2011 International Conference on Advancements in Information Technology With workshop of ICBMG 2011 IPCSIT vol.20 (2011) (2011) IACSIT Press, Singapore A Novel Design of 1.5 GHz Low-Noise RF Amplifiers in L-BAND for Orthogonal Frequency Division Multiplexing Sharmila G. + and Govindan E.G. Department of Electronics and Communication Engineering, Sri Venkateswara College Of Engineering, Pennalur, Sriperumbudur-602105. Abstract. Communication plays very important role in day-to-day life of people. Due to fast growing age, multi carrier communication is preferred over single carrier waves for better transmission. The RF power amplifier in OFDM transmitters plays a major role in amplifying the required high uency RF signal without distortions and other impairments which would decrease the usefulness of the signal. For narrowband & wideband operation, one may construct simple amplifiers whose noise figure and power gain are close to the theoretical optima allowed within an explicit power constraint.this paper introduces the design of a 1.5 GHz unconditionally stable Low-Noise RF amplifier in L-Band using Agilent s Advance Design Systems Software. The proposed design aims to provide an optimal gain of 12.715 db with low Noise Figure (NF) of 1.768 db in wideband. Keywords: Orthogonal Frequency Division Multiplexing, Low-Noise Figure, Wideband. 1. Introduction Wireless communication and its applications have travelled through rapid growth in recent years. Cellular systems, WLANS, Bluetooth as well as WPANs have undergone numerous generations of evolution in the swift development in wireless communication [1]. The radio uency (RF) front-end electronics plays an important part in high level integration of radio solutions. The low noise amplifier is one of the most critical building blocks in modern integrated radio uency solutions. The front-end low noise amplifiers have been widely used in many applications including wireless personal communication systems, Orthogonal Frequency Division Multiplexing. This paper presents a circuit topology of the Bipolar Junction Transistor Low Noise Amplifier (BJT- LNA) operating at 1.5 GHz. The circuit is constructed using AT41435 Low-Noise BJT Device. The design proposes tradeoffs between gain, noise and blocking performances [2]. Agilent's ADS software in RF and microwave simulation of circuit and system has unique advantages. Some of them are friendly interface, model base of integrity, RF performance simulation and optimization of convenience. This paper just uses Agilent's ADS software for designing the Low-Noise amplifier used in IEEE 802.11b and describes in detail the methods involved in the design and simulation of Low Noise Amplifier. In Section 2, we have analyzed the basic suitability of the device for the construction of the circuit at the desired uency range of 1.5 GHz. In the Section 3, we have analyzed and discussed the design methodology of Input Matching Network for obtaining the optimum impedance matching. Then, we design the output matching network using Microstrip-Lines in Section 4. In Section 5, we discuss the overall schematic and optimum Gain measurement at Low-Noise Figure of 1.768 db.the simulation results and future prospects of the design are presented in Section 6. 2. Device Characterization + Corresponding author. Tel.: 9444709665. E-mail address: rupinisharmila@gmail.com. 176
The design of Low-Noise Amplifier involves Device Characterization (DC Analysis), Biasing condition, Stability analysis, Design of Input and Output Matching Network, Performance Optimization and Impedance Matching. The first stage in the design process is to pick a suitable device that will give us plenty of gainmargin to allow for noise mismatching. 