A High Efficiency Doherty Amplifier With DPD for WiMAX

18 High Frequency Electronics

High Frequency Design

DOHERTY AMPLIFIER

A High Efficiency DohertyAmplifier with DigitalPredistortion for WiMAX

By Simon Wood and Ray Pengelly, Cree, Inc.,and Jim Crescenzi, Central Coast Microwave Design, LLC

The Doherty ampli-fier architecture isa well known tech-nique offering the poten-tial to improve transmit-ter efficiency especiallyfor signal protocols thatexhibit high peak-to-

average power ratios. Although the Dohertyapproach has significant efficiency advan-tages, it generally must be augmented withsome form of correction or linearity enhance-ment in the full transmitter design. Wirelessinfrastructure applications have demandinglinearity and spectral mask specifications. Thelatest WiMAX standards present a particularchallenge with their combination of very highpeak-to-average power ratios, 10 MHz orwider channel bandwidths and high linearitystandards. This article demonstrates a 2.5-2.7GHz Doherty amplifier that achieves, whenaugmented with digital predistortion, 8 wattsof WiMAX average output power at greaterthan 47% efficiency, while satisfying demand-ing spectral mask specifications.

The appeal of the Doherty amplifier config-uration is that it involves familiar core amplifi-er designs connected in a manner that main-tains high efficiency over an extended inputsignal range. Doherty development has beenenergized by the latest generations of transis-tors and is well represented in recent literature[1-5]. Applying the Doherty approach to mod-ern WiMAX signals can present unique chal-lenges. In particular, WiMAX signals combinetwo challenging features: wide video bandwidthof 10 MHz or greater, and large peak-to-averageratios of 10 dB or greater. Additionally, the lat-est WiMAX standards require that the spectral

emissions mask (SEM) at close offsets (1.5MHz) be at least 45 dB. A typical amplifier(without correction) will exceed this level by 15to 20 dB, resulting in a significant correctiondemand for the digital predistortion circuitry.

The technologies applicable to this chal-lenge have improved in three areas:

Improved GaN HEMT transistors withreduced capacitances and trapping effects

Improved digital predistortion subsys-tems and software

Doherty circuit design refinements thatreduce memory effects, increase gain andincrease bandwidth.

This article focuses on the optimization ofDoherty amplifier circuitry for WiMAX appli-cations.

Amplifier Design ApproachDoherty amplifiers can be configured with

2-, 3- or N-way combinations [8, 9, 10]. Themost commonly used Doherty architecture

This article describes aWiMAX power amplifier ,

which achieves high performance using the

latest device technologiesand design techniques

Figure 1 A basic 2-way Doherty amplifierconfiguration.

From December 2008 High Frequency ElectronicsCopyright 2008 Summit Technical Media, LLC



DOHERTY AMPLIFIER

involves two amplifiers, as shown inFigure 1. Adding additional branchescan extend the power range overwhich high efficiency is maintained.However, the 2-amplifier approach isoften preferred due to cost considera-tions (i.e., using fewer devices).Fortunately, there are multiple vari-ables besides the number of branchesthat can be adjusted to optimize per-formance.

The Doherty block diagram (Fig.1) is notable for its elegance and sim-plicity. What may not be apparent isthat details of the design can resultin large differences in performance.Operation is strongly influenced bythe coupling factor of the inputdivider/coupler and by the biasing ofthe carrier and peaking amplifierstages. The turn-on of the peakingamplifier is dependent on both inputpower level and gate bias voltage,which in turn sets the low power effi-ciency and peak power capability ofthe configuration. The peakingamplifier allows the Doherty amplifi-er to respond to the high input levelsof short duration by amplifying thesignal peaks and dynamically chang-ing the loading on the main amplifier.

The specific class of operation(Class A/B, Class B, Class C, Class F,inverse Class F, etc.) of each amplifi-er is also important. The two basicconsiderations for each stage are thebias conditions (biased on, or pinchedoff, and to what degree) and the fun-damental and harmonic impedanceterminations presented to the tran-sistors. Previous papers on Dohertydesign have advocated particularclasses of operation for the carrierand peaking amplifiers [6]. Theapproach described in this article wasto start with a waveform-engineereddesign, in which the stand-alonestage (i.e., outside the Doherty envi-ronment) was optimized in a 50-ohmsystem. Then the biases were adjust-ed for carrier and peaking functionsand the stages were inserted into theDoherty configuration. Waveformengineering [7] was used to maximize

amplifier efficiency over a powerrange from 10 dB backoff to peak(saturated) output level.

