Low Noise Amplifiers Bipolar Junctions Computer Science Essay

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As we already know that Low Noise Amplifier (LNA) is a device which is used to amplify the signals by using Bipolar Junction transistor. The aim of this report of this subject RF & Microwave circuit design (ENG 3036L) is to design a low noise amplifier using the traditional method and also using the Advanced Design System software (ADS). The report includes the calculations using Smith chart as traditional method and also using advanced system software to simulate the design to get the expecting design. The circuit contains some components like transistor, inductor, capacitors and resistors. This report ends with smith chart calculations, LNA design using ADS and the outputs.

Keywords: ADS, smith chart, LNA, transistor, capacitors, inductors, resistors.

Introduction

Low Noise Amplifiers (LNA)

Low Noise Amplifiers are the special type of electronic device which are used to amplify weak signals and filter out the noise of the input signal captured by an antenna in communication system. These LNA amplifiers are intently using in lot of applications like RF communication system including wireless computer networks and mobile phones etc. Commonly, the LNA is talented of decreasing the incoming noise and also amplifying the signal within the certain frequency range to increase the signal to noise ratio (SNR) of the communication system and also increase the quality of the received signal. Normally LNAs are located very close to the detection device to reduce the losses in transmission line because when we are using transmission lines to transmit, it should carry the entire signal successfully without any losses and also without radiating or absorbing any energy. This kind of antenna set up is broadly used in microwave system like GPS because mostly the coaxial cable transmission lines are very loss in microwave frequencies. The main purpose of using the Low Noise Amplifiers is to reduce the noise by the gain by the amplifier while the noise of the amplifier is included into the received signal. LNA has different types of features of wireless communications which includes cellular communication and wireless LANs. The LNA amplifiers are used in combination with many radio frequency functions such as mixers, voltage controlled oscillator, filters, etc.

LNA Design Pattern

The purpose of this experiment is to create a Low Noise Amplifier (LNA) by using the traditional method (calculations & Smith chart) and also using the Advanced Design System (ADS) software. The experiment should end up with a design including layout of the circuit and component values with the specifications below.

Low Noise Amplifier Specifications:

Use Philips BFG 198 transistor.

Centre frequency 900MHz, bandwidth 100 MHz

Gain: 12 dB ± 0.5 dB

Noise figure: < 1.4 dB

I/P and O/P return loss < -10 dB

Bias condition: Vce = 4 V, Ic = 10 mA

Freq S11 S21 S12 S22

800 .637 172.0 3.726 73.4 .078 52.5 .202 -100.5

900 .642 167.0 3.352 69.7 .085 54.9 .194 -106.5

1000 .648 162.5 3.027 66.1 .092 56.3 .189 -113.1

Noise parameters Rn=3.5, NFmin= 1.0dB, Гopt= 0.63<-172

Amplifier theory and Pattern

Bipolar Junction Transistor (BJT)

Bipolar Junction Transistor (BJT) is a three terminal device which is used as amplifier and switch. BJTs are also commonly known as a Bipolar Transistor because the basic construction of this transistor consists of two P-N junctions with each terminal have a name to identify the terminals known as Emitter, Base and Collector respectively. Bipolar Junction Transistor has three types of configurations such as common emitter, common base and common collector. It is also being as p type (PNP) and n type (NPN). Even BJT has two of biasing known as reverse and forward biasing.

Figure 1: NPN type BJT

In BJTs the current is supplied at the input (base) and generates larger current at the collector as output. The gain of the BJT transistor is represented as β. In this experiment BFG 198 transistor has been used with Vce = 4v and Ic = 10mA.

