Sutnan Capacitor Information

In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed.

The most common form arranges a group of semicircular metal plates on a rotary axis (“rotor”) that are positioned in the gaps between a set of stationary plates (“stator”) so that the area of overlap can be changed by rotating the axis. Air or plastic foils can be used as dielectric material. By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a linear frequency scale. Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns.

An example of a Vacuum variable capacitor. This one is a 10Kv 20 to 1000pF capacitor.A Vacuum variable uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders[1] (sleeve and plunger). These plates are then sealed inside of a non-conductive envelope such as glass or ceramic and placed under a high vacuum. The movable part (plunger) is mounted on a flexible metal membrane that seals and maintains the vacuum. A screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes. The vacuum not only increases the working voltage and current handling of the capacitor it also greatly reduces the chance of arcing across the plates. The most common usage for vacuum variables are in high powered transmitters such as those used for broadcasting, military and amateur radio as well as high powered RF tuning networks. Vacuum variables can also be more convenient since the elements are under a vacuum the working voltage can higher than an air variable the same size. By using a vacuum variable you can greatly reduce the size of the capacitor. Under some high voltage conditions you can't use anything else. When precise tuning and stability is required vacuum variables work very well.

Very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw. These so-called squeezers can’t provide a stable and reproducible capacitance, however. A variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a slider. This has practical advantages for makeshift or home construction and may be found in resonant loop antennas or crystal radios.

Small variable capacitors operated by screwdriver (for instance, to precisely set a resonant frequency at the factory and then never be adjusted again) are called trimmer capacitors. In addition to air and plastic, trimmers can also be made using a ceramic dielectric.

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The thickness of the depletion layer of a reverse-biased semiconductor diode varies with the DC voltage applied across the diode. Any diode exhibits this effect (including p/n junctions in transistors), but devices specifically sold as variable capacitance diodes (also called varactors or varicaps) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.

Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of intermodulation. At VHF/UHF frequencies, e.g. in FM Radio or TV tuners, dynamic range is limited by noise rather than large signal handling requirements, and varicaps are commonly used in the signal path.

Varicaps are used for frequency modulation of oscillators, and to make high-frequency voltage controlled oscillators (VCOs), the core component in phase-locked loop (PLL) frequency synthesizers that are ubiquitous in modern communications equipment.

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Variable capacitance is sometimes used to convert physical phenomena into electrical signals.

• In a capacitor microphone (commonly known as a condenser microphone), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in the distance between the diaphragm and a fixed plate, changing the voltage maintained across the capacitor plates.
• Some types of industrial sensors use a capacitor element to convert physical quantities such as pressure, displacement or relative humidity to an electrical signal for measurement purposes.
• Capacitive sensors can also be used in the place of switches, e.g. in computer keyboards or “touch buttons” for elevators that have no movable parts.

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China is experimenting with a new form of electric bus (capabus) that runs without powerlines using power stored in large onboard electric double-layer capacitors, which are quickly recharged whenever the electric bus stops at any bus stop (under so-called electric umbrellas), and fully charged in the terminus. A few prototypes were being tested in Shanghai in early 2005. In 2006, two commercial bus routes began to use electric double-layer capacitor buses; one of them is route 11 in Shanghai. [10]

In 2001 and 2002, VAG, the public transport operator in Nuremberg, Germany tested a bus which used a diesel-electric drive system with electric double-layer capacitors.[11]

Since 2003 Mannheim Stadtbahn in Mannheim, Germany has operated an LRV (light-rail vehicle) which uses electric double-layer capacitors.[12][13]

Other companies from the public transport manufacturing sector are developing electric double-layer capacitor technology: The Transportation Systems division of Siemens AG is developing a mobile energy storage based on double-layer capacitors called Sibac Energy Storage [14] and also Sitras SES, a stationary version integrated into the trackside power supply [15]. The company Cegelec is also developing a electric double-layer capacitor-based energy storage system[citation needed].

