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Topic: Class AB amplifier

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	Amplifier classes of operation
		The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion.
		Amplifiers biased so that collector current is cut off during one-half of the input signal are classified class B. The dc operating point for this class of amplifier is set up so that base current is zero with no input signal.
		Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of the input, which results in less total input power than the class A amplifier.
	www.tpub.com /neets/book7/25e.htm (1140 words)

	Class A (Site not responding. Last check: 2007-10-12)
		A class A amplifier is defined as one which is biased to a point where plate current in all the output devices flows for the entire 360 degrees of an input cycle, at the full, unclipped output of the amplifier.
		A class AB amplifier is one in which the grid bias is set so that plate current flows for more than half, but appreciably less that the full 360 degrees of the the input cycle, again measured at the full, unclipped output of the amplifier.
		In a class AB amplifier, as the current through the tube increases, the average voltage across the cathode resistor changes, which modulates the plate current, creating a bit of "sag" and a dynamic change in the harmonic structure of the note that changes while playing.
	www.aikenamps.com /ClassA.htm (3925 words)

	Euphonic Audio \|\| Technology: Class D Digital Amplifiers
		Class D amplifiers are highly efficient and have a great power to size ratio.
		Amplifiers in classes A, B, and AB operate their output transistors in a 'linear' mode while Class D amplifiers operate their outputs in 'switch' mode.
		A Class AB amplifier is a Class B amplifier which has a small amount of "bias" current flowing through the output transistors at all times.
	www.eaamps.com /2005/technology/classd.htm (1025 words)

	Gallium arsenide class AB output stage - Patent 4970471
		Class A amplifiers are inefficient and must dissipate large amounts of power, while totem pole circuits have substantially different gains for positive and negative output voltages and are difficult to compensate within a feedback loop.
		In a preferred embodiment of the amplifier circuit, a control loop is implemented using the gate-to-source voltage of one of the pair of output devices, the gate-to-source voltage of a device that replicates the gate-to-source voltage of the other output device, and multiple diodes used to provide a bias voltage.
		The sole figure of the drawing is a schematic diagram of an exemplary amplifier incorporating a Class AB GaAs output circuit according to the present invention.
	www.freepatentsonline.com /4970471.html (2518 words)

	The Class-A Amplifier Site - JLH Class-AB Amplifier
		Linsley Hood is quite correct when he states that the operation of transistor output stages in class AB can cause increased distortion, because of the change in the slope of the transfer characteristic around the crossover point.
		I also believe that it is advantageous for the amplifier to operate in true class A bias conditions for normal listening power levels, in that this avoids most of the ill-effects which can arise in class B, for example due to mismatched output transistor characteristics.
		In the case of the class AB amplifier circuit, this condition is met by the 100ohm potentiometer, 400uF combination connected between the bases of the two output stage transistors, since when one of these is cut-off the other is conducting and provides the necessary base-to-emitter return path.
	www.btinternet.com /~tcaas/jlhab3.htm (3724 words)

	diyAudio Forums - Class G amps
		Class G amplifier - Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required.
		Class AB plus B amplifier - Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B. Class AB amplifier - Class AB operation allows both devices to be on at the same time (like in class A), but just barely.
		Class A amplifier - Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times.
	www.diyaudio.com /forums/showthread.php?postid=79761 (1218 words)

	Amplifier classes
		Class B suffers from a fundamental problem in that the push-pull amplifier does not, in practice, move smoothly from one half of the waveform to the other half.
		Class AB amplifiers can suffer from this also, if the bias current is too low.
		This is not as efficient as the class B design which only draws current when it needs to, or the AB which is a hybrid somewhere between the two...
	www.duncanamps.com /technical/ampclasses.html (1185 words)