Series matching elements of eachstage were modified to reduce electri-cal length from the transistor to thecombining node, and harmonic termi-nations were adjusted to optimize thedesigns under Doherty operation.

It has been our experience thatcircuit modifications required afterinsertion into the Doherty configura-tion have been minimal for the inputmatching circuits, but substantial forthe output circuits. Inserting the pre-ferred stage designs as carrier andpeaking stages (optimized for stand-alone operation in a 50-ohm environ-ment), without further optimization,results in considerable performancesacrifice. Of course, this approach ispredicated on the availability of accu-rate non-linear transistor models andCAD tools such as the MicrowaveOffice non-linear (harmonic balance)simulator.

The potential for correction usingdigital predistortion is improved bytwo additional features: (1) maximiz-ing the RF bandwidth of the Dohertyamplifier, and (2) increasing the videobandwidth by minimizing the drainbias feed inductance.

Sensitivity to Input DividerCoupling Factor and Stage Bias

The input power divider for thedesign is a variation of a 2-sectionWilkinson divider (Fig. 2), with thesecond isolation resistor omitted

because of layout constraints. Thisdivider maintains a reasonable levelof isolation, while being more com-pact than if it were extended toaccommodate a second resistor. Thedelay line is added to provide thedesired quadrature relationship ofthe two outputs. The coupling isdetermined by the relative linewidths (impedances) of the twobranches. The divider block could befurther compacted by substituting acustom coupled-line design with over-coupling (possibly requiring a multi-layer printed circuit board (PCB), ora custom component). The approachemployed here is realizable with asingle layer PCB, and it allows con-siderable flexibility in setting thecoupling ratio.

The Doherty amplifier perfor-mance was simulated for variousinput couplers and peaking amplifierbias levels. All simulations are at 2.6GHz with a drain supply voltage of+28 volts DC. The devices are CreeCGH27030F GaN HEMT transistors.

The plot of Figure 3 shows theresult of simulations for two Dohertyamplifiers that are identical exceptfor the input couplers. The dark traceis for equal power to both the carrierand peaking amplifiers. The lighttrace is for an unequal power divider(approximately 1.8/4.7 dB coupling)in which approximately 3 dB more

Figure 2 Schematic of an unequalpower divider for Doherty applica-tion.

Figure 3 Simulated gain and effi-ciency vs. CW output power for twoinput couplers. Dark trace is for 3 dB(equal) coupling. Light trace is forapproximately 1.8/4.7 dB coupling(more power to carrier amplifierversus peaking amplifier).



DOHERTY AMPLIFIER

power is directed to the carrieramplifier than to the peaking ampli-fier. The unequal coupling case sup-ports significantly higher efficiencyin the backoff power range (a 4.3%efficiency improvement at 10 dBbelow the peak power is predicted aswell as more uniform gain as a func-tion of output power). The Dohertyamplifier used for this simulation isnot strictly an efficiency-optimizedamplifierrather, it was optimizedwith equal emphasis on peak power,linearity and efficiency.

The simulations of Figure 4 arefor the unequal input divider, withthe gate bias voltage to the peakingamplifier varied from 3.0 to 5.0volts. The more negative the gatevoltage, the later the peaking ampli-fier turns on as power is increased.These simulations suggest thatincreasing the gate voltage (morenegative) improves efficiency in back-off, progressively introduces moreAM/AM, butsurprisinglyalso candecrease the AM/PM. The AM/PM ispredicted to be very low at the opti-mum gate bias. These simulationsare for CW signals, and the resultswill differ for dynamic WiMAX sig-nals. An improvement in efficiencyand linearity is observed withincreasing gate voltage for the hard-ware under WiMAX drive signals. Itis also worth noting that these plots(Fig. 4) suggest that it is fruitful to

examine the effectiveness of digitalpredistortion for various peakingamplifier gate bias levels.

