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What role does Rv play in this AB class amplifier?

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This is a class B amplifier: -

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Your circuit is a class AB amplifier: -

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Rv adjusts the bias point of the two transistors so that T1 and T2 are always conducting a little bit of current - this avoids excessive cross over distortion: -

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See also this article, Crossover Distortion in Amplifiers, for more information.

Rv modifies the volt drop across the two series diodes. Remember that diodes are not just fixed 0.7 v devices. The forward volt drop can be adjusted so that the base-emitter junctions of each output transistor are conducting 1 mA or so, placing the transistors in a much more linear region of their characteristic at the expense of a sending DC current thru the transistors (an increase in power dissipation).

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  • \$\begingroup\$ how does Rv adjust the bias point? \$\endgroup\$ – ielyamani Oct 15 '15 at 12:26
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    \$\begingroup\$ It modifies the volt drop across the two series diodes - remember that diodes are not just fixed 0.7 v devices - the forward volt drop can be adjusted so that the base-emitter junctions of each output transistor are conducting a mA or so - this places the transistors in a much more linear region of their characteristic at the expense of a standing DC current thru the transistors i.e. an increase in power dissipation. \$\endgroup\$ – Andy aka Oct 15 '15 at 12:30
  • \$\begingroup\$ Your diagrams are misleading. For the first one, the Q point would be slightly below the X axis because of the \$V_{BE}\$ drop of the transistors. You need the "small bias" just to get to true "class B" operation. "Class AB" would be what you get with even more bias, in which both transistors are conducting some amount of current all of the time. \$\endgroup\$ – Dave Tweed Oct 15 '15 at 12:41
  • \$\begingroup\$ Are there equations I could look at? equations that show how different values of Rv change the bias point? \$\endgroup\$ – ielyamani Oct 15 '15 at 12:47
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    \$\begingroup\$ You can also think of Rv and the two Rs above and below it as a voltage divider. If taken together, they don't provide more than two diode drops across Rv, then the diodes won't conduct at all. In this mode, you can think of them as providing an upper limit for the bias. Also, the diodes should be thermally coupled to the transistors, to provide a degree of temperature compensation for the bias. \$\endgroup\$ – Dave Tweed Oct 15 '15 at 14:24

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