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Design And Implementation Of P- Band RF Low Noise AmplifierCHAPTER 1INTRODUCTIONLow-noise amplifier (LNA) is an electronic amplifier used to amplify possibly veryweak signals (for example, captured by an antenna) and it amplifies the signal whileintroducing a minimum amount of noise. It is usually located very close to thedetection device to reduce losses in the feed line. This active antenna arrangement isfrequently used in microwave systems like GPS, because coaxial cable feed line isvery lossy at microwave frequencies, e.g. a loss of 10% coming from few meters ofcable would cause a 10% degradation of the signal-to-noise ratio (SNR).1.1 Low Noise AmplifierIn comparison to other technologies, pHEMT is ideally, the two different materialsused for a heterojunction would have the same lattice constant (spacing between theatoms). In practice, e.g. AlGaAs on GaAs, the lattice constants are typically slightlydifferent, resulting in crystal defects. As an analogy, imagine pushing together twoplastic combs with a slightly different spacing. At regular intervals, youll see twoteeth clump together. In semiconductors, these discontinuities form deep-level traps,and greatly reduce device performance. A HEMT where this rule is violated is calleda pHEMT or pseudomorphic HEMT. This is achieved by using an extremely thinlayer of one of the materials – so thin that the crystal lattice simply stretches to fit theother material. This technique allows the construction of transistors with largerbandgap differences than otherwise possible, giving them better performance is themost cost-effective solution to date for large-scale digital applications and it enablessystem-on-a-chip owing to its capability of providing large-scale subsystems withhigh levels of integration. A number of fully integrated transceivers, up to 5GHz, arebeing implemented using standard processes; typical protocols are Bluetooth, IEEE802.11, IEEE 802.15.3, IEEE 802.11a, and Hipper- LAN. Nevertheless, the realdesign of RF circuits is still a challenge due to severe constraints on powerconsumption and noise that impose stringent margins to the design process. Accuratemodels are critical in order to reduce design cycles and to achieve first - time successin implementation. Unfortunately, available MOSFET compact models – such asCentre for Emerging Technology, Jain University 1
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Design And Implementation Of P- Band RF Low Noise AmplifierBSIM3v3, PHEMT Model, or EKV are typically not applicable for the GHz range offrequencies. Modeling of noise provides critical information in the design of RFcircuits, especially for LNA (low noise amplifier) blocks.The LNA is typically the first stage of a radio receiver and needs to provide sufficientgain while introducing the least noise possible. Since it is indispensable to understandthe physical phenomena of broadband noise and to incorporate this information intothe models, lack of understanding of MOSFET noise presents a substantial barrier tothe implementation of CMOS receivers.WIRELESS COMMUNICATIONS has thrived in the last decade, owing to explodinguser demand for information and the commensurate need for connectivity. Substantialresearch effort has focused on many application areas, such as cellular phones,cordless phones, GPS (global positioning system), and WLANs (wireless local areanet- works). The RF ICs (radio frequency integrated circuits) have been the primarydomain of GaAs or bipolar technologies since those technologies provide relativelyhigh cutoff frequencies (fT). However, as continuous scaling of PHEMT makes fTinexcess of 30GHz readily achievable in typical quarter micron technology, PHEMTbecomes an attractive alternative for RF applications in the low GHz frequency range.An LNA in a Heterodyne Receiver Figure1.1: LNA used in a Super-heterodyne receiver.The band-select filter before the LNA rejects the out-of-band interferers. The imagereject filter (preselected) after the LNA attenuates the image which is IF away fromthe desired band. LNA design is a compromise among power, noise, linearity, gain,stability, input and output matching, and dynamic range. They are characterized bythe design specifications.Centre for Emerging Technology, Jain University 2
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Design And Implementation Of P- Band RF Low Noise AmplifierThe first stage of a receiver is typically a low-noise amplifier (LNA), whose mainfunction is to provide enough gain to overcome the noise of subsequent stages (forexample, in the mixer or IF amplifier). Aside from providing enough gain whileadding as little noise as possible, an LNA should accommodate large signals withoutdistortion, offer a large dynamic range, and present good matching to its input andoutput, which is extremely important if a passive band select filter and image-rejectfilter precedes and succeeds the LNA, since the transfer characteristics of many filtersare quite sensitive to the quality of the termination.An LNA combines a low noise figure, reasonable gain, and stability withoutoscillation over entire useful frequency range.GAINThe gain G is defined as the ratio of the power delivered to the output to the poweravailable from the input. The greater the gain, the more the signal is amplified in theamplifier. Gain is usually expressed in decibels (dB).NOISE FIGUREEvery amplifier amplifies both the signal and the noise delivered to the input. Sincean amplifier is never ideal, it also adds some self-noise during the amplifying processand therefore in the amplifier output there is a sum of amplified input noise andamplifier self-noise in addition to the amplified signal. Thus, the signal-to-noiseration always decreases between amplifier input and output. This decrease isexpressed by noise figure (NF) and is calculated in decibels.INPUT AND OUTPUT MATCHINGThe input and output are each connected to the LNA with filters whose performancerelies heavily on the terminal impedance. Furthermore, input and output matching tothe source and load can maximize the gain. Input and output impedance matching ischaracterized by the input and output return loss.STABILITYStability is an issue in all amplifiers with feedback, whether that feedback is addedintentionally or results unintentionally. It is especially an issue when applied ov ermultiple amplifying stages. Stability is a major concern in RF and microwaveCentre for Emerging Technology, Jain University 3
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Design And Implementation Of P- Band RF Low Noise Amplifieramplifiers. The degree of an amplifiers stability can be quantified by a so-calledstability factor.LINEARITYAn ideal amplifier would be a totally linear device, but real amplifiers are only linearwithin limits. When the signal drive to the amplifier is increased, the output alsoincreases until a point is reached where some part of the amplifier becomes saturatedand cannot produce any more output; this is called clipping, and results in distortion.Dealing with low frequency AC and DC circuits, conventional Kirchoffs voltage andcurrent laws are used as analysis tools. Heading into higher operating frequenciesthose laws can no longer be applied without losing too much precision. Analyzing alow frequency circuit, a conductor between two elements always assumes to have thesame potential regardless where on the conductor one looks. When it comes to higherfrequencies than around 500 MHz the previous assumption may no longer be correct.The reason for this is that the wavelength of the signal becomes so small that voltageand current will propagate as waves and therefore magnitude and phase vary along theconductor.Instead of using Kirchhoffs laws one must deal with electromagnetic waves andissues like propagation constant β, phase velocity Vp and skin depth δ becomeimportant. The propagation constant and phase velocity highly depend on the mediumsurrounding the conductor and they will determine the wavelength for a specificfrequency. Since the surrounding medium is a crucial design parameter, choosing agood substrate is one of the first design steps in RF-design. Skin effect is aconsequence that also occurs due to the electromagnetic wave nature and this effectforces the majority of the energy to flow close to the surface of the conductor.Penetration of the signal into the conductor is measured in skin depth δ. When theenergy is focused in just a few percent of the conductor the result is a decrease ineffective cross-sectional area. As a consequence, loss due to higher resistance willoccur. Changing the copper thickness will have little effect on trace resistance at highfrequencies, while changes in width and length will have the greatest effect onresistance at high frequencies.Centre for Emerging Technology, Jain University 4
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Design And Implementation Of P- Band RF Low Noise Amplifier1.2 PROJECT SCOPE AND METHODOLOGYThe Low Noise Amplifier (LNA) always operates in Class A, typically at 15 -20% ofits maximum useful current. Class A is characterized by a bias point more or less atthe center of maximum current and voltage capability of the device u sed, and by RFcurrent and voltages that are sufficiently small relative to the bias point that the biaspoint does not shift.The dynamic range of the receiver, the difference between the largest possiblereceived signal and the smallest possible received signal, defines the quality of thereceiver chain. The LNA function, play an important role in the receiver designs. Itsmain function is to amplify extremely low signals without adding noise, thuspreserving the required Signal-to-Noise Ratio (SNR) of the system at extremely lowpower levels. Additionally, for large signal levels, the LNA amplifies the receivedsignal without introducing any distortions, which eliminates channel interference. An LNA design presents a considerable challenge because of its simultaneous requirement for high gain, low noise figure, good input and output matching and unconditional stability at the lowest possible current draw from the amplifier. Although Gain, Noise Figure, Stability, Linearity and input and output match are all equally important, they are interdependent and do not always work in each other‘s favor. Carefully selecting a transistor and understanding parameter trade-offs can meet most of these conditions. Low noise figure and good input match is really simultaneously obtained without using feedback arrangements. Unconditional stability will always require a certain gain reduction because of either shunt or series resistive loading.Transistor selection is the first and most important step in an LNA design.The smallest signal that can be received by a receiver defines the receiver sensitivity.The largest signal can be received by a receiver establishes the upper power levellimit of what can be handled by the system while preserving voice or data quality.Centre for Emerging Technology, Jain University 5