Amplifier Schematic and Drain Bias Design

The 2.5-2.7 GHz Doherty amplifi-er schematic is shown in Figure 5.The input and output matching cir-cuits of the carrier and peakingamplifier stages use conventionalmatching circuitry. The open circuitstubs on the input and output areplaced at critical junctions wherethey have the desired impact onsource and load impedances. The out-put circuit includes two drain biaslines per device, in order to reducevideo impedance. Additionally, thelength of these bias lines is chosen toassist in positioning the load present-ed to the devices at second harmonicfrequencies. The impedance at thesecond harmonics is approximately ashort circuit (as a stand-alone sub-circuit in a 50-ohm system), althoughthe precise angle (bias line length) isadjusted during simulation of the fullDoherty amplifier.

The motivation for limiting theinductance of the drain feed lines isto reduce memory effects, which canbe exacerbated by bias circuits. Theoutput circuit (including bias ele-

ments) impedance at video frequen-cies is also simulated. For this circuit,the magnitude of the impedance pre-sented to the drain increases mono-tonically from a few milliohms at 100kHz to 200 milliohms at 10 MHz, andapproximately 1 ohm at 50 MHz(dominated by the effective induc-tance of the bias feeds).

The typical 3rd order two toneintermodulation products as a func-tion of output power for the Dohertyamplifier design are also examinedas these relate directly to the errorvector magnitude (EVM) and relativeconstellation error (RCE) of theamplifier when driven by WiMAXsignals. The simulations of Figure 6demonstrate a typical back-off hill

Figure 4 Simulated gain and effi-ciency versus CW output power, fordifferent gate bias voltages for thepeaking amplifier (1.8/4.7 dB inputcoupler). The dark trace is for Vgs =5 volts. Voltage steps are 0.5 V.

Figure 5 Doherty amplifier schematic.

Figure 6 Simulated 2-tone inter-modulation products versus outputpower level (tone separation of 1MHz).



DOHERTY AMPLIFIER

in intermodulation distortion atabout +32 dBm average output. Notethat upper and lower IM products (atfrequencies 2F2F1 and 2F1F2) arenearly equal except in the sweetspot power range, for this case of a 1MHz difference between F1 and F2.The imbalance between the upperand lower IM products is a functionof several parameters, including theimpedances presented to the devicesat fundamental, harmonic, sum anddifference frequencies. The bias cir-cuit impacts the impedances at thedifference (video) frequencies. It isillustrative to simulate the 2-tone IMrejection at the hill power (+32 dBm)versus frequency of separationbetween the tones. The inequality ofIM products versus tone separationdiffers depending on power level, andthe plot of Figure 7 is for a +32 dBm

output level (illustrative of the hillregion). This simulation shows thatthe imbalance of 3rd and 5th orderintermodulation tones (at +32 dBmoutput) is less than 0.5 dB for toneseparations to 18 MHz. This is signif-icant for applications involving 10MHz wide WiMAX signals.

The output matching structures ofthe carrier and peaking amplifierswere adjusted while examining boththe 2-tone IM rejection and the singletone gain and efficiency versus out-put power. Critical circuit elementsincluded the matching elements atthe fundamental frequency, as well asthe bias lines for harmonic matching.All of these elements were adjustedafter integration into the Dohertyconfiguration (relative to the valuesin a stand alone 50-ohm environ-ment). The width (impedance) of the

quarter wave line connecting the car-rier and peaking amplifier outputs isa sensitive parameter, as are theopen circuit stubs near this line. Theoutput transformer was found toexhibit relatively low sensitivity.

The Doherty amplifier is shownFigure 8. The devices are CreeCGH27030F GaN HEMT transistors.The printed circuit board is RogersRO4350B material with 0.5 mmthickness. The amplifier constructionis conventional, using commonlyavailable components and materials.Only a few of the tuning pads wereused in the prototype alignment, asthe transistor modeling and simula-tions proved to be accurate.

Time Domain Simulations ofDoherty Operation

The Doherty amplifier wasdesigned from a frequency domainperspective, modifying elements toimprove efficiency, 2-tone linearity,and peak power. This processinvolved simulations commonly usedby microwave engineers, includingCW gain and efficiency versus outputpower, and two tone IM distortionversus output power. However it isalso informative to examine theresulting amplifier from the timedomain perspective. The plots ofFigures 9 and 10 are of currents atcritical nodes in the circuit (F = 2.6GHz, Vds = +28V, Idq = 170 mA for thecarrier amplifier and Vgs = 4.0 voltsfor the peaking amplifier).

