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 frequency 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 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 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 frequency 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.

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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 gain-margin to allow for noise mismatching.

2.1 DC Analysis The BJT Device used in this paper is AT-41435 biased to operate at Vce = 8V, Ic = 10 mA. The value Ibb is

calculated as 120 µA at the operating point using Ib=Ic/β as given in Fig 1. denoting the VI-Characteristic Curve of the Device [5].

m5VCE=DC.IC.i=0.010IBB=0.000120

8.000

2 4 6 80

02468

10121416

-2

18

VCE

DC.IC

.i, m

A m5

Device I-V Curves

m5VCE=DC.IC.i=0.010IBB=0.000120

8.000

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:

K > 1, |∆| < 1

(1) (2)

where

& (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.

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.91.0 2.0

0.12

0.14

0.16

0.18

0.20

0.10

0.22

freq, GHz

mag

_del

ta

m1

m1freq=mag_delta=0.123

1.500GHz

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.91.0 2.0

0.2

0.4

0.6

0.8

1.0

0.0

1.2

freq, GHz

mag

_delta

m5

Stab

Fact

1

m4

m5freq=mag_delta=0.123

1.500GHz

m4freq=StabFact1=1.098

1.500GHz

Fig. 2: Stability Analysis of AT41435 Device – Rollett’s Factor K and Delta Function ∆

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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 Zs and the output matching network transforms the 50Ω termination to the load impedance ZL. The value of unilateral Transducer Gain GTUmax 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 frequency, 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.

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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 λ0 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.

m1freq=dB(S(2,1))=11.71

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

11.6

11.8

12.0

12.2

12.4

11.4

12.6

freq, GHz

dB(S

(2,1

))

m1

m1freq=dB(S(2,1))=11.71

1.500GHz

freq1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHz

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

m2freq=nf(2)=1.866

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

1.75

1.80

1.85

1.90

1.95

1.70

2.00

freq, GHz

nf(2

)

m2

m2freq=nf(2)=1.866

1.500GHz

freq

1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHz

nf(2)

1.7241.7491.7761.8041.8341.8661.8991.9341.9712.0092.049

Fig. 6: Simulation Results for Input Matching Network

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 ZL 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.

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Fig. 7: Schematic Representation of Output Matching Network for Low-Noise RF Amplifier

m1f req=dB(S(2,1))=14.378

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

14.3

14.4

14.5

14.6

14.2

14.7

freq, GHz

dB(S

(2,1))

m1

m1f req=dB(S(2,1))=14.378

1.500GHzm2f req=nf (2)=1.571

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

1.54

1.55

1.56

1.57

1.58

1.59

1.60

1.53

1.61

freq, GHz

nf(2) m2

m2f req=nf (2)=1.571

1.500GHz

freq

1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHz

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

freq

1.450 GHz1.460 GHz1.470 GHz1.480 GHz1.490 GHz1.500 GHz1.510 GHz1.520 GHz1.530 GHz1.540 GHz1.550 GHz

nf(2)

1.5381.5441.5511.5571.5641.5711.5781.5841.5921.5991.606

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.

m1freq=dB(S(2,1))=12.715

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

12.4

12.6

12.8

13.0

13.2

12.2

13.4

freq, GHz

dB(S

(2,1

))

m1

m1freq=dB(S(2,1))=12.715

1.500GHzm2freq=nf(2)=1.768

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

1.70

1.75

1.80

1.85

1.90

1.65

1.95

freq, GHz

nf(2

)

m2

m2freq=nf(2)=1.768

1.500GHzm3freq=dB(S(2,2))=-34.262

1.500GHz

1.46 1.48 1.50 1.52 1.541.44 1.56

-40

-35

-30

-25

-45

-20

freq, GHz

dB(S

(2,2

))

m3

m3freq=dB(S(2,2))=-34.262

1.500GHz

Fig. 9: 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. Typically, the proposed LNA requires:

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• 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.

freq0.0000 Hz

10.00 MHz20.00 MHz30.00 MHz40.00 MHz50.00 MHz60.00 MHz70.00 MHz80.00 MHz90.00 MHz100.0 MHz110.0 MHz120.0 MHz130.0 MHz140.0 MHz150.0 MHz160.0 MHz170.0 MHz180.0 MHz190.0 MHz200.0 MHz210.0 MHz220.0 MHz230.0 MHz240.0 MHz250.0 MHz260.0 MHz270.0 MHz280.0 MHz290.0 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

m3f req=dB(S(2,1))=12.715

1.500GHz

1 2 3 4 5 6 7 8 90 10

-60

-40

-20

0

20

-80

40

freq, GHz

dB(S

(2,1

))m3

m3f req=dB(S(2,1))=12.715

1.500GHz

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 S11 -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 frequencies.

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8. References [1] Golmie, N., Chevrollier, N., Rebala. Bluetooth and WLAN coexistence: challenges and solutions. In: IEEE

Wireless Communication, Volume 10, Issue 6, pp.22-29, Dec. 2003.

[2] D. K. Shaeffer and T. H. Lee. A 1.5-V, 1.5-GHz CMOS low noise amplifier. In: IEEE J.Solid-State Circuits, Vol. 32, pp: 745-759, May 1997.

[3] S.F.W.M. Hatta. Design of an RF BJT-Low Noise Amplifier at 1 GHz. In: ICSE2006 Proc. 2006, Kuala Lumpur.

[4] Guillermo Gonzalez, Microwave Transistor Amplifiers - Analysis and Design. 2nd Edition, 1996.

[5] Anurag Bhargava, S. Deepak Ram Prasath, V. Periyasamy, S. Raju, V. Abhaikumar. RF Circuit Design Cook Book, 2008.

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