SINCE THE emergence of radio communication, the frequency

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1 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 10, OCTOBER An Integrated Wideband Power Amplifier for Cognitive Radio Yi-Jan Emery Chen, Senior Member, IEEE, Li-Yuan Yang, and Wei-Chih Yeh Abstract This paper presents the development of the wideband power amplifier (PA) for application to intelligent cognitive radios. The load-tracking based on the frequency-varied load pull technique is proposed for the PA design. The load impedance tracking is realized by filter network synthesis. A GHz broadband PA is demonstrated in m InGaAs pseudomorphic HEMT technology. Operated at 3.5 V, the 1dBand power-added efficiency of the PA are better than 21.4 dbm and 20%, respectively. Index Terms Cognitive radio (CR), frequency agile, loadtracking, power amplifier (PA), pseudomorphic HEMT (phemt), software-defined radio (SDR), wideband. I. INTRODUCTION SINCE THE emergence of radio communication, the frequency spectrum has been recognized as one of the most precious public resources. The frequency spectrum is carefully regulated all over the world to ensure proper usage and avoid interference. There seems to be tremendously wide frequency spectrum available at the microwave and millimeter-wave range, but most wireless applications still favor low-gigahertz operation because of non-line-of-sight (NLOS) radio propagation property and low bill-of-material (BOM) of electronic devices. However, the low-gigahertz spectrum has been crowded with many licensed applications already, and it is extremely difficult to make room for new applications, especially featuring wideband or high data rate. Cognitive radio (CR) technology was proposed recently to coexist seemly with the legacy radio based upon the discovery that the licensed spectrum is not fully exploited all the time for real-life operation. Regarded as a particular extension of the long-touted software-defined radio (SDR), CR offers a mechanism for radio spectrum pooling and model-based reasoning about users and communication contents to achieve spectrum sharing transparent to the authorized spectrum users [1]. The realization of CR systems requires wideband spectrum sensing and frequency-agile operation. The typical function Manuscript received April 4, 2007; revised July 20, This work was supported in part by the Taiwan National Science Council and by the MediaTek Wireless Research Laboratory, National Taiwan University. Y.-J. E. Chen and L.-Y. Yang are with the Graduate Institute of Electronics Engineering, Graduate Institute of Communication Engineering, and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. ( emery@cc.ee.ntu.edu.tw; r @ntu.edu.tw). W.-C. Yeh was with the Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. He is now with the United Microelectronics Corporation, Hsin-Chu, R.O.C. ( r @ntu.edu. tw). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMTT Fig. 1. Function blocks of a typical CR system. blocks of a CR system is shown in Fig. 1. The wideband sensing receiver monitors the radio within the frequency band of interest and detects the existence of legacy user signals. The transceiver will take advantage of the unused spectrum for wireless communication. Since the spectrum usage and availability vary depending on time and location, it calls for the need of the reconfigurable transceiver and wideband power amplifier (PA) [2]. There has been significant effort to develop wideband PAs. The traveling-wave and distributed amplifier topologies were widely used for broadband PA development because of the excellent characteristics in terms of bandwidth, gain flatness, and input voltage standing-wave ratio (VSWR) [3], [4]. The multistage combination technique was proposed to develop the GaAs HBT PA for the broadband wireless applications from 3.3 to 3.6 GHz [5]. The push-pull PAs can achieve wide bandwidth by use of broadband transformers or baluns [6] [8]. The shunt-feedback technique and multisection distributed matching networks were demonstrated useful for developing wideband PAs [9]. The staggered matching was shown as a viable technique to extend the bandwidth of the millimeter-wave PAs [10]. The reconfigurable output matching by using p-i-n diodes to adjust the networks was applied in the dual-mode broadband InGaP HBT PA at 0.9 and 1.8 GHz [11]. Whereas most of the wideband PAs take advantage of high supply voltage to desensitize the load impedance variation with respect to frequency, this paper presents the load-tracking technique to develop the GHz pseudomorphic HEMT (phemt) PA. Section II discusses the load-tracking technique and the realization of matching networks. The design of the two-stage wideband PA and its measurement results are described in Sections III and IV, respectively /$ IEEE

