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4 Power amplifier classes
4.Angle of flow or conduction angle
4.Electronic amplifiers use two variables: current and voltage.RF amplifiers, do not fit into this classification approach.Although, for a given appropriate source and load impedance, RF amplifiers can be characterized as amplifying voltage or current, they fundamentally are amplifying power.Common terminal
One set of classifications for amplifiers is based on which device terminal is common to both the input and the output circuit.Unilateral or bilateral
When an amplifier has an output that exhibits no feedback to its input side, it is called unilateral.Most amplifiers are bilateral to some degree, however they may often be modeled as unilateral under certain operating conditions to simplify the analysis (see the common base article for an example).Another way to classify amps is the phase relationship of the input signal to the output signal.Function
Other amplifiers may be classified by their function or output characteristics.DC servo indicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur.An RF amplifier refers to an amplifier designed for use in the radio frequency range of the electromagnetic spectrum, and is often used to increase the sensitivity of a receiver or the output power of a transmitter.Other more exotic forms of amplifier are also possible using different types of devices.Such exotic amplifiers are often used for microwave or other extremely high frequency signals.The various classes are introduced below, followed by more detailed discussion under individual headings later on.Class A amplifiers are typically more linear and less complex than other types, but are very inefficient.In Class B, there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal.Class AB2 applies to tube or transistor amplifiers in class AB where the grid or base is often more negatively biased than in AB1, and the input signal is often larger.Linearity is not good, but this is of no significance for single frequency power amplifiers.These classes use harmonic tuning of their output networks to achieve higher efficiency and can be considered a subset of Class C due to their conduction angle characteristics.More detail on the various classes is provided below.Class A Amplifier
If high output powers are needed from a Class A circuit, the power waste (and the accompanying heat) will become significant.For every watt delivered to the load, the amplifier itself will, at best, dissipate another watt.Class A designs have largely been superseded for audio power amplifiers, though some audiophiles believe that Class A gives the best sound quality, due to it being operated in as linear a manner as possible which provides a small market for expensive high fidelity Class A amps.Historically, valve amplifiers often used a Class A power amplifier simply because valves are large and expensive; many Class A designs uses only a single device.At idle (no input), the power consumption is essentially the same as at high output volume.Class B and AB
Class B amplifiers only amplify half of the input wave cycle.As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A.This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch at the "joins" between the two halves of the signal.This is called Class AB operation.Such a circuit behaves as a class A amplifier in the region where both devices are in the linear region, however the circuit cannot strictly be called class A if the signal passes outside this region, since beyond that point only one of the devices will remain in its linear region and the transients typical of class B operation will occur.The result is that when the two halves are combined, the crossover is greatly minimised or eliminated altogether.Class AB sacrifices some efficiency over class B in favor of linearity, so will always be less efficient.Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small.Some applications (for example, megaphones) can tolerate the distortion.Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage.The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be extracted by the tuned load.Class D amplifiers are much more efficient than Class AB power amplifiers.The term usually applies to devices intended to reproduce signals with a bandwidth well below the switching frequency.The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal.The resulting filtered signal is then an amplified replica of the input.The main advantage of a class D amplifier is power efficiency.Because the output pulses have a fixed amplitude, the switching elements (usually MOSFETs, but valves and bipolar transistors were once used) are switched either on or off, rather than operated in linear mode.The lower losses permit the use of a smaller heat sink while the power supply requirements are lessened too.The digital control introduces additional distortion called quantization error caused by its conversion of the input signal to a digital value.Class D amplifiers have been widely used to control motors, and almost exclusively for small DC motors, but they are now also used as audio amplifiers, with some extra circuitry to allow analogue to be converted to a much higher frequency pulse width modulated signal.Tripath have called their revised Class D designs Class T.This means that the amplifier cannot prevent the subwoofer's reactive nature from lessening the impact of low bass sounds (as explained in the feedback part of the Class AB section).The letter D used to designate this amplifier class is simply the next letter after C, and does not stand for digital.At this point the current through the transistor is zero and it is switched off.The whole circuit performs a damped oscillation.This means the voltage and the current at the transistor are symmetric with respect to time.CMOS the even harmonics of both transistors just cancel.Experiment tells that a square wave can be generated by those amplifiers and math tells that square wave do consist of odd harmonics only.So even small currents in the harmonics suffice to generate a voltage square wave.The current is in phase with the voltage applied to filter, but the voltage across the transistors is out of phase.Of course there has to be a finite voltage across the transistor to push the current across the on state resistance.On the other hand a LC series circuit with a large L and a tunable C may be simpler to implement.By reducing the duty cycle below 0.Any load mismatch behind the filter can only act on the first harmonic current waveform, clearly only a purely resistive load makes sense, then the lower the resistance the higher the current.Class F can be driven by sine or by a square wave, for a sine the input can be tuned by an L to increase gain.If class F is implemented with a single transistor the filter is complicated to short the even harmonics.Class E uses a significant amount of second harmonic voltage.In reality the impedance is mostly reactive and the only reason for it is that class E is a class F amplifier with a very simplified load network and thus has to deal with imperfections.In many amateur simulations of class E amplifiers sharp current edges are assumed nullifying the very motivation for class E and measurements near the transit frequency of the transistors show very symmetric curves, which look much similar to class F simulations.The class E amplifier was invented in 1972 by Nathan O.Some earlier reports on this operating class have been published in Russian.This section may require cleanup to meet Wikipedia's quality standards.Please improve this article if you