Proton Power Systems has created the world's first triple hybrid Forklift Truck, which uses fuel cells and battery as primary energy storage and electric double-layer capacitors to supplement this overall energy efficient storage solution.[16]

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Due to the capacitor's high number of charge-discharge cycles (millions or more compared to 200??000 for most commercially available rechargeable batteries) there were no disposable parts during the whole operating life of the device, which makes the device environmentally friendly. Batteries wear out on the order of a few years, and their highly reactive chemical electrolytes represent a serious disposal and safety hazard. This can be improved by only charging under favorable conditions, charging at an ideal rate and as rarely as possible. Electric double-layer capacitors can help in this regard, acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries only at opportune times.

Other advantages of electric double-layer capacitors compared with rechargeable batteries are extremely low internal resistance or ESR, high efficiency (up to 97-98%), high output power, extremely low heating levels, and improved safety. According to ITS (Institute of Transportation Studies, Davis, CA) test results, the specific power of electric double-layer capacitors can exceed 6 kW/kg at 95% efficiency [8]

The idea of replacing batteries with capacitors in conjunction with novel alternative energy sources became a conceptual umbrella of the Green Electricity (GEL) Initiative [2], [3], introduced by Dr. Alexander Bell. One particular successful implementation of the GEL Initiative concept was a muscle-driven autonomous solution which employs a multi-farad electric double-layer capacitor (hecto- and kilofarad range capacitors are now available) as an intermediate energy storage to power a variety of portable electrical and electronic devices such as MP3 players, AM/FM radios, flashlights, cell phones, and emergency kits.[9] As the energy density of electric double-layer capacitors is bridging the gap with batteries, it is hoped that in the near future the automotive industry will start to deploy ultracapacitors as a replacement for chemical batteries.

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The electric double-layer capacitor effect was first noticed in 1957 by General Electric engineers experimenting with devices using porous carbon electrode.[6] It was believed that the energy was stored in the carbon pores and it exhibited "exceptionally high capacitance", although the mechanism was unknown at that time.

General Electric did not immediately follow up on this work, and it was Standard Oil of Ohio that eventually developed the modern version of the devices in 1966 after accidentally re-discovering the effect while working on experimental fuel cell designs.[4] Their cell design used two layers of activated charcoal separated by a thin porous insulator, and this basic mechanical design remains the basis of most electric double-layer capacitors to this day.

Standard Oil also failed to commercialize their invention, licensing the technology to NEC, who finally marketed the results as ??upercapacitors??in 1978, to provide backup power for maintaining computer memory.[4] The market expanded slowly for a time, but starting around the mid-1990s various advances in materials science and simple development of the existing systems led to rapidly improving performance and an equally rapid reduction in cost. In 2005, the ultracapacitor market was between US $272 million and $400 million, depending on the source. It is rapidly growing, especially in the automotive sector.[4]

Recently [7], all solid state micrometer-scale electric double-layer capacitors based on advanced superionic conductors had been recognized as critical electron component of future sub-voltage and deep-sub-voltage nanoelectronics and related technologies (22 nm technological node of CMOS and beyond).

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It was stated earlier in this article that charging and discharging of electrochemical capacitors has commonly been perceived as a process much more reversible than that for batteries and hence being capable of operation at high power densities. While, in practice, this is largely true, charging of the high-area, porous-electrode structures that are required for achieving large capacitance densities (farads/g or farads/cm3) encounters limitations of rate due to the distributed electrolytic and contact resistances within the pore structure of such materials.
 
In the simplest analysis, any practical capacitor device behaves as if an ohmic resistance is in series with it, the so-called equivalent (or real) series resistance (Figure 3).

The presence of real or equivalent series resistance in the operating equivalent circuit of any capacitor introduces an ir potential drop in the process of charging or discharging and this drop depends, of course, on the charging rate (current) leading to distortion of the charging curve of accumulated charge against voltage, in time. When the distributed resistance effect also operates, as it normally does, the distortion effect becomes more complex but has been experimentally and computationally evaluated (de Levie, 1963).

