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Old 12th September 2013, 05:42 AM   #361
The Dastardly Dad of Three
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yes, all at 1A bias.

Last edited by kasey197; 12th September 2013 at 05:58 AM. Reason: 1a
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Old 12th September 2013, 09:34 AM   #362
DF96 is offline DF96  England
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Ignore higher harmonics for a while, as they are all very small and are probably just FFT artifacts.

Looking at the lower harmonics there are two things going on:
1. slight rise in third order, exactly as expected, due to re-entrant distortion with the dominant device second-order. Small amounts of degeneration can do this.
2. signficant rise in second-order for the complementary stage, which tells us that the degeneration is unbalancing the P-P stage.
Rather surprising. Are you sure there is no peak clipping? Like all feedback, degeneration can sharpen the transition to clipping. Could it be connected with the different ratio of AC signal to DC bias?
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Old 12th September 2013, 09:46 AM   #363
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I seem to remember that Douglas Self has written about this in one of his books.
I will try to find a ref.

jan
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Old 12th September 2013, 10:38 AM   #364
DF96 is offline DF96  England
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Could it be crossover distortion? Degeneration may improve high current performance, where the resistor is large compared with 1/gm, but it will have little effect at low currents. I seem to recall that, unlike the BJT CFP, there is no 'correct' quiescent bias for an FET output.
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Old 12th September 2013, 11:14 AM   #365
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Just speculating but for my money I think your initial insight is the correct one.

For a single ended stage, the addition of (even) small amounts of degeneration brings down total THD. This part is not speculation ...

But this reduction seems due mainly due to a big fall in even-harmonics vs perhaps a small rise in odd-harmonics for this small degeneration case (as you mention - H3 re-enterant distortion.)

So, in a nutshell, (small) degeneration of a SE fet gives us a fall in H2 that swamps the rise in H3 resulting in overall lower THD.

However, if you slap two SE stages together in PP fashion, there is little H2 to begin with even before one adds degeneration. So then WITH degeneration, there is an insufficient fall in H2 to counteract the rise in H3.

So addition of degeneration causes, counter-intuitively, a rise in THD. But this is just due to inherent H2 cancellation in a PP fet op stage. Not sure if this makes sense to anyone or maybe i'm just convincing myself with a spurious explanation

Well thats what I'm telling myself anyway.

I'll try and do some sims to see if that is what it is.

BTW is it just me or are the linearity of these fet op stages pretty amazing... i mean if real life comes close to sims it would suggest one can get less than .5% thd at 14 watts open-loop (no degen/no fb bla bla) without any exotic parts

Last edited by kasey197; 12th September 2013 at 11:36 AM. Reason: lots of qualifiers in brackets
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Old 13th September 2013, 06:58 AM   #366
The Dastardly Dad of Three
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I would be cautious about generalising the results to other devices
but for mosfets I did some more simulations and the above conclusion looks correct...
Thanks DF96!
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Old 13th September 2013, 08:59 AM   #367
DF96 is offline DF96  England
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You're welcome.

As I often say, the right amount of feedback is either zero or enough.
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Old 13th September 2013, 10:48 AM   #368
The Dastardly Dad of Three
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Heh this reminds me of something one of my teachers used to say a long time ago.
When you assume you make an
*** of
U and
ME


For example,

Since degeneration of a Single mosfet gave lower distortion, one makes the assumption that degeneration of two mosfets in a push pull would also give lower distortion. Turns out that that is not the case.

Similarly, in one part of the BJT vs Mosfet debate, a SE BJT gives lower THD compared to a SE mosfet. We assume therefore that a push-pull stage made of BJTs will be more linear than a PP stage made from mosfets.....

Last edited by kasey197; 13th September 2013 at 10:51 AM.
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Old 13th September 2013, 12:41 PM   #369
DF96 is offline DF96  England
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Yes. People who 'design' by fashion statement (e.g. device X needs to run at Y mA current) will keep coming up against problems caused by reality. A little understanding helps; more understanding is even better.
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