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Double Shunt Stub Impedance Matching Network based Concurrent Dual-WLAN-Band Amplifier Chandranshu Garg #1, Vivek Sharma #2, Nagendra P. Pathak #3 # Radio Frequency Integrated Circuits (RFIC) Group Department of Electronics and Computer Engineering (E&CE) Indian Institute of Technology (IIT) – Roorkee Uttarakhand, India 1 cgarg@cgarg.com 2 vivs@ieee.org 3 nagppfec@iitr.ernet.in Abstract — The paper presents design of a concurrent dual- in power dissipation, chip area and overall module cost.band microwave amplifier that operates simultaneously at the Furthermore, switching based design results in non-two WLAN frequencies around 2.44-GHz and 5.25-GHz. The concurrent operation. Hence, current need is to designamplifier employs concurrent dual-band input and outputimpedance matching networks, each of which utilizes the concurrent multi-band microwave amplifiers [4]–[5], whichconventional double open-circuited shunt-stubs structure. Such can simultaneously support the required frequencies.tuning network is designed to allow simultaneous matching of One of the ways to achieve simultaneous multi-bandarbitrarily different complex impedances of transistor to the amplification is through utilization of concurrent multi-bandstandard 50- impedance at any two desired frequencies, while impedance transforming networks at both the input and therejecting a wide-band in between them. output ends of active device. Impedance matching at both the Index Terms — Amplifier, concurrent, dual-band, double stubimpedance tuner, hybrid microstrip integrated circuit, wireless input and the output ends allows maximum power transfer,LAN. which, in turn, results in power gain at all selected frequencies. In this regard, efforts are being put in to design multi-band matching networks that match frequency- I. INTRODUCTION dependent complex impedances to the standard 50- Rapid global development of wireless communication impedance at multiple uncorrelated transmission frequencies.systems has expanded their applications into various This paper explores dual-frequency operation of thediversified fields, such as, GSM, WCDMA, GPS, Bluetooth, conventional double open-circuited shunt-stubs basedWLAN, etc. Several different communication standards have impedance transformer. The structure is shown to matchbeen introduced for such distinct areas so as to not only arbitrary frequency-dependent complex load impedances toensure their feasible performance, but also, minimize the standard resistance, simultaneously at any two, arbitrarilyinterference among their signals. Further advancements in selected, uncorrelated frequencies of interest. Thereafter, aswireless domain target implementation of multi-standard mentioned earlier, the structure’s utility is highlighted bymobile devices that allow simultaneous support for multiple realizing both the input and the output impedance matchingdistinct applications. This requires realization of multi-mode networks for implementation of a concurrent dual-bandor multi-channel, individual, radio-frequency (RF) amplifier. The fabricated prototype amplifier is designed forcomponents that will bring about reduction in both the operation around dual-WLAN-bands.components’ count and the system complexity. One of the challenging issues in effective realization of a II. CONCURRENT DUAL-BAND MATCHING NETWORKmulti-band transceiver involves realization of multi-band RFtransmit amplifiers. Various techniques have been devised to The basic concept for matching a load at any frequency isdevelop multi-band amplifiers that include wide-band to achieve zero reflection co-efficient for the power comingoperation [1], parallel architectures [2], or multi-band from the source towards the load. Design of the proposedswitching [3]. A broad-band amplifier supports multiple dual-band matching network is based on this concept and aimfrequencies by covering them within its pass-band. The next is to achieve zero reflections for two different complex loadapproach employs multiple, separate, single-band amplifiers impedances at any two arbitrary frequencies. In order toin parallel, wherein, each amplifier is optimized to one simultaneously match unequal complex load impedances atparticular band of interest. The third scheme suggests any two arbitrary frequencies, Colantonio, et. al., [6] firstswitching among distinct matching networks using CMOS or transformed complex impedances to equal real impedances atMEMS based switches, while utilizing a single active device. two desired frequencies, using dual-band shunt-stub Yet, the afore-mentioned design configurations exhibit transmission-line (TL) sections. Thereafter, dual-band two-certain issues. In particular, a wide-band amplifier section stepped impedance transformer [7] was employed toadditionally augments undesired frequency bands, match the resulting real impedances. However, such structureinterference signals, inter-modulation products and noise, doesn’t allow matching at harmonically related frequencies. Awhich, in turn, degrades amplifier’s linear performance. simplified topology of three-section stepped impedanceBesides it, the simplified parallel approach leads to increase transmission-line transformer was presented by Liu, et. al.,
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[8] for dual-band matching of unequal complex impedances. (1)However, both the above mentioned approaches share acommon constraint that the required characteristicimpedances of TL sections do not always lie in the feasible (2)range for fabrication. This limitation was mitigated byChuang [9] through the observation that concurrent dual-band . (3)matching of frequency dependant arbitrary compleximpedances requires only four design parameters. Indistributed elements based matching network design, each TL Further consider the two frequencies of interest as f1 and f2.section provides two adjustable parameters, viz., Consequently, two different load admittances, YL1 and YL2,characteristic impedance and physical length. Using physical need to be matched at the two frequencies f1 and f2,lengths of four TL sections as design parameters, the scheme respectively. Moreover, the propagation constant, β, alsoallowed their characteristic impedances to be chosen varies with operating frequency. Accordingly, six equationsarbitrarily for feasible fabrication. The approach utilized a are achieved for simultaneous impedance matching at the twotwo-section stepped impedance transmission-line transformer design frequencies. Given the values for yL1 and yL2, the(TLT) to first equalize the real parts of resulting admittances lengths l1, l2, d1 and d2 are adjusted such that all equations areto 1/50–1. Their imaginary parts were subsequently satisfied simultaneously. This will mean that load impedancescancelled out using two-section shunt-stubs, thereby, at f1 and f2 are simultaneously matched to 50-. Based onachieving dual-band matching. derived equations, a MATLAB code was developed to The proposed matching network employs series TL provide all possible solutions for feasible length parameterssections and stubs to transfer the complex impedance seen at for dual-band impedance matching.the transistor terminals to 50- at the port. The two stubs areconnected in parallel to the main line and are open-circuited. IV. PROTOTYPE CONCURRENT DUAL-BAND AMPLIFIERFig. 