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Old 30th March 2012, 10:12 AM   #31
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Quote:
Originally Posted by Bonsai View Post
jcx, I am using a beta enhanced VAS that feeds a cascode. And the whole amp has TMC and it appears to be quite stable and I have done a lot of testing on the physical amp. I prefer to see TMC as 'transitioning' the output stage (triple in my case) out of the global feedback loop at HF rather than positive feedback, or are you perhaps talking about a different mechanism?

I think in a classic Darlington connection to the VAS you talk about Homeodder then Cob indeed is an issue, but the VAS feeding the cascode is a little different. Sims show a very big improvement at HF (both with and without TMC). The whole idea of fixing the collector voltages is so that changes in Vce cannot take place and changes in Vcb are minimal anyway (a few mV) However, I'd be interested to see you findings - I might be missing something here.
Agreed it would, but there is a difference though between beta enhanced and the circuit shown by godfrey. Beta enhanced means collector of first transistor is connected to either ground or opposing voltage supply. In a Darlington the two collectors are joined then fed to a cascode. For fair comparison though youd need to keep the gain the same or try a beta enhanced cascoded vas and then compare to a darlington cascoded vas. It has to be apples compaired to apples. I recall Self does talk a little about this in his amplifier book although he only touches on why beta enhancement is better than darlington. I can think that the darlington would eventually render better results at some point as some darlingtons can have hfe s of upto 40 000 however one could use super beta transistors in beta enhanced cascoded vas.
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Old 30th March 2012, 10:56 AM   #32
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Originally Posted by Walkalone View Post
What's the best Vas stage in power amp? Simple one or two transistor? Darlington? Cascode? Complementary sysmetry?...
Of course, ''the best'' are: sounding, gain, bandwidth, speed, distortion...
For me this is the best VAS:
Memory Distortion Philosophies - Part 5 : Circuits, continued

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An other VAS stage without input-LTP is to find by post #4 about
Has anybody Burmester or AVM schematics?

Last edited by tiefbassuebertr; 30th March 2012 at 10:59 AM.
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Old 30th March 2012, 11:06 AM   #33
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I was under the impression that you were opposed to high feedback amps.
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Old 30th March 2012, 12:40 PM   #34
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Godfrey, the circuits shown by you have a lot to offer in terms of performance. Worth trying.

So essentially are there no Miller compensations caps anywhere?
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Old 30th March 2012, 01:03 PM   #35
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Originally Posted by jcx View Post
It is important to keep in mind there is no single “optimum VAS” – the rest of the amplifier circuit determines if some “enhancements” have any impact or are swamped by source/load effects
[...]
Very true!

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Originally Posted by jcx View Post
I don’t think the proponents of TMC give the stability issue enough emphasis – in fact I suspect most are ignorant of the potential gotcha in TMC loop stability from the positive feedback around the output stage

TMC is potentially dangerous in inexperienced hands -
[...]
Again, very true. But TMC is no more 'dangerous' than TPC. Both affect the PM and GM to the same extent.

Cheers,
E.

PS: What's the best VAS? Mine of course.
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Last edited by Edmond Stuart; 30th March 2012 at 01:23 PM.
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Old 30th March 2012, 01:36 PM   #36
jcx is offline jcx  United States
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I'd certainly encourage anyone using TMC to try the equivalent 2-pole with matched high frequency gain, stability (not the same component values; search for user: megajocke posts in the Cordell book thread for equations)

I suspect some would reject the 2-pole comp for appearing unstable - not realizing their TMC is "the same" on stability

as long as people fully understand the stability equivalence then they can decide on the performance differences in their amp's particular circuit topology

my sims shows matching gain intercept with TMC, conventional Miller, measured in the outer, global loop gives a higher "true" gain intercept for TMC as measured around the output Q, and less phase margin: Bob Cordell's Power amplifier book - and next page

judging TMC stability from analogy with conventional Miller, or based on step response damping will be overly optimistic

Last edited by jcx; 30th March 2012 at 01:59 PM.
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Old 30th March 2012, 01:47 PM   #37
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Originally Posted by Samuel Jayaraj View Post
Godfrey, the circuits shown by you have a lot to offer in terms of performance. Worth trying.

So essentially are there no Miller compensations caps anywhere?
Compensation is just not shown, but practical circuit would require.
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Old 30th March 2012, 03:13 PM   #38
Bonsai is offline Bonsai  Taiwan
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Edmond/jcx, what cross over frequency do you normally shoot for with TMC assuming 1281/1382 output stage and an equivalent MC ULG of say 1MHz? I have tried from 500KHz throght to nealy 1MHz without too many problems. However, I will admit I always err well on the side of caution with loop gain and ULG. Interested to know your views.
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Old 30th March 2012, 03:22 PM   #39
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It is not always possible to consider the so-called VAS (Voltage Amplification Stage) as independent of the Input Stage (IS).
Impressive results have been obtained from a very simple topology where the IS also works as a VAS (see the link to Renardson's MRJ7 given by Tiefbassuebertr).
Currently, I would like to built an amp using the push-pull VAS (differential pair loaded by a current mirror) shown by Samuel Groner fig. 56, page 48 of his "Comments on Audio Power Amplifier Design Handbook by Douglas Self". There is a common mode form of feedback from the second stage to the first stage which allows equality of current in the differential input devices. This topology allows a very high PSRR which may be a feature as important as low distortion.
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Old 30th March 2012, 03:34 PM   #40
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Originally Posted by jcx View Post
[...]
“proper” cascoding requires VAS degeneration R, returning/recycling the common base Q base current – well explained by Hawksford

really high gain, linearity VAS then deserve 2-pole compensation to make use of the gain to linearize more of the amplifier – “TMC” is an option but traditional 2-pole splitting of the Miller C gives largely equivalent results and greater improvement of input diff pair linearity
[...]
I like to add that TMC gives greater improvement of the TIS (formerly called VAS). That's why TPC and TMC give largely equivalent results (sometimes TPC is slightly better, sometimes the other way around, depending on circuit details)

As for linearizing the input stage together with TMC, this topic has been covered here: (Super TIS)

Cheers,
E
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