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87-546: The signal to be gain controlled (the detector output in a radio) goes to a diode & capacitor , which produce a peak-following DC voltage. This is fed to the RF gain blocks to alter their bias, thus altering their gain. Traditionally all the gain-controlled stages came before the signal detection, but it is also possible to improve gain control by adding a gain-controlled stage after signal detection. In 1925, Harold Alden Wheeler invented automatic volume control (AVC) and obtained

174-420: A cathode and a plate . The cathode is either indirectly heated or directly heated . If indirect heating is employed, a heater is included in the envelope. In operation, the cathode is heated to red heat , around 800–1,000 °C (1,470–1,830 °F). A directly heated cathode is made of tungsten wire and is heated by a current passed through it from an external voltage source. An indirectly heated cathode

261-505: A circuit is given by its current–voltage characteristic . The shape of the curve is determined by the transport of charge carriers through the so-called depletion layer or depletion region that exists at the p–n junction between differing semiconductors. When a p–n junction is first created, conduction-band (mobile) electrons from the N- doped region diffuse into the P- doped region where there

348-422: A current of electrons flows through the tube from cathode to plate. When the plate voltage is negative with respect to the cathode, no electrons are emitted by the plate, so no current can pass from the plate to the cathode. Point-contact diodes were developed starting in the 1930s, out of the early crystal detector technology, and are now generally used in the 3 to 30 gigahertz range. Point-contact diodes use

435-399: A decay time leads to the phenomenon of " breathing " where the background noise level gets boosted at each gap in the speech. Vogad circuits are normally adjusted so that at low levels of input the signal is not fully boosted, but instead follow a linear boost curve. This works well with noise cancelling microphones. Devices to record both sides of a telephone conversation must record both

522-404: A definite " knee " around this threshold when viewed on a linear scale, the knee is an illusion that depends on the scale of y-axis representing current. In a semi-log plot (using a logarithmic scale for current and a linear scale for voltage), the diode's exponential curve instead appears more like a straight line. Since a diode's forward-voltage drop varies only a little with the current, and

609-460: A diode is a light-emitting diode , which is used as electric lighting and status indicators on electronic devices. The most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking it in the opposite direction (the reverse direction). Its hydraulic analogy is a check valve . This unidirectional behavior can convert alternating current (AC) to direct current (DC),

696-461: A feeling, or enhance visual impact. Such distortions or "abstractions" primarily refer to purposeful deviations from photorealistic perspective or from realistic proportionality. Examples include " The Weeping Woman " by Picasso and " The Adoration of the Shepherds " by El Greco, whose human subject matters are irregularly and (as is often with physical distortions) asymmetrically proportioned in

783-444: A filter whose gain and/or delay varies with frequency, the signal suffers linear distortion. Linear distortion does not introduce new frequency components to a signal but does alter the balance of existing ones. This diagram shows the behaviour of a signal (made up of a square wave followed by a sine wave ) as it is passed through various distorting functions. The transfer function of an ideal amplifier, with perfect gain and delay,

870-492: A form of last-ditch protection against overmodulation , a properly designed vogad circuit actively controls the amount of gain to optimise the modulation depth in real time. As well as preventing overmodulation, it boosts the level of quiet signals so that undermodulation is also avoided. Undermodulation can lead to poor signal penetration in noisy conditions, consequently vogad is particularly important for voice applications such as radiotelephones . A good vogad circuit must have

957-455: A great effect on the usability of the receiver, tuning characteristics, audio fidelity, and behavior on overload and strong signals. FM receivers, even though they incorporate limiter stages and detectors that are relatively insensitive to amplitude variations, still benefit from AGC to prevent overload on strong signals. A related application of AGC is in radar systems, as a method of overcoming unwanted clutter echoes. This method relies on

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1044-411: A low work function , meaning that they more readily emit electrons than would the uncoated cathode. The plate, not being heated, does not emit electrons; but is able to absorb them. The alternating voltage to be rectified is applied between the cathode and the plate. When the plate voltage is positive with respect to the cathode, the plate electrostatically attracts the electrons from the cathode, so

