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A Novel Design of Low-Noise RF Amplifier for Orthogonal Frequency Division Multiplexing

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Bonfring International Journal of Research in Communication Engineering Volume 1, Issue Inaugural Special Issue, 2011
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  Bonfring International Journal of Research in Communication Engineering, Vol. 1, Special Issue, December 2011 5 ISSN 2250  –   110X | © 2011 Bonfring 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. One of the advantages of OFDM  system over the single carrier system is the better performance in multipath environment. For narrowband operation, one may construct simple am  plifiers 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 Low-Noise amplifier using Agilent ADS software. The  proposed design aims to provide a better gain of 12.7 dB with low Noise Figure (NF) of 1.76 dB. Keywords---   Orthogonal Frequency Division Multiplexing,  Low-Noise Figure I.   I  NTRODUCTION  IRELESS 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 frequency (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 frequency 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 the friendly interface, model base of integrity and RF  performance simulation and optimization of convenience. This  paper 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 LNA. G. Sharmila, PG Student, Department of Electronics and Communication  Engineering, Sri Venkateswara College Of Engineering, Pennalur, Sriperumbudur-602105. E-mail: rupinisharmila@gmail.com  E.G. Govindan, Professor, Department of Electronics and Communication Engineering, Sri Venkateswara College Of Engineering,  Pennalur, Sriperumbudur-602105. In this paper, in Section II, we have analyzed the basic suitability of the device for the construction of the circuit at the desired frequency range of 1.5 GHz. In the Section III, 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 IV. In Section V, we discuss the overall schematic and optimum Gain measurement at Low- Noise Figure of 1.76 dB. The simulation results and future  prospects of the design are presented in Section VI. II.   D EVICE C HARACTERIZATION  The design of Low-Noise amplifier involves DC Analysis and Device Characterization, 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 gain margin to allow for noise mismatching.  A.    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 at the operating point using I  b =I c /β as given in Fig ure 1. denoting the VI-Characteristic Curve of the Device. m3VCE=DC.IC.i=0.010IBB=0.0001208.0001234567890100246810121416-218 IBB=2.000E-5IBB=4.000E-5IBB=6.000E-5IBB=8.000E-5IBB=1.000E-4IBB=1.200E-4IBB=1.400E-4IBB=1.600E-4IBB=1.800E-4IBB=2.000E-4 VCE    D   C .   I   C .   i ,   m   A m3 Device I-V Curves m3VCE=DC.IC.i=0.010IBB=0.0001208.000 VCEIBB=2.000E-50.0000.5001.0001.5002.0002.5003.0003.5004.0004.5005.0005.5006.0006.5007.0007.5008.0008.5009.0009.50010.000IBB=4.000E-50.000DC.IC.i-22.14uA1.274mA1.300mA1.326mA1.351mA1.377mA1.403mA1.429mA1.455mA1.481mA1.506mA1.532mA1.558mA1.584mA1.610mA1.636mA1.661mA1.687mA1.713mA1.739mA1.765mA-45.36uA  Figure 1: VI-Characteristics of AT-41435 Device The device is characterized to operate at the biasing point after DC Analysis using Stability Analysis and AC Analysis is also performed followed by S-parameter and Noise Figure simulation.  B.   Stability Analysis Stability of an amplifier refers to its resistance to oscillations. This can be determined from the S parameters, matching networks, source and load terminations. In two port networks, oscillations occur when any one of the ports present a negative resistance which occurs when or A Novel Design of Low-Noise RF Amplifier for Orthogonal Frequency Division Multiplexing G. Sharmila and E.G. Govindan W  Bonfring International Journal of Research in Communication Engineering, Vol. 1, Special Issue, December 2011 6 ISSN 2250  –   110X | © 2011 Bonfring . The Stability Analysis is performed and the stability factor K =1.084 (K > 1) at required specification of 1.5 GHz is obtained as shown in Figure 2. The necessary and sufficient