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August 008l his s a highgain +35dB) low NoiseFigure 1 dB) 65 MHz bandwidth mpl i f ierdcsigncd irr -5Q h115nput/output. he wo MRF544 ransistorsTO-39case) recurrentlravailable iom http://wwvn,.rlparls.com.heyhave u.ovcrsions:he originallVlotorola evice s $5.25each: healtemate P'f device s $4.25each.Thcre s a $25nrinimumordcr.The originalclLC425opamp s non,obsolete. alionalSemiconcluctorgives he LMI16624asan "improved eplacement". ou,evert is onlyal'ailablc n aSM I'package ither the SC)IC arrorv pin or rheSO'I-235 pin package. igi-Ko,carries itherpackagebr $3.98each.'frvoo1'1hcransistors ndoneofthe opamps reneededo bu ild this amplilier. t couldbc pu t on a small PCBusingSMT partsexcept br the wo lcaded ransistors.Thc t-MI 16624 asa recommended aximumvoltagcof l2 volts absolutemarinrumol '13.2 'olts).A 12volt low dropoulvoltage egulator houldbe used br the an.rpl'usingthc I-M I6624-this would ensureheopamps eversaw rore than 2 volts. I hc ar.npsdesignedo operate t l2 voltsanyway. TheobsoleteCLC425wasnot linlited o 12Vol1s.-l'hisvror-rldea goodgeneral urpose mp o have.

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1 Functi0naly, hisamptitiersa high-input.impedancenverting ainI block, oviever.eedbackerminationiaRi owe6 he rplt impedarceto heyalue eededomatchR..Such napproachroducesess nputnoisehansimplyerminatinghe nputwith. esistorequalto3.

The amplifiercan be viewedas ahigh-input-impedance,nvertinggainblockwith a gainof -A " fFig. .0 .put.ting a resistor ogroundal the nputwouldcause he input impedanceoJUNE7, 994 NArocAppLrcATroNssslrE LECrio"rauH

BuildA 1-dB5O-C}Systenrs

:Anampsos eedback.typenputerminationtoachieYe6.d8gain nd 65.tlHzBYSTE\ 'ENC, .SMTTHConlincdr Cofi.).

Noise-Fi gufelifi ef For

.Thisarticledescribes simple wo-stage mplifierbuilt from readilyavallall le omponentshat achieves l-dB noise igureand 65_MHz and-width at a voltagegain of 86 dB. Theamplifierstarts with a low-noise om-mon emitter first stage, ollowed byan op amp to provideversatilitv andadditionalgain. Input terminitrc,n,which is providedby feedback, unc-tionsalmostnoiselessly. detailedde-scription f thebiasing ndoperationof thecircuitare ncluded,husallow-ing readerso easily ailor the amplifi-er to suit specificnput impedances,gains,andbandwidths.

. DeVelOping IOW-nOiSe amplifiersor erminatedransmrsslon-lrneenvlronmentsposesa difficult designchallenge or many engr-neers.Not onlyil the signalattenuatedby the mplifier,s'input terminatiJn,but if notcarefullydone, he terminationtself addssubstantialnoise... N9is9 n a terminated ystem s usuallyexpressedsnoise igure.Al _tnough hemany el in i i ions l noiseigureareal l tpchnica l l .vquiva lenrthey'renot ll equally asy o understand.hedefinition find he asiest ocomprehends that the noise igure is the ratio of the actual amplifier,soutputnoiseo hatportion f theoutputnoiseCue olelyo hesignai ourceandsourceesistance.hi sdefinitionsparticularly seful n simulation, sSpice eportsboth the total output noiie and the output noise ue to thesourceesistancenoiseigure s expressedn decibels)..Forexample,o-nsidernamplifierwith a 1-dB oise igure 50O).Whenmooellngheamplrtler oise sa voltage ourcen series ith he nput, he iinput-referredoise l room emperaruresonly0.46 y / y'Ez. A;ii;; i;;norse ower epresentedy 0.46 V/ / Hz o thenoise owerof a 50_!resis_to r r-esultsna netnoise dB greater han hatof a50-h psisror.