1131-632: A metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon , but other semiconducting materials such as gallium arsenide and germanium are also used. The obsolete thermionic diode is a vacuum tube with two electrodes , a heated cathode and a plate , in which electrons can flow in only one direction, from the cathode to the plate. Among many uses, diodes are found in rectifiers to convert alternating current (AC) power to direct current (DC), demodulation in radio receivers , and can even be used for logic or as temperature sensors . A common variant of

1218-415: A particular type of diode in a circuit diagram conveys the general electrical function to the reader. There are alternative symbols for some types of diodes, though the differences are minor. The triangle in the symbols points to the forward direction, i.e. in the direction of conventional current flow. There are a number of common, standard and manufacturer-driven numbering and coding schemes for diodes;

1305-463: A patent. Karl Küpfmüller published an analysis of AGC systems in 1928. By the early 1930s most new commercial broadcast receivers included automatic volume control. AGC is a departure from linearity in AM radio receivers . Without AGC, an AM radio would have a linear relationship between the signal amplitude and the sound waveform – the sound amplitude , which correlates with loudness, is proportional to

1392-499: A positively charged electroscope, but not a negatively charged electroscope. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb, later called Edison effect , and was granted a patent on application of the phenomenon for use in a DC voltmeter . About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) realized that

1479-452: A process called rectification . As rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers . A diode's behavior is often simplified as having a forward threshold voltage or turn-on voltage or cut-in voltage , above which there is significant current and below which there is almost no current, which depends on a diode's composition: This voltage may loosely be referred to simply as

1566-445: A radio carrier wave, whose amplitude or envelope is proportional to the original audio signal. The diode rectifies the AM radio frequency signal, leaving only the positive peaks of the carrier wave. The audio is then extracted from the rectified carrier wave using a simple filter and fed into an audio amplifier or transducer , which generates sound waves via audio speaker . In microwave and millimeter wave technology, beginning in

1653-438: A region on one side that contains negative charge carriers (electrons), called an n-type semiconductor , and a region on the other side that contains positive charge carriers ( holes ), called a p-type semiconductor . When the n-type and p-type materials are attached together, a momentary flow of electrons occurs from the n to the p side resulting in a third region between the two where no charge carriers are present. This region

1740-472: A small diameter metal wire in contact with a semiconductor crystal, and are of either non-welded contact type or welded contact type. Non-welded contact construction utilizes the Schottky barrier principle. The metal side is the pointed end of a small diameter wire that is in contact with the semiconductor crystal. In the welded contact type, a small P region is formed in the otherwise N-type crystal around

1827-401: A variety of other minerals as detectors. Semiconductor principles were unknown to the developers of these early rectifiers. During the 1930s understanding of physics advanced and in the mid-1930s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology. Researchers at Bell Labs , Western Electric , MIT , Purdue and in

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1914-414: A very fast attack time , so that an initial loud voice signal does not cause a sudden burst of excessive modulation. In practice the attack time will be a few milliseconds, so a clipping limiter is still sometimes needed to catch the signal on these short peaks. A much longer decay time is usually employed, so that the gain does not get boosted too quickly during the normal pauses in natural speech. Too short

2001-406: A way that is not possible in standard perspective . In optics , image/optical distortion is a divergence from rectilinear projection caused by a change in magnification with increasing distance from the optical axis of an optical system. In cartography , a distortion is the misrepresentation of the area or shape of a feature. The Mercator projection , for example, distorts by exaggerating

2088-423: A wide variety of input amplitudes and produces a generally consistent output amplitude. In its simplest form, a limiter can consist of a pair of back-to-back clamp diodes , which simply shunt excess signal amplitude to ground when the diode conduction threshold is exceeded. This approach will simply clip off the top of large signals, leading to high levels of distortion. While clipping limiters are often used as

2175-403: Is a "built-in" potential across the depletion zone. If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current flow (unless electron–hole pairs are actively being created in the junction by, for instance, light; see photodiode ). However, if the polarity of

2262-427: Is a large population of holes (vacant places for electrons) with which the electrons "recombine". When a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an immobile positively charged donor (dopant) on the N side and negatively charged acceptor (dopant) on the P side. The region around the p–n junction becomes depleted of charge carriers and thus behaves as an insulator . However,