2 2054 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 10, OCTOBER 2007 Fig. 3. Dots represent the maximum output power matching impedances of the 0.15-m phemt from 3 to 8 GHz. The impedance tracks of the desired and synthesized load networks are plotted via the dashed and solid lines, respectively. Fig. 2. Constant-power load contour of 0.15-m phemt at 5 GHz. II. LOAD-TRACKING TECHNIQUE A. Load Line Technique The load line technique is useful in determining the PA design parameters, especially when the cutoff frequency of the power transistors is much higher than the operation frequency. The load line resistance and output power of a class-a RF amplifier in the optimal power-matched condition have the values of Although the load line technique can be extended to predict the load pull contour for a power device kept in the linear regime [12], it does not reveal much about the characteristics of the power device. B. Load Pull Technique Since most power transistors have large periphery and contain significant parasitics, performing load pull measurement on the power devices at the operation frequency is preferable for RF PA development. Usually the load pull measurement is done at single frequency for tuned applications because most of wireless communications are allotted only a small chunk of frequency spectrum. Fig. 2 shows the constant power contour of the m phemt at 5 GHz when the input power is 10 dbm. C. Load-Tracking Technique The load pull technique can be extended to develop a wideband RF PA by performing a series of measurement within the frequency band of interest. The current driving capability and parasitics of the power devices are varied with respect to frequency so the optimal matching impedances will move over the frequency as well. The load matching impedance of the wideband PA can be developed to track the desired impedances within the frequency band. The maximum achievable output (1) (2) Fig. 4. Filter network consisting of port model and load-tracking network. power of the transistors at high frequency may be lower than that at low frequency so the impedance tracking may not simply follow the optimal loads at every frequency if constant power output is required. The dots in Fig. 3 represent the maximum output power matching impedances of the m phemt from 3 to 8 GHz. The total transistor gatewidth is 1200 m. The drain and gate voltages are 3.5 and 0.8 V, respectively. The is approximately 25.6 dbm and its variation across the band is less than 0.2 dbm when the input power is 10 dbm. From (1) and (2), the optimal load resistance for the low supply voltage needs to be smaller than that for the high supply voltage to reach the same output power. The impact of the load impedance variation on the output power of PAs under the low supply voltage is more severe than the high supply voltage. Therefore, the load impedance tracking is especially critical for low-voltage wideband PAs. D. Realization of Load-Tracking Networks The realization of the load-tracking network can be divided into two steps, which are: 1) filter network synthesis and 2) impedance transformation. The two-port broadband filter network with the same resistive terminations can be designed by filter synthesis techniques. One of the terminations will then be transformed to 50. The design of broadband filter networks starts with determining the load-tracking impedances within the band of interest. The dashed line in Fig. 3 represents the desired load-tracking impedances from 3 to 8 GHz for the phemt power stage. The conjugate of the load-tracking impedances can be modeled by a simple port, as enclosed in the dashed-line box in Fig. 4. For this case, the port can be well modeled by a parallel network. The resistance in the port model will be