The simulations resulting inFigure 9 show the drain currents atthe two transistors, for three outputpower levels of +36 dBm, +42 dBmand +48 dBm. Recall that the drive tothe peaking amplifier is offset (rela-tive to that to the carrier amplifier)by 90 degrees due to the delay line inthe Doherty circuitry. The peak tran-sistor currents contribute stronglyonly at the highest output power. It isinteresting to note the relatively non-sinusoidal shape of the individualwaveforms. The simulated currentsare at the die drain pads, and the

Figure 7 Simulated 2-tone IMproducts imbalance versus fre-quency separation of tones (Pout =+32 dBm).

Figure 8 The 2.5-2.7 GHz Dohertyamplifier with two 30W GaN HEMTsPCB size is 2.5 4.0 inches.

Figure 9 Simulation of drain cur-rents at the carrier (black) andpeaking (red) transistors with sin-gle-tone Pout = 36, 42 and 48 dBm.

Figure 10 Simulation of currents atDoherty output combiner node, sin-gle-tone Pout = 48 dBm; black = out-put arm, red = carrier arm, blue =peaking arm.

December 2008 25

intrinsic capacitances of thesedevices are clearly important. Thesewaveforms suggest some sort of ClassJ operation as defined by Cripps [7].These current simulations show thatthe higher order (3rd, 4th, etc.) har-monic terminations on the amplifieroutputs are non-ideal for efficiencyoptimization since the waveforms arenot squared off, but are rather softin their transitions. This is due to theemphasis on (uncorrected) linearityduring circuit optimization.

The currents shown in Figure 10are those flowing into the Dohertycombining node for the main andpeaking amplifiers, and exiting thenode for the output arm at +48 dBmoutput power. The output arm cur-rent equals the sum of the main andpeaking arm currents. The currentsadd constructively to produce a near-

ly sinusoidal waveform. It is interest-ing to observe that the role of thepeaking amplifier appears to be morea matter of complex waveform syn-thesis, as opposed to the more ele-mentary view involving Class C likecurrent pulses.

Measured Amplifier PerformanceThe design was optimized for lin-

earity and efficiency over the 2.5-2.7GHz range to support a number ofpossible WiMAX frequencies. Thesmall signal bandwidth exceeds thislimited range as shown in Figure 11.Gain is nominally 13 dB and inputreturn loss is typically 14 dB.

The efficiency and linearity of thefinal amplifier were evaluated using a10 MHz wide WiMAX signal compli-ant to 802.16e-2005. The key linearityparameters for this modulationscheme are error vector magnitude(EVM), as shown in Figure 12, andspectral emissions mask (SEM). Thespectral mask was measured using a10 MHz wide integration bandwidthin the adjacent channel, as shown inFigure 13, at 2.5, 2.6 and 2.7 GHz.The performance is optimum at 2.6GHz with trade-offs in linearity at theband edges. Efficiency versus outputpower is flat over 2.5 to 2.6 GHz drop-ping by approximately 5% at 2.7 GHz.It is clear that single frequencydesigns (optimized at either 2.5 or 2.7

GHz, not 2.6 GHz) would offer thepotential for incrementally higherefficiency.

Description of WiMAX Protocoland Digital Predistorter Design

The digital predistortion (DPD)test-bed used for correction of thisDoherty amplifier was provided byOptichron. The test-bed used is thebase-band development board withthe OP4400 chipset. A block diagramof the system is shown in Figure 14.The WiMAX signal, which is a fullycompliant 802.16e-2005 signal, isused as a stimulus to the test-bed.This signal is generated using Rohdeand Schwarz AMIQ equipment andapplied to the test-bed in digital I andQ format. When setting up the test-bed the signal is first passed throughthe OP4400 chipset with no correc-tion applied. This allows the perfor-mance of the uncorrected amplifier tobe observed. The digital signal is fedto a Texas Instruments TSW3003upconverter. The up-converted signallevel at the output of the radio boardis approximately 10 dBm and there-fore needs to be amplified to be ableto drive the Doherty amplifier to therequired power levels. It should benoted that this amplifier is locatedwithin the predistortion loop (labeledin Figure 14 as the Driving Amp),which means that its performance,

Figure 11 Measured small-signalgain and input and output returnloss versus frequency (output returnloss is dashed line).