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Design And Implementation Of P- Band RF Low Noise AmplifierThe designer should carefully review the transistor selection, keeping the mostimportant LNA design trade-offs in mind. The transistor should exhibit high gain,have a low noise figure, and offer high IP3 performance at the lowest possible currentconsumption, while preserving relatively easy matching at frequency of operation.Examination of a data sheet is a good starting point in a transistor evaluation for LNAdesign.The transistor‘s S-parameters should be published at different source/drain voltagesand different current levels for frequencies ranging from low to high values. The datasheet should also contain noise parameters, which are essential for low noise design.The designer should first look at the main design parameters as: Noise, Gain anddecide what Vds and Ids levels will produce optimal performance.The forward transducer power gain represents the gain from transistor itself with itsinput and output presented with 50 Ω impedance, the manufacturer of the transistor atmultiple frequencies and different Vds and current levels provides the S21 values.Additional gain can be obtained from source and load matching circuits. MaximumStable Gain and Maximum Power Gain (Gmax) are good indicators of additionalobtainable gain from the LNA circuit. LNA linearity is another important parameter.In amplifier design, there are a number of design techniques available in the literaturedepending on the parameter to be optimized. The most important designconsiderations in a microwave amplifier design are stability, power gain, bandwidth,and noise and DC requirements.RF performance of the LNA depends by many variables as: Frequency Stability Input and Output Matching Layout and Grounding EM Shielding Supply decouplingGeneral procedure for microwave amplifier designCentre for Emerging Technology, Jain University 6
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Design And Implementation Of P- Band RF Low Noise AmplifierAny MIC amplifier design essentially consists of the following steps. 1. Selection of a proper transistor. 2. Checking the conditional stability. 3. If transistor is unstable at the desired frequency, proper techniques are applied to make it stable. 4. Biasing is done. Bias point is selected depending on the application like low power, low noise, high power, linearity etc. 5. Different techniques are applied to optimize different parameters like noise figure, gain, power dissipation. Two parameters cannot be optimized simultaneously. Matching circuits that provide optimum performance in a microwave amplifier can be easily and quickly designed using a Smith chart.1.3 AMPLIFIER SPECIFICATIONS Parameters Required Frequency range 50M-1GHz Gain(min) 22dB(20dB) Gain flatness +/- 1dB Noise figure <4dB Power output 26dBm VSWR or return loss 1:2 input 1:2 output All port imp 50Ω Maximum RF input 10dBm Supply Voltage 15V Current, maximum 1000mA1.3 THESIS ORGANIZATIONThe thesis is organized as follows. Chapter 2 deals with Literature survey. In chapter3 background theories, Chapter 4 deals with system design i.e., stability design andmatching circuit design. The design implementation is discussed in chapter 5. Chapter6 discusses the simulation results obtained and chapter 7 concludes the thesis work.Centre for Emerging Technology, Jain University 7
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Design And Implementation Of P- Band RF Low Noise AmplifierCHAPTER 2LITERATURE SURVEYIn wireless receiver modules, the ability of LNA to meet the objective of providingsufficient amplification for subsequent stages while adding as little noise as possibleis quantified in the noise factor of the amplifier which is defined as the ratios of thesignal-to-noise ratio at the output of the amplifier to the signal-to-noise ratio at theinput. It is well known that the first amplification stage dominates the total noisefigure of the system and thus the noise optimization of this first stage is critical. In theabsence of LNA, the received signal will be very weak. It cannot be processed by thenext stage. In general the important characteristics of LNA are: high Gain, low Noisefigure, Stability, Linearity and good input output matchingAn ideal amplifier would be a totally linear device, but real amplifiers are only linearwithin limits. When the signal drive to the amplifier is increased, the output alsoincreases until a point is reached where some part of the amplifier becomes saturatedand cannot produce any more output; this is called clipping, and results in distortion.As the applications move towards frequencies then new design challenges areintroduced. At high frequencies, the noise performances of silicon bipolar junctiontransistors (BJT) are no longer satisfactory. Traditionally III-V compounds such asGaAs or lnP were used in high-speed applications, as they are capable of achievinghigh unity gain frequencies, efficiency and low voltage.The HEMT or High Electron Mobility Transistor is a form of field effect transistor(FET) that is used to provide very high levels of performance at microwavefrequencies. It offers a combination of low noise figure combined with the ability tooperate at the very high microwave frequencies. Accordingly the HEMT is used inareas of RF design where high performance is required at very high RF frequencies.The development of the HEMT took many years. It was not until many years after thebasic FET was established as a standard electronics component that the HEMTappeared on the market. The specific mode of carrier transport used in HEMTs wasfirst investigated in 1969, but it was not until 1980 that the first experimental device swere available for the latest RF design projects. During the 1980s they started to beCentre for Emerging Technology, Jain University 8
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Design And Implementation Of P- Band RF Low Noise Amplifierused, but in view of their initial very high cost their use was considerably limited.Now with their cost somewhat less, they are more widely used, even finding uses inthe mobile telecommunications as well as a variety of microwave radiocommunications links, and many other RF design applications. Figure 1.2 cross sectional schematic of HEMTHigh Mobility Electron Transistors (HEMTs) outperform MESFETs in noise figure,output power and high frequency operations.Heterojunction HEMT replaces Schottky barrier in MESFET Superior electrontransport properties due to formation of two-Dimensional electron gas (2DEG) High mobility. High transconductance. Ultra low noise.A further development of the HEMT is known as the PHEMT. PHEMTs,Pseudomorphic High Electron Mobility Transistors are extensively used in wirelesscommunications and LNA applications. PHEMT transistors find wide marketacceptance because of their high power added efficiencies and excellent low noisefigures and performance. As a result, PHEMTs are widely used in satellitecommunication systems of all forms including direct broadcast satellite television,DBS-TV, where they are used in the low noise boxes, LNBs used with the satelliteantennas. They are also widely used in general satellite communication systems aswell as radar and microwave radio communications systems. PHEMT technology isCentre for Emerging Technology, Jain University 9
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Design And Implementation Of P- Band RF Low Noise Amplifieralso used in high-speed analogue and digital IC technology where exceedingly highspeed is required.It has been shown that the input impedance matching plays an important role inachieving minimum noise figures and that an optimal source impedance exists forachieving the best noise performance. This impedance is usually different than formaximum power transfer.LNA design entails achieving a low noise figure and usuallyoptimal noise matching for a first amplifier stage. The obvious trade-off betweenminimum noise and maximum available gain has sparked much research interest intoachieving a simultaneous optimal noise and power match. A traditional approach isthe shunt-shunt feedback to modify the amplifier input impedance achieving such asimultaneous match.In 1995[4], A methodology which permits one to determine the required input andoutput termination impedances of a given transistor such that the amplifier cansimultaneously meet power gain, noise figure, and input and output VSWR constraintsis described. To support the proposed method, an amplifier design example at 2 GHzwith power gain, noise figure, and input and output VSWR constraints is presented.The applicability of the design methodology to the design of broad band low-noiseamplifier design is demonstrated with three examples.In 2003[5], A low-noise amplifier (LNA) uses low-loss monolithic transformerfeedback to neutralize the gate–drain overlap capacitance of a field-effect transistor(FET). A differential implementation in 0.18- m CMOS technology, designed for 5-GHz wireless local-area networks (LANs), achieves a measured power gain of14.2dB, noise figure (NF, 50 ) of 0.9 dB, and third-order input intercept point (IIP3) of+0.9 dBm at 5.75 GHz, while consuming16 mW from a 1-V supply. The feedbackdesign is bench marked to a 5.75-GHz cascade LNA fabricated in the sametechnology that realizes 14.1-dB gain, 1.8-dB NF, and IIP3 of+4.2 dBm, whiledissipating21.6 mW at 1.8 V.In 2007[7], various design methodologies for common-source low noise amplifiers(LNAs) in Si CMOS technologies were proposed in the past. These starts from long-channel assumptions to derive analytic design equations. This paper compares thevarious existing LNA design methodologies and verifies the long-channelCentre for Emerging Technology, Jain University 10
