe amplificationAcanbemadesubstantiallyinde- pendentoftubeconditionsandsupplyvoltages,and canbemadeconstantoverawidefrequencyrange. Thenegativeresistanceundersuchconditionsiscor- respondinglysta

48、bilized. Suchstablenegativeresistanceshaveanumberof uses.Ifplacedinparallelwithatunedcircuitas showninFig.6(a),theresultisequivalenttoreduc- ingtheequivalentresistanceofthetunedcircuit, andhenceraisingtheeffectiveQ.Thisisaformof regeneration,butunlikeordinaryregenerationthere islittleornopossibility

49、ofinstabilitybeingintro- ducedbyvariationsinthetubeorsupplyvoltages. Thustestsinanactualcaseusingatunedcircuit havingQ=100at10kilocyclesshowedthatwith AO =100andsufficientnegativeresistance toraise theeffectiveQto2000whentheplate-supplypoten- tialwas150volts,anincreaseto400voltsraisedthe effectiveQonly10percent. Anotheruseofastabilizednegativeresistanceis intheimprovementofthe(alternating-current)/ (direct-current)impedanceratioofdiodedetectors, byshuntingthenegativeresistanceacrossthediode outputasshowninFig.6(b).Thiseliminatesthe principalcauseofdistortionindiodedetectors. HighSelectiv