Figure 12 Measured Doherty amplifier EVM and effi-ciency vs. WiMAX Pout (without predistortion).

Figure 13 Measured Doherty amplifier ACPR and effi-ciency vs. WiMAX Pout (without predistortion).



DOHERTY AMPLIFIER

albeit extremely linear, is also beingcorrected by the test-bed. The result-ing modulated signal is then appliedto the amplifier under test which cre-ates some level of unwanted distor-tion. The output signal is sampledand downconverted using a commer-cially available Mini Circuits mixer.The IF signal is then processed by anAnalog Devices analog to digital con-verter (AD9233). The resulting digi-tal representation of the output sig-nal is then processed by theOptichron OP4400 chipset. At this

point a digitally predistorted signal isthen reapplied to the amplifier undertest in real time. The option existswithin the test-beds software con-trols for the user to decide whether touse a simple memoryless algorithmor whether to choose a more complexalgorithm which would help correctamplifiers with strong memoryeffects.

It was found that, at low averagepower levels, the memoryless algo-rithm was sufficient to correct thenon-linearities of the Doherty ampli-

fier down to the system noise floorassociated with the 50 dB gain driveramplifier. This was somewhat com-pensated with the use of attenuationat the output of that amplifier. At themaximum output power both algo-rithms were needed due to the largeamounts of clipping occurring at thepeaks of the WiMAX signal. At 8watts average output power thecrests were being clipped from theirtheoretical level of 100 watts by thepeak capability of the Doherty ampli-fier which was estimated to be about60 watts. This equates to more than 2dB compression assuming that thepeak-to-average power ratio (PAPR)was not increased by the digital pre-distortion algorithm!

The required SEM at 1.5 MHz off-set from the edge of the signal is 45dB. This specification was met at 8watts of average output power and at47% DC-RF efficiency for a WiMAX10 MHz channel bandwidth signalwith 11 dB peak-to-average ratio, asshown in Figure 15 . The correction inSEM with the Optichron OP4400DPD test-bed was 17 dB. The plot ofFigure 16 shows that such excellentcorrection is maintained across fre-quency (2.5 and 2.655 GHz) as wellas with average output power. It canbe seen that the distortion productsof a Doherty amplifier are relatively

Figure 14 Digital predistortion test-bed using Optichron OP4400 chipset.

Figure 15 Doherty amplifier spectral mask, with andwithout DPD correction (F = 2.5 GHz, Pave = 8 watts,Vsupply = +28 V, I = 600 mA, 48% DC-RF efficiency).

Figure 16 Doherty amplifier spectral mask vs. aver-age output power, with and without DPD correction (F =2.5, 2.655 GHz, Vsupply = +28 V).



DOHERTY AMPLIFIER

high when the output power isbacked off quite heavily from themaximum average output power. TheDPD system, however, is able to cor-rect down to the system noise floorwith better than 20 dB correctionover a wide dynamic range.

These results demonstrate thefeasibility of a compact, high efficien-cy design for WiMAX transmitters at2.5 or 2.7 GHz with up to 8 wattsaverage output power.

Summary and ConclusionsA number of design considera-

tions for Doherty amplifiers usingGaN HEMT transistors in the 2.5-2.7GHz bands have been presented. Thecritical roles of the input coupler andpeaking amplifier bias point weredescribed, as well as the rationale forinput divider design and the choice ofamplifier bias. The importance of biascircuit design and the evaluation ofsuch circuits were also covered. Thedesign optimization process wasdescribed, including the key parame-ters and tradeoffs involved. Finally,the effectiveness of digital predistor-tion for this Doherty amplifier withGaN HEMT transistors was demon-strated. The result was a fully speci-fication compliant WiMAX amplifierwith 8 watts average output level atan extraordinary 47% efficiency.

References:1. S.C. Cripps, RF Power

Amplifiers for Wireless Communi-cations, 2nd edition, Artech House,2006, pp. 290-303.