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Design And Implementation Of P- Band RF Low Noise Amplifierassumptions using a commercial .13μm CMOS technology. After demonstrating thatthe design assumptions are no longer valid, a new methodology is proposed whichenables the LNA design in a systematic way, without the drawback that it is relyingon a particular transistor model for computing the input impedance and the noisefigure. This makes the proposed technique robust to transistor model changes infuture technology nodes.In 2008[6], A narrowband LNA is designed at the center frequency of 12.7GHz with again of 10dB, bandwidth of 72MHz and noise figure of 3 to 4dB. The designmethodology required the analysis of the transistor, stability check and propermatching network selection for input and output. Ideal microwave amplifier equationsare used to carry out the analytical treatment for the design. The DC and ACsimulations for the LNA are presented in the paper. Fabrication and testing of theLNA is also discussed.In 2009[8], In this paper the aim is to design and simulate a single stage LNA circuitwith high gain and low noise using MESFET for frequency range of 5 GHz to 6 GHz.A single stage LNA has successfully designed with15.83 dB forward gain and 1.26 dBnoise figure in frequency of 5.3GHz. Also the designed LNA should be workingstably in a frequency range of 5 GHz to 6 GHz .In 2009[9], an integrated narrowband low noise amplifier in cascade topology hasbeen developed for WLAN applications. Using WINs 0.15μm PHEMT technology,the impedance matching, voltage gain, noise figure and 1dB compression point of thecircuit are analyzed and optimized under specified power consumption. Results fromthe simulation show that the maximum forward gain of the circuit is 20dB, noisefigure is 0.93dB, OP1dBis 10dBm at the center frequency of 2.4GHz, whileconsuming42mA current from a 3.3V power supply.In 2011[10], the design of a compact, two-stage, low noise, unconditionally stable,amplifier from 0.5 to 6 GHz is discussed. To achieve its wide-band characteristics, anovel matching mechanism is proposed, which consists of lumped elements andmicro-strip lines. The amplifier is designed around the HEMT FHX35LG and thePHEMT ATF-54143. The negative feedback technique is also adapted to border thefrequency band, and the whole matching network. In the covered band, the amplifierCentre for Emerging Technology, Jain University 11
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Design And Implementation Of P- Band RF Low Noise Amplifierprovides more than 22 dB gain, gain-flatness of less than ±2 dB, noise figure of lessthan 4 dB, linear output power (P -1dB) of more than 26 dBm.In 2012[11], a very low noise LNA is designed. The LNA is based on reactivefeedback and the broadband impedance matching and the flat gain are achieved. Alsoby using inductive degeneration, a noise less resistance is created. This impedance isused to match the input impedance without increasing the noise figure. The LNA isdesigned in the standard 0.18 μm CMOS technology. The noise figure (NF) isbelow0.8dB and input and output reflection coefficient are less than -10dB.Also LNAprovides 12dB power gain and consume 11.9mW from a1.2-V voltage supply.TOOL USED:AWR Microwave Office RF/microwave design software is the industrys fastestgrowing microwave design platform. Microwave Office has revolutionized thecommunications design world by providing users with a superior choice. Built on theunique AWR high-frequency design environment platform with its unique unifieddata model™, Microwave Office offers unparalleled intuitiveness, powerful andinnovative technologies, and unprecedented openness and interoperability, enablingintegration with best-in-class tools for each part of the design process.Microwave Office design suite encompasses all the tools essential for high-frequencyIC, PCB and module design, including: Linear circuit simulators Non-linear circuit simulators Electromagnetic (EM) analysis tools Integrated schematic and layout Statistical design capabilities Parametric cell libraries with built-in design-rule check (DRC).The company develops markets, sells and supports engineering software, whichprovides a computer-based environment for the design of hardware for wireless andhigh speed digital products. AWR software is used for radio frequency (RF) ,microwave and high frequency analog circuit and system design. Typical applicationsCentre for Emerging Technology, Jain University 12
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Design And Implementation Of P- Band RF Low Noise Amplifierinclude cellular and satellite communications systems and defense electronicsincluding radar, electronic warfare and guidance systems.AWRs product portfolio includes Microwave Office, Visual System Simulator (VSS),Analog Office, APLAC, AXIEM and Analyst. AWRs worldwide customers includecompanies involved in the design and development of analog and mixed signalsemiconductors, wireless communications equipment, aerospace and defense systems.High Electron Mobility Transistors (HEMTs) Heterojunction Structure Energy Bands Two-dimensional Electron Gas (2DEG)PHEMT for cryogenic low-noise amplifications recent development:State-of-the-art LNAs in Radio Astronomy ReceiversIn a PHEMT, conduction electrons are spatially separated from the donor impuritiesionized scattering is suppressed Electrons in a 2DEG exhibit very high mobilityHigh Gain: High electron mobility leads to high transconductance gm and highoperation frequency (millimeter wavelengths)Low-noise: Superior noise temperatures, especially In-P based HEMTs for low noiseand power amplifications; p-HEMT is generally recognized as the best choice. InP-HEMTs are promising in better gain and noise, but still await commercialization Amplification up to 40GHz: GaAs-HEMTs and p-HEMTs. Above 40GHz: InP-based HEMTs . 100GHz –1 THz: Superconductor-insulator- superconductor (SIS) junction devices. Above 1THz: Hot electron bolo meters (HEB).Centre for Emerging Technology, Jain University 13
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Design And Implementation Of P- Band RF Low Noise AmplifierCHAPTER 3BACKGROUND THEORYFor a fundamental understanding of the Low-Noise Amplifier design procedure, it isnecessary to introduce a series of underlying concepts. Covered topics includereflection, scattering p a r a m e t e r s , the Smith Chart, t h e quality factor, andimpedance transformation.3.1 Scattering ParametersScattering Parameters or S-parameters are complex numbers that exhibit how voltagewaves propagate in the radio-frequency (RF) environment. In matrix form theycharacterize the complete RF behavior of a network.At this point it is necessary to introduce the concept of 2-ports. It is fundamental inRF circuit analysis and simulation as it enables representation of networks by a singledevice. As the properties of the individual components and those of the physicalstructure of the circuit are effectively taken out of the equation, circuit analysis isgreatly simplified. The characteristics of the 2-port is represented by a set of four S-parameters: S11, S12, S21 and S22, which correspond to input reflection coefficient,reverse gain coefficient, forward gain coefficient and output reflection coefficientrespectively. √ √ √ √ Figure 3.1(a): Two port networkThere are alternative descriptive parameters for 2-ports, such as impedanceparameters, admittance parameters, chain parameters and hybrid parameters. Theseare all measured on the basis of short-and open circuit tests, which are hard to carryCentre for Emerging Technology, Jain University 14
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Design And Implementation Of P- Band RF Low Noise Amplifierout accurately at high frequencies. S-parameters, on the other hand, are measuredunder matched and mismatched conditions. This is why S-parameters are favored inmicrowave applications. S-parameters are both frequency-and system impedancedependent so although manufacturers typically supply S-parameter data with theirdevices it is not always applicable. Under such circumstances, it becomes necessary tomeasure the parameters. Referring to Figure3.1 (a), these measurements are carriedout by measuring wave ratios while systematically altering the termination to cance leither forward gain or reverse gain according to the following equations:A two-port network is inserted between source and load, which is shown in the circuitof Figure 3.1(a). The following may be said for any traveling wave that originates atthe source:A portion of the wave that originates at the source and is incident on the two -portdevice (a1) will be reflected (b1), and another portion will be transmitted through thetwo-port device.A fraction of the transmitted signal is then reflected from the load and becomesincident on the output port of the two-port device (a2).A portion of the signal (a2) isthen reflected from the output port back toward the load (b2), while a fraction istransmitted through the two-port device to the source. Figure 3.1(b): Two port networkIt is obvious from the above discussion that any traveling wave present in the circuitis composed of two components. For instance, the traveling wave components flowingfrom the output of the two-port device to the load consist of the portion of a2 reflectedfrom the output of the two-port device, and the portion of a1 transmitted through thetwo-port device. Similarly, the traveling wave flowing from the input of the two -portdevice back toward the source consists of the portion of a1 reflected from the inputand the fraction of a2 transmitted through the two-port device.Centre for Emerging Technology, Jain University 15
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Design And Implementation Of P- Band RF Low Noise AmplifierIf we set these observations in equation form we get the following: (3.1) (3.2)The above equations in the matrix form can be written as given below in the equation [ ] [ ]* + (3.3)From the Equation 3.3, the parameters S11, S12, S21 and S22 which representreflection and transmission coefficients, are called Scattering-parameters of the twoport network and are measured at port 1 and port 2. The matrix for these parametersis; [ ] (3.4)From Figure 3.1, The Scattering-parameters measured at the specific locations aredefined as followsWhere:S11 =the input reflection coefficient.S12 = the reverse transmission coefficient.S21 = the forward transmission coefficient.S22 = the output reflection coefficient.a1, a2 = Normalized incident voltage wave traveling towards the two-port networkb1, b2 = Normalized Reflected voltage wave reflected back from the two-port network3.2 ReflectionWhen a wave travels through an impedance discontinuity, at that junction (Figure 3.1),a fraction of the wave will be reflected. As a consequence, the counterpart (theCentre for Emerging Technology, Jain University 16
