Design Of Amplifiers In LTspice - DiVA Portal
TVE 13 027 JuniExamensarbete 15 hpJuni 2013Design of amplifiers in LTspiceAspects on the usage of spice-ware in thework of designing an electron tube amplifierPer Normann
AbstractDesign av förstärkare i LTspiceDesign of amplifiers in LTspiceTeknisk- naturvetenskaplig orietLägerhyddsvägen 1Hus 4, Plan 0Postadress:Box 536751 21 UppsalaTelefon:018 – 471 30 03Telefax:018 – 471 30 00Per NormannAmong users of guitar amplifiers there is a tendency of being enthusiastic about theusage of electron tubes in amplifiers. Further to shy away from the usage oftransistors as gain devices. This to the extent that new technology is generallyavoided. In this paper however a software tool called LTspice is used as an aid in thedesign process of an guitar amplifier. Electron tube spice models are examined andused in the process. The amplifier being designed in this paper has two types ofelectron tubes, 4 EL84 in the power section and 2 ECC83 in the pre-amplifier. Theoutput of the amplifier is 30 to 35 e: Kjell StaffasÄmnesgranskare: Martin SjödinExaminator: Martin SjödinISSN: 1401-5757, UPTEC F13 027
Table of Contents1 Introduction.41.1 History of amplification.41.2 Why electron tubes.51.3 Old technology - new tools .52 Theory.52.1 Electron tubes.52.2 The grounded cathode gain stage.72.3 Coupling capacitor.112.4 Phase inverter.112.5 Power stage.122.6 Tone control.133 Simulation and design.153.1 Electron tubes in LTspice.153.2 The grounded cathode gain stage.173.3 Pre amplifier.183.4 Power section.183.5 Amplifier.193.6 Calibrating the tone stack.194 Results.195 Design software.205.1 Tone Stack Calculator 1.3.205.2 LoadLinePlotter.205.3 LTspice IV.206 Acknowledgement.217 Literature list.218 Appendix.228.1 A – Anode characteristics diagrams.228.2 B – Frequency responses.298.3 C – Circuitry.308.4 D.333
SammanfattningBland användare av gitarrförstärkare finns en tendens att vara entusiastisköver elektronrör. Ofta till den grad att ny teknik undviks generellt isambamd med gitarrförstärkare. I den här uppsatsen undersöksmöjligheterna att använda spice-mjukvara som hjälp i designarbetet. Ianvändandet av spice-mjukvaran används så kallade spice-modeller för attemulera verkliga elektronrör. Hur bra dessa spicemodeller återger vissaegenskaper av elektronrör undersöks också.En läsare bör ha grundläggande kunskap inom elektronik,,dataanvändande, viss erfarenhet av spice-mjukvara underlättar ocksåläsandet. Allt användande av spice-mjukvara sker i programmet LTspicesom är ett så kallat freeware, dvs. det finns att tillgå fritt på internet.Utöver LTspice används annan mjukvara, tex spice modeller. All denmjukvara som används presenteras med en kortattad beskrivning hur denkan införskaffas via internet.Förstärkaren som designas här har 4 st EL84 I slutsteget och 3 st ECC83 Iförsteget. Förstäkrarens så kallade maxeffekt, maximal förstärkning innandistortion förekommer, är mellan 30 och 35 W.1 Introduction1.1 History of amplificationIn 1800 Alesandro Volta presents a galvanic element. With this inventionit is possible to create electrical voltage. Three quarters of a centery laterMaxwell, J.C put forward [1] which is commonly known as Maxwell'sequations. Maxwell's equations theoretically describes electromagneticwaves and thus paves the way for new advances in the field of electricity.In [2] Hertz, H verifies Maxwell's theories in a series of experiments.When the eighteenth century draws to an end the Italian inventorMarconi, G constructs the first signal transmitter [3] by utilizing thetheoretical achievements done by Maxwell and Hertz.When the electron tube [4] is invented by Forest, D in 1907 thetransmitted signal can be amplified on the receiving end. 1920 therefinement of the electron tube makes it possible to amplify the signalbeing sent. At the middle half of the 20th century the electron tube is themain component in gain stages in electronic devices.A theoretical transistor [5] is presented in 1948 by a group of scientists atthe Bell Laboratories. Eight years later, in 1956, the same group presents aworking transistor and recives the Nobel price for this. The transistor'sadvantages over the electron tube leads to a rapid decline in the usage ofelectron tubes. By the end of the 20:th century the usage of electron tubesis almost non-existent. There is however one field in which electron tubesstill are used frequently. In HiFi-amplifiers and guitar amplifiers theelectron tube is still popularly used. The electron tube has some featuresthat makes it popular to use in amplification of sound.4
