Series-Tuned High Efficiency RF-Power Amplifiers

Transcription

1 Downloae from orbt.tu.k on: Mar 9 8 Seres-Tune Hgh Effcency RF-Power Amplfers Vkjær Jens Publshe n: IEEE MTT-S Internatonal Mcrowave Symposum Dgest Lnk to artcle DOI:.9/MWSYM Publcaton ate: 8 Document Verson Publsher's PDF also known as Verson of recor Lnk back to DTU Orbt Ctaton (APA): Vkjær J. (8). Seres-Tune Hgh Effcency RF-Power Amplfers. In IEEE MTT-S Internatonal Mcrowave Symposum Dgest IEEE. DOI:.9/MWSYM General rghts Copyrght an moral rghts for the publcatons mae accessble n the publc portal are retane by the authors an/or other copyrght owners an t s a conton of accessng publcatons that users recognse an abe by the legal requrements assocate wth these rghts. Users may ownloa an prnt one copy of any publcaton from the publc portal for the purpose of prvate stuy or research. You may not further strbute the materal or use t for any proft-makng actvty or commercal gan You may freely strbute the URL entfyng the publcaton n the publc portal If you beleve that ths ocument breaches copyrght please contact us provng etals an we wll remove access to the work mmeately an nvestgate your clam.

2 Seres-Tune Hgh Effcency RF-Power Amplfers Jens Vkjaer DTU electro Electroscence Dept. Techncal Unversty of Denmark Kgs. Lyngby Denmark Abstract An approach to hgh effcency RF-power amplfer esgn s presente. It aresses smultaneously effcency optmzaton an peak voltage lmtatons when transstors are pushe towars ther power lmts. I. INTRODUCTION A major ecson n esgn of a hgh effcency RF-power amplfer stage s to etermne the transstor loa mpeance whch smultaneously proves the esre output power optmzes effcency an operates the transstor wthn ts voltage ratngs. Havng solve ths problem the remanng task s to esgn the corresponng mpeance transformaton an matchng crcuts for the transstor. Here we may beneft from the stll mprovng accuracy of the smulaton moels of RF power transstors. It s the purpose of ths paper to focus on the frst ssue an subsequently emonstrate how the results may support a 5W 435MHz narrow-ban RF power amplfer esgn usng venor supple transstor moels. II. SERIES TUNING IN RF POWER AMPLIFIERS Hgh powers at hgh frequences mply low mpeance levels an seres tunng becomes the natural choce of connectng to the transstor through realzable component. Seres tunng means also that the currents enterng the transstor are kept snusoal. Ths s n contrast to the classcal class A AB B an C RF-power amplfer concepts where parallel tunng forces snusoal voltages. Seres tunng s the outset for more ecate hgh effcency amplfer prncples nclung class D F an especally class E. Therefore the basc equvalent crcut setup for scussng seres tunng whch s shown n Fg. resembles the setups for class E scussons. But unlke the common approach n lterature [] [3] no attempts are mae to fn close or partly close form solutons to ths crcut problem. Instea we shall resort on numercal technques an emonstrate that the corresponng solutons have wer practcal mplcatons snce only a mnmum number of constrants are requre. Furthermore t becomes possble to ncorporate transstor seres losses n the effcency estmatons whch sgnfcantly mproves the founaton for esgn ecsons. In aton to the seres tunng whch causes the snusoal loa current of ampltue I the major assumptons behn the equvalent crcut s that the transstor s rven so har that s operaton may be escrbe by a swtch. The swtch s close when the gate to source voltage s above threshol an open when t s below. The transstor nput etals are not explctly shown n the equvalent crcut but the nput rvng s specfe through the swtch openng angle θ a. When the swtch s open the total ran current whch s the snusoal loa current plus the c supply current I charges or echarges the transstor output capactance C o. When the swtch closes an short-crcuts the capactor the ran source voltage s fxe to the voltage source V ON whch accounts for the ynamc on-voltage of the transstor. Before scussng the setup an soluton of equatons for the output loang t shoul be realze that the crcut nclues two power loss mechansms. The transstor seres loss P tr seres Ic VON = () an the swtchng loss that accompanes the closng of the swtch. If V csw enotes the capactor voltage pror to swtchng f s the operatng frequency an B c s the corresponng susceptance of the output capactor the swtchng loss becomes P = CV f = BV 4 π () tr swtch csw c csw The power balance n the crcut s gven by Ploa = Pbattery Ptr seres Ptr swtch = I V I V P DD ON tr swtch where V DD s the supply voltage. If we organze the crcut to avo swtchng losses whch s a part of the class E concept the ran effcency becomes the hghest possble (3) Ths work was conucte as the ntal step towars the esgn of a power amplfer for a p-ban ce sounng raar ESA (Europeans Space Agency ) contract no 9397/5/NL/JA. [] Fgure. Smplfe equvalent crcut for transstor loang n a seres tune amplfer /8/$5. 8 IEEE 73

