Design Of Wideband Low Noise Amplifier.
DESIGN OF WIDEBAND LOW NOISE AMPLIFIER
YONG KOK SHENG
This Report is Submitted in Partial Fulfillment of Requirement For The Bachelor Degree of Electronic Engineering (Telecommunications Electronics)
Faculty of Electronic and Computer Engineering Universiti Teknikal Malaysia Melaka
(2)
ii
DESIGN OF WIDEBAND LOW NOISE AMPLIFIER
YONG KOK SHENG
UNIVERSTI TEKNIKAL MALAYSIA MELAKA
FAKULTI KEJURUTERAAN ELEKTRONIK DAN KEJURUTERAAN KOMPUTER
BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA II
Tajuk Projek : ………
Sesi
Pengajian : 1 1 / 1 2
Saya ……….. (HURUF BESAR)
mengaku membenarkan Laporan Projek Sarjana Muda ini disimpan di Perpustakaan dengan syarat-syarat kegunaan seperti berikut:
1. Laporan adalah hakmilik Universiti Teknikal Malaysia Melaka.
2. Perpustakaan dibenarkan membuat salinan untuk tujuan pengajian sahaja.
3. Perpustakaan dibenarkan membuat salinan laporan ini sebagai bahan pertukaran antara institusi pengajian tinggi.
4. Sila tandakan ( √ ) :
SULIT*
*(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972)
TERHAD** **(Mengandungi maklumat terhad yang telah ditentukan oleh organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh:
__________________________ ___________________________________
(TANDATANGAN PENULIS) (COP DAN TANDATANGAN PENYELIA)
Tarikh: ……….. Tarikh: ……….. √ AMPLIFIER
(3)
iii
YONG KOK SHENG 11 JUNE 2012
“I hereby declare that this project report is of my own work except for the summary
and passage whereby for each which I have clearly stated its source.”
Signature : ………. Author’s name : ……….
(4)
iv
AZAHARI BIN SALLEH 15 JUNE 2012
“I hereby declare that I have read this thesis and in my opinion this project report is sufficient in term of scope and quality for the award of degree of Bachelor’s
Degree of Electronic Engineering (Telecommunication)”
Signature : ………. Supervisor’s name : ……….
(5)
v
(6)
vi
ACKNOWLEDGMENTS
I am highly appreciative of the effort of my supervisor Mr. Azahari for taking time to read through this report and his positive criticism of the project. Mr. Azahari has given me a lot of idea on the project. . He also helps me to carry out and solves the technical problems that I encountered especially helps me to troubleshoot the circuit in ADS simulation.
Next, I would like to thank my co.supervisor, Mr. Daneshkumar, who is always helpful and patient when guiding me. He has given me a lot of ideas and suggestions in order to design an LNA having such an excellent performance. Especially in ADS software, Mr. Daneshkumar taught me a lot on the basic of using ADS in the S-parameter simulation. I feel lucky and grateful upon his guidance and kindness in helping me in my project. Therefore, I would like to thank him sincerely and appreciate all the hard work by Mr. Daneshkumar.
Finally, special thanks to my family that provided me with full financial support whenever I need it. They also motivated me from time to time that played an important role as to ensure that I am on the right track in finishing the project.
Once again, I hereby want to take this opportunity to thank all of you for your concern and support throughout this project. Thank you!
(7)
vii
ABSTRACT
Today, wideband amplifier design remains one of the most challenging portions in the communication system. The conventional low noise amplifier operates on a single band, in which it is easier to design the amplifier to meet the entire specified goal. However, MOTOROLA wideband low noise amplifier which operate from 100 MHz to 1 GHz, making it harder to design as to operate at wider spectrum while maintaining low noise < 1.5 dB, great input and output return loss <-10dB, good gain > 15 dB with reverse isolation of < -20 dB with restriction of low power consumption < 1.8 V and < 10 mA, thus presenting a tougher challenge for the designer to achieve the goals for the extended frequency range. Traditional methods of tuning and tweaking will require more tedious and time consuming effort to cover the wider frequency range, thus rendering the methods to be impractical. Therefore, a more specific technical approach must be utilized to help the designers to better design the broadband amplifier. This research paper represents the amplifier requirements in wideband application and proposed a structured practical design approach which utilizes the feedback topology by using Advanced Design System (ADS) simulations in optimizing the amplifier to meet all the required specs. The best performance design is fabricated on a FR4 strip board and the performance of designed amplifier was verified using network analyzer. The fabricated LNA exhibits a signal gain of more than 17 dB, NF of less than 1 dB, IIP3 of more than 10 dBm and has been maintained unconditional stable throughout the frequency of interest.
