Low Noise Amplifier Design Project
Abstract
The following report presents the work done on the design and simulation of a low noise amplifier. The purpose of the amplifier is to amplify the received RF path of a Wireless local area network (WLAN). The design methodology required the analysis of the transistor stability and proper matching network selection. Ideal microwave amplifier equations were used to provide a starting point for the design.
The design specifications for this amplifier were not very demanding (compared to the industry) due to the nature of it being a first time design. This was the principal reason why the BJT AT 31011 was chosen over others due to the simplest configuration it offers for an amplifier design. Several Measurement techniques using Microwave Office (AWR Suite 2002), Serenade (Ansoft) for simulations, AutoCAD for the design layout and the Network Analyzer for the practical testing of the amplifier were used to verify the performance of the designed amplifier.
The built amplifier performed reasonably well for the required frequency band (2.4GHz – 2.5GHz) on the tests of gain, return loss and noise, thereby closely matching the measured readings with the simulated results.
Design Specifications
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Gain > 10 DB
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Noise Figure < 2-3 dB
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Use microstrip – matching networks
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VSWR between 1.5-2.5
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BW: 100MHz from 2.4GHz to 2.5GHz
The s-parameters of the device, the matching networks and the terminations are all used to determine stability. The stability of the amplifier or in other words its resistance to oscillate is a very important consideration in the design.
Firstly a graphical method using Smith Chart is used to determine the stability conditions of the transistor. The input and output stability circles are plotted using Microwave Office Suite for the transistor s-parameters. A frequency sweep from 1.5GHz to 3.5GHz is applied to check for unwanted oscillations around the operating frequency of 2.4-2.5GHz.
Input and Output Stability Circles
From the figure, we see that both the input and output stability circles lie completely outside the Smith Chart for the range of frequencies 1.5GHz to 3.5GHz; hence the transistor is unconditionally stable for the frequency range.
Gain & Noise Figure
Once the stable regions on the smith chart have been determined, another graphical method is used to choose a particular gain and noise figure. Desired gain and noise figure can be obtained with proper selection of the reflection coefficient of the input and output matching networks. We select an optimum GammaL point on the smith chart out of a random selection of GammaL points by checking for the best return loss performance.
Gain, Noise and Stability Circles at 2.4GHz
The overall performance of a low noise amplifier is determined by calculating the transducer gain GT, noise figure F and the input and output standing wave ratios, VSWRin and VSWRout. The optimum GammaL was obtained as GammaL = 0.4492 + j 0.677 using the Matlab program given in the appendix. The value of GammaL was selected on the 11dB circle, which corresponds to a noise figure of 2.5dB.
Design & Measurement
The design is incorporated using a microstrip matching network and shunt stubs. The length of the microstrip line and shunt stub for the input and output matching networks is calculated in Serenade for the RT/Duroid 6002 substrate with the following specifications:
• Thickness: 60 mils
• Relative Permittivity: 2.94
• Loss Tangent: 0.0012
• Highest frequency of operation: 2.9 GHz
Circuit Schematic in Serenade
Gain of amplifier = 13.2dB at 2.4GHz
VSWRin = 1.38 and VSWRout = 2.11
Fabrication
The AutoCAD drawing is transferred (using the .dxf format) and converted to a gerber file, which in turn is fed to the milling machine. The design is etched on the substrate by the machine. After the milling is complete, the excess metal (copper) is scraped away from the surface of the board. The milled circuit board is as shown below:
Amplifier
Amplifier Description
Summary of Results
Parameter
Gain
VSWRin
VSWRout
Noise
Bandwidth
Conclusion
The degree of success of this project was quite satisfactory. The amplification was a reasonably good value of 9.5 dB with a peak at the center frequency (2.45 GHz) of 10.25 dB. The VSWR was also kept within limits. There was a slight shift in the frequency band to a higher range by around 25MHz. The size of the circuit board was also maintained to 2.1” x 2.3”.
Conclusively, the time spent in studying the design process of a microwave amplifier and the designed tools learned served as a great experience and preparation for the future designing endeavors.