can (July 2007).There are a variety of amplifier designs that couple a class AB output stage with other more efficient techniques to achieve a higher efficiency with low distortion.The terms "class G" and "class H" are used interchangeably to refer to different designs, varying in definition from one manufacturer or paper to another.Class G amplifiers are a more efficient version of class AB amplifiers, which use "rail switching" to decrease power consumption and increase efficiency.Class H amplifier takes the idea of Class G one step further creating an infinite number of supply rails.This is done by modulating the supply rails so that the rails are only a few volts larger than the output signal at any given time.Switched mode power supplies can be used to create the tracking rails.Significant efficiency gains can be achieved but with the drawback of more complicated supply design and reduced THD performance.Efficiency Class H The classes can be most easily understood using the diagrams in each section below.For the sake of illustration, a bipolar junction transistor is shown as the amplifying device, but in practice this could be a MOSFET or vacuum tube device.Other arrangements of amplifying device are possible, but that given (that is, common emitter, common source or common cathode) is the easiest to understand and employ in practice.If the amplifying element is linear, then the output will be faithful copy of the input, only larger and inverted.One important limitation of a real amplifier is that the output it can generate is ultimately limited by the power available from the power supply.An amplifier will saturate and clip the output if the input signal becomes too large for the amplifier to reproduce or if operational limits for a device are exceeded.Doherty for Bell Laboratories (whose sister company, Western Electric, was then an important manufacturer of radio transmitters).The input signal is split evenly to drive the two amplifiers, and a combining network sums the two output signals and corrects for phase differences between the two amplifiers.Other classes
Several audio amplifier manufacturers have started "inventing" new classes as a way to differentiate themselves.This can easily be determined by the fact that the class name is trademarked or copyrighted.TD or Tracked Class D which tracks the waveform to more accurately amplify it without the drawbacks of traditional class D amplifiers."Class T" is a trademark of TriPath company, which manufactures audio amplifier ICs.This new class "T" is a revision of the common class D amplifier, but with changes to ensure fidelity over the full audio spectrum, unlike traditional class D designs.It operates at different frequencies depending on the power output, with values ranging from as low as 200 kHz to 1.Amplifier circuit
For the purposes of illustration, this practical amplifier circuit is described.The amplified signal from Q1 is directly fed to the second stage, Q3, which is a common emitter stage that provides further amplification of the signal and the DC bias for the output stages, Q4 and Q5.So far, all of the amplifier is operating in Class A.For the basics of radio frequency amplifiers using valves, see Valved RF amplifiers.Notes on implementation
Real world amplifiers are imperfect.Many modern amplifiers use negative feedback techniques to hold the gain at the desired value.Different methods of supplying power result in many different methods of bias.Matched inverted polarity devices are called complementary pairs.Class A amplifiers generally use only one device, unless the power supply is set to provide both positive and negative voltages, in which case a dual device symmetrical design may be used.Class C amps, by definition, use a single polarity supply.Each stage of these designs is often a different type of amp to suit the needs of that stage.It is a curiosity to note that this table is a "Zwicky box"; in particular, it encompasses all possibilities.Vsats: Very Small Aperture Terminals.Americas
Maytag AMP, an automatic washing machine introduced in 1948
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AMP,Average Manufacturer Price, a refereance drug price calculated by Centers for Medicare and Medicaid Services.This value is used to calculate Medcaid Drug Rebates paid to state Medicaid programs.All text is available under the terms of the GNU Free Documentation License.Starting July 2006, CAIDA,
a well established networking research organization based at
SDSC,
took over operational stewardship for all
NLANR machines and data.The National Laboratory for Applied Network
Research (NLANR) Project has officially ended.As we perform this audit in the next few weeks, we will
keep the community informed of any changes planned to operations
of the machines at the sites.Hosting sites will have the prerogative
at any time to decline participation in the reincarnation of any
NLANR equipment at their site, but we hope sites will find our
proposed use of sufficient relevance and merit to continue hosting
a measurement platform.What follows is the original AMP NLANR webpage.The data collected by AMP is proving to be a valuable resource for network analysis to study the network and derive performance models for various aspects of Internet traffic.The AMP monitors measure the "heartbeat" of the high performance connection (HPC) networks, dissecting the networks in over 15,000 ways every minute.Because the performance of AMP monitors is well known, and coverage is so extensive, the monitoring mesh allows engineers to quickly identify the location, extent, and duration of network events.The first AMP monitor deployment took place in December 1998.United States, as well as at strategic sites in other countries.Each uses a full mesh, with each monitor testing to all the others in the mesh.Recent expansion and development of AMP has included monitor deployments at GigaPop and backbone sites and development of a GigE interface for AMP.Stars represent sites in the AMP mesh; currently about 150 active measurement monitors are deployed.AMP has led the way to a new approach to network measurement (dense coverage with simple monitors).Consequently, there are a growing number of organizations that realize they would benefit from a performance measurement system like AMP.The AMP package will be made freely available; however, we do not intend the various users of AMP to be totally independent from one another.We will also manage the host to host testing (locating AMP software only monitors on an HPC machine).IPMP
Currently, there is no good way to make an Internet measurement that divides the delay into components related to different parts of the network and its users.NLANR researchers are proposing a new measurement protocol (the IP Measurement Protocol or just IPMP) which allows Internet devices to insert time stamps into data as it moves through the network, creating an "audit trail" for Internet data.This trail is measured in thousandths of a second and must be created at speeds that allow hundreds of millions of Internet data packets to be processed each second.They are also offering the protocol for standardisation by the Internet Engineering Task Force (IETF).It is hoped that this protocol will give users, network operators and Internet based service providers a common language with which to communicate performance issues, validate their claims and compete with one another.Specifically, it displays which components of the path changed over the course of the day.We have been collecting IPv6 performance data since October 2002.
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