3 CHEN et al.: INTEGRATED WIDEBAND PA FOR CR 2055 Fig. 5. Chebyshev filter network terminated with resistance R. Fig. 6. Impedance transformation of the termination resistance by the 1 : N transformer. Fig. 8. Gains of driver stage, power stage, and overall PA. Fig. 7. Norton transformation. used as the termination resistance for the bandpass filter design. The frequency response of the filter can be chosen from the popular implementations such as, Chebyshev, Butterworth, etc. The design procedure to synthesize the equal-terminated bandpass filter is well documented in [13] [17]. The resulting filter network can be divided into the network of the port model and the load-tracking network, as shown in Fig. 5. Since the impedances looking into the port model and load-tracking network are conjugate matched, the desired load impedance track can be obtained by separating out the port model from the filter network. After separating the port model from the synthesized filter network, the next step is to transform the right termination resistance in the filter network to 50. The impedance transformation can be done by a transformer, as illustrated in Fig. 6. The value of is given as If the transformer is not preferred in the matching network, it can be replaced by lumped reactive components using Norton transformation [19] [21]. The transformer and -type reactive network can be transformed into either a - or -type reactive network, as shown in Fig. 7. The complete load-tracking network is realized after replacing the transformer. (3) Fig. 9. IMD3 at 10- and 20-dBm output power. III. WIDEBAND AMPLIFIER DESIGN The fully integrated two-stage GHz PA was developed in the m InGaAs phemt technology. The and power density of the phemt device are 85 GHz and 290 mw/mm, respectively. The total gatewidths of the driver- and power-stage transistors were 600 and 1200 m, respectively. Both the driver and power stages were biased for class-ab operation. Although the driver stages of most RF PAs have higher gain than their power stages, the driver transistor of the presented PA was deliberately biased at deep class-ab region such that the gain of the driver stage was lower than that of the power stage. The driver gain will increase gradually when the input power rises and elevates the bias voltage. This characteristic can mitigate the gain rolloff of the power stage at high power level and extend the gain compression point of the overall PA, as shown in Fig. 8. The bias scheme may degrade the PA linearity a little bit at low output power, but the impact is not severe because most PAs have good linearity when their output power is low. At high output power, the bias voltage of the driver will be raised, so the linearity is barely deteriorated. The third-order intermodulation distortion (IMD3) of the PA is shown in Fig. 9. Even though the proposed bias scheme leads to little linearity degradation at low output power, the IMD3 at low output power is still much better than that at high output power.

4 2056 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 10, OCTOBER 2007 Fig. 10. Simplified schematic diagram of integrated wideband PA. TABLE I COMPONENT VALUES Fig. 13. Power gain and P. Fig. 11. Chip microphotograph. Fig. 14. Simulated and measured PAE at P. Fig. 12. Simulated and measured S-parameters. Fig. 15. Measured EVM at 17.4-dBm output power and output power for EVM below 5.6%. The load network of the PA was developed using the proposed load-tracking technique. The impedance track of the synthesized load network is shown as the solid line in Fig. 3. The input and interstage matching networks are designed to provide broadband conjugate impedance matching. The design procedure of the conjugate matching networks is the same as the loadtracking network. The only difference is that the load-tracking impedances are replaced by the power matching impedances. The networks were optimized after the synthesis procedure to account for circuit parasitics and reduce network complexity. Fig. 10 shows the circuit schematic diagram of the fully integrated wideband PA, and the component values are listed in Table I.

