Eftekasat: June 2009

Friday, June 19, 2009

moving display

front sideCopyright of this circuit belongs to smart kit electronics. In this page we will use this circuit to discuss for improvements and we will introduce some changes based on original schematic.
General DescriptionThis is an easy to build, but nevertheless very accurate and useful digital voltmeter. It has been designed as a panel meter and can be used in DC power supplies or anywhere else it is necessary to have an accurate indication of the voltage present. The circuit employs the ADC (Analogue to Digital Converter) I.C. CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal to digital and can drive a series of four seven segment LED displays directly. The circuits built into the IC are an analogue to digital converter, a comparator, a clock, a decoder and a seven segment LED display driver. The circuit as it is described here can display any DC voltage in the range of 0-1999 Volts.
Technical Specifications - CharacteristicsSupply Voltage: ............. +/- 5 V (Symmetrical)Power requirements: ..... 200 mA (maximum)Measuring range: .......... +/- 0-1,999 VDC in four rangesAccuracy: ....................... 0.1 %FEATURES- Small size- Easy construction- Low cost.- Simple adjustment.- Easy to read from a distance.- Few external components.
How it WorksIn order to understand the principle of operation of the circuit it is necessary to explain how the ADC IC works. This IC has the following very important features:- Great accuracy.- It is not affected by noise.- No need for a sample and hold circuit.- It has a built-in clock.- It has no need for high accuracy external components.An Analogue to Digital Converter, (ADC from now on) is better known as a dual slope converter or integrating converter. This type of converter is generally preferred over other types as it offers accuracy, simplicity in design and a relative indifference to noise which makes it very reliable. The operation of the circuit is better understood if it is described in two stages. During the first stage and for a given period the input voltage is integrated, and in the output of the integrator at the end of this period, there is a voltage which is directly proportional to the input voltage. At the end of the preset period the integrator is fed with an internal reference voltage and the output of the circuit is gradually reduced until it reaches the level of the zero reference voltage. This second phase is known as the negative slope period and its duration depends on the output of the integrator in the first period. As the duration of the first operation is fixed and the length of the second is variable it is possible to compare the two and this way the input voltage is in fact compared to the internal reference voltage and the result is coded and is send to the display.All this sounds quite easy but it is in fact a series of very complex operations which are all made by the ADC IC with the help of a few external components which are used to configure the circuit for the job. In detail the circuit works as follows. The voltage to be measured is applied across points 1 and 2 of the circuit and through the circuit R3, R4 and C4 is finally applied to pins 30 and 31 of the IC. These are the input of the IC as you can see from its diagram. (IN HIGH & IN LOW respectively). The resistor R1 together with C1 are used to set the frequency of the internal oscillator (clock) which is set at about 48 Hz. At this clock rate there are about three different readings per second. The capacitor C2 which is connected between pins 33 and 34 of the IC has been selected to compensate for the error caused by the internal reference voltage and also keeps the display steady. The capacitor C3 and the resistor R5 are together the circuit that does the integration of the input voltage and at the same time prevent any division of the input voltage making the circuit faster and more reliable as the possibility of error is greatly reduced. The capacitor C5 forces the instrument to display zero when there is no voltage at its input. The resistor R2 together with P1 are used to adjust the instrument during set-up so that it displays zero when the input is zero. The resistor R6 controls the current that is allowed to flow through the displays so that there is sufficient brightness with out damaging them. The IC as we have already mentioned above is capable to drive four common anode LED displays. The three rightmost displays are connected so that they can display all the numbers from 0 to 9 while the first from the left can only display the number 1 and when the voltage is negative the «-« sign. The whole circuit operates from a symmetrical ρ 5 VDC supply which is applied at pins 1 (+5 V), 21 (0 V) and 26 (-5 V) of the IC.
ConstructionFirst of all let us consider a few basics in building electronic circuits on a printed circuit board. The board is made of a thin insulating material clad with a thin layer of conductive copper that is shaped in such a way as to form the necessary conductors between the various components of the circuit. The use of a properly designed printed circuit board is very desirable as it speeds construction up considerably and reduces the possibility of making errors. To protect the board during storage from oxidation and assure it gets to you in perfect condition the copper is tinned during manufacturing and covered with a special varnish that protects it from getting oxidised and also makes soldering easier.Soldering the components to the board is the only way to build your circuit and from the way you do it depends greatly your success or failure. This work is not very difficult and if you stick to a few rules you should have no problems. The soldering iron that you use must be light and its power should not exceed the 25 Watts. The tip should be fine and must be kept clean at all times. For this purpose come very handy specially made sponges that are kept wet and from time to time you can wipe the hot tip on them to remove all the residues that tend to accumulate on it.DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace it. There are many different types of solder in the market and you should choose a good quality one that contains the necessary flux in its core, to assure a perfect joint every time.DO NOT use soldering flux apart from that which is already included in your solder. Too much flux can cause many problems and is one of the main causes of circuit malfunction. If nevertheless you have to use extra flux, as it is the case when you have to tin copper wires, clean it very thoroughly after you finish your work.In order to solder a component correctly you should do the following:- Clean the component leads with a small piece of emery paper.- Bend them at the correct distance from the component’s body and insert the component in its place on the board.- You may find sometimes a component with heavier gauge leads than usual, that are too thick to enter in the holes of the p.c. board. In this case use a mini drill to enlarge the holes slightly. Do not make the holes too large as this is going to make soldering difficult afterwards.- Take the hot iron and place its tip on the component lead while holding the end of the solder wire at the point where the lead emerges from the board. The iron tip must touch the lead slightly above the p.c. board.- When the solder starts to melt and flow wait till it covers evenly the area around the hole and the flux boils and gets out from underneath the solder. The whole operation should not take more than 5 seconds. Remove the iron and allow the solder to cool naturally without blowing on it or moving the component. If everything was done properly the surface of the joint must have a bright metallic finish and its edges should be smoothly ended on the component lead and the board track. If the solder looks dull, cracked, or has the shape of a blob then you have made a dry joint and you should remove the solder (with a pump, or a solder wick) and redo it.- Take care not to overheat the tracks as it is very easy to lift them from the board and break them.- When you are soldering a sensitive component it is good practice to hold the lead from the component side of the board with a pair of long-nose pliers to divert any heat that could possibly damage the component.- Make sure that you do not use more solder than it is necessary as you are running the risk of short-circuiting adjacent tracks on the board, especially if they are very close together.- When you finish your work, cut off the excess of the component leads and clean the board thoroughly with a suitable solvent to remove all flux residues that may still remain on it.