conditions for unconditional stability are: K > 1, |∆| < 1   …… ( 2.1) …… ( 2.2) where ……(2.3)  and ……(2.4)  Thus, the Device Characterization helps us to choose the  proper device meeting our specification. Figure 2: Stability Analysis of AT-41435 BJT Device indep(S_StabCircle1) (0.000 to 51.000)    S_   S   t   a   b   C   i   r   c   l   e   1 m1m1indep(m1)=S_StabCircle1=1.918 / 147.600freq=1.500000GHzimpedance = Z0 * (-0.338 + j0.260)6  Figure 3: Stability Circle for AT-41435 Device The general block diagram of RF Low-Noise Amplifier is given by Figure 4. Figure 4: Microwave Amplifier Topology The Block Diagram consists of Input Matching Network, Output Matching Network and Biasing Network and impedance matching between the same. The mode of  propagation in a microstrip lines is assumed to be quasi-transverse electromagnetic. Although radiation losses in a microstrip line are severe, the use of a thin material, having a high dielectric constant, between the top strip conductor and the ground plane of a microstrip, line reduces the radiation losses to a minimum. Microstrip lines find extensive use as passive circuit elements and as a medium in which the complete microwave amplifier can be built. The interconnection features of the microstrip line are unsurpassed. Transistor in chip or packaged form can be easily attached to the strip conductors of the strip line. Figure 5: 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. In the above Figure 5., a typical situation in which a transistor, in order to deliver maximum power to the 50 Ω load, must have the terminations Z s  and Z L [4]. 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 GT Umax is theoretically calculated as 15.12 dB where 0.38∟176◦, 0.48∟32◦  III.   D ESIGN OF I  NPUT M ATCHING  N ETWORK   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 frequency, and bias voltages with the existence of the load and output matching networks. The large signal input impedance of the power transistor consists of two parts, resistance  R in   and reactance  X  in :  Bonfring International Journal of Research in Communication Engineering, Vol. 1, Special Issue, December 2011 7 ISSN 2250  –   110X | © 2011 Bonfring Z in  = R  in  + X in ……..……… (3.1)  There is no doubt that 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. However, with an accurate and  proper design of the input matching network, the performance characteristics of the amplifier can be improved. 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. The schematic designed using distributed components is given by Figure 6 (a). (a) (b)   Figure 6: Input Matching Network of LNA a) Schematic b) Layout   If we use alumina with Γ 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 Γ ff   = 6.64.The microstrip length in the 50Ω Alumina microstrip line is   λ = 0.3984λ  0 where f=1.5GHz. The value of S(2,1) is obtained as 11.7 dB and noise figure value is evaluated as 1.8 dB as given in Figure 7. m1freq=dB(S(2,1))=11.711.500GHz 1.   461.481.501.521.541.441.5611.611.812.012.212.411.412.6 freq, GHz    d   B   (   S   (   2 ,   1   )   ) m1m1freq=dB(S(2,1))=11.711.500GHz freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHzS(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 m2freq=nf(2)=1.8661.500GHz 1.   461.481.501.521.541.441.561.751.801.851.901.951.702.00 freq, GHz    n   f   (   2   ) m2m2freq=nf(2)=1.8661.500GHz freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHznf(2)1.7241.7491.7761.8041.8341.8661.8991.9341.9712.0092.049   m3freq=dB(S(2,2))=-8.2061.500GHz 1.461.481.501.521.541.441.56-8.2-8.1-8.0-8.3-7.9 freq, GHz        d       B        (        S        (       2 ,       2        )        ) m3   m3freq=dB(S(2,2))=-8.2061.500GHz freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHzS(2,2)0.397 / -36.975 0.395 / -36.730 0.393 / -36.480 0.392 / -36.224 0.390 / -35.962 0.389 / -35.696 0.387 / -35.504 0.385 / -35.304 0.384 / -35.096 0.382 / -34.880 0.381 / -34.656 Figure 7: Simulation Results for Input Matching Network IV.   