2349-473: Is approximately proportional to the square of the input voltage, so the response is called square law in this region. Following the end of forwarding conduction in a p–n type diode, a reverse current can flow for a short time. The device does not attain its blocking capability until the mobile charge in the junction is depleted. The effect can be significant when switching large currents very quickly. A certain amount of "reverse recovery time" t r (on

2436-447: Is called the depletion region because there are no charge carriers (neither electrons nor holes) in it. The diode's terminals are attached to the n-type and p-type regions. The boundary between these two regions, called a p–n junction , is where the action of the diode takes place. When a sufficiently higher electrical potential is applied to the P side (the anode ) than to the N side (the cathode ), it allows electrons to flow through

2523-403: Is heated by infrared radiation from a nearby heater that is formed of Nichrome wire and supplied with current provided by an external voltage source. The operating temperature of the cathode causes it to release electrons into the vacuum, a process called thermionic emission . The cathode is coated with oxides of alkaline earth metals , such as barium and strontium oxides . These have

2610-400: Is mainly caused by room acoustics, poor loudspeakers and microphones, long loudspeaker cables in combination with frequency dependent loudspeaker impedance , etc. This form of distortion mostly occurs due to electrical reactance . Here, all the components of the input signal are not amplified with the same phase shift, hence making some parts of the output signal out of phase with the rest of

2697-545: Is more so a function of temperature, this effect can be used as a temperature sensor or as a somewhat imprecise voltage reference . A diode's high resistance to current flowing in the reverse direction suddenly drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage . This effect is used to regulate voltage ( Zener diodes ) or to protect circuits from high voltage surges ( avalanche diodes ). A semiconductor diode's current–voltage characteristic can be tailored by selecting

Automatic gain control - Misplaced Pages Continue

2784-407: Is not considered distortion, though the effects of quantization distortion are sometimes included in noise. Quality measures that reflect both noise and distortion include the signal-to-noise and distortion (SINAD) ratio and total harmonic distortion plus noise (THD+N). In telecommunications and signal processing , a noise-free system can be characterised by a transfer function , such that

2871-419: Is not possible if the inverse does not exist—for instance if the transfer function has flat spots (the inverse would map multiple input points to a single output point). This produces an uncorrectable loss of information. Such a situation can occur when an amplifier is overdriven—causing clipping or slew rate distortion when, for a moment, the amplifier characteristics alone and not the input signal determine

2958-411: Is only an approximation. The true behavior of the system is usually different. Nonlinearities in the transfer function of an active device (such as vacuum tubes , transistors , and operational amplifiers ) are a common source of non-linear distortion; in passive components (such as a coaxial cable or optical fiber ), linear distortion can be caused by inhomogeneities, reflections , and so on in

3045-528: Is the alteration of the original shape (or other characteristic) of a signal . In communications and electronics it means the alteration of the waveform of an information-bearing signal , such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel . Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable. For example, in noise reduction systems like

3132-1103: Is the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start pulse . The magnitude of the distortion is expressed in percent of an ideal unit pulse length. This is sometimes called bias distortion . Telegraphic distortion is a similar and older problem, distorting the ratio between mark and space intervals. With respect to audio, distortion refers to any kind of deformation of an output waveform compared to its input, usually clipping , harmonic distortion , or intermodulation distortion ( mixing phenomena) caused by non-linear behavior of electronic components and power supply limitations. Terms for specific types of nonlinear audio distortion include: crossover distortion and slew-induced distortion (SID). Other forms of audio distortion are non-flat frequency response , compression , modulation , aliasing , quantization noise , wow and flutter from analog media such as vinyl records and magnetic tape . The human ear cannot hear phase distortion , except that it may affect

3219-546: Is useful in microwave and switching circuits. Diodes, both vacuum and semiconductor, can be used as shot-noise generators . Thermionic ( vacuum-tube ) diodes and solid-state (semiconductor) diodes were developed separately, at approximately the same time, in the early 1900s, as radio receiver detectors . Until the 1950s, vacuum diodes were used more frequently in radios because the early point-contact semiconductor diodes were less stable. In addition, most receiving sets had vacuum tubes for amplification that could easily have

