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Old 6th January 2012, 05:51 PM   #131
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>Before Zobel 10nF is to much I think.
Then you have a problem.
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Old 6th January 2012, 07:14 PM   #132
dadod is offline dadod  Croatia
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Quote:
Originally Posted by Edmond Stuart View Post
>Before Zobel 10nF is to much I think.
Then you have a problem.
Maybe I was not clear when I said to much for an amp. I mean to much to expect in that place in real life(not to much only if zobel is placed close to loudspeaker), not to much for stability testing.
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Old 6th January 2012, 07:54 PM   #133
Waly is offline Waly  United Kingdom
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For whatever reasons, you seem to get better results in simulation (optimistic device models?). Do yourself a favor and repet the experiment with the amp output biased towards the rails. You may be surprised by the results.

Also do yourself a favor and determine the phase margins of the output triple. A sine transient analysis cannot tell between 60 and 5 degrees of phase margin.


Note that the Zobel and output inductors are there to protect the amp against worse case loads, think of them as safety margins. A quality amp should be stable without these.

10nF load is not that much, think of electrostatic speakers.

But then, as long as your speakers allow, I guess you can live with a marginally stable amp. Lots of people do, without even knowing about.
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Old 6th January 2012, 08:31 PM   #134
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Quote:
Originally Posted by Waly View Post
For whatever reasons, you seem to get better results in simulation (optimistic device models?). Do yourself a favor and repet the experiment with the amp output biased towards the rails. You may be surprised by the results.

Also do yourself a favor and determine the phase margins of the output triple. A sine transient analysis cannot tell between 60 and 5 degrees of phase margin.


Note that the Zobel and output inductors are there to protect the amp against worse case loads, think of them as safety margins. A quality amp should be stable without these.

10nF load is not that much, think of electrostatic speakers.

But then, as long as your speakers allow, I guess you can live with a marginally stable amp. Lots of people do, without even knowing about.

I agree with everything Walter is saying.

syn08 has done a very good research on triple EF stability problems, a good thing to read, you can find it here.

BTW: Personally I never use Zobel, output inductors nor input filter when I do stability simulations. Those has nothing to do with the "internal" stability of the amplifier.

Cheers
Stein
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Old 7th January 2012, 12:07 PM   #135
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Default Class-I

Quote:
Originally Posted by miralin View Post
Hi Edmond
[snip]
For Class-I theoretically calculated Gm is exactly equal to 1/R24 (#3 in this thread), you can verify it by simulation. [...]
Cheers Mir
Hi Mir,

However, practically, with perfectly match transistors and a highly optimized circuit, Zo still varies by about 0.5mOhm. This is of the same order as with AB2.
Regarding R24 & R25, you will need four terminal (Kelvin) high precision current sense resistors. Even 0.5% types do increase the distortion by tens of dB.

Cheers,
E.
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Old 7th January 2012, 01:07 PM   #136
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Hi Stein,

>BTW: Personally I never use Zobel, output inductors ...

Agreed.

> nor input filter when I do stability simulations....

This statement needs some explanation: not every amp likes a full power square wave, not only because one of more stages get saturated (or completely turned off), but also it over-stresses the drivers. This is most apparent in case of a MOSFET OPS (because of high input capacitances). It really makes sens to reduce the slew rate from say 500V/us to 50V/us, as you can replace the relative large (and sluggish!) driver trannies by smaller (and faster!) ones. The best way to reduce the slew rate is by means of a second order LP filter, as a 1st order is far less effective. This is because at the onset of a transition the SR is still pretty high.
Perhaps you still remember that Andy C. has already explained this on the other forum.

A better way to test an amp on stability (ringing) without an input filter was proposed by Glen K. (AFAIK). Instead of a large square wave he uses a large LF sine wave and superimposed on this a small HF square wave. With this setup you can examine the step response at various output levels -also near the supply voltage, very important!- without needlessly over-stressing the amp.

Cheers,
E.
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Last edited by Edmond Stuart; 7th January 2012 at 01:18 PM.
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Old 8th January 2012, 06:37 PM   #137
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Default gm Class-I

Quote:
Originally Posted by miralin View Post
Hi Edmond
[snip]
For Class-I theoretically calculated Gm is exactly equal to 1/R24 (#3 in this thread), you can verify it by simulation. And what about ABII?
Cheers Mir
Hi Mir,

I forgot to drop a plot, sorry. Here it is for the Class-I circuit (from page 3, without R2).
Red: emitter currents of the OP transistors.
Green: the 'internal' gm of the OPS (driver + OP trannies). Notice that the sliding bias mechanism doesn't keep it constant.
Blue: Gm of the whole amp.
Purple: Zo = 1/gm. Indeed, almost perfectly equal to R24, ~0.33 Ohms.
Cyan: Zo - 0.33. Now the true nature of this circuit is revealed: After subtraction RE from Zo, this value appears to be far from constant.
One might say that this circuit is equivalent to an amp with a large amount of NFB together with a series resistor (RE) connected to the output.
Needless to say that we shouldn't get fooled by a seemingly nice and constant Zo or gm. It's the constant value of RE that attributed to this 'constantness' (assumed that both RE's are exactly equal !).
Why is Zo equal to RE? Because the FB is taken from the emitters of the OP trannies instead of the output proper. See 2nd figure. Looks pretty insane, don't you think so?

>And what about ABII?

In AB2 the (differential) FB is taken directly from the output (as it should be). As a result, Zo is far lower. Theoretically a few uOmhs, practically around 1mOmm.

Cheers,
E.
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goed verliezen dan dooft het licht…(H.M. van Randwijk)

Last edited by Edmond Stuart; 8th January 2012 at 06:39 PM. Reason: added 'differential'
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Old 8th January 2012, 11:29 PM   #138
ontoaba is offline ontoaba  Indonesia
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Quote:
Originally Posted by Edmond Stuart View Post
....
The cross-connection reduces the feed-through of switching currents (of the sense trannies) into the main signal path. As a result, less induced distortion from these trannies. However, it has hardy any effect on the Ip*In product.
.....
E.
But there still small current continuity gap feed from input signal when crossing, you could improve it if and make cross conduction or another additional circuitry on it that able to create negative(opposite) current gap that naturally created by class B or AB circuitry. And looks like cross connection need to work in class AB to do this task.
Of course not necessary, because less than 0.005% THD couldn't be heard.
All you need to improve is stability.
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Old 8th January 2012, 11:37 PM   #139
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Default D2S

It's possible to linearize the 'internal' gm of a class-AB OPS. See:
Michael Williams, "Making a linear difference to square law fets", EW+WW, Jan. 1994, pp.82-84.
It's about the so called D2S amp*, which relies on the fact that the difference of two equal quadratic functions returns a linear function.

Cheer,
E.

*No, it doesn't run on Deuterium Sulfide.
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Last edited by Edmond Stuart; 8th January 2012 at 11:47 PM. Reason: added: 'equal'
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Old 9th January 2012, 01:15 AM   #140
ontoaba is offline ontoaba  Indonesia
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Hi, Edmond, any link? Ok I'll try with googing search.

Quote:
Originally Posted by dadod View Post
Hi Edmond,
I don't know how much capacitance is realistic to have before Zobel network but with 100nF it is not stable, with 10nF it is. Next test is with parasitic inductances(10uH I think that is realistic) and 1nF before Zobel and 2uF after and no problem there.
Damir
Use this test, instead. It is at least more realistic and easier. Both in test is using AB2.
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