2.1 DC Analysis The BJT Device used in this paper is AT-41435 biased to operate at V ce = 8V, I c = 10 ma. The value I bb is calculated as 120 µa at the operating point using I b =I c /β as given in Fig 1. denoting the VI-Characteristic Curve of the Device [5]. DC.IC.i, ma 18 16 14 12 10 8 6 4 2 0-2 0 Device I-V Curves 2 4 6 8 VCE VCE= 8.000 DC.IC.i=0.010 IBB=0.000120 Fig. 1: Voltage-Current Characteristics of BJT AT41435 Device 2.2 Stability Analysis: Stability Analysis is performed to verify the immunity of the device against oscillations and the stability factor K at required specification of 1.5 GHz is evaluated as 1.098.The optimum Noise Figure value for the device is given by 1.571 db. The necessary and sufficient conditions for unconditional stability are: where K > 1, < 1 (1) (2) & (3) The value of K obtained using the theoretical analysis is 1.1 and the value obtained using ADS is 1.098 which shows K > 1 and the value of obtained using theoretical analysis is 0.14 and the value obtained using ADS is 0.123 as shown in Fig 2. K > 1, < 1 shows the device is unconditionally stable. Thus, the Device Characterization helps us to choose the proper device meeting our specification. 0.22 0.20 mag_delta 0.18 0.16 0.14 0.12 mag_delta=0.123 0.10 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 1.2 m4 1.0 StabFact1 mag_delta 0.8 0.6 0.4 mag_delta=0.123 0.2 0.0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 m4 StabFact1=1.098 Fig. 2: Stability Analysis of AT41435 Device Rollett s Factor K and Delta Function 177
The general block diagram of RF Low-Noise Amplifier is given by Fig 3. The Block Diagram consists of Input Matching Network, Output Matching Network and Biasing Network and impedance matching between the same. Fig. 3: Topology of Microwave Amplifier showing Input and Output Matching Network The need for matching networks arises because, in amplifiers in order to deliver maximum power to a load or to perform in a certain desired way, must be terminated properly at both the input and output ports. The input matching network is designed to transform the generator impedance (50Ω) to the source impedance Z s and the output matching network transforms the 50Ω termination to the load impedance Z L. The value of unilateral Transducer Gain GT Umax is theoretically calculated as 15.12 db where 0.38 176, 0.48 32 and simulated results are as shown in Fig 4. Fig. 4: Evaluation of Maximum Unilateral Transducer Gain for AT41435 Device 3. Design of Input Matching Network The input matching network can be designed to match the large signal input impedance of the RF power device with the 50Ω source impedance. Therefore, the large signal input impedance of the RF transistor should be estimated at the nominal input power, operating uency, and bias voltages with the existence of the load and output matching networks. The input matching network improves the net input power delivered to the RF device. The amplifier circuit was simulated again after adding the input matching circuit using ADS as shown in Fig 5. to improve the performance characteristics of the amplifier. This design has presented and discussed the main guidelines for synthesizing the input matching circuits for the LNA RF amplifier to achieve the improved performance. 178
Fig 5. Schematic Representation of Input Matching Network for Low-Noise RF Amplifier If we use alumina with r = 9.6 and H = 25 mils to build the amplifier we find that a characteristics impedance of 50Ω is obtained with W= 39.78 mils and ff = 6.64.The microstrip length in the 50Ω Alumina microstrip line is λ = 0.3984 λ 0 where f=1.5ghz.the value of S(2,1) is obtained as 11.71 db and noise figure value is given as 1.866 db as given in Fig 6. 12.6 =11.71 12.4 12.2 12.0 11.8 11.6 S(2,1) 12.289 / -51.375 12.173 / -52.336 12.057 / -53.291 11.942 / -54.239 11.826 / -55.180 11.710 / -56.115 11.635 / -57.037 11.561 / -57.955 11.486 / -58.868 11.411 / -59.777 11.337 / -60.681 11.4 =1.866 2.00 1.724 1.749 1.776 1.804 1.834 1.866 1.899 1.934 1.971 2.009 2.049 1.95 1.90 1.85 1.80 1.75 Fig. 6: Simulation Results for Input Matching Network 1.70 4. Design of Output Matching Network The output matching network for the LNA is designed to transform the impedance of 50Ω to the load impedance Z L or to the load reflection coefficient. The output matching network for the Low-Noise Amplifier operating at 1.5 GHz is designed using ADS as shown in Fig 7. The value of S(2,1) is given by 14.37 db and noise figure is given by 1.51 db as shown in Fig 8. 179
