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Old 2nd September 2011, 12:47 PM   #231
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
I observe no such oscillations in clipping until I add your Schottky diodes.
That is correct. But replacing them with a 10p capacitor is sufficient to create oscillations.
This means that the stability is very marginal.

And if the timestep is decreased to 100ns, it oscillates with C=0.

The overall loop margin is probably much too small, and because the conduction of the zeners shifts the closed loop gain towards unity, that's sufficient to cause oscillations.
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Old 2nd September 2011, 12:58 PM   #232
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Stranger still, LTSpice doesn't complain when I give a resistor a
negative value (-4.7R) and abuse as base stoppers in the X...
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Old 2nd September 2011, 01:26 PM   #233
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Quote:
Originally Posted by kenpeter View Post
Stranger still, LTSpice doesn't complain when I give a resistor a
negative value (-4.7R) and abuse as base stoppers in the X...
Waow, that's a really weird one: and it really works, the phase of the current is reversed.
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Old 2nd September 2011, 07:23 PM   #234
Elvee is online now Elvee  Belgium
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Probably time to make some proper loop stability analysis.

A quick and dirty check shows that the margin, under quiescent and unstressed conditions is ~15°. It is a bit short, to say the least.

When you make a pact with the X-devil, you can't get away without letting go something dear in exchange for the improved linearity and loop gain

I didn't opt for a X-quad in the Circlophone, yet (or because?) I already have a 15~20yrs experience with this topology.
But when you do your homework, it can be very rewarding. And Eva's version is a good tradeoff between performance, complexity and ease of use.
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File Type: gif LoopXit.gif (51.5 KB, 416 views)
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Old 3rd September 2011, 01:10 AM   #235
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Add 47R in series with CDOM, and eliminate 33pf + 47R in the return path.
What are considered basic victory conditions for this test of stability?

I know 180 deg at 0db (unity gain) would be an obvious fail, but how
much gain we allowed at 120deg? 150?
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Old 3rd September 2011, 07:26 PM   #236
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by kenpeter View Post
Add 47R in series with CDOM, and eliminate 33pf + 47R in the return path.
What are considered basic victory conditions for this test of stability?

I know 180 deg at 0db (unity gain) would be an obvious fail, but how
much gain we allowed at 120deg? 150?
Closed loop response is not very telling.

What you really need is an open loop plot.

That said, it doesn't look bad: the amplitude has no peaking, although the slope increase at ~5Mhz could be of concern but combined with the phase response, there is no obvious danger signal.

For a quantitative evaluation, a real Bode plot is necessary.

The way I do it is simplified and unorthodox, but in this case, and if you understand what you are doing, it is more than sufficient for a quick evaluation.

Insert the stimulus at a single point of passage of the signal (ie, if it's cut there nothing can pass elsewhere), and make sure the source has a much lower impedance than the receiver, that way you can dispense with the complete transfer matrix required in a general case.

That's what I did on the pic above, in #234.

But that won't reveal anything, except marginal phase, since the analysis is made under quiescent conditions: oscillations occur only near saturation, and/or at unity gain.
To chase them and iron out any possible instability, you need to explore those conditions/combinations of conditions too.

But obviously, if stability is already marginal under gentle conditions, it will be worse under stress.
Which is why the static stability has to be addressed before anything else is attempted.
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Old 4th September 2011, 06:15 PM   #237
Bigun is offline Bigun  Canada
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sorry if this is a dumb question - but there's two very different design in this thread - the circlophone and the kenpeter design - what is the connection - the Kenpeter design looks unrelated to the circlophone ????
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Old 4th September 2011, 10:25 PM   #238
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What difference? Maybe I'm not seeing it?
Both have secondary feedback in quadrature
to the noble loop. And for the same reasons...

Maybe you are confused cause Elvee's peasant
loop senses current from the outsides. And my
example senses this same current from within.
Otherwise same exact thing, completely different.

Besides, its all the art of rounding corners of the
schematic. Them hard simulated turns, all fun and
games till someone puts out an electron! Running
in the hallway with scissors, xquads or whatever...

Earlier in this thread, my peasant loop returned
this feedback all the way back to the LTP's tail,
exactly same as Elvee's. But I later found that a
much shorter peasant loop functions just as well.

Last edited by kenpeter; 4th September 2011 at 10:46 PM.
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Old 4th September 2011, 11:27 PM   #239
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+/-25V rails, check!
2n3055 finals, check!
2n3019 VAS, check!
10K+470R Input and GNF Z, check!
22x voltage gain, check!
Peasant loop in quadrature, check!
Active tail modullation, check!
No transistor ever turns off, check!
Nice round corners, check!
PNP LTP, check! Fell off the BC556B
wagon somewhere, but easily fixed...

No similarity whatsoever...

Last edited by kenpeter; 4th September 2011 at 11:36 PM.
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Old 5th September 2011, 02:51 AM   #240
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by kenpeter View Post
What difference? Maybe I'm not seeing it?
Both have secondary feedback in quadrature
to the noble loop. And for the same reasons...
that's great - because it looks to be less complex. If I understand then, the front end no longer controls the sliding bias, it's function is purely to be a voltage error amplifier.
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