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lt cdr data 22nd May 2005 07:40 AM

how many ways to stablise?
is it just....

cap around VAS collector -base

cap from output of VAS to ground

cap from output of VAS to -ve of input stage?

That's all I have really seen, but I have also seen stability caps around the output stage transistors, too!!


AndrewT 22nd May 2005 08:22 AM

Read Dr Cherry, for ideas on differentiated loops.
For good sound most designers avoid miller compensation (VAS cb)
Also JLH shows; LTP cap bypass on collector load, LTP c -c cap bypass, LTP e-e cap bypass, output Thiel network, VAS to -ve input CR //C
Many show a //cap across the global NFB resistor.
Leach & many others use a very small miller comp, I suspect to minimise its effect on sound and also to boost speed.

Jontie 23rd May 2005 07:32 AM


A capacitor from the VAS collector to ground is not a good idea. It needs to be at least 10 nF, as opposed to the 50-200pF required when used collector-to-base (Miller compensation). The VAS and the LTP then need to run at a much higher current in order to drive this larger capacitance. The late Peter Baxandall said that this compensation scheme was "In all respects, sub-optimal";)


lt cdr data 23rd May 2005 10:49 AM

well people say miller comp. is sub optimal, so I suppose all methods are, what matters is how it sounds and I have heard a good amp using it.

AndrewT 23rd May 2005 07:30 PM

and not just PJ Baxandall.
I believe the main reason that D. Self's designs are supposedly poor in the sound quality stakes is due to almost total reliance on Miller Comp. He for one and many in the "kit" market adopt Miller Comp because it is foolproof in achieving stability.

amplifierguru 24th May 2005 12:46 AM

Couldn't have said it better myself, Andrew T!

Often, if you're stuck with a Vas pole (after all it's a high gain stage) it's reasonable to use a small Miller C to 'define' it ,say 10pF and then use feedback comp and/or other pole zero cancellation. A fully complementary Vas can drive a fairly large C to ground without significant distortion AND it doesn't degrade PSRR.

Workhorse 24th May 2005 02:13 AM

We always implement a compensation cap from Collector of VAS to Base of Inverting input Transistor of differential pair, this give some what less phase shift at high frequencies and boosts the slew rate too.......

amplifierguru 24th May 2005 02:24 AM

Yes kanwar, this is a reliable strategy with low impact on slewing rate and THD. I used this in my PA amp range of the '70's! The downside is that it bypasses the output stage so reduces loop NFB around this most distorting stage.

AndrewT 24th May 2005 06:38 AM

yes Amplifier guru, all the more reason to look to nested loops.

Any design details coming forward?

amplifierguru 24th May 2005 06:54 AM

Hi Andrew T.

Yes certainly the nested approach is all about juggling poles and zeros - but, of course, we need them to be stable and defined and they in turn need to ensure their negative impact on their local block is minimal in terms of PSRR, THD, slewing rate, ....

Yes I would like to do that but I am also of the view that the forum's strength is it's mulling of ideas rather than offering up complete solutions. I'm still thinking this one out.


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