3306-493: The Dolby system , an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject to electrical noise , then it is symmetrically "undistorted" after passing through a noisy communication channel, reducing the noise in the received signal. Distortion is also used as a musical effect , particularly with electric guitars . The addition of noise or other outside signals ( hum , interference )

3393-469: The Shockley diode equation article. In detector and mixer applications, the current can be estimated by a Taylor's series. The odd terms can be omitted because they produce frequency components that are outside the pass band of the mixer or detector. Even terms beyond the second derivative usually need not be included because they are small compared to the second order term. The desired current component

3480-535: The UK intensively developed point-contact diodes ( crystal rectifiers or crystal diodes ) during World War II for application in radar. After World War II, AT&T used these in its microwave towers that criss-crossed the United States, and many radar sets use them even in the 21st century. In 1946, Sylvania began offering the 1N34 crystal diode. During the early 1950s, junction diodes were developed. In 2022,

3567-651: The diode law (named after the bipolar junction transistor co-inventor William Bradford Shockley ) models the exponential current–voltage (I–V) relationship of diodes in moderate forward or reverse bias. The article Shockley diode equation provides details. At forward voltages less than the saturation voltage, the voltage versus current characteristic curve of most diodes is not a straight line. The current can be approximated by I = I S e V D / ( n V T ) {\displaystyle I=I_{\text{S}}e^{V_{\text{D}}/(nV_{\text{T}})}} as explained in

Automatic gain control - Misplaced Pages Continue

3654-488: The dynamic range ; the result can be a reduced musical quality if the signal is not re-expanded when playing, as in a companding system. Some reel-to-reel tape recorders and cassette decks have AGC circuits. Those used for high-fidelity generally don't. Most VCR circuits use the amplitude of the vertical blanking pulse to operate the AGC. Video copy control schemes such as Macrovision exploit this, inserting spikes in

3741-515: The harmonics (overtones) added to a pure sinewave fed to the system. Harmonic distortion may be expressed in terms of the relative strength of individual components, in decibels , or the root mean square of all harmonic components: Total harmonic distortion (THD), as a percentage. The level at which harmonic distortion becomes audible depends on the exact nature of the distortion. Different types of distortion (like crossover distortion ) are more audible than others (like soft clipping ) even if

3828-418: The propagation path. Amplitude distortion is distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions. Harmonic distortion adds overtones that are whole number multiples of a sound wave's frequencies. Nonlinearities that give rise to amplitude distortion in audio systems are most often measured in terms of

3915-550: The retinal photoreceptors adjust gain to suit light levels. Further on in the visual system, cells in V1 are thought to mutually inhibit, causing normalization of responses to contrast, a form of automatic gain control. Similarly, in the auditory system , the olivocochlear efferent neurons are part of a biomechanical gain control loop. As in all automatic control systems, the temporal dynamics of AGC operation may be important in many applications. Some AGC systems are slow to react to

4002-499: The semiconductor materials and the doping impurities introduced into the materials during manufacture. These techniques are used to create special-purpose diodes that perform many different functions. For example, to electronically tune radio and TV receivers ( varactor diodes ), to generate radio-frequency oscillations ( tunnel diodes , Gunn diodes , IMPATT diodes ), and to produce light ( light-emitting diodes ). Tunnel, Gunn and IMPATT diodes exhibit negative resistance , which

4089-416: The signal-to-noise ratio is lower than it could be. To produce the least noisy recording, the recording level should be set as high as possible without being so high as to clip or distort the signal. In professional high-fidelity recording the level is set manually using a peak-reading meter. When high fidelity is not a requirement, a suitable recording level can be set by an AGC circuit which reduces

4176-426: The stereo imaging . In most fields, distortion is characterized as unwanted change to a signal. Distortion in music is often intentionally used as an effect when applied to an electric guitar signal in styles of rock music such as heavy metal and punk rock . In the visual arts a distortion is any change made by an artist to the size, shape or visual character of a form in order to express an idea, convey

4263-434: The "unilateral conduction" across a contact between a metal and a mineral . Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894. The crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard , who invented a silicon crystal detector in 1903 and received a patent for it on 20 November 1906. Other experimenters tried