. In^put rerminat ionsas igni f icantroblemncountercdnbui ld ingaow_noise-figure mplifier.Terminating he nput resistiveiy imits the mrnimum :achier,'ableoise igure to 3 dB (thecombination f noise rom the sourceanJ ,termination esistorsmakes he noisepower wice hat of thesource esrstor ,alone). oiseiessor almostnoiselesslermination anbe doneeitherwith :inductorsand capacitors ver a narrow bandwidth,or wilh feedbackovera ,broadbandwidth.

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EI .EGIROTI IGESIGl IIANALOGAPPLICATIONS

have he samevalueas the resistor.I f the res is tor s insteadPlacedaround the amplifier, the feedbackacts o reducehe nput mpedancena manner identical o the Miller ef-fect:Ri . :Rr / (A.+1)Fo r example,f Rt : 19 k and A" :199, he nput impedances 50O.The advantages that feedbacktermination llowsusinga value orR. much larger than the source m-pedance .. I Kr s much arger han

---...............--

LC 'VV-NO I SE-F I G U RE AMPLIFIEFt

amplifier's closed-loopain is to as- : sistanceof about 3 O and an f, of 1 : This determines he ratio of R.sume that the closedJoopnput im- ; GHz. Stability s assuredwhen eed- and Rn,,given that you know V0",,The simplestway to analyze he

pedanceequals the source imped-, back sappliedf the first gainstage : the base-emitter oltageof Q1.Noieance.This means ha t voltagemea- sets the dominant eedback-loop that Rb1s in parallelwith the nputsuredattheamplifier nput s halfof t imeconstant. , impedince, ind allowance mustthe source oltage(V"), hus theout- : For the second tage, he opamp , thereforebe made or this when set-putvoltage is: i used must have very-broadband- : ting R,".Sinceonly small changes n: width and ow noise.The CLC425s i Vu, cause arge swings n the collec-V":-(A"V.)/2 , an excellent hoice or the second, tor currentof Q1 , he negativeeed-, stage amplifier, with its 1.9-GHz back n the oopwill forceV, to equalresulting n a net closedJoopainof , gain-Dendwidth roduct and 1.05 V,andV".-A. /2 ]n an input - impedance- 'V / aHz input o i tage oise. t rhe Witd hal f the supplyvol tagematchedcondition.Notetha tthecor-, LC425'sminimumrecommended dropped crossR. , Ql'has fa r toorect way to change he amplifier's i gain, the second tage bandwidih s much gain for broad bandwidth acclosed-loopain sto change oth he : 300MHz. i operation.To set the ac gain of Q1,openJoopgain and the feedback e- i Thedominantloopimeconstant resistor R. would be split into twosistor by the same factor. Just : is setby the Millermultiplication f pieces R"fand R"r),and R., is by -changing the feedback resistor or : the base-collectorapacitancef Ql : passedwilih al-p.Fiapacitor'(Fig.31.

R" , hen he currentnoise ue o R,will be much smaller han the cur-rentnoise due o R. when summed tthe amplifier input (remember hatvoltagenoisegoesup as the square

the amplifier oise. : bemidway etweenhesupplies6V . tance:The transistor used n the first , in this case).Then. he resistorval-

root "of resisLnce, - *hite cuireni . 2 lit.,]lo',t*o .Tuil diagramhowshathe mplifier0nsistslhrogain tages-olandhenoise goes down in the .u-" t".t- , - ::lf_:Tll'"e op'alhpoiseonhibutionustbe ow nougho hatO dominate!he otalion). : ampllrrerolse'

and R.) and he operational mplifier : levels at Y" and V, are the sameas : commonemitter amplifier dependsmust be low enough in noise that , the dc voltage at Vr. For maximum : on both the collectorcurrent in theQ1's noise contribution dominates signal swings,V,, V' and V" should : transistor and the collector resis-