The above effect causes limitation of rates at which the capacitor can be charged or discharged and, for ac modulated charging, it also introduces a frequency-dependent phase angle (normally -90o) between the modulated applied voltage and the resultant charging current. This also applies to other, non-constant charging modes.

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An important class of electrochemical capacitors utilizes the co-called double-layer capacitance that arises at all electrode interfaces with electrolyte solutions or ionic melts. The concept and model of the double layer arose in the work of von Helmholtz (1853) on the interfaces of colloidal suspensions and was subsequently extended to surfaces of metal electrodes by Gouy, Chapman, and Stern, and later in the notable work of Grahame around 1947. Models of the double layer are shown in Figure 2, with their capacitor-like structures.
Helmholtz envisaged a capacitor-like separation of anionic and cationic charges across the interface of colloidal particles with an electrolyte. For electrode interfaces with an electrolyte solution, this concept was extended to model the separation of "electronic" charges residing at the metal electrode surfaces (manifested as an excess of negative charge densities under negative polarization with respect to the electrolyte solution or as a deficiency of electron charge density under positive polarization), depending in each case, on the corresponding potential difference between the electrode and the solution boundary at the electrode. For zero net charge, the corresponding potential is referred to as the "potential of zero charge".

In response to positive or negative electric polarization of the electrode relative accumulations of cations or anions develop, respectively, at the solution side of the charged electrode. If, for energetic reasons, the ions of the electrolyte are not faradaically dischargeable (that is no electron transfer can occur across the interface ("ideally polarizable electrode", for example a mercury electrode, Grahame 1947 and Parsons 1954), then an electrostatic electrical equilibrium is established at the interface resulting in a "double layer" of separated charges (electrons or electron deficiency at the metal side and cations or anions at the solution side of the interface boundary), negative and positive, across the interface.

The difference of potential extends beyond the immediate layer of solvated ions in the compact, capacitor-like (Helmholtz) region, out into solution, so that a further diffuse region capacitance (the diffuse-layer capacitance "Cdiff") arises. It combines with that of Helmholtz's region "CH" in series. (See the Appendix for further details.)

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A different kind of capacitance can arise at electrodes of certain kinds, for example ruthenium dioxide, when the extent of faradaically admitted charge depends linearly, or approximately linearly, on the applied voltage. For such a situation, the electrode behavior is equivalent to, and measurable as, a capacitance. This capacitance can be large but it is faradaic and not electrostatic (that is non-faradaic) in origin. This is hence an important difference from the nature of double-layer capacitance, so it is called "pseudocapacitance". This kind of pseudocapacitance can originate when an electrochemical charge-transfer process takes place to an extent limited by a finite quantity of reagent or of available surface. Several examples of pseudocapacitance can arise, but the capacitance function is usually not constant and, in fact, is usually appreciably dependent on potential or state of charge.
However, when the process is surface limited, and is proceeding in several one-electron stages, a broad range of significant capacitance values arises as is found with ruthenium dioxide electrodes where the pseudocapacitance is almost constant (within 5%) over the full operating voltage range. Some other metal oxides behave similarly but only over smaller operating voltage ranges. The ruthenium dioxide pseudocapacitance provides one of the best examples of electrochemical (pseudo)capacitance as, in addition to the almost constant capacitance over a wide voltage range, its reversibility is excellent, with a cycle life over several hundred-thousand cycles. Furthermore, the pseudocapacitance can increase the capacitance of an electrochemical capacitor by as much as an order of magnitude over that of the double-layer capacitance. However, its cost prevents its large-scale use so that it has been employed mainly in military applications.

