4. CONCLUSION

A systematic study of the effect of a metal contact layer’s reflec-tion on the effective coupling coefficient has been presented. In a2nd-order DFB-LD, the grating provides vertical radiation by 1st-order diffraction. The metal top contact layer can provide powerreflection, and part of the reflected power is coupled back into theguided waves. This process has an effect on the effective couplingcoefficient �eff. In this paper, examples were given to demonstratethe Au contact’s effect on �eff, and this effect’s dependence uponthe contact’s location and the grating’s dutycycle. Because �eff iscomplicatedly related to the optical field of the radiation mode inthe grating region, the contact’s location and dutycycle are veryimportant parameters that can determine the character and extentof the contact reflection’s effect on �eff. Im(�eff) can be enlargedseveral times if tTC and the dutycycle are at optimized values,which is practically useful to fabricate single longitudinal-modeDFB-LD.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foun-dation under grant no. 10374085.

REFERENCES

1. W. Streifer, D.R. Scifres, and R.D. Burnham, Analysis of grating-coupled radiation in GaAs:AlGaAs lasers and waveguides, IEEE JQuantum Electron QE-12 (1976), 422–428.

2. W. Streifer, R.D. Burnham, and D.R. Scifres, Radiation losses indistributed feedback lasers and longitudinal mode selection, IEEE JQuantum Electron QE-12 (1976), 737–739.

3. W. Streifer, D.R. Scifres, and R.D. Burnham, Coupled wave analysisof DFB and DBR lasers. IEEE J Quantum Electron QE-13 (1977),134–141.

4. A.M. Shams-Zadeh-Amiri, J. Hong, X. Li, and W.P. Huang, Second-and higher-order resonant gratings with gain or loss, Part II: Designingcomplex-coupled DFB lasers with second-order gratings, IEEE JQuantum Electron QE-36 (2000), 1431–1437.

5. A.M. Shams-Zadeh-Amiri, J. Hong, X. Li, and W.P. Huang, Second-and higher-order resonant gratings with gain or loss, Part I: Green’sfunction analysis, IEEE J Quantum Electron QE-36 (2000), 1421–1430.

6. R.G. Baet, K. David, and G. Morthier, On the distinctive features ofgain coupled DFB lasers and DFB lasers with second-order grating,IEEE J Quantum Electron QE-29 (1993), 1792–1797.

7. T. Makino and J. Glinski, Effects of radiation loss on the performanceof second-order DFB semiconductor lasers, IEEE J Quantum ElectronQE-24 (1988), 73–82.

8. R.F. Kazarinov and C.H. Henry, Second-order distributed feedbacklasers with mode selection provided by first-order radiation losses,IEEE J Quantum Electron QE-21 (1985), 144–150.

9. A. Hardy, D.F. Welch, and W. Streifer, Analysis of second-ordergratings, IEEE J Quantum Electron QE-25 (1989), 2096–2105.

10. Y. Yamamoto, T. Kamiya, and H. Yanai, Improved coupled modeanalysis of corrugated waveguides and lasers, IEEE J Quantum Elec-tron QE-14 (1978), 245–258.

11. K. Kawano and T. Kitoh, Introduction to optical waveguide analysis:solving Maxwell’s equations and the Schrodinger equation, Wiley,New York, 2001.

12. C.H. Chen, L.H. Chen, and Q.M. Wang, Coupling coefficients ofgain-coupled distributed feedback lasers with absorptive grating, Elec-tron Lett 32 (1996), 1288–1290.

© 2004 Wiley Periodicals, Inc.

A 2-WATT BALANCED POWERAMPLIFIER MMIC FOR KU-BANDSATELLITE COMMUNICATIONS

Jeong-Geun Kim,1 Hyun-Min Park,1 Jae-Young Kim,2

Sun-Ik Jeon,2 and Songcheol Hong1

1 Department of Electrical EngineeringKorea Advanced Institute of Science and Technology373-1 Guseong-Dong, Yuseong-GuDaejeon, 305-701, Korea2 ETRI161 Gaejong-Dong, Yuseong-GuDaejeon, 305-350, Korea

Received 22 January 2004

ABSTRACT: A Ku-band power amplifier MMIC has been developedusing 0.25-�m GaAs pHEMT technology. To achieve small chip sizeand simple drain-bias connection, a bus-bar power combiner is used.Also, balanced-power amplifier topology is used to obtain good input/output return losses. The small-signal gain is about 15 dB and the gainvariation is less than 1 dB from 12 to 17 GHz. Good input/output returnlosses are achieved at less than �15 dB due to the balanced topology.P1dB of 32.6 dBm and PAE of 23.5% are achieved at 14 GHz. The effec-tive chip area is 4.2 � 3.2 mm. Because the power amplifier is imple-mented using the balanced topology with the bus-bar power combiner,compact size, high output power, and good input/output return losses