As it is recommended start working by identifying the components and separating them in groups. There are two points in the construction of this project that you should observe:First of all the display IC’s are placed from the copper side of the board and second the jumper connection which is marked by a dashed line on the component side at the same place where the displays are located is not a single jumper but it should be changed according to the use of the instrument. This jumper is used to control the decimal point of the display.If you are going to use the instrument for only one range you can make the jumper connection between the rightmost hole on the board and the one corresponding to the desired position for the decimal point for your particular application. If you are planning to use the voltmeter in different ranges you should use a single pole three position switch to shift the decimal point to the correct place for the range of measurement selected. (This switch could preferably be combined with the switch that is used to actually change the sensitivity of the instrument).Apart from this consideration, and the fact that the small size of the board and the great number of joints on it which calls for a very fine tipped soldering iron, the construction of the project is very straightforward.Insert the IC socket and solder it in place, solder the pins, continue with the resistors the capacitors and the multi-turn trimmer P1. Turn the board over and very carefully solder the display IC’s from the copper side of the board. Remember to inspect the joints of the base of the IC as one row will be covered by the displays and will be impossible to see any mistake that you may have made after you have soldered the displays into place.The value of R3 controls in fact the range of measurement of the voltmeter and if you provide for some means to switch different resistors in its place you can use the instrument over a range of voltages.For the replacement resistors follow the table below:0 - 2 V ............ R3 = 0 ohm 1%0 - 20 V ........... R3 = 1.2 Kohm 1%0 - 200 V .......... R3 = 12 Kohm 1%0 - 2000 V ......... R3 = 120 Kohm 1%When you have finished all the soldering on the board and you are sure that everything is OK you can insert the IC in its place. The IC is CMOS and is very sensitive to static electricity. It comes wrapped in aluminium foil to protect it from static discharges and it should be handled with great care to avoid damaging it. Try to avoid touching its pins with your hands and keep the circuit and your body at ground potential when you insert it in its place.Connect the circuit to a suitable power supply ρ 5 VDC and turn the supply on. The displays should light immediately and should form a number. Short circuit the input (0 V) and adjust the trimmer P1 until the display indicates exactly «0».
Parts R1 180k R2 22k R3 12k R4 1M R5 470k R6 560 Ohm C1 100pF C2, C6, C7 100nF C3 47nF C4 10nF C5 220nF P1 20k trimmer multi turnU1 ICL 7107 LD1,2,3,4 MAN 6960 common anode led displays
If it does not workCheck your work for possible dry joints, bridges across adjacent tracks or soldering flux residues that usually cause problems.Check again all the external connections to and from the circuit to see if there is a mistake there.- See that there are no components missing or inserted in the wrong places.- Make sure that all the polarised components have been soldered the right way round. - Make sure the supply has the correct voltage and is connected the right way round to your circuit.- Check your project for faulty or damaged components.