3 η opt loa P = ON DD = P P = V V. (4) battery tr swtch Wthout seres losses ether ths result agrees wth the eal class E promse of % effcency. In realstc amplfers the effcency n (4) represents an upper boun for any esgn. III. MODEL SETUP Fg. summarzes the assume wave-shapes of the ran current an the ran voltage V I () t = I + I cos( ϕ θ ) ϕ = ω t (5) ϕ V + I ( ϕ) ϕ < ϕ < θ ON a Bc ( ϕ) = I I = ϕ+ [ sn( ϕ θ) + sn θ] Bc Bc VON θa < ϕ < π. The nstance where the swtch opens s taken as tme orgn above so the swtch open pero of length θ a starts from zero. Wthn ths pero the voltage gets an optmum f an when the current turns negatve at phase θ m. We shall avo lengthy scussons of pathologcal cases an assume that ths happens as sketche n Fgure. so the voltage peaks at phase m cos ( I / I). (6) θ = θ + π (7) The voltage expresson (6) proves a bass for formulatng three basc constrants that always must be met. Frst the mean voltage must equal the battery voltage V( ϕ) ϕ = V( I I θ θa Bc VON) = VDD. (8) Takng the funamental frequency ran voltage an current components as phasors the loang conton s expresse r r r V = Z I = ( R + jx ) I (9) L L L where R L an X L are the real an the magnary part of the loa mpeance. Worke out n etals ths gves two loang constrants for n-phase an quarature components respectvely π π V( ϕ)cos ϕϕ = VI( I I θ θa Bc) () [ θ θ ] = I RL cos + XLsn V( ϕ)sn ϕϕ = V Q( I I θ θa Bc) () [ snθ cosθ ] = I R X L L Expresson etals are summarze below n the appenx Fgure. Dran current an voltage waveshapes n the swtchng equvalent crcut of Fgure. By equatons (8) () an () we have establshe three relatons among a set of nne varables an parameters { I I R X B V V } θ θ. () a L L c ON DD Clearly some of the components are fxe constants n a esgn task but stll more constrants are requre to solve the transstor loang problem. Relevant canates are Output Power : P = R I / (3) out L Effcency : η = R I / I V (4) L DD Maxmum Current : I = I + I (5) max Maxmum Voltage : V = V ( θ ) (6) max m No Swtchng Loss : V = V ( θ ) V =. (7) csw a ON The last conton s a prerequste for class E operaton but n lterature t s commonly followe by the atonal requrement that the current through the swtch must be zero at the swtchng nstant. Ths contons s referre to as optmal swtchng although t s har to follow the ratonale behn the term as the conton whch reas " Optmal " Swtchng : I ( θ ) = I + I cos( θ θ ) = a a (8) oes not nfluence the effcency of the amplfer. There are however two mplcatons of (8). The nce property s that t becomes a remey n settng up close form esgn equatons for class E amplfers. The ba property s that t unnecessarly worsens the well known raw-back of class E esgns [3] p.7 namely that the maxmum ran voltage approaches or excees the transstor voltage ratngs f we ten to operate them towars ther maxmum power capabltes. Snce the strategy n the present work s to solve the esgn equatons numercally there are no nees for conserng the conton n (8) here. IV. NUMERICAL SOLUTION To emonstrate a numercal soluton to the amplfer loang problem we conser the task of esgnng a 5W 435MHz narrowban amplfer usng the MRF373A LDMOS transstor from Freescale wth 8V supply voltage. The transstor may be use to 75W an t has a ran voltage ratng of /8/$5. 8 IEEE 74