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Design And Implementation Of P- Band RF Low Noise Amplifierincident wave) will lose some of its magnitude. Naturally, this is an undesirablephenomenon in any application where power conservation is critical. The extent ofincident power loss is related to the similarity of the impedances as seen in bothdirections from the junction. So the objective, In order to maximize the powertransfer, is to optimize the impedance match. Further information on that subjectfollows in Chapter 3.There are a number of performance parameters that‘s how to what extent theimpedances are matched. Firstly, the Reflection Coefficient which by definition is theratio of the reflected wave to the incident wave (Equation 3.5), but can also isexpressed in terms of impedances. It is a complex entity that describes not only themagnitude of the reflection, but also the phase shift.Note that this is the load reflection coefficient with respect to the source impedance. Itis also commonly expressed with respect to the characteristic impedance (Z0 ). Whenthe load is short-circuited, maximum negative reflection occurs and the reflectioncoefficient assumes minus unity. In contrast, when the load is open-circuited,maximum positive Figure 3.2: Simple circuit showing the impedance discontinuity junction and measurement location of Γ L.Reflection occurs and the reflection coefficient assumes plus unity. In the ideal case,when Z Lis perfectly matched to ZS, there is no reflection and the reflection coefficientis consequently zero.A closely related parameter is the Voltage Standing Wave Ratio (VSWR), which iscommonly talked about in transmission line applications. As the incident andreflected wave travel in opposite directions the addition of the two generates astanding wave, see Figure 3.2. The VSWR is defined as the ratio of the maximumvoltage to the adjacent minimum voltage of that standing wave (Equation3.8).Knowing the domain of the reflection coefficient, it follows that when there is noCentre for Emerging Technology, Jain University 17
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Design And Implementation Of P- Band RF Low Noise Amplifierreflection as in a perfectly matched system; VSWR assumes its minimum and idealvalue of 1.0:1.The Return Loss (RL) is simply the magnitude of the reflection coefficient in decibels3.3 The Smith ChartBy using the smith chart, RF circuit problems including noise factor optimization,stability analysis and impedance matching circuits etc can be found. Among all theRF circuit problems above, designing of impedance matching circuits is very hard andimportant. Figure 3.3: the complete Smith ChartThe center point in Smith chart represents normalized impedance Z = 50 Ώ which isthe load in case of perfectly matched circuit. At the extreme left side of smith chartCentre for Emerging Technology, Jain University 18
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Design And Implementation Of P- Band RF Low Noise Amplifierthere is a point represents short circuit that means Z = 0 Ώ and the in extreme rightside there is one point which represents open circuit it means Z = ∞ Ώ . Pointselsewhere on unity circle represents pure resistance values and points on arcs willrepresents reactance values.In its most common form, the chart is made up out of two overlaid grids: the constantresistance circles and the constant reactance circles. The Cartesian coordinate systemwithin the Smith Chart is used to plot the reflection coefficient. Further more thereare three varieties of the Smith Chart: with impedance grid (Z Smith Chart), withadmittance grid(Y Smith Chart) and the two combined (ZY Smith Chart).As theradius of the chart is unity, it is implied that all plotted values, whether they areimpedances or admittances, must be normalized with respect to a reference. Thisreference is usually the characteristic impedance of the system which usually is 50Ω.In impedance chart all circles are started from the right side. A large circle meansdecreasing resistance and it is noted as R. It does not matter where you are on thesame circle; always resistance value is same on this circle. There is another reactancecurve in the smith chart which starts from the right hand side and stretch out likeincreasing arcs is the reactance (jx).the bigger the arch is the smaller the reactancevalue.The impedance chart is shown in the below figure Figure 3.4: Constant Resistance CirclesCentre for Emerging Technology, Jain University 19
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Design And Implementation Of P- Band RF Low Noise AmplifierThe admittance chart is shown in the below figure 3.7. Figure 3.5: Constant Reactance and suceptance CirclesAlong the horizontal line in the middle, the reactance is always zero because there isonly resistive part. (R = 0).At this horizontal line end of the right side is open (R = ∞)and the left side circuit is shorted (R = 0). Admittance chart (Y) is just likeimpedance. It is simply inverse of Z (Y = 1/Z) .graphically it is possible by rotatingthe smith chart 180 degree around.Centre for Emerging Technology, Jain University 20
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Design And Implementation Of P- Band RF Low Noise Amplifier Figure 3.6: Combination of Z and Y Smith ChartAn impedance value can also be turned 180 degree around to find the admittancevalue. When both impedance and admittance chart shows in one figure than it iscalled normalized impedance and admittance coordinates smith chart. It is oftenreferred to as a ZY smith chart. Figure 3.8 shows the combination of impedance andadmittance smith chart.Admittance chart contains both real and imaginary part same as impedance hasY = G±jB.WhereG = ConductanceB = SusceptanceMany sources and loads have values greater than 50-ohm (ZS = 50+j100, ZL =100+j100).The smith chart cannot represent this value so the smith chart showsnormalized impedance values. To transform to a normalized value first we have toknow the characteristic impedance value Z0 (50 Ohm, 75 Ohm) then simply dividedthe actual value of ZS or ZL with characteristic impedance Z0 i.e. z = ZS/Z0 or z =ZL/Z0.3.4 The Quality FactorThe Quality Factor (Q)is a descriptive parameter of the rate of energy loss in completeRLC networks or simply individual inductors or capacitors. For the latter, Q is ameasurement of how lossy the component is, that is how much parasitic resistancethere is. So it follows that in applications where loss is undesirable, high Qcomponents are advantageous. Additionally the Q factor is directly related to thebandwidth, where higher corresponds to narrower bandwidth. The equations forcalculating Q are: (3.9) (3.10) (3.11)Centre for Emerging Technology, Jain University 21
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Design And Implementation Of P- Band RF Low Noise Amplifier (3.12)3.5 Impedance TransformationAs previously stated, in order to maximize power transfer from source to load,matching impedances is required. Specifically, in a circuit as seen in Figure3.1 wherethe source and load impedances are fixed, the objective is to design the inputmatching network so that ZS matches Z1 and the output matching network so thatZL matches Z2. In other words Z1 and Z2 respectively, are transformed toperceptually match the input and output impedances of the transistor. Figure 3.7: Different Direction of movements on Smith Chart and the corresponding elements.According to the Maximum Power Theorem, the maximum power transfer will occurwhen the reactive components of the impedances cancel each other, that is when theyare complex conjugates. This is suitably called conjugate matching.To achieve the conversion with an impedance matching network of passivecomponents, there are primarily three options. Firstly, there is the L-match .ItsCentre for Emerging Technology, Jain University 22
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Design And Implementation Of P- Band RF Low Noise Amplifieradvantage is the simplicity, but that is simultaneously its down side as well because ithas only two degrees of freedom. Since there are only two component values to set,the L-match is restricted to determining only two out of the three associatedparameters: impedance transformation ratio, centre frequency and Q. To acquire athird degree of freedom, it is therefore desired to cascade another L-match stage. Bydoing so, another two types of impedance transformation matches are encountered:the π-match and the T-match.The above figure 3.9 shows the different ways of placing the elements dependingupon the direction of movement on smith chart. The advantages with the T-and π-match configurations do not end with an additional degree of freedom. But because oftheir topology they can absorb parasitic reactance present in source or load.Specifically the T-match will absorb parasitic inductance where as the π-match willabsorb parasitic capacitance. In addition it is also possible to achieve significantlyhigher Q compared to an L-match configuration.3.6 StabilityWhen embarking on any amplifier design it is very important to check on the stabilityof the device chosen, otherwise the amplifier may well turn into an oscillator. Themain way of determining the stability of a device is to calculate the Rollett‘s stabil ityfactor (K), which is calculated using a set of S-parameters for the device at thefrequency of operation.The conditions of stability at a given frequency are |Γin| < 1 and |Γout| < 1,and musthold for all possible values ΓL &ΓS obtained using passive matching circuits. We cancalculate two Stability parameters K & |∆| to give us an indication to whether a deviceis likely to oscillate or not or whether it is conditionally/unconditionally stable.WhereS11 = Input reflection coefficientCentre for Emerging Technology, Jain University 23
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Design And Implementation Of P- Band RF Low Noise AmplifierS22 = Output reflection coefficientS12 = Reverse transmission gainS21= Forward transmission gainThe parameters must satisfy K > 1 and |∆| < 1 for a transistor to be unconditionallystable. Once we have calculated the K factor and find the device to be unconditionall ystable we can calculate the Maximum available gain (MAG):-Where K is on the limit of unity the above equation reduces down to:- ( √ ) (3.14) (3.15)The equations for calculation of the stability circles are:- (3.16)This gives the location of the centre of the input stability circle .This gives the radius of the input stability circle. | | (3.17)Similarly for the outputThis gives the location of the centre of the output stability circle (3.18) (3.19) | | (3.20)This gives the radius of the output stability circle.The Figure below shows the form of stability circles in relation to a Smith chart.In the above example the active device would be showing unconditional stability, asthere is no intersection of the stability circles on the Smith Chart. All devices withS11 and S22 < 1 must be stable with a load impedance of 50 ohms therefore; theCentre for Emerging Technology, Jain University 24
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Design And Implementation Of P- Band RF Low Noise Amplifiercentre of the Smith chart must always be a stable region. In the above example thedevice will be stable for all possible matches on the input or output of the activedevice. Figure 3.8 stability circles.However, in the case where S11 or S22 > 1 and the stability circle cover the centre ofthe Smith chart then this region is unstable the following diagram shows the regionsof instability.The Figure below (Figure 3.9) shows the areas of instability with S11 <> 1 & S22<1. Figure 3.9 Areas of instabilityCentre for Emerging Technology, Jain University 25