1.2 Why electron tubesFrom the emerging of the transistor till present days the development oftransistors in integrated circuit has been extensive. It is now possible toemulate electron tube like amplifiers by the help of software tools incomputers. This is however a subject of strong opinions. Throughout theguitar amplifier community the fact whether an emulated electron tube ison par with or even close to the real thing seems to be a big no no. Theinterested reader is encouraged to join an on-line forum or just readcomments regarding this subject. While this text is written, 2013-05-26,the Google search transistor vs. tube renders over 22 million hits.The idea that the electron tube is irreplaceable has a huge affect onamplifier market. Designers and vendors of amplifiers have to comply withthis supply and demand situation. With a market craving for electron tubeamplifiers, new inventions is not likely to be profitable in years to come. Inthe race of cutting costs to raise profit many amplifier producersmanufacture transistor amplifiers. These transistor amplifiers are oftenpresented as a low cost alternative to electron tube amplifiers. In additionto this misleading marketing of these devices is often used. Guitaramplifiers with transistor technique are often labelled in such a way thatthe user is lead to believe that the amplifier is a electron tube amplifier.Valvestate, Tubetrans, Valvetronix are just a few of these somewhatmisleading names, note that in British English electron tubes arecommonly known as valves.1.3 Old technology - new toolsOther aspects of guitar amplifiers should however be able to benefit fromnew technology. No mater if a computer emulated electron tube amplifieris a far cry from the real thing or not software tools can be of aid in thedesign of a tube amplifier. In this paper the underlying technique ofelectron tube amplification is investigated to give an insight in how acommon electron tube amplifier works. This knowledge is then used todesign an electron tube amplifier in the software tool LTspice.2 TheoryIn this section a survey of the components in an electron tube amplifier ispresented. The survey is meant to give the reader a fair idea of how eachpart of how a standars electron tube amplifier works. The text is alsomeant to display and explain customary construction solution. A reader ofthis section should have basic knowledge of alternating current electronics.2.1 Electron tubes2.1.1 The triodeOkasura, S. et. al. surveys the historical usage and development of electron5
tubes [6]. An in depth examination of the triode is presented. A commonway of constructing and vend contemporary triodes is in pairs mounted invacuum tubes. One of the predominant electron tubes is the ECC83 whichis two EBC91 triodes package in one vacuum tube. Due to the wide spreadusage in Hi-Fi and guitar amplifiers the ECC83 is one of few types ofelectron tubes still being produced in large numbers.To get a triode to amplify a signalthe triode is connected to the signalwire, to ground and to wires thatsupply heater current. The basicstructure of the triode stage ispresented in Illustration 1.In a triode the amplification isobtained by inducing andcontrolling an electric field betweenthe anode and the cathode. Thecathode is heated to lower thethreshold for the cathode to emitelectrons. Electrons are prone tostream from the negative cathode tothe positive anode. The electricstream from the cathode to theanode in the triode induces aIllustration 1: The in-signal is receivedcurrent. By controlling the electricat the grid connection 1. The anodefield the current through the triodeconnection 2 sends the amplified outis indirectly controlled. The controlsignal. The cathode connection 3 ismechanism is achieved by applyinggrounded over a resistor.a negative voltage at the girdbetween the anode and the cathode.The way the anode, cathode and grid voltages are balanced is oftenreferred to as bias. Low negative bias voltage at the grid makes it easy forelectrons to flow from the anode to the cathode. Conversely, a highnegative bias voltage at the grid prevents the electrons flow from the anodeto the cathode. This behaviour allows for variations in the grid voltage toappear at the anode. By superimposing an electric signal on the biasvoltage at the grid the signal is transferred from the grid to the anode. Ingain stages a triode amplifies the signal appearing at the anode. The EBC91triode's amplification factor is 100.2.1.2 The beam power pentodeThe beam power pentode is similar to the triode in many ways. In a powerpentode there are three grids instead of the triode's single grid. In orderfrom the cathode to the anode the first grid is the control grid, it functionslike the grid in the triode. The second grid is the screen grid that focus theelectron beam so that more of the jumping electrons make it to the anode.The third is suppressor grid that hinders electrons to jump back to thescreen grid. This is necessary because the screen grid is often kept at6