Figure 2: Amplifier Block Diagram

The following parameters were obtained from the data sheet of the Philips BFG 198 transistor;

S11 = 0.61  178 NFmin = 1.5dB

S21 = 3.0  78 opt = 0.48  134

S12 = 0.09  37 rn = 0.15 (normalized)

S22 = 0.28  -69 NFi = 2.0dB

Explanation of matching and biasing networks

Amplifier biasing

Generally, two types of biasing are in Bipolar Junction Transistors which are forward biasing and reverse biasing. In this experiment Collector feedback biasing has been used to do the experiment. In this type of biasing, there is a resistor will be connected in between base and collector as you can see that from the below diagram. The design of this circuit is helped to get the feedback from the collector and passing through the base to increase the forward biasing which is called collector feedback bias. The main purpose of this biasing is to maintain a constant Q point and also it is not a complicated circuit.

Figure 4: Collector feedback bias.

The technique of the biasing is to make the Q point to be stable all the time with temperature changing even and also when the collector current (Ic) increases the current gain (β) also increases.

Fixed biasing

Fixed bias basically consists of a resistor which is connected between the collector supply voltage and with the base. Even though this basic arrangement is not stable because if the temperature of the transistor increases for any of the reasons like current through the transistor or due to the increase in ambient temperature the collector current also will be increased. This raise in current will move the q point (quiescent point) away from its current desired position. So it will also affect the amplifier gain and could result in distortion.

Vcc = IBRB + Vbe Vcc = ICRC + Vce

IB = (Vcc - Vbe)/RB IC = βIB

Voltage Divider Bias

Voltage divider bias is a process of appropriate biasing amplifier that helps in order to maintain a stable Q point even with a various external factors. The need of the voltage divider bias is to maintain a constant Q point which is balance the change in external factors thereby keeping a stable Q point. Voltage divider bias is a method of suitably biasing amplifier by connecting two resistors (noticed as R1 and R2) in series. This voltage divider bias is a combination of fixed bias and self bias. The operating point of a transistor can be achieved as an independent of β with a suitable selection of the resistors R1 and R2.

This is approximately if the case,

( \beta + 1 ) R_E >> R_1 \parallel R_2

After comparing all the biasing methods, it was suitable for this LNA design because self bias is firm to several temperatures than the other bias means the change in temperatures will not affect the Q point. Also it is very convenient in practical or designs wise to perform the circuit diagram is above (figure ).

Design using Smith Chart

What is Smith Chart?

The Smith chart is invented by Phillip H. Smith (1905-1987). Smith chart is a graphical design to assist in solving problems with transmission lines and matching circuits. The use of the Smith chart is increasing in the field of radio frequency (RF). The Smith chart can be used to represents many parameters including impedance, admittance, noise figure etc.

The features of the Smith chart

Display a sequence of normalized impedance, admittance or reflection coefficient in a circle of unity radius commonly known as a Smith Chart

Complex-valued sequences can be entered through the keyboard, or through copy/paste from a opened local text file

Options of viewing with the impedance chart, admittance chart or both

Reporting parameters to any data point on the chart

Smith chart plotting

Calculate Noise Circle for 2dB

Calculate Gain Circle for 12.5dB, 13dB and 13.5dB

Drawn Smith Chart

Determine Shunt & Series Component from Smith Chart

Advanced Design System (ADS) Software

Introduction to ADS software

Advanced Design System (ADS) is an electronic designed software system for RF, microwave and high speed data links. Advanced Design System has lot of advantages like speed, understandable arrangement, easy to use etc. ADS software is very convenient to use and also it helps each and every step of the process such as schematic capture, layout, time domain circuit simulation, frequency domain circuit simulation.

Circuit design

Selecting the transistor

In order to design the Low Noise Amplifier, the first step is creating a new project when the schematic window is opened. As given specifications the Philips BFG 198 transistor is selected. The transistor is chose from the library system in the menu bar and the name of the transistor is typed in that dialogue box as BFG 198 in the library system with Vcc = 4V and Ic = 10mA. The library window is shown below when the transistor is selected.

Designing the circuit

The first thing is selected the Tlines-Microstrip palette and from this select Msub which is placed in the schematic window. The Msub is selected to set up the parameters for the microstrip lines. Then it is double clicked to change the parameters as H= 1.58, Er = 2.55, T = 0.03 and tanD = 0.01.