5 CHEN et al.: INTEGRATED WIDEBAND PA FOR CR 2057 TABLE II SUMMARY OF WIDEBAND PAS Off-chip baluns not included. IV. MEASUREMENT RESULTS The chip size of the PA is 2.29 mm 0.83 mm and the microphotograph is shown in Fig. 11. The supply voltage is 3.5 V and the gate bias voltages of the driver and power stages are 0.9 and 0.8 V, respectively. The measured -parameters are shown in Fig. 12. The input return loss is better than 9.2 db. Fig. 13 shows the measured power gain and. The measured power gain is approximately 20.6 db with 1-dB variation between GHz, and the varies from 21.4 to 22.9 dbm. The measured power-added efficiency (PAE) at is better than 20%, as shown in Fig. 14. The orthogonal frequency-division multiplexing (OFDM) is very popular for modern wireless communication and has been adopted in many emerging applications, so the 64-QAM OFDM signal is used for error vector magnitude (EVM) measurement. Due to the limitation of the signal source, the EVM measurement was performed up to 6 GHz. The output power of the PA at which the EVM is maintained below 5.6% is shown in Fig. 15. The channel power varies between dbm. When the channel power is kept constant at 17.4 dbm, the EVM varies from 4.8% to 7.4%. The characteristic of the PA, accompanied by those of the other wideband PAs, is summarized in Table II. V. CONCLUSION The CR is an emerging wireless application to seemly coexist with the licensed radio and exploit the frequency spectrum resource efficiently. The frequency-agile operation of the CR systems calls for the need of wideband RF PAs. Since the low supply operation escalates the impact of load impedance variation with respect to frequency, the load-tracking technique is proposed for wideband PA development. The 3.5-V GHz integrated PA was developed in m InGaAs phemt technology. The measured and PAE are better than 21.4 dbm and 20%, respectively. ACKNOWLEDGMENT The authors would like to thank Prof. C.-K. Tzuang, National Taiwan University, Taipei, Taiwan, R.O.C., for the fruitful discussion. The help on the PA measurement from Prof. H. Wang and Dr. C.-H. Wang, both with National Taiwan University, is also highly appreciated. The chip was fabricated due to the support of the Taiwan National Chip Implementation Center, Hsin-Chu, Taiwan, R.O.C., which is gratefully acknowledged. REFERENCES [1] J. Mitola III, Cognitive radio for flexible mobile multimedia communications, in IEEE Int. Mobile Multimedia Commun. Workshop Dig., 1999, pp [2] J. Laskar, R. Mukhopadhyay, Y. Hur, C. H. Lee, and K. Lim, Reconfigurable RFICs and modules for cognitive radio, in IEEE Silicon Monolithic Integrated Circuits RF Syst. Dig., 2006, pp [3] J. J. Xu, W. Yi-Feng, S. Keller, S. Heikman, B. J. Thibeault, U. K. Mishra, and R. A. York, 1 8-GHz GaN-based power amplifier using flip-chip bonding, IEEE Microw. Wireless Compon. Lett., vol. 9, no. 8, pp , Aug [4] M. J. Schindler, J. P. Wendler, M. P. Zaitlin, M. E. Miller, and J. R. Dormail, A K=Ka-band distributed power amplifier with capacitive drain coupling, IEEE Trans. Microw. Theory Tech., vol. 36, no. 12, pp , Dec [5] M. Hirata, T. Oka, M. Hasegawa, Y. Amano, Y. Ishimaru, H. Kawamura, and K. Sakuno, Fully-integrated GaAs HBT power amplifier MMIC with high linear output power for 3 GHz-band broadband wireless applications, Electron. Lett., vol. 42, no. 22, pp , Oct [6] A. Vasylyev, P. Weger, and W. Simburger, Ultra-broadband GHz monolithically-integrated CMOS power amplifier, Electron. Lett., vol. 41, no. 23, pp , Nov [7] L. Jong-Wook, L. F. Eastman, and K. J. Webb, A gallium nitride push pull microwave power amplifier, IEEE Trans. Microw. Theory Tech., vol. 51, no. 11, pp , Nov [8] L. Jong-Wook and K. J. Webb, Broadband GaN HEMT push pull microwave power amplifier, IEEE Microw. Wireless Compon. Lett., vol. 11, no. 9, pp , Sep [9] A. Sayed and G. Boeck, Two-stage ultrawide-band 5-W power amplifier using SiC MESFET, IEEE Trans. Microw. Theory Tech., vol. 53, no. 7, pp , Jul