1 shows the impedance transformer structure that hasbeen used for dual-band matching. Besides, the afore- This section details out design and implementation ofmentioned drawback of infeasible characteristic impedances concurrent dual-band microwave amplifier, supporting theis mitigated by considering the four physical lengths of both two commercially popular IEEE 802.11 WLAN (WirelessTL sections and stubs as design parameters. Hence, designer Local Area Network) frequencies around 2.44-GHz andcan arbitrarily set characteristic impedances of all TL 5.25GHz. Design employs the conventional double open-sections. Such consideration not only allows dual-band circuited shunt stubs impedance transformer for simultaneousmatching of unequal complex impedances, but also, feasible dual-frequency impedance matching at both the input and thefabrication of microstrip transmission-line sections. output ends of the active device. The circuit schematic of the proposed amplifier is depicted in Fig. 2.Fig. 1. Concurrent dual-band impedance matching network. Consider the network as shown in Fig. 1. Let YL be the load Fig. 2. Circuit schematic for the concurrent dual-band amplifier.admittance, which is converted to YB by the first series TLsection of length l2 and an open-circuited shunt stub of length The amplifier is realized using hybrid microstrip integratedd2. This admittance is further transformed into standard technology (HMIC). Hence, all surface mount devicesadmittance Y0 by another series TL section of length l1 and an (SMDs) are mounted on the commercial NH9320 substrate,open-circuited shunt stub of length d1. For ease of analysis which is a Poly-Tetra-Fluoro-Ethylene (PTFE)/glass/ceramicand feasible fabrication, characteristic impedances of all TL dielectric. The substrate is characterized by the dielectricsections and stubs are set to standard 50-. constant (r) of 3.2 and the substrate height (h) of 60-mil. Considering, the normalized admittances with respect to Y0 Matching networks consist of only distributed elements ofas yL, yB and yA, the transmission line theory leads to microstrip TL sections.following design relations:
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Furthermore, the active device, used in the design, is TABLE IIATF54143 from AVAGO Technologies. The device is a low- OPTIMIZED MATCHING NETWORK PHYSICAL LENGTHSnoise enhancement mode pseudomorphic high electron Matchingmobility transistor (E-pHEMT). Both the drain and the gate l1 (mm) l2 (mm) d1 (mm) d2 (mm) NetworksDC bias circuits of the amplifier employ a common topology, Input 11.5 4.5 15.3 11which consists of a short-circuited, quarter-wavelength, high-impedance, TL stub. The high-impedance lines provide DC Output 8.7 7.8 13.85 11.9paths for their respective supply voltages, while, acting asopen circuits for ac signals in the circuit, thereby, avoiding Fig. 3 shows a fabricated prototype of the proposedunwanted ac coupling. Apart from that, coupling capacitors amplifier, using concurrent dual-WLAN-band impedanceare employed at both the input and the output ends of the matching networks at both the input and the output ends oftransistor to couple only RF power, while, blocking DC the E-pHEMT active device.signals. Non-linear model of the selected active device,including package parasitic effects, is used in all circuitsimulations using Advanced Design System (ADS). The DCbias point selected for the amplifier design is VDS = 3.0V andVGS = 0.59V. The chosen bias point operates the transistor inclass-A operation. Source-pull and load-pull characterizations of the biasedtransistor identify the required matching source and loadimpedances for the maximum power transfer at 2.44-GHz and5.25-GHz. These impedances are listed in Table I. Thescattering (S-) parameters for the transistor confirm that theimpedances do not lie in the unstable region of operation and, Fig. 3. Fabricated concurrent dual-WLAN-band amplifier.therefore, stabilization of the device is validated. TABLE I V. EXPERIMENTAL RESULTS MATCHING SOURCE AND LOAD IMPEDANCES Laboratory setup for measurement of amplifier’s concurrent Frequency Required Matching Impedances () dual-WLAN-frequency response on Network Analyzer is (GHz) illustrated in Fig. 4. Source Load 2.44 19.95 – j51.25 43.50 – j36.65 5.25 127.90 + j163.25 82.30 – j10.30 Using the complex matching impedances as target loads,design parameters for the input and the output matchingnetworks are obtained through the MATLAB code. The codeis written to solve the design equations (1) to (3) forconcurrent dual-frequency complex impedance matchingthrough the conventional double open-circuited shunt stubsstructure. These design relations are derived in the previoussection. The inputs to the program are the two designfrequencies along with corresponding source and loadmatching impedances. Moreover, substrate parameters, suchas, the dielectric constant, etc., are also provided in order totake their effects into account while performing computationsat the two frequencies of interest. The program provides allpossible solutions in terms of the physical lengths of the Fig. 4. Complete setup for measurement of dual-band amplifier’sseries TL sections and the open-circuited shunt-stubs. The S-parameter response on the Network Analyzer.circuit physical design parameters are further tuned throughADS harmonic and EMDS simulations to increase power Measured small-signal scattering (S11, S22, and S21)transfer at both the desired frequencies. Table II shows parameter characteristics of the fabricated dual-bandresulting design parameters for the input and the output amplifier are displayed in Fig. 5(a), (b) and (c).matching networks.
