High power amplifiers wikipedia
Robert W. Bob Carver is an American designer of audio equipment based in the Pacific Northwest. Educated as a physicist and engineer , he found an interest in audio equipment at a young age. He applied his talent to produce numerous innovative high fidelity designs since the s. Two magazines accepted the challenge. In , Stereophile magazine challenged Bob to copy a Conrad-Johnson Premier Four the make and model was not named then, but revealed later amplifier at their offices in New Mexico within 48 hours.
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Linear amplifier
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity , or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiberoptic cables which carry much of the world's telecommunication links.
There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers.
In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. In semiconductor optical amplifiers SOAs , electron - hole recombination occurs. In Raman amplifiers , Raman scattering of incoming light with phonons in the lattice of the gain medium produces photons coherent with the incoming photons. Parametric amplifiers use parametric amplification. The principle of optical amplification was invented by Gordon Gould on November 13, Gould co-founded an optical telecommunications equipment firm, Optelecom Inc.
Huber and Steve Alexander of Ciena invented the dual-stage optical amplifier [7] US Patent 5,, that was a key to the first dense wave division multiplexing DWDM system, marking the start of optical networking. Almost any laser active gain medium can be pumped to produce gain for light at the wavelength of a laser made with the same material as its gain medium. Such amplifiers are commonly used to produce high power laser systems. Special types such as regenerative amplifiers and chirped-pulse amplifiers are used to amplify ultrashort pulses.
Solid-state amplifiers are optical amplifiers that uses a wide range of doped solid-state materials Nd:YAG , Yb:YAG, Ti:Sa and different geometries disk, slab, rod to amplify optical signals. The variety of materials allows the amplification of different wavelength while the shape of the medium can distinguish between more suitable for energy of average power scaling.
Doped fiber amplifiers DFAs are optical amplifiers that use a doped optical fiber as a gain medium to amplify an optical signal. The signal to be amplified and a pump laser are multiplexed into the doped fiber, and the signal is amplified through interaction with the doping ions. Amplification is achieved by stimulated emission of photons from dopant ions in the doped fiber. The pump laser excites ions into a higher energy from where they can decay via stimulated emission of a photon at the signal wavelength back to a lower energy level.
The excited ions can also decay spontaneously spontaneous emission or even through nonradiative processes involving interactions with phonons of the glass matrix.
These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification. The amplification window of an optical amplifier is the range of optical wavelengths for which the amplifier yields a usable gain. The amplification window is determined by the spectroscopic properties of the dopant ions, the glass structure of the optical fiber, and the wavelength and power of the pump laser.
Although the electronic transitions of an isolated ion are very well defined, broadening of the energy levels occurs when the ions are incorporated into the glass of the optical fiber and thus the amplification window is also broadened.
This broadening is both homogeneous all ions exhibit the same broadened spectrum and inhomogeneous different ions in different glass locations exhibit different spectra.
Homogeneous broadening arises from the interactions with phonons of the glass, while inhomogeneous broadening is caused by differences in the glass sites where different ions are hosted.
Different sites expose ions to different local electric fields, which shifts the energy levels via the Stark effect. In addition, the Stark effect also removes the degeneracy of energy states having the same total angular momentum specified by the quantum number J.
The gain spectrum of the EDFA has several peaks that are smeared by the above broadening mechanisms. The net result is a very broad spectrum 30 nm in silica, typically. The broad gain-bandwidth of fiber amplifiers make them particularly useful in wavelength-division multiplexed communications systems as a single amplifier can be utilized to amplify all signals being carried on a fiber and whose wavelengths fall within the gain window.
An erbium-doped waveguide amplifier EDWA is an optical amplifier that uses a waveguide to boost an optical signal. A relatively high-powered beam of light is mixed with the input signal using a wavelength selective coupler WSC. The input signal and the excitation light must be at significantly different wavelengths. The mixed light is guided into a section of fiber with erbium ions included in the core.
This high-powered light beam excites the erbium ions to their higher-energy state. When the photons belonging to the signal at a different wavelength from the pump light meet the excited erbium ions, the erbium ions give up some of their energy to the signal and return to their lower-energy state. A significant point is that the erbium gives up its energy in the form of additional photons which are exactly in the same phase and direction as the signal being amplified.
So the signal is amplified along its direction of travel only. This is not unusual — when an atom "lases" it always gives up its energy in the same direction and phase as the incoming light. Thus all of the additional signal power is guided in the same fiber mode as the incoming signal.
An optical isolator is usually placed at the output to prevent reflections returning from the attached fiber. Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser. Noise figure in an ideal DFA is 3 dB, while practical amplifiers can have noise figure as large as 6—8 dB. As well as decaying via stimulated emission, electrons in the upper energy level can also decay by spontaneous emission, which occurs at random, depending upon the glass structure and inversion level.