4350-417: The 1930s, researchers improved and miniaturized the crystal detector. Point contact diodes ( crystal diodes ) and Schottky diodes are used in radar, microwave and millimeter wave detectors. Rectifiers are constructed from diodes, where they are used to convert alternating current (AC) electricity into direct current (DC). Automotive alternators are a common example, where the diode, which rectifies

4437-549: The AC into DC, provides better performance than the commutator or earlier, dynamo . Similarly, diodes are also used in Cockcroft–Walton voltage multipliers to convert AC into higher DC voltages. Since most electronic circuits can be damaged when the polarity of their power supply inputs are reversed, a series diode is sometimes used to protect against such situations. This concept is known by multiple naming variations that mean

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4524-622: The Edison effect could be used as a radio detector . Fleming patented the first true thermionic diode, the Fleming valve , in Britain on 16 November 1904 (followed by U.S. patent 803,684 in November 1905). Throughout the vacuum tube era, valve diodes were used in almost all electronics such as radios, televisions, sound systems, and instrumentation. They slowly lost market share beginning in

4611-461: The THD measurements are identical. Harmonic distortion in radio frequency applications is rarely expressed as THD. Non-flat frequency response is a form of distortion that occurs when different frequencies are amplified by different amounts in a filter . For example, the non-uniform frequency response curve of AC-coupled cascade amplifier is an example of frequency distortion. In the audio case, this

4698-431: The damaging voltage spikes that would otherwise occur. (A diode used in such an application is called a flyback diode ). Many integrated circuits also incorporate diodes on the connection pins to prevent external voltages from damaging their sensitive transistors . Specialized diodes are used to protect from over-voltages at higher power (see Diode types above). Distortion In signal processing , distortion

4785-620: The depletion region from the N-type side to the P-type side. The junction does not allow the flow of electrons in the opposite direction when the potential is applied in reverse, creating, in a sense, an electrical check valve . Another type of junction diode, the Schottky diode , is formed from a metal–semiconductor junction rather than a p–n junction, which reduces capacitance and increases switching speed. A semiconductor diode's behavior in

4872-460: The diode but with laser light would be the optical isolator , also known as an optical diode, that allows light to only pass in one direction. It uses a Faraday rotator as the main component. The first use for the diode was the demodulation of amplitude modulated (AM) radio broadcasts. The history of this discovery is treated in depth in the crystal detector article. In summary, an AM signal consists of alternating positive and negative peaks of

4959-411: The diode's forward voltage drop or just voltage drop , since a consequence of the steepness of the exponential is that a diode's voltage drop will not significantly exceed the threshold voltage under normal forward bias operating conditions. Datasheets typically quote a typical or maximum forward voltage (V F ) for a specified current and temperature (e.g. 20 mA and 25  ° C for LEDs), so

5046-466: The effective modulation depth of the signal, distorting the sound. Communications receivers may have more complex AVC systems, including extra amplification stages, separate AGC detector diodes, different time constants for broadcast and shortwave bands, and application of different levels of AGC voltage to different stages of the receiver to prevent distortion and cross-modulation. Design of the AVC system has

5133-458: The external voltage opposes the built-in potential, recombination can once again proceed, resulting in a substantial electric current through the p–n junction (i.e. substantial numbers of electrons and holes recombine at the junction) that increases exponentially with voltage. A diode's current–voltage characteristic can be approximated by four operating regions. From lower to higher bias voltages, these are: The Shockley ideal diode equation or

5220-488: The fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected

5307-450: The first superconducting diode effect without an external magnetic field was realized. At the time of their invention, asymmetrical conduction devices were known as rectifiers . In 1919, the year tetrodes were invented, William Henry Eccles coined the term diode from the Greek roots di (from δί ), meaning 'two', and ode (from οδός ), meaning 'path'. The word diode however

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5394-432: The gain as the average signal level increases. This allows a usable recording to be made even for speech some distance from the microphone of an audio recorder. Similar considerations apply with VCRs . A potential disadvantage of AGC is that when recording something like music with quiet and loud passages such as classical music, the AGC will tend to make the quiet passages louder and the loud passages quieter, compressing