pic microcontroller

(Redirected from PIC micro)
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PIC microcontrollers in DIP and QFN packages

16-bit 28-pin PDIP PIC24 microcontroller with a metric ruler
PIC is a family of Harvard architecture microcontrollers made by Microchip Technology, derived from the PIC1640[1] originally developed by General Instrument's Microelectronics Division. The name PIC initially referred to "Peripheral Interface Controller".[2][3]
PICs are popular with developers and hobbyists alike due to their low cost, wide availability, large user base, extensive collection of application notes, availability of low cost or free development tools, and serial programming (and re-programming with flash memory) capability.
Microchip announced on February 2008 the shipment of its six billionth PIC processor.[

Wednesday, June 17, 2009

types of amplifiers

Amplifier Types

Amplifier Summary
There are two types of commonly used amplifiers for instrumentation and test equipment: single-ended and differential. Both of these amplifiers have benefits and drawbacks in the way they are used in measurement systems.

Single-ended

Good for measurements between any point and chassis ground
Susceptible to noisy environments
Same signal common reference for multiple channels
Cannot be used for "above ground" measurements

Differential

Amplifier the difference between two points
Less susceptible to noisy environments (CMR)
Can be used for "above ground" measurements up to the CMV
Some signal common reference for multiple channels
Possible crosstalk with wide voltage differences between channels

Isolated

Amplifies the difference between any point and iso-common
Commons are isolated from chassis ground, earth ground and other commons
Less susceptible to noisy environments (IMR/CMR)
Can be used for "above ground" measurements up to the IMV/CMV
No crosstalk, even with wide voltage differences between channels

Single-Ended
A single-ended amplifier has only one input, and all voltages are measured in reference to signal common. In fact, single-ended is a misnomer, since the input voltage is measured relative to signal ground. With this amplifier, Vout is equal to Vin multiplied by the gain of the amplifier. This type of input is frequently used in devices such as oscilloscopes. A feature of single-ended amplifiers is that only one measurement point is needed. The following is a diagram for a single-ended amplifier:

One of the drawbacks of this amplifier type is the fact that in a multi-channel system, signal common (defined as the common point supplying power for the analog circuitry) can be common to all channels. Another disadvantage is that it is susceptible to noise (internal or external interference in the form of unpredictable voltages) on the input.

Differential
A differential amplifier has two inputs, and amplifies the difference between them. The voltage at both inputs is measured with respect to signal common.The following is a diagram for a differential amplifier:

Calculating the gain for a differential is more complex than a single-ended one. There are two gains associated with a differential amplifier, differential gain (Gd) and common gain (Gc). The output of a differential amplifier is described by the following:

Vout=[(Va-Vb) x Gd] + [Vavg x Gc] where Vavg=(Va + Vb)/2

In an ideal differential amplifier Gc would be zero, and the output of the amplifier would simply be the amplified difference between Va and Vb. Unfortunately, ideal differential amplifiers do not exist in practice, therefore Gc should be as small as possible. The ratio of the differential gain to the common gain becomes important since the goal is to make the second term in the above gain equation negligible. This is referred to as the Common Mode Rejection Ratio (CMRR) and leads to the Common Mode Rejection (CMR) specification that is usually used. The CMR specification is defined as follows:

CMR=20log(CMRR)=20log(Gd/Gc)

The goal when designing such an amplifier is to make the CMR as high as possible. A higher CMR indicates a differential amplifier that is less susceptible to voltages common to both inputs (noise). Another benefit of a high CMR is the ability to accurately measure a small voltage difference between two points that are both at a higher voltage potential. Since CMR decreases as the frequency of a signal increases, it is usually specified at a particular frequency (i.e., 60 Hz).

Another important specifications associated with differential amplifiers is Common Mode Voltage (CMR). This is defined as the maximum voltage allowed at each input with respect to signal common. It is important to realize that the CMV of an amplifier can be much higher than the measurement range of that amplifier. Generally, a higher CMV allows an amplifier to be used in a wider range of applications.