Frequency Dependent Components

The MLINs and MTEEs are selected from the microstrip palette to connect components together and those are positioned 3mm Ã- 3mm except those connected to ports. The other MLINs and MTEEs are positioned 4.4mmÃ-5mm wide and long respectively.

Biasing Networks

In this circuit design the bias is created of two resistors and 2 low pass filters. To produce a high impedance RF path, the base and collector each have their own 1st order filter followed by a large value shunt capacitor which gives the low impedance path to ground. The element values obtained from the smith chart by calculating.

Optimizing the LNA design

Optimizing is completed by double clicking on each of the component but the capacitors only elected to be discrete optimize.

The goals are set for the gain and noise figure. The S21 is set for the gain and the noise figure is set in the appropriate box in S-parameters by clicking on by using the noise tab. All the required items were selected from the palettes Optim/ Start/ Yield or from the Simulation S- param palette.

Simulation Result Requirements

Selecting a transistor with a package

The transistor model is replaced with a package which package is selected from the library as like previous. The diagram below shows the replacement model of the transistor. As from the diagram shows normally the transistor package model has 4 connections instead of 3 because the real RF transistor has 2 emitter for low inductance path to ground.

G:\RF 12.JPG

Creating a layout

To generate a schematic, from the layout menu Place Components from Schem is selected. All the components are surrounded by red boxes on the schematic and also there is layout window is opened. The process is very easy that placing the components from schematic window to layout window like just selected the component in the schematic window and moves the cursor into the layout window. The first component is placed at port 1 because which is allowed the layout to be built up from the left. There are some items cannot be placed in the layout window since they are not required to be on the microstrip board which are battery and ground.

Simulation Results

Properties

S-Parameters

Scattering parameters or S-parameters are reflection and transmission coefficients between the wave's incident and reflection. These S parameters are describing fully about the characteristics of a device with a certain linear conditions at microwave frequency range. These are characterized by magnitude, decibel and phase. S- Parameters are always voltage ratio of the waves so the expression in decibel is 20 log(Sij).

VSWR (Voltage Standing Wave Ratio)

Voltage standing wave ratio represents the ratio of a line's maximum voltage to minimum voltage,

When impedance terminates the transmission line then it does not match the characteristic impedance of a transmission line. It cannot be absorbed all of the power of the termination but small amount of power will reflect back down the transmission line. So the incident (forward signal) signal mix with the reflected (reverse signal) signal to create a voltage standing wave pattern on the transmission line which ratio between the maximum to minimum voltage is called voltage standing wave ratio.

Maximum Gain

The LNA gain is normally set based on the signals strengths. So by alternating the gain of the LNA by measuring the differences between the RF receivers' first received signal strength reading and the second received signal strength reading will show the inter modulation interference is present. Normally, the RF receivers contain a first received signal strength indicator which is measure the strength of the wideband signal also it has the second received signal strength indicator which is couples after BPF and measure the narrowband signal's strength. The maximum power gain is basically very hard to get but it can be achieved upon the parameters characteristics. LNA is better when it has higher gain.

Minimum Noise Figure

Generally noise figure is determined by using the simple noise figure. Simple noise figure is the noise figure of a device finished with the certain value of source impedance. Basically the noise figure varies with the source reflection changes. So the noise figure of a device is to be minimized the relationship between noise figure and source impedance has to be known.

First print out (Incorrect)

Second print out (Incorrect)

Third print out (Correct)

Layout

Conclusion

As a conclusion from this experiment that the LNA design was designed with the given specifications. First of all the transistor was selected according to the biasing which was collector feedback (self bias) biased transistor known as Philips BFG 198. The calculations of Smith chart was measured and the design has been done with the help of the software ADS (Advanced Design System) which was very convenient for simulations. According to the specifications given some simulation waveforms were obtained like S (11), S (22), S (21), VSWR, Max gain, NF min and NS circle. The micro strip outline was created and it was also very convenient to build. The optimization and simulation were done more than three times to get the accurate result as predicted. The outline of the LNA design using ADS software as this experiment was very useful to understand the subject and to get familiar with that particular software regarding the LNA.

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