(8)
viii
ABSTRAK
Masa kini, rekaan penguat hinggar rendah kekal satu bahagian yang paling mencabar dalam bidang telekomunikasi. Penguat hinggar rendah konvensional beroperasi dalam satu jalur sempit sahaja, di mana ia lebih mudah untuk direka untuk mencapai semua gol yang ditentukan. Walau bagaimanapun, penguat hinggar rendah jalur lebar Motorola yang beroperasi dari 100 MHz hingga 1 GHz, membuatnya lebih sukar dalam mereka proses kerana ia perlu beroperasi pada spectrum yang lebih luas pada masa yang sama ia perlu mengekalkan hinggar yang rendah < 1.5 dB,
dan < -10 dB, penguatan > 15 dB dengan pengasingan < -20 dB dengan sekatan penggunaan kuasa yang rendah < 1.8 V and < 10 mA. Kaedah tradisional penalaan akan memerlukan banyak usaha dan memakan masa untuk meliputi julat frekuensi yang lebih luas, maka menyebabkan kaedah tradisional untuk menjadi tidak praktikal. Oleh itu, pendekatan teknikal yang lebih khusus mesti digunakan untuk membantu pereka untuk mereka penguat hinggar rendah jalur lebar yang baik. Kertas kajian ini menbentangkan keperluan dalam perekaan penguat hinggar rendah jalur lebar dan mencadangkan pendekatan perekaan praktikal dengan menggunakan topologi penguat suap balik dengan menggunakan Advance Design System (ADS) simulasi dalam mengoptimumkan penguat untuk memenuhi semua spesifikasi yang ditentukan. Rekaan yang memberi prestasi terbaik akan difabrikasikan menggunakan papan FR4 dan ukuran praktikal reka bentuk diuji dengan menggunakan penganalisa vektor rangkaian untuk mengesahkan prestasi LNA. LNA yang difabrikasikan mempamer penguatan lebih daripada 17 dB, hinggar kurang daripada 1 dB, IIP3 lebih daripada 10 dBm dan kekal stabil sepanjang frekuensi operasi.
(9)
ix
TABLE OF CONTENTS
CHAPTER TITLE PAGE
RROJECT TITLE
CONFIRMATION REPORT STATUS ii
DECLARATION iii
SUPERVISOR CONFIRMATION iv
DEDICATION v
ACKNOWLEDGEMENT vi
ABSTRACT vii
ABSTRAK viii
TABLE OF CONTENTS ix
LIST OF TABLES xii
LIST OF FIGURES xiii
LIST OF ABBREVIATIONS xv
LIST OF APPENDICES xvi
I INTRODUCTION 1
1.1 Project Background 1
1.2 Problem Statement 3
1.3 Objective 3
1.4 Scope of work 4
(10)
x II LITERATURE REVIEW 6
2.1 Introduction 6
2.2 Design Consideration 7
2.3 Transistor Biasing 9
2.4 Noise Figure 11
2.5 Two Port Power Gain 13
2.5.1 Operating Power Gain 14 2.5.2 Transducer Power Gain 14 2.5.3 Available Power Gain 15
2.6. S-Parameter 15
2.7 Stability 17
2.8 Stabilization Techniques 18
2.9 Linearity 19
2.9.1 Input 1dB Gain Compression 19 2.9.2 Intermodulation distortion (IP3) 20
2.10 Condition of matching 21
2.11 LNA Architecture 22
2.11.1 RLC Feedback 22
2.11.2 Inductive Feedback 23
2.12 Related Work 23
III PROJECT METHODOLOGY 25
3.1 Introduction 25
3.2 Project Flow 26
3.3 Design specification 28
3.4 Transistor selection 28
3.5 Stability checking 29
3.6 DC biasing 30
(11)
xi
IV RESULTS AND ANALYSIS 34
4.0 Introduction 34
4.1 Stability Analysis 34
4.2 DC Biasing 38
4.3 Two Port Power Gain 40
4.4 Matching Network 43
4.5 Noise Figure 48
4.6 Feedback LNA Design 49
4.7 PCB Layout 51
4.8 Measurement Results versus
Simulation Results 54
4.8.1 Return Loss, S11 54
4.8.2 Gain, S21 55
4.8.3 Output Return Loss, S22 55
4.8.4 Reverse Isolation, S12 56
4.8.5 Noise Figure 56
4.8.6 Input Third Order Inception Point, IIP3 57
IV CONCLUSION AND FUTURE WORKS 58
5.1 Conclusion 58
5.2 Future Works 59
REFERENCES 60
(12)
xii
LIST OF TABLES
NO TITLE PAGE
1.1 Wideband LNA design specification 2
2.1 Types of biasing network and formula 9
2.2 Comparison of the reported LNA with other LNA design 24
3.1 Wideband LNA design specification 28