positive voltages.In this paper the EL84 is used in the power section of the amplifier. TheEl84 is a widely spread pentode that is still being manufactured because ofthe popularity in Hi-Fi and guitar amplifiers.2.2 The grounded cathode gain stageThe grounded cathode gainstage is a fundamental buildingblock in electron tubeamplifiers, it is characterised bythe triode and the topology ofthe stage. In Illustration 2 theschematics of a groundedcathode gain stage is outlined.The main idea of the groundedcathode stage is to amplify asmall input signal into a largeroutput signal. An ideal gainstage amplifies the in-signalwith a linear frequencyresponse and no distortion.However, in guitar amplifiers aIllustration 2. A grounded cathode gainstage.gain stage is often deliberatelydesigned to distort theamplified signal and alter the frequency response.2.2.1 Triode characteristicsA data sheet displaying the static anode characteristics is often used whena triode gain stage is designed. There seem to be many different namesused for this diagram, in this paper the diagram will be called static anodecharacteristics diagram.An anode characteristics diagram holding the data of the ECC83 electrontube is displayed in Appendix A1. The diagram displays the current Ia andvoltage drop Ua from the anode to the cathode. The curved lines shows thevoltage drop Ugc from the grid to the cathode. These lines are oftenreferred to as grid curves. These entities are central while setting up acommon cathode gain stage.2.2.2 The anode load lineIn a grounded cathode gain stages there is a resistor Ra connected to theanode of the triode. With a voltage applied on Ra, the triode and Ra are inseries. If there is no current flowing through the triode there can be novoltage drop over the Ra, consequentially by Ohm's law, all the voltagedrop is over the triode. Assuming that a point A can be defined in the staticanode characteristics diagram at A (Ua Umax, Ia1 0), Appendix A2. Umaxis commonly known as plate voltage. If all available current flows through7
the triode all the voltage drop will be over Ra. By applying Ohm's law it ispossible to calculate Ia,I a Ua.Ra(1)With no current flowing through the triode a point B (Ua 0, Ia Ua/Ra)can be defined in the anode characteristics diagram, Appendix A2.Although these two extreme points are not seen in any functioning triodestage they are useful when a triode stage is design. Since Ohm's law islinear a straight line can be interpolated between these points. By mergingthese ideas a function (2) can be put together for calculations of the anodeload line in a grounded cathode gain stage.I a (U ) U a URa(2)The dashed line in Appendix A2 forms an exemplified load line of a ECC83triode grounded cathode gain stage. The load line is central in the analysisof a electron tube circuit and hence the triode stage. As seen in (2) theanode load line of a triode stage can be adjusted simply by altering Ra orUa. The load line and the grid curves intersects, see Appendix A2. Theseintersections shows what Ua and Ia will be for different Ugc.2.2.3 The cathode load lineLike Ra on the anode side of the triode there is a cathode resistor Rcconnected on the cathode side. Similar to the anode load line it is possibleto draw a load line for the cathode. If all current is flowing through Rc thevoltage drop from the grid to the cathode in the triode can be calculated byOhm's law (1). Adjusting the cathode load line is usually done after the setup of the anode load line. This is assumed here, hence Ia is assumed to beknown. A point C ( Ugc -Ia Rc , Ia Ua/Ra) can now be marked in thestatic anode characteristics diagram in Appendix A2. The cathode load linehas a linear part, but it is not linear close to 0 V. The grid curves in theanode characteristics diagram have a wobbly behaviour at 0 to 50 V, anodevoltage. This suggests that it is not possible to interpolate a straightcathode load line in this interval. Ohm's law is used to calculate a secondpoint D in the static anode characteristics diagram in Appendix A2 withvalues chosen outside the low anode voltage range, D (Ugc -Ia Rc,Ia 0.5). By putting together these ideas a way of describing the cathodeload line can be defined,8
U gc U a Rc.Ra(3)Graphically the same can be done by interpolating a line between point Cand D in the anode characretistics diagram. An interpolation between Cand D forms the cathode load line. The blue dashed dotted line inAppendix A2 exemplifies a cathode load line. The intersection between theanode and the cathode loadlines is the bias point.2.2.4 Bias pointThe bias point is the operation point of the triode. When the operatingpoint is set in an grounded cathode gain stage it is possible to determineamplification and other features of the stage. By shifting the bias point it ispossible to induce more or less gain. Note that the bias point of thegrounded cathode gain stage is determined by the values of Ua Ra and Rc.2.2.5 Stationary amplificationBijl's equation states that,μ r a g m ,(4)where μ is the amplification factor, ra is the inner anode resistance and gmis the transconductance of the triode. Bijl's equation is convenient becauseit states a dependency that make it possible to calculate ra. If ra is knownthe amplification of the grounded cathode gain stage can be calculated. μ islisted in most electron tube specifications. It can also be derived from astatis anode characteristics diagram,μ Ua U gc .(5)IaThe transconductance gm is an entity that models the triodes ability toconvert a voltage change into a current change. m is to indicate themaximum achivable gain of the gain stage. If Ua is kept constant gm is,g m Ia U gc (6)UaThe inner resistance ra is deriven by keeping Ugc constant,9