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Design And Implementation Of P- Band RF Low Noise AmplifierCalculations of stability circles are tedious, prone to error and in addition they shouldbe plotted across a range of frequencies – low frequency to at least the ft of the activedevice. This because a match may be clears of a region of instability at the pass-bandfrequencies but may be in a region of instability at another out of band frequency. Acommon problem of FET devices is that they are conditionally stable and have thestability circles clipping the outer edge of the Smith chart. This means that if an openor short circuit is applied to the input then the device may well oscillate. This may notbe a problem where a broadband load is applied but in case of antennas for examplethey often are open/short circuit at very low frequencies and together with the veryhigh gain of the active device at low frequencies often leads to oscillation.As shown in figure 3.12 there is still some potential instability with applied highimpedance at around 10GHz. However the resulting matching circuit will have someloss at this frequency so the impedance at this frequency will always be lower. . Figure 3.10 shows several methods of stabilizationIf when the circuit is re-simulated with RF bias networks and matching applied, thereis some additional stabilizing (a) is the addition of a series resistor to ensure that nomatch is capable of intersecting an input stability circle to tend to clip the outer edgeof the Smith chart. That is why some devices especially FET‘s readily oscillat e whenan open or short circuit is applied to the input of the device. A note of cautionhowever that is the addition of a resistor will greatly increase the noise figure of thedevice as the resistor acts as a noise generator.The second method (b) involves adding a fairly high value shunt resistor across theoutput of the device. The DC block ensures that the DC bias to the drain/collector isnot upset.Centre for Emerging Technology, Jain University 26
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Design And Implementation Of P- Band RF Low Noise AmplifierThe third method (c) involves the use of a quarter wavelength piece of transmissionline connected to a resistor usually 50 ohms. At the resonant frequency the quarterwavelength transmission line being an open circuit at one end will be transformed to ashort circuit at the resistor end. Therefore, at the resonant frequency the resistor willbe effectively shorted to ground ensuring a 50ohm load to the device. This method isgenerally required for high frequency problems with devices having very high Ft‘s.Additionally 50-ohm resistors can be added to the bias networks to ensure that a 50-ohm resistor is connected at low frequencies where the gain of the device is at itshighest.3.7 Gains for Two-Port NetworksThe ratio between the signal outputs of a system to signal input of a system is calledgain. For LNA design there are three power gain definitions appears in the literature. Transducer power gain (GT) Operating power gain (GP) Available power gain (GA)3.7.1 Transducer Power Gain (GT)The ratio of the power delivered to the load and the power available from the source iscalled Transducer power gain. The equation for transducer power gain is givenbelowGT = PPower delivered to the load / PPower available from the source.The equation for transducer power gain is given below3.7.2 Operating Power Gain (GP)The ratio between powers delivered to the load and the power input to the network iscalled Operational Power Gain.Gp = PPower delivered to the load / PPower input to the network.Centre for Emerging Technology, Jain University 27
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Design And Implementation Of P- Band RF Low Noise AmplifierThe equation for Operating Power Gain is given below3.7.3 Available Power Gain (GA)The ratio between the power available from the network and power from the sourceGA = P Available form the network / PAvailable form the sourceThe equation of Available Power Gain is given belowBeside these three gain definitions, there are three additional gain definitions that canbe use for LNA design. · Maximum unilateral transducer power gain (Gumx) · Maximum transducer power gain (Gmax) · Maximum stability gain (Gmsg)3.7.4 Maximum Unilateral Transducer Power Gain (Gumx)Gumx is the transducer power gain with assumption of S12 to be zero and the sou rce-load impedances are conjugate matched to the LNA, i.e. Gs = S*11 and GL = S*12.3.7.5 Maximum Transducer Power Gain (Gmax)Gmax is the simultaneous conjugate matching power gain , when input and outputboth are conjugate matched.GS = G*in and GL = G*out when S12 is small and Gumxis close to Gmax.WhereK= Stability ( √ )Centre for Emerging Technology, Jain University 28
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Design And Implementation Of P- Band RF Low Noise Amplifier3.7.6 Maximum Stability Gain (Gmsg)Gmsg is the maximum of Gmax when stability k is greater than one is still satisfied. Itis defined as the ratio of magnitude of to the magnitude of .3.8 NoiseObjects capable of allowing the flow of electrical current will exhibit noise. Thisoccurs as some electrons will have a random motion, causing fluctuating voltage andcurrents. As noise is random it can only be predicted by statistical means, usually witha Gaussian probability density Function as shown below:- Figure 3.11 Gaussian probability density FunctionAs noise is random then it‘s mean value will be zero, hence we use mean squarevalues, which are measurements of the dissipated noise power. The effective noisepower of a source is measured in root mean square of rms values. Ie Vn=Vn2 (rms)3.8.1 Noise power spectral density – describes the noise content in a 1Hz bandwidth.Units are V2/Hz and denotes as Svn(f). The graph below shows how Svn(f) is defined.3.8.2 Equivalent Noise Bandwidth (NBW) - is defined as the frequency span of anoise power curve with an amplitude equal to the actual peak value, and with thesame integrated area. In other words the NBW describes the bandwidth of a ‗brickwall‘ system with the same noise power as the actual system (f1 is set such that thearea of the ‗brick wall‘ is ~ equal to the whole function). The graph below shows acouple of examples.Centre for Emerging Technology, Jain University 29
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Design And Implementation Of P- Band RF Low Noise Amplifier Figure 3.12 shows examples for whole functionsThe main constituents of noise in a system, is due to Shot, Thermal, Burst, Avalancheand Flicker noise.3.8.3 Shot noise – This noise is generated by current flowing across a P-N junctionand is a function of the bias current and the electron charge. The impulse of charge qdepected as a single shot event in the time domain can be Fourier transformed into thefrequency domain as a wideband noise ie Figure 3.13 shows shot noise in time and frequency domain3.8.4 Thermal noise – In any object with electrical resistance the thermal fluctuationsof the electrons in the object will generate noise ievn2 = 4kTRV 2 / Hz Where k = Boltzmann s constant (1.38x10 -23 J/K)The spectral density of thermal noise is flat with frequency and is known as whitenoise.3.8.5 Burst noise – occurs in semiconductor devices, especially monolithic amplifiersand manifests as a noise crackle.Centre for Emerging Technology, Jain University 30
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Design And Implementation Of P- Band RF Low Noise Amplifier3.8.6 Avalanche noise – occurs in Zener diodes are reversed biased P-N junctions atbreakdown. This noise is considerably larger than shot noise, so if zeners have to beused as part of a bias circuit then they need to be RF decoupled.3.8.7 Flicker noise – This noise occurs in almost all electronic devices at lowfrequencies and takes the form of:- Figure 3.14 shows Flicker noise frequency domainFlicker noise is usually defined by the corner frequency FL.Equivalent Noise Model Figure 3.15 shows Equivalent noise modelWhen analyzing a circuit we transform the many possible sources of noise (generatingnoise currents and voltages) to an equivalent noise source at the input of the circuit ie3.8.8 Noise figure (NF) is a measure of degradation of the signal-to-noiseratio (SNR), caused by components in a radio frequency (RF) signal chain. The noisefigure is thus the ratio of actual output noise to that which would remain if the deviceitself did not introduce noise.The noise factor of a system is defined as: (3.27)Centre for Emerging Technology, Jain University 31
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Design And Implementation Of P- Band RF Low Noise AmplifierThe noise figure is defined as: NF=10log(F) (3.28)The noise figure is the noise factor expressed in dB where and are alsoin decibels (dB):WhereNF= Noise figureSNRin = Signal to Noise ratio at the input of a circuit or systemSNRout = Signal to Noise ratio of the circuit or system at output.Or Noise Figure is simply defined by (3.30)There are three key parameters that are needed for the noise figure analysis of an RFamplifier. Minimum noise figure NFmin, that depends on the biasing condition and operating Frequency of the device. Equivalent noise resistance Rn Optimum reflection coefficient Γopt3.8.9 The Friis formula for noise factorFriiss formula is used to calculate the total noise factor of a cascade of stages, eachwith its own noise factor and gain. The total noise factor can then be used to calculatethe total noise figure. The total noise factor is given as (3.31)where and are the noise factor and available power gain, respectively, of the n-thstage. Note that both magnitudes are expressed as ratios, not in decibels.An important consequence of this formula is that the overall noise figure of a radioreceiver is primarily established by the noise figure of its first amplifying stage.Subsequent stages have a diminishing effect on signal-to-noise ratio. For this reason,Centre for Emerging Technology, Jain University 32
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Design And Implementation Of P- Band RF Low Noise Amplifierthe first stage amplifier in a receiver is often called the low-noise amplifier (LNA).The overall receiver noise figure is then-Where is the overall noise factor of the subsequent stages. According to theequation, the overall noise figure is dominated by the noise figure of theLNA, if the gain is sufficiently high.3.8.10 The Friis formula for noise temperatureFriiss formula can be equivalently expressed in terms of noise temperature:3.9 VSWRThe SWR is usually defined as a voltage ratio called the VSWR, for voltage standingWave ratio. For example, the VSWR value 1.2:1 denotes maximum standing waveamplitude that is 1.2 times greater than the minimum standing wave value. It is alsopossible to define the SWR in terms of current, resulting in the ISWR, which has thesame numerical value. The power standing wave ratio (PSWR) is defined as thesquare of the VSWR.The VSWR is related to the reflection coefficient as:Where ρ = the magnitude of the reflection coefficient.It is also defined as the superposition of forward travelling wave and reflectedtravelling wave when the transmission line is terminated with other than itscharacteristic impedance. (3.35)Where Γ is the reflection coefficient which is the ratio of reflected wave to theforward wave.Centre for Emerging Technology, Jain University 33