impedance. , width can easily be compensated r i R., portion of the collector resis-IN|SIDE THE AMPLIFIER : reducedby addingbasecollect r ca- : tance,he midbandacgaincanbe set

the openJoopgain by themselves interacting with the source mped-r At dc, the loop will have very highwill result in a poorlymatchednput , ance. f needed,he amplifier'sband- r gain,andby adjusting he sizeof theAs shown n simplified orm, the : pacitanceo Ql. i as desired.amplifier s easy o implement n two : BIASING Tt{E AMPLIFIER r Ideally, he open-loopainof thegain stages Fig. 2). Therc needs o : The easiestway to bias his cir- i overall amplifier shouldbe ndepen-be enoughgain n the first stage Q1 : cuit is to assume hat the dc voltage : dent of temperature.The gain of a

is mlutethe tempenthe cturesistconcienthisarouagaso tsupV,:

andemlWithsamforYwe clng:Rr, ture

gain stagemust havevery low para- : ues needed o support his assump- Q2i1 -gm R"sitic base esistance nd relatively tibnarecomputed,taningwith V.;high f,. One transistor that meets : wheregm : I"/V,, and V,: thermalthese equirementss the MRF544, V. :V"" , [ {R, t Rbr) /R5r l6V vohage.T/q.which ossessesparas i t ic ase e- . .^ l t r r ; , {q T) t4. . - rz . v fA/ l f l " ( thecol lectorcur rentofQl '[ [ |

-: , - -*, . iu ' : '1"tErECrRoMcEsrcNNArocpplrc{rloNssu. 'un.o- ,-, . ;fi i(}t;\ r^"t'; l' lo

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Rf

.:tooac :Q1,:b1'-s).theset

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LOVI'-NOISE-FI G T.'FT AMPLTFIEFT' amplifier.Once heopen-loopai n s: set. he value or R, seasy o deter-: mtne:

Rr: R,"(A" 1)for whateverR,- s desired.lf thevalueof R,gets oo ow , tcan contnbute o th e amplifier,sn-pur.norse.ery-low_noise-figuream -pltrrers n this topologyneedhighgaln.". Thedominant oleof th eampli_l ler sse lby hebase_colector aUacitanceof Q1,with higherorder polesmostlycaused y the CLC425 mpli_ler. lo maxrmizehe overallband_width, he CLC4Z5houldbesetnearIt s. mtnlmum recommended arn,with R, : 50 0O andR, = s0 O.;ihisputs he CLC425n a noninver t inggainof 11,and maximizests band-width (about 300MHz). The rest ofthegalnmustcome ro m e1 :? ln he.comptetemr,,t,r,"no*@ Jt openloopain.asicallr.heempe.alur pends n hev!. muttiptieruit arounaez.@etficientot 01

sgain sclntmlledy v| which e'

: is madedirectlyproportionalo abso_r lute temperatureT, then the qainof, the first stagewill be ndependent f: temperature. his emperature om_pensalionanbeachievedy settingrneco ect0r urrentwitha Lempera_: ture-sensitiyeource., MakingV, dependentna uan-sistor's.V'does prettygood ob ofcontroltlng he tempera(ure oeffi_i clent of Q1's gain. To accomplish. th is . he V""mul t ip l ier i rcu i t-bu j l t' aroundQ2 shouldbe used Fig. J,ogain).RorzndRn " henar echr_,senso that Vr is mid*ay between he,suppl ies:: V, : V*, [(Ro, R*)/Ror] - 6V; _This determineshe ratio of Ro,. and Ro* given that V_,. the base_. g.Ti!t"i voltage of Q2.- s known.wtth theequationor V, having hesame or m as he previous quition

grourd-referenced* multiplier ir_eulr s hat he gainand heclosed-: toop nput mpedance f the amplifi-

: ACPAHTSELECTION, Closed-loopai n an d input rm _poance aybeset"ndependentlyn. thisamp)ifier y choosing ppropri_ate valuesof A . the onpn--|.'.", o;.

