What role does Rv play in this AB class amplifier?
Best Answer
This is a class B amplifier: -
Your circuit is a class AB amplifier: -
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: -
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).
No, class J has a single transistor, just as class B amplifier + crazy RF magic to increase efficiency. This paper is pretty clear about it:
A class-J amplifier is biased as a conventional “deep” class-AB PA
that will be considered hereafter in the analytical development as
class-B. Thus, a class-J PA demonstrates very similar drain current
waveform to its class-B counterpart.
Judging from the single diagram you post, run away from those web pages.
It seems they've managed to make a simple concept difficult to understand, and got it wrong in the process. What it appears that diagram is trying to show is how one of the transistors operates. However, what they are showing is class B operation, not class AB.
In class B, each transistor only conducts for exactly half the cycle, as shown. In class AB, there is a little crossover between the two transistors. Exactly in the middle, they are both conducting some. In the diagram you show, point Q should really be moved to the left a little, and the "output signal" biased up a little.
Added
Yes, your new diagram now shows true class AB behavior.
Best Answer
This is a class B amplifier: -
Your circuit is a class AB amplifier: -
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: -
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).