5481-413: The gain with greater granularity, in specific detection cells. Many radar countermeasures use a radar's AGC to fool it, by effectively "drowning out" the real signal with the spoof, as the AGC will regard the weaker, true signal as clutter relative to the strong spoof. An audio tape generates a certain amount of noise . If the level of the signal on the tape is low, the noise is more prominent, i.e.,

5568-403: The inverse function F can be found, and used intentionally to distort either the input or the output of the system, then the distortion is corrected. An example of a similar correction is where LP/ vinyl recordings or FM audio transmissions are deliberately pre-emphasised by a linear filter , the reproducing system applies an inverse filter to make the overall system undistorted. Correction

5655-404: The late 1940s due to selenium rectifier technology and then to semiconductor diodes during the 1960s. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, and in musical instrument and audiophile applications. In 1874, German scientist Karl Ferdinand Braun discovered

5742-412: The metal point during manufacture by momentarily passing a relatively large current through the device. Point contact diodes generally exhibit lower capacitance, higher forward resistance and greater reverse leakage than junction diodes. A p–n junction diode is made of a crystal of semiconductor , usually silicon, but germanium and gallium arsenide are also used. Impurities are added to it to create

5829-660: The need for gain changes, while others may react very rapidly. An example of an application in which fast AGC recovery time is required is in receivers used in Morse code communications where so-called full break-in or QSK operation is necessary to enable receiving stations to interrupt sending stations mid-character (e.g. between dot and dash signals). Diode A diode is a two- terminal electronic component that conducts current primarily in one direction (asymmetric conductance ). It has low (ideally zero) resistance in one direction and high (ideally infinite) resistance in

5916-421: The order of tens of nanoseconds to a few microseconds) may be required to remove the reverse recovery charge Q r from the diode. During this recovery time, the diode can actually conduct in the reverse direction. This might give rise to a large current in the reverse direction for a short time while the diode is reverse biased. The magnitude of such a reverse current is determined by the operating circuit (i.e.,

6003-418: The other. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. It has an exponential current–voltage characteristic . Semiconductor diodes were the first semiconductor electronic devices . The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and

6090-416: The output y ( t ) {\displaystyle y(t)} can be written as a function of the input x {\displaystyle x} as When the transfer function comprises only a perfect gain constant A and perfect delay T the output is undistorted. Distortion occurs when the transfer function F is more complicated than this. If F is a linear function , for instance

6177-419: The output. Can be found only in dispersive media . In a waveguide , phase velocity varies with frequency. In a filter, group delay tends to peak near the cut-off frequency , resulting in pulse distortion. When analog long distance trunks were commonplace, for example in 12 channel carrier , group delay distortion had to be corrected in repeaters . As the system output is given by y(t) = F(x(t)), then if

6264-414: The output. Many symmetrical electronic circuits reduce the magnitude of even harmonics generated by the non-linearities of the amplifier's components, by combining two signals from opposite halves of the circuit where distortion components that are roughly the same magnitude but out of phase. Examples include push-pull amplifiers and long-tailed pairs . In binary signaling such as FSK , distortion

6351-483: The peak inverse voltage rating for application in high voltage rectifiers), and required a large heat sink (often an extension of the diode's metal substrate ), much larger than the later silicon diode of the same current ratings would require. The vast majority of all diodes are the p–n diodes found in CMOS integrated circuits , which include two diodes per pin and many other internal diodes. The symbol used to represent

6438-480: The pulse which will be ignored by most television sets, but cause a VCR's AGC to overcorrect and corrupt the recording. A voice-operated gain-adjusting device or volume-operated gain-adjusting device (vogad) is a type of AGC or compressor for microphone amplification. It is usually used in radio transmitters to prevent overmodulation and to reduce the dynamic range of the signal which allows increasing average transmitted power. In telephony , this device takes

6525-420: The radio signal amplitude, because the information content of the signal is carried by the changes of amplitude of the carrier wave . If the circuit were not fairly linear, the modulated signal could not be recovered with reasonable fidelity . However, the strength of the signal received will vary widely, depending on the power and distance of the transmitter , and signal path attenuation . The AGC circuit keeps

6612-474: The receiver on weaker signals as low gain can worsen signal-to-noise ratio and blocking ; therefore, many designs reduce gain only for stronger signals. Since the AM detector diode produces a DC voltage proportional to signal strength, this voltage can be fed back to earlier stages of the receiver to reduce gain. A filter network is required so that the audio components of the signal don't appreciably influence gain; this prevents "modulation rise" which increases