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Design And Implementation Of P- Band RF Low Noise AmplifierThe way to improve VSWR of the system is to use impedance matching deviceswhere change in the impedance occurs.It relates to the magnitude of the voltage reflection coefficient and hence to themagnitude of S22 for the output port and S11 for the input port. VSWR at the input is given by the formula VSWR at the input is given by the formula3.9.1 Reflection coefficient:Reflections occur as a result of discontinuities, such as an imperfection in anotherwiseUniform transmission line, or when a transmission line is terminated with other thanits Characteristic impedance. The reflection coefficient Γ is defined thus:Γ is a complex number that describes both the magnitude and the phase shift of thereflection. The simplest cases, when the imaginary part of Γ is zero, are: Γ = − 1: maximum negative reflection, when the line is short-circuited, Γ = 0: no reflection, when the line is perfectly matched, Γ = + 1: maximum positive reflection, when the line is open-circuited.For the calculation of VSWR, only the magnitude of Γ, denoted by ρ, is of interest.Therefore, we define ρ=|Γ|.3.9.2 Return loss:Return loss or Reflection loss is the reflection of signal power resulting from theinsertion of a device in a transmission line or optical fiber. It is usually expressed as aratio in dB relative to the transmitted signal power.Centre for Emerging Technology, Jain University 34
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Design And Implementation Of P- Band RF Low Noise AmplifierIf the power transmitted by the source is PT and the power reflected is PR, then thereturn loss in dB is given byOptical Return Loss is a positive number; historically ORL has also been referred toas a negative number. Within the industry expect to see ORL referred to variably as apositive or negative number.This ORL sign ambiguity can lead to confusion when referring to a circuit as havinghigh or low return loss; so remember:- High Return Loss = lower reflected power =large ORL number = generally good. Low Return Loss = higher reflected power =small ORL number = generally bad.In metallic conductor systems, reflections of a signal traveling down a conductor canoccur at a discontinuity or impedance mismatch. The ratio of the amplitude of thereflected wave Vrto the amplitude of the incident wave Vi is known as the reflectioncoefficient Γ. (3.40)When the source and load impedances are known values, the reflection coefficien t isgiven by where ZS is the impedance toward the source and ZL is the impedancetoward the load.Return loss is simply the magnitude of the reflection coefficient in dB. Since power isproportional to the square of the voltage, then return loss is given byThus, a large positive return loss indicates the reflected power is small relative to theincident power, which indicates good impedance match from source to load.When the actual transmitted (incident) power and the reflected power are known (i.e.through measurements and/or calculations), then the return loss in dB can becalculated as the difference between the incident power Pi (in dBm) and the reflectedpower Pr (in dBm).Centre for Emerging Technology, Jain University 35
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Design And Implementation Of P- Band RF Low Noise Amplifiers11/s22 relationship to impedance matching:3.9.3 Input return lossInput return loss is a scalar measure of how close the actual input impedance of thenetwork is to the nominal system impedance value and, expressed in logarithmicmagnitude, is given byBy definition, return loss is a positive scalar quantity implying the 2 pairs ofmagnitude (|) symbols. The linear part is equivalent to the reflected voltage magnitudedivided by the incident voltage magnitude.3.9.4 Output return lossThe output return loss has a similar definition to the input return loss but applies tothe output port (port 2) instead of the input port. It is given by3.10 Information on P BandP Band: 0.2-1.0 GHz. Because lower frequencies are trapped by the ionosphere, onlyfrequencies above 100 MHz are available for satellite communications. The VHF andUHF range is principally used for mobile satellite communications because the designof the satellite and terminal hardware is relatively straightforward and wellunderstood. For example, the receive antenna can be a simple Yagi or wire helix. Thesize and cost of terminals can also be reduced by using higher powers that are easierto generate on board the satellite because of the low frequency being used. Thepropagation of the longer wavelengths is also useful because they diffract more easilyaround obstacles and are able to penetrate buildings. The main restriction in the use ofthese relatively low frequencies is the competition provided by a large number ofexisting terrestrial radio applications in these bands, which restricts the frequencyrange available for satellite communications.Centre for Emerging Technology, Jain University 36
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Design And Implementation Of P- Band RF Low Noise AmplifierCHAPTER 4SYSTEM DESIGN4.1 Selection of the type of the designIn AWR there are two types of devices, (1) S-parameter & (2) normal device. S-parameter device is an in-built device with S-parameters loaded from the data sheet.There is no need of applying external bias to it, because it has fixed S -parameters (i.e.fixed biasing).On the other hand normal device is just like any transistor device towhich any bias value can be applied. For the LNA design, S-parameter device ischosen in general.S-parameters do not use open and short circuit conditions to characterize a linearelectrical network; instead of matched loads are used. This termination is much easierto use at high frequency than open and short circuit termination. Moreover thequantities are measured in terms of power.S-parameters are mostly used for network operating at RF and micro wave frequencywhere signal power and energy considerations are more easily quantified than currentand voltages.4.2 Selection of the transistorSelection of the transistor is the crucial stage in LNA design. Any tr ansistor has itsNF maximum available gain (MAG) and minimum intrinsic noise figure (NF min). Soafter adding the matching and biasing sections, we cannot achieve gain more thanMAG and Noise figure less than NF min. To achieve a gain over 20 dB, a 2-stage LNAis designed. As we know, noise figure of the first stage is very crucial in the overallnoise figure, because the noise figure of the next stages is reduced by a factor equal tothe total gain till that stage.From the comparison from many transistors like ATF 34143,ATF 54143 etc., it isobserved that Transistor ATF53189 has the Noise Figure of 0.85 dB at bias point of4V, IDS=135 mA 2 GHz and associated Gain of 17.5dB. It has the similar parameterCentre for Emerging Technology, Jain University 37
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Design And Implementation Of P- Band RF Low Noise Amplifiervalues as the specifications of the project. Hence Transistor ATF53189 has beenchosen.4.3 General Amplifier Design ProceduresNow that we have picked our device, stabilized it and checked it‘s maximum availablegain we can begin the process of designing the LNA. This process consists of thefollowing steps:- 1) Evaluate the Rollett‘s stability factor to identify the possibility of instabilities depending on source and load matching. 2) Determine Bias conditions and circuit. 3) If a specified gain is required at a single frequency then the gain circles can be plotted on a Smith chart and the associated source match can be read off and the corresponding load match calculated. Careful consideration must be taken to the position of the source match in relation to the stability circles. 4) If a specified noise figure and gain at a frequency is required then the noise circles need to be added to the gain circles from (ii). The source match required will be the intersection of the required gain & noise circles. Again careful consideration must be given to the position of the source match in relation to the stability circles. 5) Once the required source impedance has been chosen the corresponding output match required for best return loss can be calculated.4.3.1 Stability DesignStability design should be the next step in LNA design. Unconditional stability of the circuit is the goal of the LNA designer. Unconditional stability means that with any load present to the output or output of the device, the circuit will not become unstable – will not oscillate. Instabilities are primarily caused by three phenomena: internal feedback of the transistor, external feedback around the transistor caused by external circuit, or excess gain at frequencies outside of the band of operation.Centre for Emerging Technology, Jain University 38
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Design And Implementation Of P- Band RF Low Noise Amplifier S-parameters provided by manufacturer of the transistor will aid in stability analysis: numerical and graphical. Numerical analysis consists of calculating a term called Rollett Stability Factor (K-factor). When K-factor is greater than unity, the circuit will be unconditionally stable for any combinations of source and load impedance. When K-factor is less than unity, the circuit is potentially unstable and oscillation may occur with a certain combination of source and /or load impedance present to the transistor.The K-factor represents a quick check for stability at given biasing condition. Asweep of the K-factor over frequency for a given biasing point should be performed toensure unconditional stability outside of the band of operation.The designer‘s goal is to design an LNA circuit that is unconditionally stable for thecomplete range of frequencies where the device has a substantial gain.Different stabilization methods of LNA: The first one consists of resistive loading of the input. This method, although capable of improving the stability of the circuit, also degrades the noise of the LNA and is almost never used. Output resistive loading is preferred method of circuit stabilization. This method should be carefully used because it effects are lower gain and lower P1dB point (thus IP3 point). The third method uses collector to base resistor-inductor-capacitor (RLC) feedback to lower the gain at the lower frequencies and hence improve the stability of the circuit. The fourth method consists of filter matching, usually used at the output of the transistor, to decrease the gain at a specific narrow bandwidth frequency. This method is frequently used for eliminating gain at high frequencies, much above the band of operation.Centre for Emerging Technology, Jain University 39