	Is the Vox AC-30 really class A? (Site not responding. Last check: 2007-10-12)
		If the amplifier is designed such that the plate current flows for more than half, but appreciably less than a full cycle, it is then a class AB amplifier.
		In a class AB amplifier each tube amplifies more than half the signal, in order to reduce the distortion that occurs at the zero crossings of the waveform, which is called "crossover distortion".
		If this were a true class A amplifier, the two output phase currents (the green and purple traces) would be centered around the quiescent point (the red trace) and they would just hit the zero current axis at the same time the output starts to clip.
	www.aikenamps.com /VoxAC30classA_2.html (1528 words)

	The Class-A Amplifier Site - JLH Class-AB Amplifier
		The topology is very similar to the Class-A circuit with the exception that the output stage operates in push-pull, therefore the amplifier can continue to deliver the necessary power (within the limits of the power supply and output transistors) when the load requires currents above the Class-A bias level.
		The article gives an insight into JLH’s thought process when designing amplifiers and further confirms that his designs are based on both subjective listening and objective measurement (contrary to some of the suggestions that I have seen).
		The amplifier can be biased into full Class-A operation and, as with the 1969 Class-A amplifier, this circuit could be modified to operate from dual supply rails.
	www.tcaas.btinternet.co.uk /jlhab.htm (231 words)

	Electronic amplifier - Wikipedia, the free encyclopedia
		Amplifier design is always a compromise of numerous factors, such as cost, amount of power consumed, devices that have real-world imperfections, and the need to match the amplifier to the input signal as well as the output load.
		Class D amplifiers were 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.
		Class D and Class E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols, but a Class D amplifier merely converts an input waveform into a continuously pulse-width modulated (square wave) analog signal.
	en.wikipedia.org /wiki/Class_AB_amplifier (5484 words)

	EDN Access -- 05.25.95 Class D amplifiers provide high efficiency for audio systems
		Such amplifiers are class D switching amplifiers, which are analogous to conventional class A, B, AB, and C circuits, in which each switching element is in its linear mode of conduction for a large percentage of each cycle of an audio input signal.
		Early attempts to develop switching amplifiers with vacuum tubes were limited by the tubes' large voltage drops and low current capabilities, which reduced the amplifiers' efficiencies and limited their output power.
		The half-bridge class D amplifier using constant-frequency, duty-cycle modulation (Fig 2), sums the square-wave output of the switching power transistors with the audio input to provide negative feedback.
	www.edn.com /archives/1995/052595/11df3.htm (2265 words)

	Audio DesignLine \| Design and analysis of a basic class D amplifier
		Class AB amplifiers are very similar to class B amplifiers, but their performance is improved by the addition of two diodes that eliminate the crossover region and allows both transistors to be turned on at the same time.
		Class D amplifiers are not new, but recent advances in the technology of semi-conductor devices have regenerated interest in developing them.
		Class D amplifiers consist mainly of 3 stages: the input switching stage, the power amplification stage, and the output filter stage.
	www.audiodesignline.com /howto/175006800 (4381 words)

	The Class-A Amplifier Site - JLH Class-AB Amplifier
		This arrangement of the output transistors was of particular interest to the author, since the first three stages of such an amplifier could be substantially the same as those used in the previously described class A design, of which the performance was known.
		In fact, the system could be constructed on the basis of the class A design, with the quiescent current reduced to a much lower level, and a pair of suitably biased back-to-back emitter followers interposed between the output and the loudspeaker load.
		Also, since the last voltage amplifier stage is not required to deliver significant power, it can be optimised for voltage gain, with an increase in the available negative feedback.
	www.btinternet.com /~tcaas/jlhab2.htm (2366 words)

	Tech Tips: Class A vs. Class AB guitar amps. \| Sweetwater.com
		In a Class A circuit, a positive voltage is applied to the Grid, which controls the flow of electrons.
		In a Class "AB" design a negative "bias" voltage is applied to the grid, which will cause the tube to "shut off" when the audio waveform is below a certain point.
		- Not as "responsive" as a Class "A" amp.
	www.sweetwater.com /expert-center/techtips/d--01/12/2005 (719 words)