6699-456: The receiver's output level from fluctuating too much by detecting the overall strength of the signal and automatically adjusting the gain of the receiver to maintain the output level within an acceptable range. For a very weak signal, the AGC operates the receiver at maximum gain; as the signal increases, the AGC reduces the gain. It is usually disadvantageous to reduce the gain of the RF front end of

6786-449: The relatively large signal from the local user and the much smaller signal from the remote user at comparable loudnesses. Some telephone recording devices incorporate automatic gain control to produce acceptable-quality recordings. As is the case with many concepts found in engineering, automatic gain control is also found in biological systems, especially sensory systems. For example, in the vertebrate visual system , calcium dynamics in

6873-520: The same thing: reverse voltage protection, reverse polarity protection, and reverse battery protection. Diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices. They are usually reverse-biased (non-conducting) under normal circumstances. When the voltage rises above the normal range, the diodes become forward-biased (conducting). For example, diodes are used in ( stepper motor and H-bridge ) motor controller and relay circuits to de-energize coils rapidly without

6960-429: The series resistance) and the diode is said to be in the storage-phase. In certain real-world cases it is important to consider the losses that are incurred by this non-ideal diode effect. However, when the slew rate of the current is not so severe (e.g. Line frequency) the effect can be safely ignored. For most applications, the effect is also negligible for Schottky diodes . The reverse current ceases abruptly when

7047-507: The stored charge is depleted; this abrupt stop is exploited in step recovery diodes for the generation of extremely short pulses. Normal (p–n) diodes, which operate as described above, are usually made of doped silicon or germanium . Before the development of silicon power rectifier diodes, cuprous oxide and later selenium was used. Their low efficiency required a much higher forward voltage to be applied (typically 1.4 to 1.7 V per "cell", with multiple cells stacked so as to increase

7134-463: The thermionic diodes included in the tube (for example the 12SQ7 double diode triode ), and vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes (such as selenium rectifiers ) that were available at that time. In 1873, Frederick Guthrie observed that a grounded, white-hot metal ball brought in close proximity to an electroscope would discharge

7221-837: The two most common being the EIA / JEDEC standard and the European Pro Electron standard: The standardized 1N-series numbering EIA370 system was introduced in the US by EIA/JEDEC (Joint Electron Device Engineering Council) about 1960. Most diodes have a 1-prefix designation (e.g., 1N4003). Among the most popular in this series were: 1N34A/1N270 (germanium signal), 1N914/ 1N4148 (silicon signal), 1N400x (silicon 1A power rectifier), and 1N580x (silicon 3A power rectifier). The JIS semiconductor designation system has all semiconductor diode designations starting with "1S". The European Pro Electron coding system for active components

7308-496: The user has a guarantee about when a certain amount of current will kick in. At higher currents, the forward voltage drop of the diode increases. For instance, a drop of 1 V to 1.5 V is typical at full rated current for silicon power diodes. (See also: Rectifier § Rectifier voltage drop ) However, a semiconductor diode's exponential current–voltage characteristic is really more gradual than this simple on–off action. Although an exponential function may appear to have

7395-587: The width of the depletion region (called the depletion width ) cannot grow without limit. For each electron–hole pair recombination made, a positively charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is created in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination. At this point, there

7482-419: Was already in use, as were triode , tetrode , pentode , hexode , as terms of multiplex telegraphy . Although all diodes rectify , "rectifier" usually applies to diodes used for power supply , to differentiate them from diodes intended for small signal circuits. A thermionic diode is a thermionic-valve device consisting of a sealed, evacuated glass or metal envelope containing two electrodes :

7569-502: Was introduced in 1966 and comprises two letters followed by the part code. The first letter represents the semiconductor material used for the component (A = germanium and B = silicon) and the second letter represents the general function of the part (for diodes, A = low-power/signal, B = variable capacitance, X = multiplier, Y = rectifier and Z = voltage reference); for example: Other common numbering/coding systems (generally manufacturer-driven) include: In optics, an equivalent device for

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