Differential amplifiers are quite common, since they do not have the advantage of single-ended amplifiers. They are useful for "above ground" measurements, as long as the CMV of the amplifier is not exceeded. They are also useful in environments where there is potential noise. One of the drawbacks of the standard differential amplifier is that in a multi-channel system, signal ground is often the same for all channels.

Isolated Inputs
An isolated input has a signal common (iso-common) that is isolated from the power supply for the analog circuitry. An additional feature is that in a multi-channel system, each channel's iso-common is independent and isolated from other channels. Isolated inputs can be either single-ended or differential, although a single-ended isolated input will have similar specifications to a differential one. An isolated differential input is useful with DC bridges, where an excitation voltage must be supplied. With isolated amplifiers, the terms Isolation Mode Voltage (IMV) and Isolation Mode Rejection (IMR) are used interchangeably with CMV and CMR. Isolated single-ended amplifiers, for example, have the same noise reducing characteristics as a non-isolated differential amplifier. The following is a diagram for an isolated single-ended amplifier:

As the diagram illustrates, the isolated common on the input of the amplifier is not referenced to the output common of the analog circuitry. In fact, the iso-common can actually be used for "above ground" measurements, up to the limitation of the CMV. In this respect, an isolated single-ended amplifier is very much like a standard differential one.

Many Astro-Med instruments make use of both transformer and optically-coupled isolated inputs, which offer such benefits as channel-to-channel isolation, noise reduction, and higher CMV in more sensitive ranges. With isolated amplifiers, the amplification stages are isolated from other circuitry. There is also no resistive path from the iso-common of any channel to chassis ground, no to any other iso-common.

amplifier

For the British rock band of the same name, see Amplifier (band).

Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal. The "signal" is usually voltage or current. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

In popular use, the term usually refers to an electronic amplifier, often as in audio applications to operate a loudspeaker that is being used in a PA system to make the human voice louder or play recorded music. Amplifiers may be classified by the input (source) they are designed to amplify (such as a guitar amplifier to perform with an electric guitar), or named for the device they are intended to drive (such as a headphone amplifier), or by the frequency range of the signals (Audio, IF, RF and VHF amplifiers for example), or grouped by whether they invert the signal (inverting amplifiers and non-inverting amplifiers, or by the types of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).

A related device that emphasizes conversion of signals of one type to another (for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.

The quality of an amplifier can be characterized by a number of specifications, listed below.

[edit] Gain

The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(P_out /(P_in)). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance).

Example: an audio amplifier with a gain given as 20dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical).

[edit] Bandwidth

The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The "satisfactory performance" may be different for different applications. However, a common and well-accepted metric are the half power points (i.e. frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths (otherwise called "frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or "plus of minus 1dB" (roughly the sound level difference people usually can detect).

A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points <> 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.

[edit] Efficiency

Efficiency is a measure of how much of the input power is usefully applied to the amplifier's output. Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy so in turn less heat.

[edit] Linearity

An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.

Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain.

Linearization is an emergent field, and there are many techniques, such as feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the non-linearities.

[edit] Noise

This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. The metric for noise performance of a circuit is Noise Factor. Noise Factor is the ratio of input signal to that of the output signal.

[edit] Output dynamic range

Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.^[1]

[edit] Slew rate

Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.

[edit] Rise time

The rise time, t_r, of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is approximated by:

t_r * BW = 0.35, where t_r is rise time in seconds and BW is bandwidth in Hz.

[edit] Settling time and ringing

Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is called the settle time, and is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output that cycles above and below its final value, leading to a delay in reaching final value quantified by the settling time above.

[edit] Overshoot

In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value.

[edit] Stability factor

Stability is a major concern in RF and microwave amplifiers. The degree of an amplifiers stability can be quantified by a so-called stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its two-port parameters.

[edit] Electronic amplifiers

Main article: Electronic amplifier

There are many types of electronic amplifiers, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Critical components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows.

[edit] Power amplifier

The term "power amplifier" is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier: see power amplifier classes.

[edit] Vacuum tube (valve) amplifiers

Main article: valve amplifier

The glow from four "Electro Harmonix KT88" brand power tubes lights up the inside of a Traynor YBA-200 guitar amplifier

According to Symons, while semiconductor amplifiers have largely displaced valve amplifiers for low power applications, valve amplifiers are much more cost effective in high power applications such as "radar, countermeasures equipment, or communications equipment" (p. 56). Many microwave amplifiers are specially designed valves, such as the klystron, gyrotron, traveling wave tube, and crossed-field amplifier, and these microwave valves provide much greater single-device power output at microwave frequencies than solid-state devices (p. 59).^[2]

[edit] Transistor amplifiers

Main articles: transistor, bipolar junction transistor, Audio amplifier, and MOSFET

The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification (the "forward gain") is determined by the external circuit design as well as the active device.