	Class AB Operation
		A simple class AB amplifier is shown in figure 1-5.
		Class AB amplifiers have better efficiency and poorer fidelity than class A amplifiers.
		Class AB amplifiers are usually defined as amplifiers operating between class A and class B because
	www.tpub.com /content/neets/14180/css/14180_19.htm (347 words)

	Elysian Audio: Rapture Audio System - Bass Amplifier (Site not responding. Last check: 2007-10-12)
		A standard Class AB Amplifier is one of the most general.
		Class D, although sometimes termed as a digital amplifier, is not digital but rather pulse width modulation (PWM).
		Despite any efforts to make the amplifier and signal chain as pure as possible, it is the job of the bass driver itself to reproduce the signal as precisely as possible.
	www-personal.engin.umich.edu /~jglettle/section/rapture/bassamp.html (600 words)

	Sunfire Power Amplifier
		Bipolar transistors are used in the amplifier section, and these solid state devices are known for their ability to drive low impedance speakers, but unless the power supply can deliver the necessary current and voltage, this capability will not be realized.
		In a class A amplifier, at idle, four times the maximum available output is constantly being dissipated as heat.
		This is why high power class AB amplifiers and even modestly powered class A amplifiers often have massive heat sinks (fins) on the sides of the amplifier.
	www.mindspring.com /~jonfoulkes/SunfireAmpReview.html (1367 words)

	[No title] (Site not responding. Last check: 2007-10-12)
		For Class-AB amplifiers, beyond a few watts of output power, some type of heat sink is required, with its associated cost and increased space requirements.
		While the 45% efficiency of the Class-D amplifier may seem low, again keep in mind that the test signal is a music clip with a 16-dB crest factor.
		Other applications for Class-D amplifiers are in thermally-limited equipment, where the Class-D's high efficiency allows the heat sink to be eliminated or, with the same heat sink, a higher output power can be obtained.
	archive.chipcenter.com /analog/c018.htm (1275 words)

	Audio DesignLine \| Designing Class-D Audio Amplifiers into Portable Applications
		The higher the efficiency of the amplifier the less power is wasted as heat from the battery.
		While selection of an audio amplifier is very important to the design of your circuit, the layout of the circuit on the PC board can determine the success or failure of the circuit to operate according to specs.
		Routing the input traces is important since any noise that is coupled onto the inputs is amplified by the amplifier gain and may be heard as undesirable distortion or noise.
	www.audiodesignline.com /showArticle.jhtml?articleID=189602183 (2153 words)

	The GEM: Class-A//AB Amplifier
		Being class-A there are no output stage conduction crossovers, and, if properly constructed there is no need for the stabilisation components or the series output choke that so often introduce NFB control delay to a real world amplifier's output terminals when dynamic loudspeakers are being driven.
		The JLH not only amplifies percussion transients and spoken sibilants cleanly, it is also silent behind voices and notes, such that an artificial brightness or smear does not affect the reproduction of detail, and thus its output is instantly recognisable as being correct.
		Generally though the resulting amplifier was not temperature stable through different audio duty cycles; or their biasing arrangements had an audible impact upon transients; or they were unacceptably complex.
	www.zen22142.zen.co.uk /Circuits/Audio/gem100.htm (3268 words)

	Stereo-amp design team exterminates noise sources - 5/22/2006 - EDN (Site not responding. Last check: 2007-10-12)
		Conventional wisdom might say that the introduction of Class-D amplifiers eliminated a lot of this concern by providing output stages that are substantially more efficient at moderate to high drive levels than traditional Class AB amps.
		So if the amplifier isn't driving a lot of power into the load a lot of the time, the impact on battery life may be small.
		One is concern over EMI: the design team may worry that the Class D amplifier—which after all is a source of high-voltage pulses—will create too much electrical noise, or the designers may feel ill-equipped to measure and model the problem.
	www.edn.com /article/CA6336585.html?ref=nbebn (1118 words)

