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TU4C 3LOW VOLTAGE ELECTRONICS FOR PORTABLE WIRELESS APPLICATIONS: ANINDUSTRIAL PERSPECTIVE

Mike Gol ioRockwel l -Col ns400 Col l ins Road NECedar Rapids , IA 52498ABSTRACT

This paper provides an overview of lowvoltage/low powe r electronics developm entsand issues for portable RF wirelessapplications. Issue s discu ssed include:semiconductor materials, devicetechnologies, device modeling, circuitapproaches, and system architectures.Finally, issues that impact the lower limitsachievable in DC power and voltagereductions are discusse d.

INTRODUCTIONThe basic components required forwireless applications are undergoing arevolutionary change in terms of DC powerconsumption. Figure 1shows the historicaldevelopment of commercial GPS DC powerrequiremen ts as a function of the year ofintroduction of the product. Overapproxim ately the sam e period of time, thesupply voltage for handheld cellulartelephone products has seen a similarleciine from o olts.

O C P O W E R R E Q U I R E M E N T S F O R G P S H A N D H E L DP R O D U C T S B Y Y E A R

15 -- S U P P L Y POWER ( m W )... -- U P P L Y V O L T A G E V )

g 5 --2

0 0I 9 9 0 1991 1992 1983 1994 1985 1886 I987 I 8 8 8

Y E A R r I N T R O D U C T I O NFigure 1. DC power consumption batteryvoltage for hand heldGPS units.

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There are several drivers for this powerconsum ption revolution, but the main driversare a desire for longer battery lifetimesaccompanied by a simultaneous desire forsmaller, lighter batteries. At present, thesingle largest volume and weightcomponent in most portable wirelessproducts is the battery, yet the consumerfinds the available talk and s tandby times forbatteries to b e undesirably short. To achievedesired improvements requires that thepower consumption of the individualcom ponen ts, and thus, the overall hand heldunit be reduced.Although the reductions in supply voltageand power consumption of wirelessproducts has been dramatic, furtherreductions will be increasingly difficult toachieve. The constraints implied by the lowvoltage imperative need to be examined asthey apply to eve ry aspect of RF/microwavecomponen development

MATERIALS TECHNOLOGY ISSUESLow voltage/low powe r operation requiresimproved efficiency, low parasitic resistanceand precise control of on-voltage. Each ofthese req uiremen ts is im proved through theuse of appropriate materials and materialstructures. Imp rovem ents in efficiency andreductions in parasitic resistance can be

achieved by using materials that exhibitincrea sed carrier mobility and velocity. Therequ irement for precise control of the on-voltage clearly favors a bipolar (vs. a FET)structure. When other considerationsindicate that a FE T structure is appropriate,

0-7803-4471-5/98/ 10.00 998 IEEE 998 IEEE MTT S Digest

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however , hete ost uct u e buffer layersimprove the on-voltage control of FETs.From a purely technical point of view, theabove discussion argues for the use of I l l-Vheterost uct u e devices. But thesearguments ignore the overwhelmingimportance of cost and schedule to thedevelopment of many commercial wirelessproducts. It is a dramatically superiorprocess maturity and low cost that hasallowed Silicon devices to continue toprovide competitive performance fastdevelopment cycle times and inexpensiveparts for RF wireless applications.There is no short cut for the developmentof experience and maturity with advancedIll-V material structures. As dema nd fordiminishing power consum ption RF productscontinues to grow, h owev er, experience willbe gained and an increasing nu mbe r of I l l-Vheterostructure devices will gain entry intowireless products.

DEVICE TECHNOLO GY ISSUESDevice technology decisions affect manyof the same performance metrics that areaffected by material decisions. For

example, efficiency, parasitic resistance andon-voltage control are all affected by thecho ice of device. Additional dev iceconsiderations include linearity, breakdownvoltage/power density tradeoffs and singlevs. dual polarity sup ply requirements.Table 1presents a listing of d evice typesthat exhibit particular advantage ordisadvan tage with respect to the importantcharacteristics for low voltage wirelessapplications. As is readily seen from thetable, no particular device type excels in allareas and all device types exhibit at leastsome significant disadvantage s.Again, as in the case of materialconsidera tions, all of the technicalconsiderations must b e weighed against the

demanding cost and schedule requirementsfor wireless product developm ent.TABLE 1. Important performance

characteristics for low voltage wirelessPERFORMANCEMETRICHigh EfficiencyLow ParasiticResistanceOn-VoltageControlLinearityHigh Breakdownwith High PowerDensitySingle PolaritySupply

ipplications.DEVICESWITHADVANTAGEHEMTsHEMTsBJTs HBTsepi-MESFETsand HEMTsHBTs, HEMTs

doped chnnlMESFETsHBTs, BJTsMOSFETs

DEVICESADVANTAGEBJTsBJTsMOSFETsHEMTsMESFETsMOSFETs

WITH DIS-

conventionalMESFETsMESFETsHEMTs

MODEL ING ISSUESAlthough device m odels for low voltage RFapplications are required to predict thesame performance figures (gain, saturatedpower, harmonic distortion, efficiency, etc.)as those required for other applications, thelow voItage/low pow er operation does p lace

additional constraints on mo deling activities.In particular, operation at low current leadsto the ne ed for mo re accurate subthreshold,more realistic breakdown and improvedtemperature models.Low current biasing m eans that the deviceis operated in the subthreshold regionwhere many mod els fail. Harm onic contentprediction of devices operated in this regionexhibit particularly poor correlation tomea sured data.When devices are operated nea r the onsetof breakdown, leakage currents and devicenoise increase. These increase s can havesignificant effects on PA design, but are notwell predicted by DC breakdo wn models.