Another type of material exhibiting pseudocapacitive behavior that is highly reversible is the family of conducting polymers such as polyaniline or derivatives of polythiophene. These are cheaper than ruthenium dioxide but are less stable, giving only thousands of cycles (still quite attractive) over a wide voltage range. (See the Appendix for a more detailed discussion of the pseudocapacitance.)

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In order to describe "electrochemical capacitors" and to explain their function and applications, it is necessary first to consider the nature of an ordinary electrostatic capacitor or a "condenser" as it used to be called, and thence the meaning of the term electrical capacitance.

The nature of electricity took a long time to be understood, from the early experiments on electrostatic electricity in the mid-18th century, for example by Galvani, through the time of the invention of the first electric battery by Alessandro Volta (Volta's "Pile") in 1800, on to Faradays's and Davy's monumental discoveries on the chemical origin of electricity generated by Volta's pile. At first, two "kinds" of electricity were postulated: "animal electricity", as in the works of Galvani on stimulation of the frog's leg nerve by contact between two dissimilar metals and later, "Voltaic electricity" generated chemically from a Volta pile of zinc and silver or copper plates separated by paper wetted with an acid or salt solution (Conway, 2000).

In parallel with these discoveries were extensive works on electrostatic electricity generated for example by the rubbing of naturally occurring amber or by the so-called Wimshurst machine (a rotating circular plate, containing insets of amber-like material, rubbing against charge-collector plates connected overall to a Leyden Jar or a spark-gap). It was from this direction of research on electricity that the invention of the electric condenser arose, referred to as the "Leyden Jar", and capable of storing electric charge generated by a Wimshurst machine. Such a jar had the "capacity", depending on its dimensions and materials of construction, of storing electric charge by bringing it together in a condensed way (hence the term "condenser") on the surfaces of a Leyden Jar at a certain two-dimensional charge density.

The principle of design and operation of the Leyden Jar and all subsequent regular condensers or capacitor devices, is as follows. Two metal surfaces that constitute electrodes are separated at some small distance either in air (or vacuum) or on each side of a liquid or solid film, referred to as the "dielectric", a term first used by Michael Faraday . For a given separation of the electrode plates, the capacitance developed per unit area of the two plates depends on the properties of the dielectric between the plates characterized by its so-called dielectric constant.

In the case of the Leyden Jar (Figure 1), the material (glass) of the jar itself serves a the dielectric medium and the contact plates were metal foils wrapped, inside and out, around the cylindrical surfaces of the jar. The electrical contact to the inner surface foil was by means of a conducting electrolyte solution (or originally by ordinary water itself) in which was immersed a conducting metal electrode for electrical contact. The device was charged by joining two wires from the inside electrode and the outside foil to an electrostatic machine of the Wimshurst type. In later experimentation, the Leyden Jar capacitor was connected to the electrodes of a Volta's pile or battery for charging. This was the first-generation capacitor for storage of electric charge.

The nature of electric charge remained elusive until much later (1897) when J.J. Thomson identified and characterized the fundamental entity of electric charge as the "electron", present ubiquitously in all atoms of the Universe and identified, in his experiments, by means of experiments on gases at low pressures in gas-discharge tubes (Crookes tubes or neon lights). The electron charge was determined independently by Townsend and by Millikan (Glasstone, 1940), and was shown to be equivalent to Faraday's constant for the relation between extent of passage of charge and extent of chemical change (as related by Faraday's Laws) caused by electrolysis of conducting solutions, when calculated on a "per gram-atom" or "gram-equivalent" basis.

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Electrochemical capacitors provide a mode of electrical charge- and energy-storage and delivery, complementary to that by batteries. The first electrochemical capacitor device was disclosed in a General Electric Co. patent in 1957 to Becker but was of a crude nature, employing porous carbon. Later work by Sohio (1969) described a so-called "electrokinetic capacitor" utilizing porous carbon in a non-aqueous electrolyte which enabled it to be charged up to about 3 V, though the operation of the device was not "electrokinetic" in nature, a misnomer. In 1971, Trasatti and Buzzanca recognized that the electrochemical charging behavior of ruthenium dioxide films was like that of capacitors. Between 1975 and 1980, the present author and his co-workers, under contract with the then Continental Group Inc., carried out extensive fundamental and development work on the ruthenium oxide type of electrochemical capacitor (Conway, 1997) which behaves as a surface- redox pseudocapacitance (see below). The whole field has burgeoned since about 1990 and is very active in fundamental, and R&D directions.