Many common active devices in transistor amplifiers are bipolar junction transistors (BJTs) and metal oxide semiconductor field-effect transistors (MOSFETs).

Applications are numerous, some common examples are audio amplifiers in a home stereo or PA system, RF high power generation for semiconductor equipment, to RF and Microwave applications such as radio transmitters.

Transistor-based amplifier can be realized using various configurations: for example with a bipolar junction transistor we can realize common base, common collector or common emitter amplifier; using a MOSFET we can realize common gate, common source or common drain amplifier. Each configuration has different characteristic (gain, impedance...).

[edit] Operational amplifiers (op-amps)

Main articles: operational amplifier and instrumentation amplifier

An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain. Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves.

[edit] Fully differential amplifiers (FDA)

Main article: fully differential amplifier

A fully differential amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins.

[edit] Video amplifiers

These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p etc.. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented.

[edit] Oscilloscope vertical amplifiers

These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.

[edit] Distributed amplifiers

Main article: Distributed Amplifier

These use transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.

[edit] Microwave amplifiers

[edit] Travelling wave tube (TWT) amplifiers

Main article: Traveling wave tube

Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.

[edit] Klystrons

Main article: Klystron

Very similar to TWT amplifiers, but more powerful and with a specific frequency "sweet spot". They generally are also much heavier than TWT amplifiers, and are therefore ill-suited for light-weight mobile applications. Klystrons are tunable, offering selective output within their specified frequency range.

[edit] Musical instrument (audio) amplifiers

Main articles: instrument amplifier and audio amplifier

An audio amplifier is usually used to amplify signals such as music or speech.

[edit] Other amplifier types

[edit] Carbon microphone

One of the first devices used to amplify signals was the carbon microphone (effectively a sound-controlled variable resistor). By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications; analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones were also used as amplifiers in telephone repeaters for long distance service.

[edit] Magnetic amplifier

Main article: magnetic amplifier

A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the hundreds of kilohertz.

[edit] Rotating electrical machinery amplifier

A Ward Leonard control is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy. Changes in generator field current result in larger changes in the output current of the generator, providing gain. This class of device was used for smooth control of large motors, primarily for elevators and naval guns.

Field modulation of a very high speed AC generator was also used for some early AM radio transmissions.^[3] See Alexanderson alternator.

[edit] Johnsen-Rahbek effect amplifier

The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen-Rahbek effect was not understood until the semiconductor era.

[edit] Mechanical amplifiers

Mechanical amplifiers were used in the pre-electronic era in specialized applications. Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.

[edit] Optical amplifiers

Main article: Optical amplifier

Optical amplifiers amplify light through the process of stimulated emission. See Laser and Maser.

[edit] Miscellaneous types

There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

kurshoff laws

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For other laws named after Gustav Kirchhoff, see Kirchhoff's laws. Not to be confused with Kerckhoffs' principle.

Kirchhoff's circuit laws are two equalities that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff. Widely used in electrical engineering, they are also called Kirchhoff's rules or simply Kirchhoff's laws (see also Kirchhoff's laws for other meanings of that term).

Both circuit rules can be directly derived from Maxwell's equations, but Kirchhoff preceded Maxwell and instead generalized work by Georg Ohm.

The current entering any junction is equal to the current leaving that junction. i₁ + i₄ = i₂ + i₃

This law is also called Kirchhoff's point rule, Kirchhoff's junction rule (or nodal rule), and Kirchhoff's first rule.

The principle of conservation of electric charge implies that:

At any point in an electrical circuit that does not represent a capacitor plate, the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point.

Adopting the convention that every current flowing towards the point is positive and that every current flowing away is negative (or the other way around), this principle can be stated as:

$\sum_{k=1}^n {I}_k = 0$

n is the total number of currents flowing towards or away from the point.

This formula is also valid for complex currents:

$\sum_{k=1}^n \tilde{I}_k = 0$

This law is based on the conservation of charge whereby the charge (measured in coulombs) is the product of the current (in amps) and the time (which is measured in seconds).

[edit] Changing charge density

Physically speaking, the restriction regarding the "capacitor plate" means that Kirchhoff's current law is only valid if the charge density remains constant in the point that it is applied to. This is normally not a problem because of the strength of electrostatic forces: the charge buildup would cause repulsive forces to disperse the charges.