3.2 Comparison between FET and BJT transistor 29
4.1 Calculated K-factor (before stabilize) 35
4.2 K-factor swept across operating frequency 36
4.3 K factor swept across operating frequency after stabilization 37 4.4 And for different frequency 43
(13)
xiii
LIST OF FIGURES
NO TITLE PAGE
1.1 Block diagram of the superheterodyne receiver 1
2.1 Block Diagram of basic LNA 7
2.2 Illustration of signal to noise in a system 11
2.3 Noise in cascaded stage 13
2.4 Input and output circuit for 2-port network 13
2.5 Two-port network showing incident waves and reflected waves 15
2.6 Stability Circles 17
2.7 Instability region for S11<1 and S11>1 18
2.8 Definition of 1dB compression point for a nonlinear amplifier 19
2.9 IIP3 Concept 20
2.10 IIP3 Measurement in Network Analyzer 21
2.11 Basic Diagram of Matching Network 21
2.12 Feedback Circuit for Stability Increment 22
3.1 LNA design flow 26
3.2 ADS setup to find the best biasing point 31
3.3 Series inductor moves clockwise (left) and series capacitor moves 33 counter-clockwise (right) along circles of constant resistance
3.4 Shunt inductor moves counter-clockwise (left) and shunt capacitor 33 moves clockwise (right) along circles of constant conductance
4.1 Stability checking of BFP640 using ADS 35
4.2 Stabilization of transistor using RC feedback and output resistive load 36
4.3 BFP 640 Characteristic Curves 38
(14)
xiv
4.5 Gain across all biasing point 39
4.6 Biasing Network along with DC block and RF chock 40
4.7 Two port power gain of BFP640 42
4.8 Smith Chart for input matching 44
4.9 Input Matching Network 44
4.10 Smith Chart For Output Matching 45
4.11 Output Matching Network 46
4.12 LNA Design with Matching Network 46
4.13 Comparison between before and after optimization for (a) , 47 (b) , (c) , (d) , (e) noise figure and (f) stability factor
4.14 Noise figure and noise figure minimum 48
4.15 Schematic design for feedback LNA 49
4.16 Input and Output return loss for schematic design 50
4.17 Forward gain and reverse isolation for schematic design 50 4.18 Noise figure and stability factor for schematic design 50
4.19 Schematic with transmission line 52
4.20 Layout for LNA design 53
4.21 LNA Prototype 53
4.22 Comparison between Simulation and Measurement for 54
Return Loss, S11
4.23 Comparison between Simulation and Measurement for Gain, S21 55
4.24 Comparison between Simulation and Measurement for 55
Output Return Loss, S22
4.25 Comparison between Simulation and Measurement for 56
Reverse Isolation, S12
4.26 Comparison between Simulation and Measurement for noise figure 56
4.27 Comparison between Simulation and Measurement for 57
(15)
xv
LIST OF ABBREVIATIONS
LNA - Low Noise Amplifier RF - Radio Frequency
IF - Intermediate Frequency BJT - Bipolar Junction Transistor
E-PHEMT - Enhancement Mode Pseudomorphic HEMT PHEMT - Pseudomorphic HEMT
WLAN - Wireless Local Area Network DC - Direct current
VSWR - Voltage Standing Wave Ratio ADS - Advanced Design System FET - Field-Effect Transistor Si - Silicon
GaAs FET - Gallium arsenide Field-Effect Transistor CMOS - Complementary Metal–Oxide–Semiconductor MMIC - Monolithic Microwave Integrated Circuit MESFET - Mmetal Semiconductor Field Effect Transistor HEMT - High Electron Mobility Transistor
(16)
xvi
LIST OF APPENDICES
NO TITLE PAGE
(17)
CHAPTER 1
INTRODUCTION
1.1 Project Background
The receiver can be divided into Front-End and Back-End. The front-end converts RF (radio frequency) to IF (intermediate frequency). The back-end converts IF to baseband. For digital radios, the back-end will also provide analog to digital conversion. This allows the microprocessor to perform digital signal processing on the received signal and convert it to useful information.