A great deal of scientific and technological research has been reported in the scientific literature since about 1990. An extensive and detailed account of this has been given in the author's monograph on "Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications" (1999).

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The capacitance of a capacitor is proportional to the area of the contact plates and the dielectric constant of the medium between the plates, and it is inversely proportional to the separation between the plates (see the Appendix). In relation to electrochemical capacitors, to be discussed below, the capacitance of small dielectric capacitors is very small being on the order of microfarads or nanofarads (millionth or billionth of a farad, respectively) for small devices on the order of mm or cm in dimensions. By having very thin insulating films, on the order of 10 to 100 nanometers, formed anodically on the plate of a two-electrode capacitor, substantially larger specific capacitances (that is per cm2) can be attained. Such devices are called ?a href="art-c04-electr-cap.htm">electrolytic capacitors?because the thin dielectric oxide films are formed on the plates by an anodic electrolysis procedure applied at metals such as aluminum, tantalum, titanium, niobium, etc. Such capacitors are still of the dielectric type (the dielectric medium being here the thin, insulating oxide film, usually having a relatively high dielectric constant) and should not be confused with the "electrochemical" capacitor type of device which is the topic of this article.

Electrochemical capacitors are a special kind of capacitor based on charging and discharging the interfaces of high specific-area materials such as porous carbon materials or porous oxides of some metals. They can store electric charge and corresponding energy at high densities in an highly reversible way, as does a regular capacitor, and hence can be operated at specific power densities (in watts/kg) substantially higher than can most batteries. Their capacitance for a given size of the device is thus much higher, by a factor of 10,000 or so, than those achievable with regular capacitors. For this reason proprietary names such as "Supercapacitors" or "Ultracapacitors" have been coined to describe their performance.

While they function formally like rechargeable batteries in storing or delivering electric charge, their mechanisms of charge storage are quite different, in most cases, from those operating in batteries. Thus, electrochemical capacitors are not substitutes for batteries but rather are to be regarded as complementary to them for charge storage or delivery. They can offer advantageously fast charging or discharging rates over most batteries of comparable volume but their energy density is usually less, by a factor of 3 to 4, than that of batteries. Their high power or power densities, however, enables them to be employed in interesting complementary ways in hybrid systems with batteries.

An important difference between charging a capacitor and charging a battery is that there is always an intrinsic increase of voltage on charge (or decrease on discharge) of a capacitor as the charge per cm2 is increased or decreased. In contrast, an ideal battery has a constant voltage during discharge or recharge except as the state of charge approaches 0 or 100%. Although practical batteries exhibit some dependence of cell voltage on state of charge, especially lithium-intercalation batteries, the latter for fundamental reasons arising from intercalation. (See the Appendix for further details.)

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Electronics is the study of the flow of charge through various materials and devices such as semiconductors, resistors, inductors, capacitors, nano-structures and vacuum tubes. Although considered to be a theoretical branch of physics, the design and construction of electronic circuits to solve practical problems is an essential technique in the fields of electronic engineering and computer engineering.

The study of new semiconductor devices and surrounding technology is sometimes considered a branch of physics. This article focuses on engineering aspects of electronics.

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Electronic systems are used to perform a wide variety of tasks. The main uses of electronic circuits are:

1. The controlling and processing of data.
2. The conversion to/from and distribution of electric power.

Both these applications involve the creation and/or detection of electromagnetic fields and electric currents. While electrical energy had been used for some time prior to the late 19th century to transmit data over telegraph and telephone lines, development in electronics grew exponentially after the advent of radio.