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Design And Implementation Of P- Band RF Low Noise Amplifier Short circuit quarter wave lines designed for problematic frequencies, or simple capacitors with the same resonant frequency as the frequency of oscillation (or excessive gain) can be used to stabilize the circuit. The final stabilization method can be realized with a simple emitter feedback inductor. A small inductor can make the circuit more stable at higher frequencies. But if the source inductance is increased, the K-factor at higher frequencies eventually falls below 1. This effect limits the amount of source inductance that can safely be used. To get the best LNA stability performances have to accommodate the full range of expected variations in operating parameters as: Component package parasitic Component values Temperature Supply voltage Most common causes for LNA instability are: Insufficient RF decoupling between supply lines of the amplifier bias. Parasitic inductance in GND connections. Excess in-band and/or out-of-band Gain. Electro-Magnetic coupling and Feedback. Always check stability of your LNA well beyond band-of-interest checking for both, small-signal stability and for large-signal stability. Use stability circles on Smith Chart (for both, source and load) to verify legitimacy of chosen Zin and Zout impedances.In the design of transistor amplifiers it is always very important to pay attention to thestability of the design. A common way to make potentially unstable transistorsunconditionally stable is to use resistive loading. Normally this technique is used inthe design of broadband amplifiers only because both the noise and gain performancesare significantly degraded. However, today there is a strong demand for broadbandCentre for Emerging Technology, Jain University 40
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Design And Implementation Of P- Band RF Low Noise Amplifiercircuits. There are four different ways to improve the stability with resistive loads;series loading atthe input, series loading at the output, shunt loading at the input, andshunt loading at the output. The load resistors that provide unconditional stability forthe transistor are normally found by studying the stability circles. It is well knownthat the design equation for simultaneous conjugate match, i.e. the condition formaximum transducer power gain, is often very useful.4.3.2 Noise Matching and Input Return Loss (IRL)The next step in LNA design consists of Noise Match and Input Return Loss (IRL). IRL defines how well the circuit is matched to 50 Ω matching of the source. A typical approach in LNA design is to develop an input matching circuit that terminates the transistor with conjugate of Gamma optimum (Γopt), which represents the terminating impedance of the transistor for the best noise match.In many cases, this means that the input return loss of the LNA will be sacrificed.The optimal IRL can be achieved only when the input-matching network terminatesthe device with a conjugate of S11, which in many cases is different from theconjugate of Γopt To design an LNA for minimum Noise Figure, determine (experimentally or from the data sheet) the source resistance and bias point that produce the minimum Noise Figure for that device. Then force the actual source impedance to ―look like‖ that optimum value with all stability considerations still applying. If the Rollet stability factor (K) is calculated to be less than 1 (K is defined as a figure of merit for LNA stability), then you must be careful in choosing the source and load-reflection coefficients. A typical method used in designing input matching network is to display noise circles and gain/loss circles of the input network on the same Smith chart. This provides a visual tool in establishing an input matching network for the best Input Return Loss and noise trade off.Centre for Emerging Technology, Jain University 41
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Design And Implementation Of P- Band RF Low Noise Amplifier4.3.2.1 Using Noise Data from Datasheets Generally for microwave transistors following a datasheet the minimum Noise Figure (Fmin) at higher frequencies is based on measurements, while the F mins at lower frequencies are extrapolated. Fmin represents the true minimum Noise Figure of the device when the device is presented with an impedance matching network that transforms the source impedance, typically 50Ω, to impedance represented by the reflection coefficient Γopt. The designer must develop a matching network that will present Γopt to the device with minimal associated circuit losses. To accomplish this have to minimize the number of components needed on the LNA input. The Noise Figure of the completed amplifier is equal to the Noise Figure of the device plus the losses of the matching network preceding the device. The Noise Figure of the device is equal to F min only when the device is presented with Γopt. If the reflection coefficient of the matching network is other than Γopt, then the Noise Figure of the device will be greater than Fmin The losses of the matching networks are non-zero and they will also add to the noise figure of the device creating a higher amplifier noise figure. The losses of the matching networks are related to the Q of the components and associated printed circuit board loss. Γopt is typically fairly low at higher frequencies and increases as frequency is lowered. For FET devices larger gate width devices will typically have a lower Γopt as compared to narrower gate width devices. Typically for FETs, the higher Γopt usually infers that an impedance much higher than 50Ω is required for the device to produce F min. At VHF frequencies and even lower L Band frequencies, the required impedance can be in the vicinity of several thousand ohms.Centre for Emerging Technology, Jain University 42
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Design And Implementation Of P- Band RF Low Noise Amplifier Matching to such high impedance requires very hi-Q components in order to minimize circuit losses.4.3.2.2 Different types of Input MatchingThere are few topologies for matching of GaAsFETs, each of them having pros andcons.This low insertion loss and simple input match circuit, works well up to UHFfrequencies. There are not too many choices of tuning and a match from 50 ohms toGamma Optimum (best Noise Figure) depends on the FET‘s internal stray capacitancefrom gate to the ground . This input match gives a high performance below 500MHz. Can get the best Noise Figure, Gamma Optimum that can be reached. Because the input impedance of the GaAs FET below 500MHz is high, the Q or Bandwidth of the input circuit can be varied with little impact on the Noise Figure. The tapped L input is fine for VHF range high performance LNAs.Centre for Emerging Technology, Jain University 43
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Design And Implementation Of P- Band RF Low Noise Amplifier It has very low loss, input is grounded, but the circuit has very wide bandwidth. The Pi input matching network works from low frequencies up to few GHz, and virtually can match any impedance. In the Pi matching network the second capacitor is tuned at a very low value, due to the FET input stray capacitance. This method requires high quality capacitors. The stub circuit is versatile and capable of matching almost of any input impedance, but the assembly is very large below 1GHz and relative difficult to supply the bias voltage. The microstrip stub circuit is limited to microwave frequencies, is narrowband, but has great repeatability.Centre for Emerging Technology, Jain University 44
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Design And Implementation Of P- Band RF Low Noise Amplifier Just a small inductance in the gate lead can match the impedance in the 1GHz to 2GHz region. The circuit is simple and has low loss, broad bandwidth and excellent Noise Figure.4.3.2.3 Output Matching The last step in LNA design involves output matching of the transistor. An additional resistor, either in series or parallel, has been placed on the collector of the transistor for circuit stabilization. Conjugate matching has been exclusively used for narrow band LNA design to maximize the gain out of the circuit. With additional IP3 requirement forced on the LNA, the trade-off between IP3 and gain must be considered. Linearity matching is widely known by high-power amplifier designers. The so-called load pulling is used to establish IP3 and gain impedance contours. The load pulling can be realized by using the non-linear Spice model ofthe transistor with simulation software. Harmonic balance can be used for establishing two-tone environment. In order to improve the gain and noise response of the final stage in the cascade design, we need to provide the RL = ROUT*.4.3.3 Real issues in LNA design An LNA is a design that minimizes the Noise Figure of the system by matching the device to its noise matching impedance, or Gamma optimum (Γopt). Gamma optimum (Γopt) occurs at impedance where the noise of the device is terminated. All devices exhibit noise energy. To minimize this noise as seen from the output port, one must match the input load to the conjugate noise impedance of the device.Centre for Emerging Technology, Jain University 45
