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Amplifier - Class-AB

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					Amplifier – Class-AB
In spite of the high distortion level of Class-B amplifier, and the high inefficiency of a
Class-A amplifier, a hybrid of the both makes a Class-AB amplifier without any of their
flaws when they function on their own.

Class-A’s advantage of being very low in distortion level is acquired by giving a
quiescent current to the output level power transistors, while at the same time providing
high power efficiency by conducting only a low current in doing so. The usual practice is
to trim the current level to the state that the power transistors are conducting current at
the level they would when they start to conduct fully for their parts in their respective half
waves.

Class-AB amplifier with trimmable quiescent current without thermal tracking:




Although it seems that a Class-AB amplifier does not produce any crossover distortion at
all, a careful adjustment at the conducting level of the power transistors is crucial. More
over, tracking the varying bias of each transistors as they vary with temperature, humidity
and time, is a challenging task.

Class-AB amplifier with trimmable quiescent current with thermal tracking:




Many designs has been proposed and implemented but the basic idea is just the same, that
is to use a thermally coupled transistor B-E junction to give the majority of bias voltage
to the B-E inputs of the output power transistors. A variable current regulated buffer is
then added to the bias transistor to provide the exact current needed to make the power
transistors conduct slightly.

				
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