2. J. Kim, J. Cha, I. Kim, and B.Kim, Optimum operation of asym-metrical cell-based linear Dohertypower amplifiersuneven powerdrive and power matching, IEEETrans. Microwave Theory &Techniques, vol. 53, no. 5, May 2005,pp. 1802-1809.

3. J. Gajadharsing, O. Bosma, andP. Van Western, Analysis and designof a 200W LDMOS based Dohertyamplifier for 3-G base station, IEEEMTT-S International Microwave Sym-

posium Digest, 2004, pp. 529-532.4. M.J. Pelk, W.D.E. Neo, J.

Gajadharsing, R. Pengelly, L.C.N. deVreede, A High Efficiency 100WGaN Doherty Amplifier for Base-Station Applications, IEEETransactions on Microwave Theoryand Techniques, July 2008, pp. 1582-1591.

5. U. Andre, J. Crescenzi, R.Pengelly, A. Prejs, S. Wood, HighEfficiency, High Linearity GaNHEMT Amplifiers for WiMAXApplications, High FrequencyElectronics, June 2007, Vol. 6, No. 6.,pp. 16-29.

6. J. Kim, B. Kim, Y. Yun Woo,Advanced Design of Linear DohertyAmplifier for High Efficiency usingSaturation Amplifier, 2007 IEEEMTT-S International SymposiumDigest, pp. 1573-1576.

7. S.C. Cripps, RF PowerAmplifiers for Wireless Communi-cations, 2nd edition, Artech House,2006, pp. 68-73, 122-124.

8. R.S. Pengelly, N-way RF poweramplifier with increased backoffpower and power added efficiency,U.S. Patent 6 700 444, Mar. 2, 2004.

9. R.S. Pengelly and S.M. Wood,N-way RF power amplifier circuitwith increased back-off capabilityand power-added efficiency usingunequal input power division, U.S.Patent 6,737,922, May 18, 2004.

10. R.S. Pengelly and S.M. Wood,N-way RF power amplifier circuitwith increased back-off capabilityand power-added efficiency usingselected phase lengths and outputimpedances, U.S. Patent 6,791,417,Sep. 14, 2004.

Author InformationSimon Wood has a Bachelor of

Engineering degree from theUniversity of Bradford, UK. Sincejoining Cree Inc. in 2000, he hasdesigned amplifiers using SiC MES-FET, Si LDMOS, and more recentlyGaN HEMT devices. He has beenManager of Product Development atCree since January 2006. In his pro-

fessional activities, he has writtennumerous magazine articles and haspresented papers and led workshopsat international conferences. Heserved as secretary on the steeringcommittee for IMS2006 in SanFrancisco and is currently acting onthe RWS Technical Paper Committee.He holds five United States patentsin amplifier design. Simon can bereached at [email protected]

Ray Pengelly gained his BSc. andMSc. degrees from SouthamptonUniversity, England in 1969 and1973, respectively. Since August1999, Ray has been employed by CreeInc. Initially, he was the GeneralManager for Cree Microwave respon-sible for bringing Crees widebandgap transistor technology to thecommercial marketplace. On theacquisition of UltraRF, Sunnyvale in2001, he became Chief TechnicalOfficer enabling his skills inmicrowave semiconductor technology,based on 30 years of experience, to beutilized. From September 2005 hebecame responsible for new businessdevelopment of wide bandgap tech-nologies for RF and microwave appli-cations for Cree in Durham, NC andmost recently has been involved withthe release of GaN HEMT transistorsand ICs for general purpose andtelecommunications applications.Ray has written over 100 technicalpapers, 4 technical books, holds 12patents and is a Fellow of the IETand Senior Member of the IEEE.

Jim Crescenzi received the BSdegree at UC Berkeley in 1961 andthe MS and PhD degrees at theUniversity of Colorado in 1962 and1969. He founded Central CoastMicrowave Design in 2005, where heprovides consulting services primari-ly for microwave power amplifiersand subsystems. Jim is a Life Fellowof the IEEE for contributions to thedevelopment of microwave ampli-fiers, integrated circuit technology,and miniature receivers for defenseapplications. He can be reached [email protected].

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A High Efficiency Doherty Amplifier With DPD for WiMAX