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2.44-GHz and 5.25-GHz, respectively. Hence, as required, the amplifier passes the desired WLAN frequencies while rejecting the undesired frequencies. V. CONCLUSION The paper presents concurrent dual-band impedance matching characteristics of conventional double open-circuited shunt- stubs structure. The impedance transformer is shown to match frequency-dependent complex load impedances to standard 50- line at any two distinct frequencies. This is established through design of a concurrent dual-WLAN-band amplifier that utilizes the proposed structure for both the input and the (a) output impedance matching. Measured performance of the fabricated amplifier exhibits the required dual-band response with a wideband rejection in between the two operating WLAN frequencies of 2.44-GHz and 5.25-GHz. ACKNOWLEDGEMENT Authors wish to acknowledge assistance and support for their work through research fund grant from SERC, DST. REFERENCES [1] P. S. Wu, T. W. Huang, and H. Wang, “An 18-71 GHz multi- band and high gain GaAs MMIC medium power amplifier for millimeter-wave applications,” 2003 IEEE MTT-S Int. Microwave Symp. Dig., vol. 2, pp. 863-866, June 2003. [2] S. Zhang, J. Madic, P. Bretchko, J. Mokoro, R. Shumovich, (b) and R. McMorrow, “A novel power-amplifier module for quad- band wireless handset applications,” IEEE Trans. Microwave Theory & Tech., vol.51, no.11, pp. 2203- 2210, June 2003. [3] A. Fukuda, H. Okazaki, and S. Narahashi, “A novel compact reconfigurable quad-band power amplifier employing RF- MEMS switches,” 36th European Microwave Conf., pp. 344- 347, September 2006. [4] A. Cidronali, N. Giovannelli, I. Magrini, and G. Manes, “Compact concurrent dual-band power amplifier for 1.9GHz WCDMA and 3.5GHz OFDM wireless systems,” 38th European Microwave Conf., pp. 1545-1548, October 2008. [5] D. T. Bespalko, and S. Boumaiza, “Concurrent dual-band GaN power amplifier with compact microstrip matching network,” Microwave & Optical Technology Letters, vol. 51, no. 6, pp. 1604-1607, March 2009. [6] P. Colantonio, F. Giannini, and L. Scucchia, “A new approach to design matching networks with distributed elements,” 15th Int. Conf. on Microwaves, Radar & Wireless Communications (MIKON), vol. 3, pp. 811-814, May 2004. [7] C. Monzon, “A small dual-frequency transformer in two (c) sections,” IEEE Trans. Microwave Theory & Tech., vol. 51, no. 4, pp. 1157-1161, April 2003.Fig. 5. Measured small-signal |S11|, |S22| and |S21| parameters for [8] X. Liu, Y. Liu, S. Li, F. Wu, and Y. Wu, “A three-section dual-the designed dual-band amplifier (plots depict 1-GHz to 7-GHz band transformer for frequency-dependent complex loadfrequency range with major steps of 500-MHz). impedance,” IEEE Microwave & Wireless Components Letters, vol. 19, no. 10, pp. 611-613, October 2009. [9] M. L. Chuang, “Dual-band impedance transformer using two- Plots for the input (|S11|) and the output (|S22|) reflection section shunt stubs,” IEEE Trans. Microwave Theory & Tech.,coefficients establish dual-band performance of the two vol. 58, no. 5, pp. 1257-1263, May 2010.impedance matching networks. Moreover, measured transfercharacteristic of |S21| parameters are 5.6 dB and -5.3 dB at
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