The LNA is a key component in the front-end of receiver section. The main function of LNA is to amplify possibly very weak signal without picking up excessive noise, which will enhance the receiver signal-to-noise ratio (SNR).
(18)
2
Another important attributes of the LNA includes low noise figure, reasonable gain and stability over the designated frequency band without oscillating while operating at very low power levels. For large signal, the LNA amplifies the signal without introducing any distortions. Hence, the sensitivity of the receiver is highly dependent on the LNA as it defines the smallest signal that a receiver can detect. [9]
In this project, a high performance wideband low noise amplifier which operates from 100 MHz to 1 GHz is presented. This wideband requirement means that it will be harder for the designer to build the individual receiver section. The receiver subsection needs to be able to receive signals from every critical band across radio frequency while allowing the user to selectively choose the desired frequency band and convert it to baseband. To achieve that, the LNA needs to provide a constant gain, low noise high linearity and unconditionally stable to ensure the whole receiver chain to meet the standard specification. The technical requirements of the wideband LNA design is shown in Table 1.1 below.
Table 1.1: Wideband LNA Design Specification
Parameter Specs
Operating Frequency (MHz) 100 – 1000
Noise figure (dB) < 1.5
Gain (dB) > 15
Input Third Order Inception, IIP3 (dBm) > 5
Stability Factor (K) > 1
Current (mA) < 10
Voltage (V) < 1.8
(19)
3
1.2 Problem Statement
Portable two ways radio size has been shrinking over time, the employ of multiband radios which consist of several narrow band discrete circuits are larger because inside of housing must contain two or three separate transceivers. Besides, driving a slightly larger housing is the additional filtering and shielding between the transceivers in order to avoid the interfering effect between different transceivers.[1] To solve this problem, the combination of several narrow bands LNA circuit into single wideband LNA circuit is proposed. Wideband LNA design presents a considerable challenge, as we know conjugate matching will give maximum gain only over relatively narrow bandwidth, while designing for less max gain will improve the gain bandwidth, but the input and output part of the amplifier will be poorly matched.[2] For these reason feedback technique is proposed to simultaneously achieve improvement in bandwidth and also on its gain, noise figure and return loss. The conventional LNA suffer from the problem that the input matching network can be tuned for low noise figure or low VSWR (conjugate match) but not both parameter simultaneously.[3] The negative feedback technique can be used in wideband amplifier to provide a flat gain response and to reduce the input and output VSWR. It also controls the amplifier performance due to variations in the S parameters from transistor to transistor and furthermore, in-band stability is also improved by employing negative feedback. [4]
1.3 Objective
The objective of this project is to design low noise amplifiers for MOTOROLA application by using feedback and cascaded technique, then analysed on their noise figure, matching and gain at the operating frequency. Finally the best performance low noise amplifier will be fabricated as final result.