6 2058 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 55, NO. 10, OCTOBER 2007 [10] K. Fujii and H. Morkner, 1 W power amplifier MMICs for mm-wave applications, in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2004, vol. 3, pp [11] Z. Haitao, G. Huai, and L. Guann-Pyng, A novel tunable broadband power amplifier module operating from 0.8 GHz to 2.0 GHz, in IEEE MTT-S Int. Microw. Symp. Dig., Jun. 2005, pp [12] S. C. Cripps, RF Power Amplifiers for Wireless Communications. Norwood, MA: Artech House, [13] W.-K. Chen, Broadband Matching Theory and Implementation, 2nd ed. Teaneck, NJ: World Sci., [14] R. M. Fano, Theoretical limitations on the broadband matching of arbitrary impedances, J. Franklin Inst., vol. 249, pp , , [15] D. C. Youla, A new theory of broadband matching, IEEE Trans. Circuit Theory, vol. CT-11, no. 1, pp , Mar [16] M. E. Van Valkenburg, Analog Filter Design. New York: Oxford Univ. Press, [17] A. B. Williams and F. J. Taylor, Electronic Filter Design Handbook, 4th ed. New York: McGraw-Hill, [18] D. J. Mellor, Improved computer-aided synthesis tools for the design of matching networks for wideband microwave amplifiers, IEEE Trans. Microw. Theory Tech., vol. MTT-34, no. 12, pp , Dec [19] Y.-P. Wu, Equalizer and amplifier design of broadband impedance matching, Nat. Sci. Council, Taipei, Taiwan, R.O.C., Tech. Rep., [20] Y.-I. Huang, Ultra-wideband CMOS low noise amplifier, M.S. thesis, Graduate Inst. Electron. Eng., Nat. Taiwan Univ., Taipei, Taiwan, R.O.C., [21] R. Rhea, Filter Techniques. Raleigh, NC: SciTech/Noble, Tektronix, where he was responsible for electronic test and measurement solutions. From 2000 to 2002, he was with National Semiconductor, where he was involved with RF transceiver and RF PA integrated circuit (IC) design. In 2002, he joined the Georgia Institute of Technology, as a member of the research faculty, where he explored the device-to-circuit interactions of advanced SiGe technology. Since 2003, he has been with the National Taiwan University, Taipei, Taiwan, R.O.C., where he is currently an Assistant Professor. He has authored or coauthored over 50 refereed journal and conference papers. His recent research has focused on RF IC/module development, RF PA design, gigascale interconnect, system-on-package integration, and low-temperature polysilicon (LTPS) IC deign. Dr. Chen serves on the IC Implementation Review Committee, National Chip Implementation Center, Taiwan, R.O.C., and the National Committee of the R.O.C. for the International Union of Radio Science (URSI). He was the corecipient of the 2000 IEEE Microwave Theory and Techniques Society (IEEE MTT-S) International Microwave Symposium (IMS) Best Paper Award. Li-Yuan Yang was born in Kinmen, Taiwan, R.O.C., in He received the B.S. degree in mechanical engineering from National Taiwan University, Taipei, Taiwan, R.O.C., in 2005, and is currently working toward the M.S. degree in electrical engineering at the Graduate Institute of Electronics Engineering, National Taiwan University. His research focuses on high-efficiency RF PAs and their linearization techniques. Yi-Jan Emery Chen (S 97 M 01 SM 07) received the B.S. degree in electrical engineering from National Taiwan University, Taipei, Taiwan, R.O.C., in 1987, the M.S. degree in electrical and computer engineering from the University of California at Santa Barbara, CA, in 1991, and the Ph.D. degree in electrical and computer engineering from the Georgia Institute of Technology, Atlanta, in From 1992 to 1993, he was a Software Engineer with Seimens Telecommunication, where he was involved with working on synchronous optical networks (SONET) equipment development. From 1993 to 1996, he was with Wei-Chih Yeh was born in Hsin-Chu, Taiwan, R.O.C., in He received the B.S. degree from National Central University, Taoyuan County, Taiwan, R.O.C., in 2004, and the M.S. degree from National Taiwan University, Taipei, Taiwan, R.O.C., in 2007, both in electrical engineering. He is currently involved with RF integrated circuit (RFIC) design with the United Microelectronics Corporation (UMC), Hsin-Chu, R.O.C.