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Old 30th August 2011, 10:55 AM   #1
GregH2 is offline GregH2  Australia
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Default Darlington or Sziklai (compound) pair output stages?

Hi all,

Which do you prefer and why? On paper the sziklai pair is superior - lower distortion, fewer high order harmonics and better temperature stability. However, in real life the driver transistor of the sziklai pair can oscillate on switch off requiring compensation capacitors and perhaps degrading sonics.

Theory aside, what do you prefer in a practical circuit and why?

Look forward to your thoughts!
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Old 30th August 2011, 12:46 PM   #2
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Its easy enough to make a non-switching output stage that always maintains
a hundred or so milliamps in bypass of the load. CFP would never need to see
the "driver switch off" condition you so worried about.

I prefer Aleph (BJT MOSFET) or Allison (BJT BJT) super pair for my outputs.
1st BJT turned backwards (emitter coupled) as a depletion mode vreference.

Last edited by kenpeter; 30th August 2011 at 12:50 PM.
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Old 30th August 2011, 01:02 PM   #3
GregH2 is offline GregH2  Australia
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Thanks Ken,

Correct me if I'm wrong, but aren't you referring to class a output stages? I am talking about their use in class ab outputs. Guess i wasn't clear. Please explain your concept further if I have misinterpreted you as I woud like to understand.

Cheers,

Greg.
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Old 30th August 2011, 02:10 PM   #4
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I could easily make these curves cross a lot lower. if you need to
see them squashed flatter against the 200mA reserve current...

Is it AB or Aa? Hard to tell from this graph of transistor currents.
But if you were to look at my Schottky diodes, you then see AB!
Only diodes without recovery probs turn off, transistors never...

AB like this would not bother your siziklai pairs...
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Old 30th August 2011, 02:57 PM   #5
Bigun is offline Bigun  Canada
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You should read more from Doug Self on CFP output for some of the subtleties. For example, although thermal runaway is less of an issue because the drivers have firm control over the outputs, the optimal Class AB bias current (temperature dependent) needs to be controlled much more accurately than regular EF output, for minimum distortion.

And I think Doug found that low signal level distortion for CFP was worse than for EF even though large signal linearity favours the CFP. I believe the reason is that the change in gain around cross-over is more abrupt than EF.

So my take on it is use CFP for Class A only, such as the drivers to an EF stage like the Roender amplifier - or you need some kind of adaptive bias as Kenpeter shows.
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Old 30th August 2011, 03:03 PM   #6
AndrewT is offline AndrewT  Scotland
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Hi BG,
I think your interpretation of D.Self is correct.
It seems to fit with the general consensus we hear around here.
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Old 30th August 2011, 03:08 PM   #7
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CFP outputs are trickier as far the stability is concerned.They are more prone to oscillation when driving capacitive loads. With careful design of compensation circuitry this can be overcome.
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Old 30th August 2011, 05:03 PM   #8
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to the same topic also this thread exists:
Why collector/drain output stages are so rare???
by my post #59 are additional URLs for more informations.
Go also to
Abacus rieder 60-120 amplifier

Last edited by tiefbassuebertr; 30th August 2011 at 05:06 PM.
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Old 31st August 2011, 12:06 AM   #9
GregH2 is offline GregH2  Australia
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Ah thanks all,

That thread makes good reading.
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Old 31st August 2011, 05:46 AM   #10
GregH2 is offline GregH2  Australia
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Does anyone have any thoughts on which topology would be preferable for highly biased class AB, say 0.5A of bias?
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