Fig. 7: Schematic Representation of Output Matching Network for Low-Noise RF Amplifier S(2,1) 5.431 / -43.001 5.394 / -44.406 5.356 / -45.820 5.316 / -47.244 5.276 / -48.678 5.235 / -50.120 5.216 / -51.525 5.197 / -52.939 5.176 / -54.362 5.154 / -55.793 5.131 / -57.233 1.538 1.544 1.551 1.557 1.564 1.571 1.578 1.584 1.592 1.599 1.606 =14.378 nf (2)=1.571 14.7 1.61 1.60 14.6 1.59 14.5 14.4 1.58 1.57 1.56 14.3 1.55 1.54 14.2 1.46 1.48 1.50 1.52 1.54 1.44 1.56 1.53 1.46 1.48 1.50 1.52 1.54 1.44 1.56 Fig. 8: Simulation Results of Output Matching Network for 1.5 GHz LNA 5. Performance Optimization of Low-Noise Amplifier The input and output matching network combined with the active biasing network of the BJT-LNA is designed using ADS and impedance matching is performed to obtain the optimum gain of 12.715 db and gain ripple of 1.02dB with noise figure of 1.768 db. The simulated results are given by Fig 9. =12.715 =1.768 db(s(2,2))=-34.262 13.4 1.95-20 13.2 1.90-25 13.0 12.8 12.6 1.85 1.80 1.75 db(s(2,2)) -30-35 12.4 1.70-40 12.2 1.65 Fig. 9: Simulation Results of Low-Noise Amplifier at 1.5 GHz -45 An LNA design presents a great challenge because of its simultaneous requirement for high gain, low noise figure, good input and output matching and unconditional stability at the lowest current draw from the amplifier [3]. Although gain, noise figure, stability, linearity and input and output match are all equally important, each of these parameters are independent and rarely work. Typically, the proposed LNA requires: 180
Low supply voltage, High gain, Low noise figure Low current consumption, hence ultra-low power consumption Unconditionally stable, Good Input return loss High isolation, Low cost Most of these conditions can be met by carefully selecting a transistor, choosing the right component values and understanding parameter trade-offs. Low noise figure and good input match can be simultaneously obtained using feedback configurations. High gain apart from producing inter-modulation distortion, can lead to instability. Unconditional stability requires a certain gain reduction. 6. Simulation Results and Future Prospects The designed LNA requires a 15V supply voltage. The circuit is designed and simulated using Advanced Design System Software from Agilent Technologies. At l.5 GHz, the proposed BJT-LNA has a low noise figure (NF) of 1.768 db and optimum gain of 12.715 for wideband as shown in Fig 10. 0.0000 Hz 10.00 MHz 20.00 MHz 30.00 MHz 40.00 MHz 50.00 MHz 60.00 MHz 70.00 MHz 80.00 MHz 90.00 MHz 100.0 MHz 110.0 MHz 120.0 MHz 130.0 MHz 140.0 MHz 150.0 MHz 160.0 MHz 170.0 MHz 180.0 MHz 190.0 MHz 200.0 MHz 210.0 MHz 220.0 MHz 230.0 MHz 240.0 MHz 250.0 MHz 260.0 MHz 270.0 MHz 280.0 MHz 290.0 MHz S(2,1) 28.162 / 169.250 27.829 / 166.322 27.488 / 163.401 27.141 / 160.488 26.790 / 157.583 26.433 / 154.687 26.072 / 151.801 25.708 / 148.926 25.342 / 146.062 24.973 / 143.210 24.603 / 140.371 24.231 / 137.543 23.860 / 134.729 23.489 / 131.927 23.118 / 129.138 22.748 / 126.362 22.380 / 123.598 22.014 / 120.847 21.650 / 118.109 21.289 / 115.382 20.931 / 112.667 20.575 / 109.963 20.223 / 107.270 19.875 / 104.587 19.530 / 101.913 19.189 / 99.249 18.851 / 96.593 18.518 / 93.944 18.188 / 91.303 17.862 / 88.668 f req= =12.715 40 20 0-20 -40-60 -80 0 1 2 3 4 5 6 7 8 9 10 Fig. 10: Simulation Results of Wide-Band Low-Noise RF Amplifier Table 1. presents the summary of Simulation Results of 1.5 GHz Low-Noise Amplifier. Table 1. Simulation Results of 1.5 GHz Low-Noise Amplifier PARAMETER VALUE RF Frequency 1.5 GHz Power Supply 15 V Noise Figure 1.7 db Gain 12.715 db S 11-2.97 db The amplifier can be further modified and improved to form Two-Stage Amplifier for achieving better gain. When a two stage amplifier is to be designed, the source and load must be matched to the conjugate of the output reflection coefficient of the first stage and input reflection coefficient of the second stage. The impedance to be presented to the output of the first stage is transferred to the impedance to be presented for the second stage directly to minimize the length of the transmission line. Thus, there are many design challenges involved while designing the two-stage amplifier design. 7. Conclusion The design proposed is efficiently used in the Wireless Communication applications for amplifying the Wideband RF signals at 1.5 GHz with better gain of 12.715 db and Low Noise Figure of 1.768 db. The design of RFIC remains a huge challenge due to strong constraints in power consumption and noise. Thus, bipolar junction transistors were the first solid-state active device to provide better practical gain and low noise figure at microwave uencies. 181
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