Figure 4 Effective coupling coefficients vs. duty cycle for tTC � 6� and6.25�: (a) Re(�eff); (b) Im(�eff)

342 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 4, August 20 2004

can be achieved simultaneously. © 2004 Wiley Periodicals, Inc.Microwave Opt Technol Lett 42: 342–344, 2004; Published online inWiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20297

Key words: balanced power amplifier; Bus-bar power combiner; Ku-band PA; GaAs pHEMT

1. INTRODUCTION

Recently, as the satellite communication market has grown, high-power amplifiers (HPAs) with broad bandwidth and flat-gain per-formance are required at the Ku-band [1, 2]. The power amplifieris one of the largest and the most expensive chips. HPA MMICwith small chip size is necessary to reduce the manufacturing cost.To achieve high output power, power transistors have to be com-bined. Therefore, the powercombiner method is very important forhigh output power and small chip size. To implement small chiparea and simple bias connection, the bus-bar power combiner wasproposed [3, 4]. When a power amplifier is packaged or assembledwith other circuitries, such as a phase shifter, VCO, or antenna (forexample), the performance of the input/output return losses isimportant. In the case of a single-ended power amplifier, it has aslightly higher output power and gain performance because there isno loss due to an additional coupler. However, narrow bandwidthand poor input/output return losses are shown [5, 6]. To improvethe performance of input/output return losses, the balanced topol-ogy is widely used.

In this paper, a 2-watt balanced power amplifier MMIC with abus-bar power combiner is presented using 0.25-�m GaAspHEMT technology.

2. CIRCUIT DESIGN

We have designed a two-stage balanced power amplifier MMIC. A0.25 � 1200 �m transistor is used as a basic cell. Load-pull/source-pull simulation is performed using HP ADS to obtain theoptimum load and the source impedances. This transistor showsoutput power of 26.5 dBm and gain of 13 dB under the biascondition of VDS � 7 V and IDS � 120 mA at the Ku-band. Toachieve output power above 33 dBm, eight transistors need to becombined at the output stage. The power amplifier is combinedwith a bus-bar power combiner for compact size and simple drainbias. The bus-bar power combiner has its own asymmetry, ascompared to a binary tree combiner such as the Wilkinson com-biner. In other words, the mismatches of phase and amplitude canoccur at the combined output stage of each transistor. However, aslong as the length of the drain bias line is from 1/16� to 1/4�, thedegradation due to those mismatches in the power amplifier will benegligible [3]. As the length of the drain bias line is increased to1/4�, the imbalances of phase and amplitude are decreased. How-ever, in this case, a large chip size is required. Therefore, thecompromise between output power and chip size is needed. Thewidth of the bus-bar track is designed to match 50� easily andreduce the chip size in consideration of the maximum current

density of the metal line eliminating the electromigration of themetal line at high drain current.

The balanced topology with a Lange coupler was used toimprove input/output return losses. Because the Lange coupleroccupies a small chip area and shows broad bandwidth perfor-mance, it is widely used in MMIC technology [5, 6]. Broad-bandwidth and flat-gain performances were achieved using multi-stage high-pass and low-pass matching networks. The small-signalgain was optimized by using a linear model. The power andfrequency characteristics, such as P1dB and PAE, were estimatedby using a nonlinear model. To suppress odd-mode oscillation, theresistors were added to eliminate odd-mode oscillation betweenthe combined transistors. Also, RC bias circuits were included atthe gate and the drain-bias line in order to prevent bias-circuitoscillation. The unconditionally stable condition of the poweramplifier was verified using the K-factor at various bias conditionsfor the maximum oscillation frequency.

TABLE 1 Device Characteristics

0.25 � 1200 �m GaAs pHEMT

fT 65 GHzVBD �19 VIDSS 285 mA/mmIMAX 510 mA/mmGMAX 375 mS/mm

Figure 1 Microphotograph of the fabricated power amplifier (chip size:4.2 � 3.2 mm). [Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

Figure 2 Measured S-parameters. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 4, August 20 2004 343

3. DEVICE TECHNOLOGY

A Ku-band balanced power amplifier MMIC was fabricated usingTriquint’s 2MI 0.25-�m GaAs pHEMT technology. A modifiedMarteka model was supported as a transistor model. The typicalproperties of 0.25 � 1200 �m GaAs pHEMT are summarized inTable 1. The technology provides a SiNx MIM capacitor with 300pF/mm2, a TaN resistor of 50 �/� and two interconnection metals.The wafer is thinned by 100 �m using a backside via. Figure 1shows a microphotograph of the fabricated power amplifier. Thechip size is 4.2 � 3.7 mm and the effective chip area is 4.2 �3.2 mm.