(20)
4 1.4 Scope of work
Scope of this project can be divided into four parts which are: a) Literature review
Include the study of the characteristics of the LNA especially on the gain, noise figure, stability which must be taken into consideration in LNA design. b) Design and simulation
LNA which operated at frequency band start from 100 MHz to 1 GHz is designed and simulated using Advanced Design System (ADS) and optimization will be performed to get the best result.
c) Fabrication
The fabrication of the low noise amplifier will be done using microstrip FR4 will be used to fabricate the low noise amplifier.
d) Test analysis and measurement
The performance of the designated amplifier circuit is measured using the vector network analyzer and will be compared with the simulation result.
1.5 Thesis Outline
The thesis is separate into five chapters. Chapter 1 is the brief introduction of the project where the problem statement, objective and scope of works are mentioned clearly in this chapter.
Chapter 2 is the Literature review where the basic of RF Fundamental, Introduction of LNA and LNA architecture are explained.
Chapter 3 involve the LNA design process include the transistor selection, stability checking, DC biasing, matching networks, simulation and layout design.
(21)
5
In chapter 4, this chapter will analysis the simulation results and fabrication process and lastly testing important parameter of the circuit.
Finally in chapter 5, the conclusion and future work willsummarizes the result of the LNA designed. It also conclude the type of LNA that gives the best performance and most suitable to be used in Motorola product’s application.
(22)
CHAPTER II
LITERATURE REVIEW
2.1 Introduction
Recently, the growing demand in high performance wireless technology had boosted the development of LNA. The low noise amplifier is key block in any receiving system due to the receiver sensitivity is generally determined by its gain and noise figure. Besides, the noise figure for the follow amplifier stages in receiver system are reduced by providing a LNA with low noise figure and high gain, thus it is vital for LNA to amplify the received signal without introducing internal noise.
Most of the LNA are designed using BJT, GaAs FET, CMOS, PHMET and MESFET type of transistors. These are widely used in RF communication applications such as wireless communication, radar communication and also mobile communication. A low voltage, low power consumption, ability to operate at a wide range of temperatures and better performance are always being an advantage for a good LNA design for RF applications.
A lot of research is done before going through the project. This is definitely very important since some basic knowledge must be known before starting a project. Internet sources and books on RF were used to search for information on LNA design. Apart from good understanding on RF theory, knowledge on the wideband LNA methodology is also needed in order to design a broadband LNA
(23)
7
Basically, the research is done on the steps for a good design and what are the trade-offs involved. Datasheets for transistors are also reviewed from various sources to understand the characteristics of a good transistor. The S-parameter characteristic and the biasing information given by the manufacturer in the form of s2p file for various transistors are collected and simulated by using the Advance Design System (ADS). This will give a better understanding on the performance of the transistor for a certain operating frequency.
A BJT is a good choice for the LNA designers, because it gives higher gain with acceptable low noise figure, even though a GaAs FET gives extremely low noise compared to BJT. The problems with GaAs FET are that the designers will face significantly stability problem. This is because FET will oscillate at frequencies where bypassing and grounding becomes a challenge of its own. Hence, the trade-off involve between the choosing of both type of transistor must give prior consideration.
2.2 Design Consideration
The most important design considerations in a low noise amplifier design are stability, noise, power gain, bandwidth and DC requirements. As show in Figure 2.1, a LNA must have DC biasing circuit to biasing the selected transistor, input and output matching network for maximum power transfer in the circuit.