ageat Vm s Vs/2.This esults n theIollowing elationship:A": A. /2',i/hereA, is the closedJoop ain

The desired losed-loopai nde_rrmlnes he openJoopgain of the :

whereA I s hegaino e t, andA" rhegainof theCLC425,ndA, : A"/11assumngA2 : 11 Rr: 500O an dR,: 50 )).

er depend n he supply oltage.Anarternate pproach ould be ro re -place he Q2 V* muhipliercircurrwjth a fixedvoltage eferenced fl:: lhf positive uppiy nd epJace.R.,anoK 2wtth thermislors ossessinga-negattveemperature oefficientol about 3300 pm/"C.

fo r Vo , ndassuminghatV*, - V*r,; we cansimplifyconstraints y miJi_lng:: Ro, R,andR": Ro, :One disadvantage f tempera_ture compensatinghe bias with a .

FINALTZII{G TH E DESIGNFrom he desired ccharacLerrs_tics (gainan d nput mpedance), eehosehe feedbackesisroran d heopenJoop ain of the amplifier. Op_erationat dcdictateshevllue ofRi,Rn, and Rrr. If the two transistorsare un atthe same ollectorcurrent.LhismakesR" - R", r R"r.What e_malns s to selecta value or the col_lector current of Ql, which will de_termineR.,,R",,an dR...Q1's 6llecior urr6'ntargelyde_termrnesh noise igureof theam_plrlrer. nalyzingheamplifier orseis simplified with the issumptronthat noise from Q1 and Johirsonnoise rom R" ,dominateheamolif i_er noise. efeiring hese oiseerrnsto.the input of the amplifier, fournoNe ourcesesult:o e^2 (2qy,2)/I.

, ate. yil e.s f A., the open-loop iin, ,andR' rhe feedbackesistor. ran .byconsideringheclosedJoopain. .. . I hevoltageabeled . in Figure1 is the voltage t V,"mujtipliea " the amplitier'i op""f8"p g"ii.'iitf,'" 'amplllrer nput mpedance atchesthesourcempedance,he n he volt- .

due to shot noise in e1's collectorcurrent.JIINE27,199{At\tAtCX; IJPUCATTONSSSLTELECT?ONIC @DESIGN

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suchas nputs.It's essential hat thepowersup-ply beblpassed o a greatextent us-ing both a F to lGpF tantalum anda0.1-pFceramiccapacitor.Very shortlead engthsare vital on the l-pF ce-ramic capacitors n order to avoidadding nductance ndpossiblymov-ing a resonancen-band.ANACTUALDESIGNThe component alues n Figure3 were arrived at by assuminga re-quiredclosedJoop ain of 36 dB (63V/V) andn,":mo. The ,"se tby eed-back s a little higher than 50 Q toallow or theparallelcontributionofRn, o the amplifier nput mpedance.Room-temperature easurementson heamplifiershowquite good ev-els of performance see he tdble).The amplifier exhibits no peakingand olls off smoothly o the 6FMHz3-dBpoint.Moreover,performancemea-surementsof noise igure and nput

= . - _

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, t i" ' : 2qlrdue to shot noise n Q1'sbasecur-rent.' e,u2:4krrrdue oJohnsonnoisen Q1'sparasiticbase esistance.o .rr': (4kT)/&rdue o Johnson oise n Ror.These noises are summed asnoise power (or by summing thesquares of the individual noises),converting the noise-current ermsronoise oltages y using henet m-pedance t theamplifier nput, R./2:

+ (i.,+ i"blrxR./2),For a1-dBnoise-figure mplifier, heresultinE oisemustbe ess han0.46nY/{Ei.A value or I"canalso edecidedonempirically, tthebench. emem-bering that the collector voltage ofQ1 s fixed by the feedback oop at 6V in this case), he collectorcurrentof Q1canassumedo be he currentthat f lowswith 6 V across heresis-tanceR"r * R"r. Given he collectorcurrent and he gainof Q1 referredto as A1 earlier),you can select hesize or R",. f the noise igure is toohigh, the collectorcurrent canbe tn-creasedby decreasingR", and R.,proportionately,beingcarefulnot toincreasehe base-currentho (noisetoo much.DESIGN RECAPStart by deciding what closed-loop gain (A,) and input impedance(R,") ar e needed n your system.Thise determine he open-loop arnof the amplifier (A") and the feed-back esistorvalue(Rr):R,: R' (A "+ 1)Remembero increaseRr o accountfor the effect of Ro,.The secondstage bandwidthmust be maximized or stability, sothe CLC425should be set near itsminimum ecommendedainof *11:Rz: 5ooX, Rr 5oXtrilll]rELF]CTRONlc)I:SIGNNALOGAI'T'LICAI'IONSSSUEJUNE7,1994

LOVV-NOISE-F IG U R E AMPLIF IEFIThe emain inghoicesfcompo,nentvaluecanbe simplifiedwith the :following ssumptions: :

D _ D - L ' 'D - D D _ Dr ! \2 - r l t i 1 ! \ t - ] !h t

.The value of Ro, s chosen o:makethe cvalue f V"abouthalfof :the supply voltage by meansof the :V"" multiplier formedwith Ql: ,V"= Vo. ,(R, , Ro, ) /Ro, i

Finally, the value of I" s chosenbased n noise equirements, hichdetermines , . e-r. ", s thenpickedto set hecorrect ai n nQLIf the amplifier'soutput doesn'tneed o drivea terminatedransmis-sion ine,R, may be removed. hi swill double he apparentgain of theamplifier,and may require decreas-ing the feedback esistor and open-loopgain by a factor of two to com-pensate.Remember o leave he accoupling capacitoron the output toblock he GV dc evel.Building the amplifier sn't par-ticularly difficult if good RF con-struction tchniquesare followed.RF isolation n the supplies etweenQl and the CLC425sn't required.Lead engthsmusi be kept short, es-pecially on high-impedance odes

f At ow emperatures,heamplilier's oiseigure ncreasesecausel exhibit!morc aae-c0renlt shotnoi3e.llis ncreasen sholnoises due o a reduclionnOl'sbeta t oweremperaturs&

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Hols. Fl9.r6 tn . 50 dn S9st6i (dB)

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ti nanlso he mptifer'semperaturc.compenr"tedi".inJ,.l,er{iliffiiiliEiiliffi2t retatvety onstaloveremoeralure.impedanceover temperature illus-trate the effectivenessof the tern-perature-compenat ed biasing(Figs. t and 5).A1so,he amplifier,ibandwidth remains virtually con-stant at 65MHz over emperature._ The increase n noise igure atlow tmperatures s due to the rrr-crease n Q1's base-currentshot Motchenbacher,., and Fitchen,i.:":!:!.J.?s!i!).AsthetempraF.,LowNoisi Lrii'"iit" o"rts".rure{rops,so does I s beta,which Ne w york: John Wiley and So"ns,results n an ncreasenthe base-cur- 1g?8.rent shotnoise. he minimumnorsefigureachievable.withhi samplifier Roberge, James,Operatzonaloepenosnsunlcrent et a n e1 . Amplifiers: Theorttaid praclice.,^_ i:.rgTll9i, rn , rhe amptifier.is New york: John Wiley and Sons,domlnated y heharmonicsontrib_ 197.5-uted by Ql. At an output powerof 0

ness tRaytheon ndJimRiphahn r lComlinear. any onversationsith ,Kevin resulred n a numberof im-p r o v e m e n t s ,h i l eJ i m b u i l t a n dmeasuredhecircuit. UHefcrences:

dBm, he base+mittervoltageof Qlchanges yaboutSmVp-p, isultingin-39dB csecond-,nd 61dBc hird--harmonic levels at a fundamentalfrequencyof 10MHz.Although low-noise mplifierswith input ermination anbe diffi-cult ro design, he topologydis-cussedn thisarticle s airlv easv owork with. Feedbacknput-rcrmino-tion provides an almoit noiseless,well