However, a charge build-up can occur in a capacitor, where the charge is typically spread over wide parallel plates, with a physical break in the circuit that prevents the positive and negative charge accumulations over the two plates from coming together and cancelling. In this case, the sum of the currents flowing into one plate of the capacitor is not zero, but rather is equal to the rate of charge accumulation. However, if the displacement current dD/dt is included, Kirchhoff's current law once again holds. (This is really only required if one wants to apply the current law to a point on a capacitor plate. In circuit analyses, however, the capacitor as a whole is typically treated as a unit, in which case the ordinary current law holds since exactly the current that enters the capacitor on the one side leaves it on the other side.)

More technically, Kirchhoff's current law can be found by taking the divergence of Ampère's law with Maxwell's correction and combining with Gauss's law, yielding:

$\nabla \cdot \mathbf{J} = -\nabla \cdot \frac{\partial \mathbf{D}}{\partial t} = -\frac{\partial \rho}{\partial t}$

This is simply the charge conservation equation (in integral form, it says that the current flowing out of a closed surface is equal to the rate of loss of charge within the enclosed volume (Divergence theorem)). Kirchhoff's current law is equivalent to the statement that the divergence of the current is zero, true for time-invariant ρ, or always true if the displacement current is included with J.

[edit] Uses

A matrix version of Kirchhoff's current law is the basis of most circuit simulation software, such as SPICE.

[edit] Kirchhoff's voltage law (KVL)

The sum of all the voltages around the loop is equal to zero. v₁ + v₂ + v₃ + v₄ = 0

This law is also called Kirchhoff's second law, Kirchhoff's loop (or mesh) rule, and Kirchhoff's second rule.

The directed sum of the electrical potential differences around any closed circuit must be zero.

Similarly to KCL, it can be stated as:

$\sum_{k=1}^n V_k = 0$

Here, n is the total number of voltages measured. The voltages may also be complex:

$\sum_{k=1}^n \tilde{V}_k = 0$

This law is based on the conservation of energy whereby voltage is defined as the energy per unit charge. The total amount of energy gained per unit charge must equal the amount of energy lost per unit charge. This seems to be true as the conservation of energy states that energy cannot be created or destroyed; it can only be transformed into one form to another.

[edit] Electric field and electric potential

Kirchhoff's voltage law as stated above is equivalent to the statement that a single-valued electric potential can be assigned to each point in the circuit (in the same way that any conservative vector field can be represented as the gradient of a scalar potential).

This could be viewed as a consequence of the principle of conservation of energy. Otherwise, it would be possible to build a perpetual motion machine that passed a current in a circle around the circuit.

Considering that electric potential is defined as a line integral over an electric field, Kirchhoff's voltage law can be expressed equivalently as

$\oint_C \mathbf{E} \cdot d\mathbf{l} = 0,$

which states that the line integral of the electric field around closed loop C is zero.

In order to return to the more special form, this integral can be "cut in pieces" in order to get the voltage at specific components.

This is a simplification of Faraday's law of induction for the special case where there is no fluctuating magnetic field linking the closed loop. Therefore, it practically suffices for explaining circuits containing only resistors and capacitors.

In the presence of a changing magnetic field the electric field is not conservative and it cannot therefore define a pure scalar potential—the line integral of the electric field around the circuit is not zero. This is because energy is being transferred from the magnetic field to the current (or vice versa). In order to "fix" Kirchhoff's voltage law for circuits containing inductors, an effective potential drop, or electromotive force (emf), is associated with each inductance of the circuit, exactly equal to the amount by which the line integral of the electric field is not zero by Faraday's law of induction.

resistor

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Electronic symbol
Resistor
Three resistors
Type	Passive
(Europe) (US)
This box: view • talk

Partially exposed Tesla TR-212 1kΩ carbon film resistor


		Potentiometer

Resistor		Variable resistor

Resistor symbols (Europe, the IEC)


		Potentiometer

Resistor		Variable Resistor

Resistor symbols (American)

Axial-lead resistors on tape. The tape is removed during assembly before the leads are formed and the part is inserted into the board.

Three carbon composition resistors in a 1960s valve (vacuum tube) radio.

A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current through it in accordance with Ohm's law:

V = I R

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).

The primary characteristics of a resistor are the resistance, the tolerance and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance depends upon the materials constituting the resistor as well as its physical dimensions; it's determined by design.

Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power.