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In Reply to: RE: Thank you, exactly. posted by morricab on February 12, 2025 at 07:08:36:
Distortion is created if the transfer function is anything other than a straight line (with slope = gain). For a push-pull output stage, if there is a little wiggle in the line around the zero crossing that creates cross-over distortion. The larger the deviation, the higher the distortion and sharper wiggles create higher frequency harmonics. Feedback can only suppress any open loop distortion by the amount of loop gain at the frequency of interest. So, higher frequency harmonics are suppressed less because amplifier loop gain is always rolling off as frequency increases, and this applies to all distortions not just cross-over.
One danger, I think, is that biasing too hard into class B could create a 'dead-zone' at the cross over point where there is a small region with no transconductance and that will give low, or no, loop gain in that region and, hence, no suppression. It would be interesting to study but I don't get much spare time and I'd rather listen!
Conversely, class AB output stage should have twice the slope in the region where both devices are conducting (class A region) and a signal transitioning between A & B regions will be distorted by that slope change and there will also be a halving of loop gain in the class B region. So open loop distortion is created by that transition and the amount of suppression is modated. This is the so-called 'gm doubling' effect that is in the literature. Most recently it was tackled by Jason Stoddard in his blog, he came up with something he calls a 'continuity bias'to get around that problem. But, IIRC, that biasing burns significant power that is still lower than staying class A all the time but much more than a traditional AB.
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Follow Ups
- RE: Thank you, exactly. - 13th Duke of Wymbourne 02/12/2512:24:48 02/12/25 (4)
- If you have a "dead zone" - Steve O 14:50:08 02/12/25 (3)
- How do you classify taxonomists? - 13th Duke of Wymbourne 11:18:59 02/13/25 (1)
- Hadn't considered that until now - Steve O 15:41:18 02/13/25 (0)
- RE: If you have a "dead zone" - morricab 06:37:17 02/13/25 (0)
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