r a Ua Ia .(7)U gcIt should be mentioned that these three entities vary slightly depending onthe triodes bias point. A reading from the static anode characteristicsdiagram is often a better choise than using values listed in tubespecifications. As mentioned these three entities, μ, ra and gm are readablein the static anode characteristics diagram. An example, the amplificationfactor is read by checking what ΔUa is when ΔUgc 1.The amplification of a grounded cathode gain stage can be derived byassuming that the triode is a perfect amplifier with the gain factor μ. If theamplifier leads a signal current down through any impedance present inthe gain stage, all these impedances forms a voltage divider. The out signalin a grounded cathode stage is taken across Ra. To properly calculate theamplification A all impedances down to ground has to be considered [7],A μ R a i 01Ri .(8)Since the difference between the grid and the cathode are amplified anyimpedance placed in series on the cathode side will appear to be amplified,μ*Rc. Three more impedances forms the total impedance; Ra, ra, and Rc.The amplification of the grounded cathode stage isA μ R aR a r a μ R c Rc(9)The minus sign is a result of the fact that an increased grid voltage willlower the anode voltage. In practice this means that the grounded cathodegain stage not only amplifies the in-signal, the in-signal is also inverted. Itis possible to calculate the stationary amplification A by using (4) and (9).2.2.6 Grid leak resistorWhen a triode is operating the grid is indirectly heated by the heatedcathode. The heat leads to emission of electrons from the grid. This loss ofelectrons makes the grid slightly more positive charged. Reduced negativegrid voltage lowers the threshold for electrons to jump from anode tocathode. This results in more current flowing from the anode to thecathode, hence worsen the heat problem even more. To maintain the gridvoltage at a controlled level electrons must be replenished in the grid. Agrid leak resistor Rgl connected between ground and grid will provide aleakage
misleading names, note that in British English electron tubes are commonly known as valves. 1.3 Old technology - new tools Other aspects of guitar amplifiers should however be able to benefit from new technology. No mater if a computer emulated electron tube amplifier is a far cry from the real thing or not software tools can be of aid in the
RF/IF Differential Amplifiers 5 Low Noise Amplifiers 7 Low Phase Noise Amplifiers 10 Gain Blocks 11 Driver Amplifiers 13 Wideband Distributed Amplifiers 13 Power Amplifiers 15 GaN Power Amplifiers 18 Variable Gain Amplifiers 19 Analog Controlled VGAs 19 Digitally Controlled VGAs 20 Baseband Programmable VGA Filters 21 Attenuators 22
RF/IF Differential Amplifiers 9 Low Noise Amplifiers 9 Gain Blocks and Driver Amplifiers 10 Wideband Distributed Amplifiers 11 Linear and Power Amplifiers 12 GaN Power Amplifiers 13 Active Bias Controllers 13. Variable Gain Amplifiers 14. Analog Controlled VGAs 14 Digitally Controlled VGAs 14 Baseband Programmable VGA Filters 14. Attenuators 15
Semiconductor optical amplifiers (SOAs) Fiber Raman and Brillouin amplifiers Rare earth doped fiber amplifiers (erbium – EDFA 1500 nm, praseodymium – PDFA 1300 nm) The most practical optical amplifiers to date include the SOA and EDFA types. New pumping methods and materials are also improving the performance of Raman amplifiers. 3
Introduction to LTspice Acknowledgment: LTspice material based in part by Devon Rosner (6.101 TA 2014), Engineer, Linear Technology . WINE(Linux) workarounds-PCBnetlist Convert a schem a tic to a PCB
51.3 Types of Semiconductor Optical Amplifiers SOAs can be classified as either subthreshold or gain clamped. Subthreshold amplifiers are lasers operated below threshold, and gain-clamped amplifiers are lasers operated above threshold but used as amplifiers. Subthreshold SOAs can be further classified according to whether optical feedback .
Optical Amplifiers vs Regenerators (1 of 2) Transparent: Regenerators specific to bit rate and modulation format used; O-Amps are insensitive Easily upgraded: A system with optical amplifiers can be more easily upgraded to higher bit rate without replacing the amplifiers Optical amplifiers
sistor amplifiers. Thirty years ago, transistor amplifiers were all the rage and valve amplifiers had become passe. Anybody who was anybody was discarding their large, hot and fragile valve amplifiers for small, cool, and — generally far more expensive — transistor amplifiers. Mullard and Philips had released their OC16's, which
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