One way of looking at an electronic system is to divide it into 3 parts:

• Inputs – Electronic or mechanical sensors (or transducers). These devices take signals/information from external sources in the physical world (such as antennas or technology networks) and convert those signals/information into current/voltage or digital (high/low) signals within the system.

• Signal processors – These circuits serve to manipulate, interpret and transform inputted signals in order to make them useful for a desired application. Recently, complex signal processing has been accomplished with the use of Digital Signal Processors.

• Outputs – Actuators or other devices (such as transducers) that transform current/voltage signals back into useful physical form (e.g., by accomplishing a physical task such as rotating an electric motor).

For example, a television set contains these 3 parts. The television's input transforms a broadcast signal (received by an antenna or fed in through a cable) into a current/voltage signal that can be used by the device. Signal processing circuits inside the television extract information from this signal that dictates brightness, colour and sound level. Output devices then convert this information back into physical form. A cathode ray tube transforms electronic signals into a visible image on the screen. Magnet-driven speakers convert signals into audible sound.

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An electronic component is any physical entity in an electronic system whose intention is to affect the electrons or their associated fields in a desired manner consistent with the intended function of the electronic system. Components are generally intended to be in mutual electromechanical contact, usually by being soldered to a printed circuit board (PCB), to create an electronic circuit with a particular function (for example an amplifier, radio receiver, or oscillator). Components may be packaged singly or in more or less complex groups as integrated circuits.

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Most analog electronic appliances, such as radio receivers, are constructed from combinations of a few types of basic circuits. Analog circuits use a continuous range of voltage as opposed to discrete levels as in digital circuits. The number of different analog circuits so far devised is huge, especially because a 'circuit' can be defined as anything from a single component, to systems containing thousands of components.

Analog circuits are sometimes called linear circuits although many non-linear effects are used in analog circuits such as mixers, modulators, etc. Good examples of analog circuits include vacuum tube and transistor amplifiers, operational amplifiers and oscillators.

Some analog circuitry these days may use digital or even microprocessor techniques to improve upon the basic performance of the circuit. This type of circuit is usually called "mixed signal."

Sometimes it may be difficult to differentiate between analog and digital circuits as they have elements of both linear and non-linear operation. An example is the comparator which takes in a continuous range of voltage but puts out only one of two levels as in a digital circuit. Similarly, an overdriven transistor amplifier can take on the characteristics of a controlled switch having essentially two levels of output.

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Digital circuits are electric circuits based on a number of discrete voltage levels. Digital circuits are the most common physical representation of Boolean algebra and are the basis of all digital computers. To most engineers, the terms "digital circuit", "digital system" and "logic" are interchangeable in the context of digital circuits. In most cases the number of different states of a node is two, represented by two voltage levels labeled "Low"(0) and "High"(1). Often "Low" will be near zero volts and "High" will be at a higher level depending on the supply voltage in use.

Computers, electronic clocks, and programmable logic controllers (used to control industrial processes) are constructed of digital circuits. Digital Signal Processors are another example.

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Mixed-signal circuits refers to integrated circuits (ICs) which have both analog circuits and digital circuits combined on a single semiconductor die or on the same circuit board. Mixed-signal circuits are becoming increasingly common. Mixed circuits are usually used to control an analog device using digital logic, for example the speed of a motor. Analog to digital converters and digital to analog converters are the primary examples. Other examples are transmission gates and buffers.

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Heat generated by electronic circuitry must be dissipated to prevent immediate failure and improve long term reliability. Techniques for heat dissipation can include heatsinks and fans for air cooling, and other forms of computer cooling such as water cooling. These techniques use convection, conduction, & radiation of heat energy.

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Noise is associated with all electronic circuits. Noise is defined^[1] as unwanted disturbances superposed on a useful signal that tend to obscure its information content. Noise is not the same as signal distortion caused by a circuit.

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