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Old 4th March 2009, 05:48 PM   #1
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Default An attempt for clarification.

Power amplifiers are classified by the position of the operating point on the transfer characteristic of the output device.

In class B the device conducts about 50% of the cycle, commonly a pair in push-pull arrangement, both shut off when no signal is present, giving low power consumption, high efficiency (78%) and high maximum current drive capability. This operating mode, however, introduces significant time errors and distortion to the signal as it moves through the zero crossover point. Switching distortion is created by completely turned off and turned on devices. It depends on several variables, that is output topology, device speed, current, temperature and the driver`s ability to provide controlling charge. The switching speed is determined by the time required for the charge carriers to travel across the semiconductor region. Cross-conduction (simultaneous conduction) allows brief transient currents to flow. The rapid change of control voltage triggers high slew rates and peak currents, resulting in distortion. The so-called "non-switching" amplifiers maintaining a small quiescent current are considered class B. An absence of switching distortion does not mean an absence of crossover distortion.

In class AB the device conducts less than 100% but more than 50% of the cycle, the operating point is near the cut-off region. Crossover distortion is caused by nonlinear transfer and by switching effects and a nonsmooth transition between devices. Class AB is an over-biased class B, giving class A operation up to a certain power level, over that, the device begins to cut off and distortion rises. Crossover distortion implies most disagreeable odd and high order harmonics, increasing with falling signal amplitude, followed by much higher level of intermodulation products, increasing with signal complexity. An excessive amount of negative feedback is needed to reduce crossover distortion because of the poor loop gain in the area. Numerous attempts have been made to alleviate it by other correction techniques, like error feedforward and active error feedback.

In Class A the whole signal waveform is conducted by the device, therefore one device is adequate, like in the single-ended unbalanced circuit, where the signal is referred to ground and is asymmetric with reference to ground, or in a balanced circuit, two devices operating in phase opposition, where the signal is balanced to ground and is symmetric with reference to ground. At full output, the device current varies from twice the quiescent current to almost zero.
Class A means operation in a linear region, no traces after merged halves of unequal amplification, no time displacements, no phase errors and no emaciated harmonics. There`s no other way to obtain such purity in performance.
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Old 4th March 2009, 07:46 PM   #2
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Yes there is a way is called Super-A (polarization bias with variable with signal input )
The patent is JVC, if I am not mistaken Nelson Pass did experiments.

In this system the current bias changes according to the input signal amplitude. but consumes more than Class AB. I have the diagram a device marketed in my country in the 70's



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An excessive amount of negative feedback is needed to reduce crossover distortion because of the poor loop gain in the area. Numerous attempts have been made to alleviate it by other correction techniques, like error feedforward and active error feedback
No, I have seen Class AB amplifiers, Low feedback (20dB in all frequencies) and Low distortion(0,01), distortion well below its speaker
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