Photons are emitted spontaneously in all directions, but a proportion of those will be emitted in a direction that falls within the numerical aperture of the fiber and are thus captured and guided by the fiber. Those photons captured may then interact with other dopant ions, and are thus amplified by stimulated emission.
The initial spontaneous emission is therefore amplified in the same manner as the signals, hence the term Amplified Spontaneous Emission. ASE is emitted by the amplifier in both the forward and reverse directions, but only the forward ASE is a direct concern to system performance since that noise will co-propagate with the signal to the receiver where it degrades system performance. Counter-propagating ASE can, however, lead to degradation of the amplifier's performance since the ASE can deplete the inversion level and thereby reduce the gain of the amplifier and increase the noise produced relative to the desired signal gain.
Noise figure can be analyzed in both the optical domain and in the electrical domain. For the electrical measurement method, the detected photocurrent noise is evaluated with a low-noise electrical spectrum analyzer, which along with measurement of the amplifier gain permits a noise figure measurement.
Generally, the optical technique provides a more simple method, though it is not inclusive of excess noise effects captured by the electrical method such multi-path interference MPI noise generation. In both methods, attention to effects such as the spontaneous emission accompanying the input signal are critical to accurate measurement of noise figure.
Gain is achieved in a DFA due to population inversion of the dopant ions. The inversion level of a DFA is set, primarily, by the power of the pump wavelength and the power at the amplified wavelengths.
As the signal power increases, or the pump power decreases, the inversion level will reduce and thereby the gain of the amplifier will be reduced. This effect is known as gain saturation — as the signal level increases, the amplifier saturates and cannot produce any more output power, and therefore the gain reduces.
Saturation is also commonly known as gain compression. To achieve optimum noise performance DFAs are operated under a significant amount of gain compression 10 dB typically , since that reduces the rate of spontaneous emission, thereby reducing ASE. Another advantage of operating the DFA in the gain saturation region is that small fluctuations in the input signal power are reduced in the output amplified signal: smaller input signal powers experience larger less saturated gain, while larger input powers see less gain.
The leading edge of the pulse is amplified, until the saturation energy of the gain medium is reached. In some condition, the width FWHM of the pulse is reduced. Due to the inhomogeneous portion of the linewidth broadening of the dopant ions, the gain spectrum has an inhomogeneous component and gain saturation occurs, to a small extent, in an inhomogeneous manner.
This effect is known as spectral hole burning because a high power signal at one wavelength can 'burn' a hole in the gain for wavelengths close to that signal by saturation of the inhomogeneously broadened ions.
Spectral holes vary in width depending on the characteristics of the optical fiber in question and the power of the burning signal, but are typically less than 1 nm at the short wavelength end of the C-band, and a few nm at the long wavelength end of the C-band. The depth of the holes are very small, though, making it difficult to observe in practice.
The absorption and emission cross sections of the ions can be modeled as ellipsoids with the major axes aligned at random in all directions in different glass sites. The random distribution of the orientation of the ellipsoids in a glass produces a macroscopically isotropic medium, but a strong pump laser induces an anisotropic distribution by selectively exciting those ions that are more aligned with the optical field vector of the pump.
Also, those excited ions aligned with the signal field produce more stimulated emission. The change in gain is thus dependent on the alignment of the polarizations of the pump and signal lasers — i. In an ideal doped fiber without birefringence , the PDG would be inconveniently large.
Fortunately, in optical fibers small amounts of birefringence are always present and, furthermore, the fast and slow axes vary randomly along the fiber length. A typical DFA has several tens of meters, long enough to already show this randomness of the birefringence axes.
These two combined effects which in transmission fibers give rise to polarization mode dispersion produce a misalignment of the relative polarizations of the signal and pump lasers along the fiber, thus tending to average out the PDG. The result is that PDG is very difficult to observe in a single amplifier but is noticeable in links with several cascaded amplifiers. The erbium-doped fiber amplifier EDFA is the most deployed fiber amplifier as its amplification window coincides with the third transmission window of silica-based optical fiber.
Both of these bands can be amplified by EDFAs, but it is normal to use two different amplifiers, each optimized for one of the bands. The principal difference between C- and L-band amplifiers is that a longer length of doped fiber is used in L-band amplifiers. The longer length of fiber allows a lower inversion level to be used, thereby giving emission at longer wavelengths due to the band-structure of Erbium in silica while still providing a useful amount of gain. EDFAs have two commonly used pumping bands — nm and nm.
The nm band has a higher absorption cross-section and is generally used where low-noise performance is required. The absorption band is relatively narrow and so wavelength stabilised laser sources are typically needed. The nm band has a lower, but broader, absorption cross-section and is generally used for higher power amplifiers. A combination of nm and nm pumping is generally utilised in amplifiers. Gain and lasing in Erbium-doped fibers were first demonstrated in —87 by two groups; one including David N.