	Stereo Filter-Free Class-D Audio Amplifier with Stereo Class-AB Headphone Drive - TPA2000D4 - TI Product Folder
		The amplifier uses TI’s second-generation modulation technique, which results in improved efficiency and SNR, and also allows the device to be connected directly to the speaker without the use of the LC output filter commonly associated with class-D amplifiers (this will result in an EMI which must be shielded at the system level).
		The headphone amplifier is a stereo single-ended (SE) class-AB amplifier which requires two external resistors per channel to set the gain.
		The MODE pin selects which amplifier is active; the unused amplifier is placed in shutdown to reduce supply current.
	focus.ti.com /docs/prod/folders/print/tpa2000d4.html (612 words)

	Westek Electronics - RF class AB power amplifier
		THE new Ophir model 5074 RF class AB power amplifier from Westek Electronics has a bandwidth from 30 to 512MHz, covering the military VHF/UHF bands, and is suited to applications including EW and satellite communications.
		Alternatively, the amplifier is available in module form (model 5303074) suitable for cold-plate mounting and requiring only a single 28 volt dc power supply.
		The Ophir amplifiers are suitable for operation in ambient temperatures from 0°C to +50°C, up to 95% RH, and to altitudes of 3000 metres.
	www.ferret.com.au /articles/94/0c016694.asp (225 words)

	Class AB Audio Amplifier measures 1.25 x 1.25 x 0.4 mm., National Semiconductor Cor
		The company's line of Boomer audio amplifiers enhances and enriches the entire range of sound -- from voice, ring sounds and music -- that can be amplified in a mobile phone.
		National manufactures Class AB and Class D mono and stereo speaker amplifiers, mono and stereo headphone amplifiers, and audio subsystems for desktop and notebook computers, multimedia displays and portable electronic devices.
		Mar 6, 2006 - Class D Amplifiers are designed for car audio.
	news.thomasnet.com /fullstory/482783 (1108 words)

	THD Flexi
		While Class-A amplifiers do have a strong following due to their smooth, even character, there are many players who need, or even crave the immediacy, clarity and “punch” of a grid-biased class-AB amplifier.
		We hear every day from guitarists who purchased 3 and 4 channel guitar amplifiers, only to discover that, aside from varying gain levels, the channels all sound and, equally importantly, feel more or less the same, dashing their dreams of flexibility to pieces.
		First off, it is a single-channel amplifier, but one that can be switched and blended among a number of different sounding and different feeling voices.
	www.stevesmusiccenter.com /THDFlexi.html (977 words)

	Amplifier Class-XD™
		Back to the catalog, on page 60, I found a new integrated amplifier from Cambridge Audio, the Azur 840A, a $1,375, 120W effort that “Redefines Affordable Hi-End” by employing a new amplifier class of output stage mode—class XD™.
		The whitepaper doesn’t show the actual amplifier schematic and I haven’t been able to locate a copy of their UK Patent (number GB 0505024.0), so I don’t know how this sliding current trick is accomplished.
		The MAX9756 is a 2.3W amplifier that includes an ALC (automatic level control) in its small package.
	www.tubecad.com /2006/12/blog0088.htm (1829 words)

	The Tube CAD Journal: SS Variation
		It is an old trick to add pull-down resistors to IC Op-Amp outputs, which forces the Op-Amp's normally Class-B output stage to work in Class-A by drawing extra current through its top transistor and thus improving its sonic characteristics.
		This extra current draw would have to be matched by the amplifier's top output devices and it would also force the amplifier to cutoff the conduction of its bottom devices as it shifts its driver's DC output voltage higher to increase the top devices' conduction.
		This would make the ultimate accessory for our solid-state brethren, as it would force the amplifier to work in single-ended Class-A mode up to 4 watts of output, after which the amplifier would return to push-pull operation.
	www.tubecad.com /articles_2002/Mixed_Amplifiers/page7.html (240 words)

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