4 Fgure 3. Part (a) an (b) hol the numercal solutons to the loang problem (8) () () subject to output power (3) an maxmum voltage (6) constrants. The amplfer effcences an swtche voltages n part (c) are post-calculate by (4) an the frst equaton n (7). 7V. To stay safe we shall lmt the maxmum ran voltage to 8% of the ratng. The transstor has an output capactance of 49pF an the ynamc on-voltage s set to 4.5V. Thereby three of the quanttes n () are ntally fxe B =.34 S V = 4.5 V V = 8 V. (9) c ON DD In the soluton process the swtch openng angle θ a s swept as an nepenent varable an the numercal process must solve for the remanng fve unknowns I I θ R L an X L. Beses the three basc contons (8) () an () the output power (3) an the maxmum voltage (6) requrements are enforce usng P = 5 W V = 56 V. () out max The actual soluton process s unertaken by the fsolve equaton solver routne from the optmzaton toolbox n MATLAB an we get the results that are shown n Fg.3. The horzontal axes span the whole range of openng angles θ a where the solutons are meanngful real-value quanttes. The unknowns that are etermne by the numercal process are shown n Fg.3 (a) an (b). On bass of the solutons the resultant effcency an the voltages V csw whch are shortcrcute by the swtch are calculate an shown n Fg.3(c). It s obvous that there s a smultaneous maxmum n effcency an mnmum n V csw. For a seres-tune amplfer wth maxmum effcency the soluton curves prove the followng esgn ata θ a max. η = 47 η = 73.6% R = 3.7 Ω X = 3.35 Ω. L L () The result shows that the penalty for stayng wth a safe maxmum ran voltage s a reucton n effcency from the upper boun n (4) whch gves η = 4.5/8 =.84 84%. () opt V. AMPLIFIER DESIGN SUMMARY The schematc of the fnal amplfer s shown n Fg.4 by the corresponng smulaton setup for ADS (Avance Desgn System from Aglent). The transstor moel to be employe comes from the esgn lbrary that may be ownloae from Freescale. Seres tunng s enforce n the crcut by seres connectng nuctors n the sgnal path rectly to the transstor termnals leas (short broa transmsson lnes). At the output se of the transstor L o C o an C o transform the external loa to the ran loa mpeance n (). Before the nput matchng crcut was establshe the correctly loae crcut was rven by a snusoal current whch was ajuste to prove the esre output power n smulaton. By ths step we mplctly ncorporate the openng angle n the practcal Fgure 4. ADS smulaton setup for a narrowban power amplfer that s loae for maxmum effcency accorng to (). Inset shows actual crcut /8/$5. 8 IEEE 75

5 Fgure 5. Measure (heavy lnes) an smulate (thn lnes) frequency responses of the crcuts n Fg.4. esgn process. It s one by recorng an subsequently by power matchng to the corresponng large sgnal nput mpeance through the L C an C crcut. The frequency characterstcs for output power gan an nput matchng acheve by ths approach are summarze by Fg.5 an Table I. TABLE I. AMPLIFIER PERFORMANCE P out [W] Eff. % I DD [A] Measure Smulate (ADS) Swtch Moel VI. DISCUSSIONS AND CONCLUSIONS Conserng the three levels of methos an result that are summarze by Fg.5 an TABLE they are n remarkably goo agreement compare to RF power amplfers smulaton stanars so the seres tunng approach whch was presente n ths paper has proven useful n practce. There s room for mprovement n the seres tune swtch moel of the transstor loang. Ths becomes clear f we conser the wave-shapes of the smulate ran voltages an currents n Fg.6. It s seen that the assumpton of a constant ynamc on-voltage s reasonable. The swtchng n the smulaton however s not nstant an the ynamc on-voltage s not completely constant. It shoul be nvestgate how we may compensate for a fnte swtchng pero an make a more refne on-voltage escrpton lke t was one n an earler Fgure 6. Smulate current an voltage wave-shapes (thn lnes) compare wth the smlar swtch moel wave-shapes (heavy lnes). The snusoal currents can harly be stngushe. successful attempt to cope wth bpolar power-amplfes n a smlar way [4]. Regarng smulatons where the supple moel seems to overestmate realtes t shoul be kept n mn that the prevalng stuaton a few years ago was that practcally no RF power amplfer esgner truste any form of smulatons. It s emonstrate above that the qualty of the smulaton moels have reache a level where they successfully may contrbute to the esgn process here by translatng smplfe theoretcal esgn crtera nto useful crcut matchng parameters. APPENDIX VOLTAGE COMPONENTS IN (8)() AND () V ( I I θ θ B V ) = V + a c ON ON I θ a + I a + a π Bc [ { θ sn θ cos θ cos( θ θ )}] (3) V I( I I θ θa Bc) = I [ ( cos sn I θa + θa θa ) + (4) πbc 4 { θ snθ cos( θ θ ) + cosθ + 4snθ sn θ }] a a a a V Q( I I θ θa Bc) = I [ ( sn cos I θa θa θa) + (5) πbc 4 {θ cosθ sn( θ θ ) + 3snθ 4 snθ cos θ }]. a a a a REFERENCES [] V. Krozer C.C.Hernanez J.L.Vazques Roy J.Vkjaer J.Dall Development of an Arborne Ice Sounng Raar Front-En Proc.4th Europ.Raar Conf. pp. 6-8 Munc Germany Oct.7. [] F.H.Raab Iealze operaton of the Class-E tune power Amplfer IEEE Trans.Crcuts Syst. vol CAS-4 pp Dec.977. [3] S.C.Crpps RF Power Amplfers for Wreless Communcatons Boston:Artech House 999 pp [4] J.Vkjaer A Descrbng Functon Approach to Bpolar RF-Power Amplfer Smulaton IEEE Trans.Crcuts Syst. vol CAS-8 no.8 pp Aug /8/$5. 8 IEEE 76