(24)
8
LNA operate in class A mode, characterized by a bias point that is about at the center of maximum voltage and current of the bias supply for the transistor. By referring to data sheet, the biasing point for the LNA should have high gain, low noise figure, linear, good input and output matching and unconditionally stable at the lowest current drain from the supply. In this project, the current drain and voltage supply for the design are restricted to maximum of 10mA and 1.8V respectively.
Unconditional stability of the circuit is another important parameter in designing of LNA, this characteristic means that the device does not oscillate over a range of frequencies with any combination of source and load impedance.
The next step would be the input and output matching. For input matching, two main criteria can be match for, either match for best noise match or great Input Return Loss (IRL) where IRL defines how well the circuit is matched to 50 Ω of the source. A typical approach in designing a LNA is to develop a input matching network terminates the transistor with conjugate of Gamma optimum (Γopt), which imply the matching the impedance of the transistor for the greatest noise match. In most cases, this means that the input return loss (IRL) of the Low Noise Amplifier will be sacrificed. The optimal IRL only can be achieved when the input-matching circuit terminates the device with a conjugate of S11, which is different from the conjugate of Γopt. Hence, there must always be a trade-off between both criteria. For output matching, conjugate matching has been exclusively used to maximize the gain of the circuit.
S11 and S22 are measures of the input and the output match. A value of -10 dB means matching to within 20 %. Therefore the target value for S11 and S22 is set to be below -10 dB over the entire bandwidth.
For the Third-Order-Intercept-Point (IIP3 and OIP3) and the gain is controlled by some external factors such as the linearizing inductor, and bypassing component which also play a significant role in the performance of the LNA itself.
(1)
1.2 Problem Statement
Portable two ways radio size has been shrinking over time, the employ of multiband radios which consist of several narrow band discrete circuits are larger because inside of housing must contain two or three separate transceivers. Besides, driving a slightly larger housing is the additional filtering and shielding between the transceivers in order to avoid the interfering effect between different transceivers.[1] To solve this problem, the combination of several narrow bands LNA circuit into single wideband LNA circuit is proposed. Wideband LNA design presents a considerable challenge, as we know conjugate matching will give maximum gain only over relatively narrow bandwidth, while designing for less max gain will improve the gain bandwidth, but the input and output part of the amplifier will be poorly matched.[2] For these reason feedback technique is proposed to simultaneously achieve improvement in bandwidth and also on its gain, noise figure and return loss. The conventional LNA suffer from the problem that the input matching network can be tuned for low noise figure or low VSWR (conjugate match) but not both parameter simultaneously.[3] The negative feedback technique can be used in wideband amplifier to provide a flat gain response and to reduce the input and output VSWR. It also controls the amplifier performance due to variations in the S parameters from transistor to transistor and furthermore, in-band stability is also improved by employing negative feedback. [4]
1.3 Objective
The objective of this project is to design low noise amplifiers for MOTOROLA application by using feedback and cascaded technique, then analysed on their noise figure, matching and gain at the operating frequency. Finally the best performance low noise amplifier will be fabricated as final result.
(2)
1.4 Scope of work
Scope of this project can be divided into four parts which are:
a) Literature review
Include the study of the characteristics of the LNA especially on the gain, noise figure, stability which must be taken into consideration in LNA design.
b) Design and simulation
LNA which operated at frequency band start from 100 MHz to 1 GHz is designed and simulated using Advanced Design System (ADS) and optimization will be performed to get the best result.
c) Fabrication
The fabrication of the low noise amplifier will be done using microstrip FR4 will be used to fabricate the low noise amplifier.
d) Test analysis and measurement
The performance of the designated amplifier circuit is measured using the vector network analyzer and will be compared with the simulation result.
1.5 Thesis Outline
The thesis is separate into five chapters. Chapter 1 is the brief introduction of the project where the problem statement, objective and scope of works are mentioned clearly in this chapter.
Chapter 2 is the Literature review where the basic of RF Fundamental, Introduction of LNA and LNA architecture are explained.
Chapter 3 involve the LNA design process include the transistor selection, stability checking, DC biasing, matching networks, simulation and layout design.