4. MEASUREMENT RESULTS

To measure the performance of the fabricated power amplifier, atest fixture was used in consideration of heat dissipation and toeliminate bias-circuit oscillation. The power amplifier was testedusing GSG probes on a Cascade probe station. All measurementswere carried out under CW operation condition of VDS � 7 V andIDS � 1.3 A. The S-parameters were obtained using an HP 8764Evector network analyzer. Figure 2 shows the measured S-param-eters of the power amplifier. The small-signal gain is about 15 dBfrom 12 to 17 GHz and the gain variation is less than 1 dB.Input/output return losses are less than �15 dB in the operatingbandwidth. The power amplifier shows the performances of broadbandwidth, flat gain, and good input/output return losses. Thelarge-signal characteristics were measured using two powermeters, HP E4418B and HP 437B. RF input was driven using anAnritsu MG3695A synthesized signal generator. To drive suffi-cient power to the input port, the driver amplifier was used. Thedrive amplifier shows PSAT of 28 dBm and gain of greater than 19dB from 14.0 to 14.5 GHz. Figure 3 shows the measured outputpower, gain, and PAE versus input power at various frequencies.The fabricated power amplifier shows P1dB of 32.6 dBm, G1dB of15.6 dB, and PAE of 23.5% at 14 GHz. The measurement resultsof the fabricated power amplifier are summarized in Table 2.

5. CONCLUSION

We have demonstrated a 2-watt balanced power amplifier MMICat the Ku-band using commercially available 0.25-�m GaAspHEMT technology. To achieve compact size and simple biasconnection, a bus-bar power combiner was used. The small-signal

gain are about 15 dB and input/output return losses are less than�15 dB from 12 to 17 GHz. The power amplifier shows P1dB of32.6 dBm and PAE of 23.5% at 14 GHz. The compact chip size of4.2 � 3.2 mm is achieved by using a bus-bar power combiner. Thisis the first demonstration of the balanced power amplifier withbus-bar power combiner at the Ku-band to the authors’ knowledge.

ACKNOWLEDGMENTS

The author would like to thank Jeong-Ho Lee, Chung-Hwan Kim,and Jae-Jin Lee at Teltron for their helpful discussions and en-couragement. This work was supported by KOSEF under the ERCprogram through the MINT research center at Dongguk Univer-sity.

REFERENCES

1. M. Cardullo, C. Page, D. Teeter, and A. Platzker, High-efficiencyX-Ku-MMIC power amplifier, IEEE MTT-S Int Microwave Symp, SanFrancisco, CA, 1996, pp. 145–148.

2. J.I. Upshur, P.E. Goettle, B.D. Geller, R.K. Gupta, and F. Phelleps, Afully monolithic broadband power amplifier for Ku-band communica-tions satellite applications, 35th Midwest Symp Circ Syst 1992, pp.1020–1023.

3. S.P. Marsh, D.K.Y. Lau, R. Sloan, and L.E. Davis, Design and analysisof an X-band MMIC “bus-bar” power combiner, IEEE Symp High-PerfElectron Dev Microwave Optoelectron Applic, 1999, pp. 164–169.

4. M. McCullagh, J. Moult, R. Davies, B. Wallis, and S. Wadsworth, Anoverview of high-frequency circuit on the GMMT GaAs HBT process,IEEE Symp High-Perf Electron Dev Microwave Optoelectron Applic,1995, pp. 148–154.

5. H.M. Le, Y.C. Shih, V.D. Hwang, T.Y. Chi, K.J. Kasel, and D.C. Wang,An X-band high-efficiency MMIC power amplifier with 20-dB returnlosses, IEEE J Solid-State Circ (1991), 1383–1389.

6. M.K. Siddiqui, A.K. Sharma, L.G. Callejo, and R. Lai, A high-powerbroadband monolithic power amplifier for Ka-band ground terminal,IEEE MTT-S Int Microwave Symp, Anaheim, CA, 1999, pp. 951–954.

© 2004 Wiley Periodicals, Inc.

REMARKS ON “A 2D TLM MODEL FORELECTROMAGNETIC WAVEPROPAGATION IN TELLEGEN MEDIA”

Tom G. MackaySchool of MathematicsUniversity of EdinburghJames Clerk Maxwell BuildingKing’s BuildingsEdinburgh EH9 3JZ, United Kingdom

Received 21 January 2004

ABSTRACT: Cabeceira et al. recently presented a time-domain analysisof wave propagation in a Tellegen medium [1]. Their analysis is ill-foundedbecause (i) the existence of any Tellegen medium is not tenable within mod-ern electromagnetic theory; and (ii) the constitutive relations employed are

Figure 3 Measured Pout, gain and PAE as a function of the input power.[Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

TABLE 2 Summary of the Fabricated Power Amplifier

Gain 15 dBInput/output return losses �15 dBP1dB 32.6 dBmPAE 23.5%Chip Size 4.2 � 3.7 mm

(effective area: 4.2 � 3.2 mm)

344 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 42, No. 4, August 20 2004