D ESIGN OF O UTPUT M ATCHING  N ETWORK   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 50 ohm line is matched to the load impedance to be presented at the output of the LNA.The output matching network for the Low-Noise Amplifier operating at 1.5 GHz is designed using ADS as follows: Figure 8: Schematic for Output Matching Network The value of S(2,1) is given by 14.3 dB and noise figure is given by 1.5 dB as shown in Figure 9.  Bonfring International Journal of Research in Communication Engineering, Vol. 1, Special Issue, December 2011 8 ISSN 2250  –   110X | © 2011 Bonfring m1freq=dB(S(2,1))=14.3781.500GHz 1.461.481.501.521.541.441.5614.314.414.514.614.214.7 freq,GHz      d     B     (     S     (     2 ,     1     )     )   m1m1freq=dB(S(2,1))=14.3781.500GHzm2freq=nf(2)=1.5711.500GHz 1.461.481.501.521.541.441.561.541.551.561.571.581.591.601.531.61 freq,GHz     n     f     (     2     )   m2m2freq=nf(2)=1.5711.500GHzm3freq=dB(S(2,2))=-17.9231.500GHz 1.461.481.501.521.541.441.56-20-18-16-22-14 freq,GHz      d     B     (     S     (     2 ,     2     )     )   m3m3freq=dB(S(2,2))=-17.9231.500GHz freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHzS(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 freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHznf(2)1.5381.5441.5511.5571.5641.5711.5781.5841.5921.5991.606freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHzS(2,2)0.186 /134.778 0.174 /131.874 0.162 /128.697 0.150 /125.181 0.139 /121.241 0.127 /116.765 0.116 /111.263 0.105 /104.808 0.095 /97.152 0.086 /88.035 0.079 /77.285 Figure 9: Simulation Results for Output Matching Network of 1.5 GHz LNA V.   P ERFORMANCE O PTIMIZATION OF L OW -N OISE A MPLIFIER   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 Figure 10.  m1freq=dB(S(2,1))=12.7151.500GHz 1.46 1.48 1.50 1.52 1.541.44 1.5612.412.612.813.013.212.213.4 freq,GHz     d    B     (     S     (    2 ,    1     )     )   m1m1freq=dB(S(2,1))=12.7151.500GHzm2freq=nf(2)=1.7681.500GHz 1.46 1.48 1.50 1.52 1.541.44 1.561.701.751.801.851.901.651.95 freq,GHz    n     f     (    2     )   m2m2freq=nf(2)=1.7681.500GHzm3freq=dB(S(2,2))=-34.2621.500GHz 1.46 1.48 1.50 1.52 1.541.44 1.56-40-35-30-25-45-20 freq,GHz     d    B     (     S     (    2 ,    2     )     )   m3m3freq=dB(S(2,2))=-34.2621.500GHz  Figure 10: Simulation Results of Low-Noise Amplifier at 1.5 GHz 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 in each other's favour. Typically, the proposed LNA requires: 1.   Low supply voltage 2.   Low current consumption, hence ultra-low power consumption 3.   High gain 4.   Low noise figure 5.   Unconditionally stable 6.   Input return loss 7.   High isolation 8.   Low cost Most of these conditions can be met by carefully selecting a transistor, choosing the right component values and understanding parameter tradeoffs. Low noise figure and good input match can be simultaneously obtained using feedback configurations. High gain at gigahertz frequencies, in addition to producing intermodulation distortion, can lead to instability. Unconditional stability requires a certain gain reduction. VI.   S IMULATION R  ESULTS AND F UTURE P ROSPECTS  The designed LNA requires only a 1.5 V supply voltage. The circuit is designed and simulated using Advanced Design System Software from Agilent Technologies. ADS can be used for virtual prototyping, debugging or in aiding manufacturing. To enhance engineering productivity and shorten time-to-market, ADS offers a high level of design automation and applications intelligence. At l.5 GHz, the  proposed BJT-LNA has a low noise figure (NF) of 1.7 dB, with input return loss of -2.9 dB [5]. 1.46 1.47 1.48 1.49 1.50 1.51 1.52 1.53 1.541.45 1.55-3.2-3.1-3.0-2.9-2.8-3.3-2.7 freq, GHz        d       B        (        S        (       1 ,       1        )        )   m4m4freq=dB(S(1,1))=-2.9721.500GHz   m4freq=dB(S(1,2))=-24.6101.500GHz 1.46 1.47 1.48 1.49 1.50 1.51 1.52 1.53 1.541.45 1.55-24.610-24.605-24.600-24.595-24.590-24.615-24.585 freq, GHz        d       B       (       S       (       1 ,       2       )       ) m4m4freq=dB(S(1,2))=-24.6101.500GHz  Figure 11: Simulation Results showing Input Return Loss Table 1 presents the summary of Simulation Results of 1.5 GHz Low-Noise Amplifier   
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