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Design And Implementation Of P- Band RF Low Noise Amplifier Otherwise the noise will be reflected back from the load to the device and amplified. While this gives a minimum noise figure, it often results in slightly reduced gain as well as possibility increasing the potential instabilities. Noise match often comes close to S11 conjugate (S11*) under non-feedback conditions. As a result, the input impedance to the amplifier will not be matched to 50 ohms. Γopt, as presented in data sheets, is the actual measured load at which the minimum noise figure is found. Designing an effective low noise amplifier (LNA) requires a high-performance transistor. But most suitable devices are potentially unstable at microwave frequencies, leading to oscillation. Fortunately, resistive loading at the input or output of the transistor can prevent oscillation at the frequency of interes t for all passive source and load termination but stability at other frequencies remains problematic, and out-of-band oscillations are possible.A further complication on LNA design is that the input load of the amplifier is usuallyless than ideal. It is either connected to an antenna, which can change its impedancewith changing the environment, or to a filter, which by very physics of a reflectivenetwork will have very bad match out of band. These mismatches could cause thedevice to become unstable out of band and some cases in band. As the gain of thedevice increases, the difficulties in yielding a stable design become increasingly morechallenging.To avoid overloading the LNA, an input filter is commonly used. Since the device isnot matched to S11*, the input of the LNA will not be 50 ohms. This can causedistortions in the pass band of the filter when connected to the input of the LNA, asfilter are intended to operated in their characteristic impedance, typically 50 ohms.Printed inductors or transmission lines are free as compared to SMT inductors, whichtypically cost 10 to 25 times as much as resistors or capacitors in volume. Printing aninductor is easy and results in highly repeatable results. Printed inductors usuallyexhibit poor Q due to the lossy dielectric, and, if a ground plane exists, they are nomore than a high impedance transmission line.Centre for Emerging Technology, Jain University 46
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Design And Implementation Of P- Band RF Low Noise AmplifierAs shown a transmission line can replace an inductor to some degree, but inductorsand high impedance transmission lines have a different trajectory on the Smith Chart.High impedance transmission line can be made to look more like printed inductors incases where the backside of the PCB is suspended away from a grounded chassis. Thisis accomplished by removing the backside ground plane of the PCB directly under theprinted inductor.In this case beware of digital noise coupling into the input of the LNA from circuitryon the opposite side. The next concern is what load impedance to match. Remember matching to the conjugate of S22* is only valid if the input is conjugate matched. Since S12 is non-zero, whatever load is present to the input will cause the output load change. Another issue is stability, especially if a filter is going to be used at the input. The output port can potentially give difficulties since the input is very restricted by its match.Real components differ from ideal ones in several respects. First, real componentshave a price associated with them. There is a trade-off between price and performanceof these parts. The competitiveness of todays markets often forces designers to useinexpensive components in their designs. Real discrete components have a finite resistance called Equivalent Series Resistance (ESR). The ESR introduces losses that result in lower gain and noise figure. Although typically only a few tenths of an ohm in value, ESR will affect the matching networks. Discrete components also have a Q value, measured at a particular frequency that can contribute to unwanted resonance. High-Q networks are sensitive to variations in process, voltage, temperature, and component value. A components Series Resonant Frequency (SRF) is the frequency where it will behave erratically.Centre for Emerging Technology, Jain University 47
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Design And Implementation Of P- Band RF Low Noise Amplifier If an inductor is operated at or above its SRF, it might behave as a capacitor. To avoid this, select components where the SRF is much higher than the operating frequency. Also, leaded through –hole parts have leads that add series inductance to a design, and surface-mount parts have pads that add shunt capacitance to a circuit.4.4 Gain & Noise ParametersUsing the S-parameters of the device it is possible to calculate the overall transducergain which consists of three parts, the gain factor of the input (source) matchingnetwork, the active device and the output (load) matching network: - (4.1) (4.4) (4.2) (4.5) (4.3) (4.6)Overall Transducer gain =10LOG10 (Gs .Go .GL )4.5 Constant Noise circlesFormula for calculation of noise circles:- | |WhereF= required noise figureFmin= Optimum noise figureRN =Equivalent noise resistance of transistorΓopt = Reflection coefficient to achieve optimum noiseCentre of noise figure circle = ΓoptN+1And the radius of the noise figure circle isCentre for Emerging Technology, Jain University 48
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Design And Implementation Of P- Band RF Low Noise Amplifier √ ( | |)4.6 LAYOUT RULESThere are certain rules which were followed in the layout design No two devices must be connected back to back. Use MLIN between components. Use MTEE$ / MCROSS$ whenever there is a junction, Extend the junction arms using MLIN. Grounding must be done using via. Break schematic in to Sub circuits.Centre for Emerging Technology, Jain University 49
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Design And Implementation Of P- Band RF Low Noise AmplifierCHAPTER 5DESIGN & IMPLEMENATION5.1 DESIGN FLOWThis design flow will explore the design parameters space of integrated inductively-low noise amplifiers (LNA), under the constraint of matched input impedance, ispresented. It is based on AWR microwave simulation tool and can be easilyautomated. The method is applied to the design of a (0.05-1) GHz LNA with less than4dB noise figure (NF) for a fixed bias current.Centre for Emerging Technology, Jain University 50
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Design And Implementation Of P- Band RF Low Noise Amplifier5.1.1 Transistor selectionPHEMT (Pseudomorphic High Electron Mobility Transistor) transistors find widemarket acceptance because of their excellent low noise figures and performance. Thetransistor is also selected depending up on the frequency range, noise figure and gainthat are given in the specification.The data sheet provides the device‘s small signal S parameters, which must be rea dinto the simulator in order to calculate the transistor parameters. The S Parameters ofthe transistor with different bias points are available with the transistor datasheet. Thetransistor with the bias point of -4v, 60mA is selected, which can give betterperformance at this bias point.5.1.2 Stability checkWhen embarking on any amplifier design it is very important to check on the stabilityof the device chosen, otherwise the amplifier may well turn into an oscillator. Themain way of determining the stability of a device is to calculate the Rollett‘s stabilityfactor (K), which is calculated using a set of S-parameters for the device at thefrequency (0.5 to 18GHz) of operation. For the device to be unconditionally stable,the stability circles should lie outside the circle for |S 11 |<1 and |S 22 |<1.5.1.3 STABILITY ENHANCEMENTResistive loading A transistor can be stabilized by adding small series resistors or large shunt resistors to its input or/and output. These lossy elements ensure that the transistor cannot be presented with impedances inside the instability regions, irrespective of what source and load impedance are connected.Series Resistance Stabilization MethodSteps: 1. Convert Transistor S-parameters to Z-Parameters 2. Add series resistance to real part of Transistor Z22Centre for Emerging Technology, Jain University 51
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Design And Implementation Of P- Band RF Low Noise Amplifier 3. Convert composite Z-Parameters to S-Parameters 4. Check K5.1.4 INPUT MATCHING CIRCUITThe matching for lowest possible noise figure over a band of frequ encies requires thatparticular source impedance be presented to the input of the transistor. The noiseoptimizing source impedance is called as Gopt, and is obtained from themanufacturer‘s data sheet. Figure 5.1(a) input matching circuitThe above circuit shows the input matching circuit. The input matching circuit ismatched to noise optimizing source impedance 0.78∟30 called as Gopt, and isobtained from the manufacturer‘s data sheet. Figure 5.1(b) layout of input matching circuitCentre for Emerging Technology, Jain University 52
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Design And Implementation Of P- Band RF Low Noise AmplifierThe 1st stage has no matching on the output and as we require a good output returnloss we should match to S22*. Note S22 will now have been modified by adding theinput matching circuit and will have to design the matching circuit to be the conjugateof S22modified (This is because S22 is looking into the device and the conjugate willlooking towards the matching circuit.The process of layout involves the interconnection of MLINS, Tees between theelements. The layout obtained for the input matching network shown in schem atic5.8(a) is shown in above figure 5.8(b).The layout has certain constraints that the widthand the length of MLIN, MTEE, should be not less than 0.025mm.5.1.5 OUTPUT MATCHING CIRCUIT Figure 5.2(a) Schematic of output matching circuitThe above figure 5.12(a) shows the schematic of output matching network followedby interconnecting the MLINS and MTEE for layout purpose. In order to improve thegain and noise response of the second stage we need to provide the RL (7.6891 -7.0605i).In order to improve the gain and noise response of the final stage we need to providethe RL = ROUT* given by: ( )Centre for Emerging Technology, Jain University 53
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Design And Implementation Of P- Band RF Low Noise AmplifierThe value of S1 of the output matching is calculated and layout is shown belowThis composite design using ideal elements was then optimized. Finally, the idealelements were replaced with vender elements and the design was again optimized.After this step, the layout process was started.The layout process involved interconnecting appropriate bends, tees, and MLINs tothe optimized LNA design with vender elements. Figure 5.2(b) layout of output matching circuitThe layout of output matching network is shown in the above figure 5.2(b). The longlength MLIN and inductor shown in the fig 5.2(b) is used to take RF output.5.2 IMPLEMENTATION OF THE DESIGNThe design implementation requires adding microstrip lines between the lumpedelements and placing of micro strip Tee at the junction. The junction arm has to beextended using MLINS. The grounding must be done using via.The design is implemented on FR4 substrate with the relative permittivity =4.4 witha height of 1.6mm. The substrate thickness is chosen to be 0.035mm, Rho=1.The below figure shows the sub circuit of final schematic of an amplifier, the first subcircuit represents the input matching network. The second block shows the first stageof stabilized transistor. The inter-stage matching network is represented in thirdblock. The fourth block represents second stage and the output matching network isrepresented in the last block.Centre for Emerging Technology, Jain University 54
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