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Device current levels can vary b y an orderof magnitude at low current bias whentypical tempe ra u re variations areconsidered. Tem perature mod els that areadequate for higher current bias levels, mayfail to predict low current variationsadequately.

CIRCUIT TECHNOLO GY ISSUESIn general, RF circuit performance is notimproved by the reduction of voltage orcurrent. A typical low voltage, low currentamplifier exhibits a significant reduction ingain and linearity as either supply voltage orcurrent is reduced. In addition, lowvoltage/low power circuit designers face

difficult issues related to decreasingimpedance levels of PA devices, limits todevice stacking and difficult power-

Figure 2. Illustration of the low impedanceproblem caused by l ow voltage operation.Low voltage devices require higher peakcurrents to achieve equivalent Pout. This isaccomplished, in practice, by increasing thedevice periphery. Figure 2illustrates theproblems encountered in circuit design ifdevice sizes are no t scaled up when voltageis decreased. Larger devices, however,exhibit lower impedance that must bematched. Thus, low voltage power amp lifier

parts have high transformation ratios.Typical 1- 4 Watt power amplifier parts forcommercial cellular phones exhibit outputimpedance of less than a few o h m s . High Q

matching elements provide some advantagein achieving required transformations butcome at high cost and redu ced integration.SYSTEM D ESIGN ISSUES

Communication systems continue torequire greater bit rates and bandwidthswhich leads to greater required linearity andhigher frequency opera tion. Similarly,greater functionality requirements leads togrea ter circuit complexity. Each of thesetrends makes reductions in powerconsumption more difficult and places evenmore emphasis on the importance of lowvoltage/low power consump tion design.Other system trends, such as themovement toward digital modulation, canease the low voltage/low power designissues. Because digitally mod ulatedproducts are pulsed (vs. CW) the powerconsumption of the RF componentsrepresents a smaller percentage of theoverall product power requirements.Some proposed system innovations wouldhave a dramatic effect on the war on powerconsumption changing the requirements(and the technologies of choice) entirely.Micro-cell telephone systems, for example,would dramatically reduce the batteryrequ emen s for cellular phone systems.Power consumption of the PA for such asystem becomes inconsequential. Incontrast, direct-to-satellite systems presentsignificant challenges due to the highinstantaneo us output powe r requirem ents ofthe PA and low minimum noise figure of thefront end LNA.These system architecture issues are likelyto have greater impact on the ultimatebattery reduction limits than any of thematerial, device or circuit issues discussed.

LIMITS TO VOL TAGE REDUCTION

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Since the reduction of battery size andweight is the goal of low voltage/low powerelectronics strategies, it is important toexamine some fundamental issues relatedto ba tteries and the RF circuits they power.Issues that limit DC power requirementsfor RF circuits are fundamentally differentthan those that limit DC power requirementsfor associated digital circuits. D igital circuitryis required to store and analyze informationthat is encoded in a binary manner. Thiscan be accomplished theoretically by thepresence or absence of a small charge(single electron). Although practicalconsiderations make a single electronmem ory improbable, and movem ent of evenone electron into and out of storage still

requires energy, it is clear that binary datacan be manipulated with extremely smallamounts of energy. The systemarchitecture does not impose arbitrarypower requirements on the strength of thedigital signal.In contrast, the RF portion of radios isrequired to transmit and/or receive signalsover a distance. Because power is lost inthe radiation process, RF circuits must beable to handle power levels that aredetermined by the propagation media andtransmitter-to-receiver separation. Thus, forRF circuitry, a reduction in voltage must beaccompanied by increased efficiency and/orincreased current. Since many portableunits are already operating at efficiencylevels near theoretical limits, voltagereductions nearly always involve increasedcurrent requirements.As battery current requirements areincreased, the internal resistance of thebattery becomes a limiting factor in the totalpower the battery is capable of delivering.Figure 3 presents the maximum powercapability for a battery as a function ofnominal voltage and internal resistance.Although internal resistance is a function of

the chemistry, number and size of thebattery cells, the values used in Figure 3 aretypical for nickel-metal-hydride batteries incommon use for cell phones today and thetrends plotted in the figure will hold for allbatteries. It is clear from the figure that asbattery voltages are reduced below -3 volts,the ma ximum power available from them isreduced to levels on the order of thatrequired from a portable cellular PA alone.When efficiency and other circuitrequirements are considered, battery power?vels are inadequate.

BATTERY PEAK PO WE R FO R VARIO US NO MINAL VO L TAG E AN0INTERNAL RESISTA NCE VAL UES

I O 0 0

i , 4 5NOMINAL VOLTLOE (V)

Figure 3. Maximum battery power definedas nominal voltage x short c ircuit current) asa function of nominal battery voltage plotted

for typical in ternal resistance values.SUMMARY

Reductions in voltage and powerconsumption is important to thedevelopment of portable wireless productsthat are smaller, lighter and require lessbattery maintenance. Reductions can beachieved by careful consideration andimprove men t in the cho ice of semiconductormaterial, device type, models, and circuittopology. Further reductions may come withthe emergence of innovative systemarchitectures. The ultimate limit todecreasing battery voltage, however, will bedetermined by transmitter powerrequirements and achievable internalresistance of batteries.

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