(3)
In chapter 4, this chapter will analysis the simulation results and fabrication process and lastly testing important parameter of the circuit.
Finally in chapter 5, the conclusion and future work willsummarizes the result of the LNA designed. It also conclude the type of LNA that gives the best performance and most suitable to be used in Motorola product’s application.
(4)
CHAPTER II
LITERATURE REVIEW
2.1 Introduction
Recently, the growing demand in high performance wireless technology had boosted the development of LNA. The low noise amplifier is key block in any receiving system due to the receiver sensitivity is generally determined by its gain and noise figure. Besides, the noise figure for the follow amplifier stages in receiver system are reduced by providing a LNA with low noise figure and high gain, thus it is vital for LNA to amplify the received signal without introducing internal noise.
Most of the LNA are designed using BJT, GaAs FET, CMOS, PHMET and MESFET type of transistors. These are widely used in RF communication applications such as wireless communication, radar communication and also mobile communication. A low voltage, low power consumption, ability to operate at a wide range of temperatures and better performance are always being an advantage for a good LNA design for RF applications.
A lot of research is done before going through the project. This is definitely very important since some basic knowledge must be known before starting a project. Internet sources and books on RF were used to search for information on LNA design. Apart from good understanding on RF theory, knowledge on the wideband LNA methodology is also needed in order to design a broadband LNA
(5)
Basically, the research is done on the steps for a good design and what are the trade-offs involved. Datasheets for transistors are also reviewed from various sources to understand the characteristics of a good transistor. The S-parameter characteristic and the biasing information given by the manufacturer in the form of s2p file for various transistors are collected and simulated by using the Advance Design System (ADS). This will give a better understanding on the performance of the transistor for a certain operating frequency.
A BJT is a good choice for the LNA designers, because it gives higher gain with acceptable low noise figure, even though a GaAs FET gives extremely low noise compared to BJT. The problems with GaAs FET are that the designers will face significantly stability problem. This is because FET will oscillate at frequencies where bypassing and grounding becomes a challenge of its own. Hence, the trade-off involve between the choosing of both type of transistor must give prior consideration.
2.2 Design Consideration
The most important design considerations in a low noise amplifier design are stability, noise, power gain, bandwidth and DC requirements. As show in Figure 2.1, a LNA must have DC biasing circuit to biasing the selected transistor, input and output matching network for maximum power transfer in the circuit.
(6)
LNA operate in class A mode, characterized by a bias point that is about at the center of maximum voltage and current of the bias supply for the transistor. By referring to data sheet, the biasing point for the LNA should have high gain, low noise figure, linear, good input and output matching and unconditionally stable at the lowest current drain from the supply. In this project, the current drain and voltage supply for the design are restricted to maximum of 10mA and 1.8V respectively.
Unconditional stability of the circuit is another important parameter in designing of LNA, this characteristic means that the device does not oscillate over a range of frequencies with any combination of source and load impedance.
The next step would be the input and output matching. For input matching, two main criteria can be match for, either match for best noise match or great Input Return Loss (IRL) where IRL defines how well the circuit is matched to 50 Ω of the source. A typical approach in designing a LNA is to develop a input matching network terminates the transistor with conjugate of Gamma optimum (Γopt), which imply the matching the impedance of the transistor for the greatest noise match. In most cases, this means that the input return loss (IRL) of the Low Noise Amplifier will be sacrificed. The optimal IRL only can be achieved when the input-matching circuit terminates the device with a conjugate of S11, which is different from the conjugate of Γopt. Hence, there must always be a trade-off between both criteria. For output matching, conjugate matching has been exclusively used to maximize the gain of the circuit.
S11 and S22 are measures of the input and the output match. A value of -10 dB means matching to within 20 %. Therefore the target value for S11 and S22 is set to be below -10 dB over the entire bandwidth.
For the Third-Order-Intercept-Point (IIP3 and OIP3) and the gain is controlled by some external factors such as the linearizing inductor, and bypassing component which also play a significant role in the performance of the LNA itself.