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Old 25th April 2012, 10:07 PM   #121
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Quote:
Originally Posted by keantoken View Post
I'm guessing this means a heavily buffered and/or error corrected output stage? Better stability and speed?
Hi Keane,

Nope. I was referring to the front end, which has three distinct features:
1. Low subtractor error of the IPS.
2. Well defined standing current of the VASes (or TISes)
3. No 'fighting VAS' issues (they can't fight, as there's only one Cdom)
Please, see also post 116 and 120.

Cheers,
E.
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Old 25th April 2012, 10:44 PM   #122
Steven is offline Steven  Netherlands
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Quote:
BTW, do you have a link to the Lender VAS (I've never heard of it )
@steven: VAS stage in your zero-feedback design?
The most linear stage for large voltage swings - Lender's Circuit
Borbely-Lender True Symmetry

Cheers, Steven
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The Analog Art shows no sign of yielding to the Dodo's fate. The emergence and maturation of monolithic processing finesse has perhaps lagged a bit behind the growth of the Binary Business. But whereas digital precision is forever bounded by bits, there is no limit excepting Universal Hiss to the ultimate accuracy and functional variety of simple analog circuits. - Barry Gilbert, 1973
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Old 26th April 2012, 11:20 AM   #123
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Default a masterpiece of poetry

Thanks for the links Steven.
Of course, I know of this circuit, though under a different name. I thought, erroneously!, it was named after Borbely. This circuit differs in many ways from the super TIS. The NPN/PNP combination you mentioned for example, serves a different purpose. Digging further into the links, I found this schematic of the Lender VAS.
Next, I decided to have a closer look at the origin of this incorrect implementation, and stumbled on a folded cascode configuration (see below). This circuit comes more closely to the super TIS. However, the author dismissed the idea:

'Firstly, there would appear to be a lack of overall open-loop gain because the common-base stages Q4, Q5 do not give any current gain.'
It's precisely this 'lack' of gain that makes the super TIS stable! (at DC and HF)

'Secondly, there is no obvious way to apply the Miller dominant-pole compensation that is so very useful in linearizing a VAS.'
Oh really? Apparently, the author has never heard of an 'input-inclusive-compensation '.
Perhaps he is too much indoctrinated by his own 'blameless' ideas.

Then I dug a bit further and found a highly amusing discussion about IPS subtraction errors between Fred Dieckmann and another guru whose "semiconductor physics background is beyond reproach" (his words, not mine), which culminated in a masterpiece of poetry.

Once again, thank you so much for the links!

Cheers,
E.
Attached Images
File Type: png Folded cascode.png (24.8 KB, 230 views)
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Old 26th April 2012, 01:36 PM   #124
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Quote:
Originally Posted by Steven View Post
Hi Edmond,

I know Yamaha uses them in a completely different (traditional) way. It was just because Baxandall pairs are almost never used in commercial designs and some discussion arose about stability issues, that I wanted to show these applications of the Baxandall pair.
BTW Already in 2004 these stability issues were discussed on this forum, both as TIS (VAS) and as cascode stage: Baxandall Super Pair. But going through that thread I just saw that also Yamaha's use of it was already mentioned.

Cheers, Steven
The biggest problem here is the simulators, these always show baxandalls as oscillating, but in real life they can be stabilized. When you look at for example the thread you mention youll see every comment there is based on simulators, why didnt anyone actually go and build it. Does anyone really believe yamaha sell amps that are oscillating ?? I too have used traditional baxandalls with success in some circuits, theres another member here providing diy power amp schematics with baxandalls for builds, and none of his builders have mentioned any problems as yet, from what I gather 87 builds so far. Yamaha are still using them and many more recent Analog Devices opamps are full of them, look at AD quadcore technology.
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Old 26th April 2012, 02:19 PM   #125
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Default oscillations

Hi Alex,

My sims don't show any oscillations. At only one occasion -under very unrealistic conditions- I observed peaking at 20MHz. That was with an output completely left open, i.e. without any load. However, as soon as I put a small cap (22pF or so) at the output, the peaking was gone.
Nevertheless, I've blocked the pos. FB of the base currents (at UHF) by means of 470pF caps, just to be on the safe side; better safe than sorry. Most likely one may omit these caps (C10 & C11).

Cheers,
E.

PS: Did you read the poem?
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Last edited by Edmond Stuart; 26th April 2012 at 02:49 PM.
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Old 26th April 2012, 02:54 PM   #126
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Quote:
Originally Posted by Edmond Stuart View Post
Hi Alex,

My sims don't show any oscillations. At only one occasion -under very unrealistic conditions- I observed peaking at 20MHz. That was with an output completely left open, i.e. without any load. However, as soon as I put a small cap (22pF or so) at the output, the peaking was gone.
Nevertheless, I've blocked the pos. FB of the base currents (at UHF) by means of 470pF caps, just to be on the safe side; better safe than sorry. Most likely one may omit these caps (C10 & C11).

Cheers,
E.

PS: Did you read the poem?
Edmond the easiest way I found to keep from trouble was having the second transistor having much lower beta, I see you have it as well, I dont think youll have problems either although your design is a little different, I was refering to using the baxandall in traditional way.

Btw, I tried a sim of your design with just a triple outputstage, no problems with stability and 4 ppm distortion at THD20 40v p-p, some members find this dissapointing ??
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Old 26th April 2012, 03:25 PM   #127
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Hi Alex,

4ppm with TMC, I suppose, right?
A BJT- or a MOS-OPS?
As for the reported instability, I wouldn't be surprised if it was due to some error.
I'll just wait till he has nailed down the cause.

Cheers,
E.
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Old 26th April 2012, 03:34 PM   #128
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Same as post 103, except with triple EF BJT, lowest THD Ive seen from an amp with conventional outputstage.
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Old 26th April 2012, 03:46 PM   #129
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That's an amazingly good result, even better than Bob's famous HEC MOSFET amp (that's my benchmark)
The dawn a new class of power amps?
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Last edited by Edmond Stuart; 26th April 2012 at 03:54 PM.
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Old 27th April 2012, 11:32 AM   #130
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Default similar circuits

Well guys, I just got a PM from Glen calling my attention to this thread.
Basically, it covers the same idea. Thx Glen.

Cheers,
E.
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