Payne , R. Mears , I. M Jauncey and L. Desurvire, P. Becker, and J. Alexander at Ciena Corporation. Thulium doped fiber amplifiers have been used in the S-band — nm and Praseodymium doped amplifiers in the nm region. However, those regions have not seen any significant commercial use so far and so those amplifiers have not been the subject of as much development as the EDFA. However, Ytterbium doped fiber lasers and amplifiers, operating near 1 micrometre wavelength, have many applications in industrial processing of materials, as these devices can be made with extremely high output power tens of kilowatts.
Semiconductor optical amplifiers SOAs are amplifiers which use a semiconductor to provide the gain medium. Recent designs include anti-reflective coatings and tilted wave guide and window regions which can reduce end face reflection to less than 0.
Class-D amplifier
LDMOS laterally-diffused metal-oxide semiconductor [1] is a planar double-diffused MOSFET metal—oxide—semiconductor field-effect transistor used in amplifiers , including microwave power amplifiers, RF power amplifiers and audio power amplifiers. The fabrication of LDMOS devices mostly involves various ion-implantation and subsequent annealing cycles. In , the efficiency of LDMOS power amplifiers was boosted using typical efficiency enhancement techniques, such as Doherty topologies or envelope tracking. From Wikipedia, the free encyclopedia. World Scientific.
Audio power amplifier
In electronics , power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier 's characteristics and performance. The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period. A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse width modulation output is obtained from the stage. Additional letter classes are defined for special purpose amplifiers, with additional active elements or particular power supply improvements; sometimes a new letter symbol is used by a manufacturer to promote its proprietary design. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. The angle of flow is closely related to the amplifier power efficiency. In the illustrations below, a bipolar junction transistor is shown as the amplifying device. The active element remains conducting [2] all of the time.
Headphone amplifier
In electronics , the figures of merit of an amplifier are numerical measures that characterize its properties and performance. Figures of merit can be given as a list of specifications that include properties such as gain , bandwidth , noise and linearity , among others listed in this article. Figures of merit are important for determining the suitability of a particular amplifier for an intended use. The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels.
Audio power
An amplifier , electronic amplifier or informally amp is an electronic device that can increase the power of a signal a time-varying voltage or current. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain : the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one. An amplifier can either be a separate piece of equipment or an electrical circuit contained within another device.
Power amplifier classes
A radio frequency power amplifier RF power amplifier is a type of electronic amplifier that converts a low-power radio-frequency signal into a higher power signal. Design goals often include gain , power output, bandwidth, power efficiency, linearity low signal compression at rated output , input and output impedance matching, and heat dissipation. Some classes are class A , class AB, class B , class C , which are considered the linear amplifier classes. In these classes the active device is used as a controlled current source. The bias at the input determines the class of the amplifier.
Instrument amplifier
A guitar amplifier or amp is an electronic device or system that strengthens the weak electrical signal from a pickup on an electric guitar , bass guitar , or acoustic guitar so that it can produce sound through one or more loudspeakers , which are typically housed in a wooden cabinet. A guitar amplifier may be a standalone wood or metal cabinet that contains only the power amplifier and preamplifier circuits, requiring the use of a separate speaker cabinet—or it may be a "combo" amplifier, which contains both the amplifier and one or more speakers in a wooden cabinet. Guitar amplifiers can also modify the instrument's tone by emphasizing or de-emphasizing certain frequencies, using equalizer controls, which function the same way as the bass and treble knobs on a home hi-fi stereo, and by adding electronic effects ; distortion also called "overdrive" and reverb are commonly available as built-in features.
Energy amplifier
Technical specifications and detailed information on the valve audio amplifier , including its development history. Valves also known as vacuum tubes are very high input impedance near infinite in most circuits and high-output impedance devices. The characteristics of valves as gain devices have direct implications for their use as audio amplifiers , notably that power amplifiers need output transformers OPTs to translate a high-output-impedance high-voltage low-current signal into a lower-voltage high-current signal needed to drive modern low-impedance loudspeakers cf. Capacitors have little effect on the performance of amplifiers. Interstage transformer coupling is a source of distortion and phase shift, and was avoided from the s for high-quality applications; transformers also add cost, bulk, and weight. The following circuits are simplified conceptual circuits only, real world circuits also require a smoothed or regulated power supply, heater for the filaments the details depending on if the selected valve types are directly or indirectly heated , and the cathode resistors are often bypassed, etc.
Magnetic amplifier
An Intermediate power amplifier IPA is one stage of the amplification process in a radio transmitter which usually occurs prior to the final high power amplification. In very high power transmitters, such as 10 kilowatts and above, multiple IPAs are combined to provide enough drive for the final. An exciter , an even lower power transmitter, provides a similar service to the IPA by driving it; although an exciter usually encompasses other important functions, such as choosing the frequency of the RF. This broadcasting-related article is a stub. You can help Wikipedia by expanding it.
Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the s and s, initially for receivers and low power stages of transmitters, transmitter output stages switching to transistors somewhat later. Specially constructed valves are still in use for very high power transmitters, although rarely in new designs. Tetrode and pentode valves have very flat anode current vs.
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