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Old 1st April 2012, 09:04 PM   #2311
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Quote:
Originally Posted by sonnya View Post
The use of tracking cascode should be done with care as i have seen oscillations just above or in conjuction with 180 degree phase shift.

But if remember right, then there is a lot of measurements/simulation of different type of VAS stage floating around in here done by a french guy 7 - 8 years ago.
Peufeu memory distortion plus magic resistor?
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Old 1st April 2012, 09:04 PM   #2312
sonnya is offline sonnya  Denmark
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Just to show the difference.
Attached Images
File Type: jpg phasetest1.jpg (155.6 KB, 429 views)
File Type: jpg phasetest2.jpg (248.1 KB, 404 views)
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Old 1st April 2012, 09:12 PM   #2313
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Just to show the difference.
OMG these two convinced me, again some work ahead hehe
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Old 1st April 2012, 09:12 PM   #2314
sonnya is offline sonnya  Denmark
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I think it was him, but there is a lot more.

1) phasemargin
2) and gain in the vas stage / or impedance.
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Old 1st April 2012, 09:17 PM   #2315
sonnya is offline sonnya  Denmark
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The first pic if a copy of Marantz HDAM buffer and the second is a slightly modified, more standard diamond buffer.

But i would allways add a little series resistor to prevent oscillations in real world, also on mosfets.
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Old 1st April 2012, 09:45 PM   #2316
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Sims are very useful tool to avoid-eliminate major flaws and helps converging to the best solution by optimizing/correcting the parameters. Still the basic ideas of topology and solution is up to a designer. In reality all well designed circuits has to be tested throughout especially on stability issues, since these are parts-layout dependable. And if you go to tracking cascodes, current sources ie. high impedance local circuits in a big scale, where oscillations and instability are just around the corner, than a designer must really have a good knowledge and a lot of practical experiences.

Quote:
Originally Posted by sonnya View Post
But i would allways add a little series resistor to prevent oscillations in real world, also on mosfets.
Sometimes practical results (measurements & sound) are better without them.

Last edited by Lazy Cat; 1st April 2012 at 09:47 PM.
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Old 1st April 2012, 09:48 PM   #2317
sonnya is offline sonnya  Denmark
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Agree!
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Old 2nd April 2012, 06:16 AM   #2318
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Now to the house keeping. I do believe that the weakest part of this fine circuit and the very reason why it does not simulate particularly well is the poor relations between the impedance of the current drive and the feedback currents. The basic amp is basically fed from a 1Kohm current source..which is way too low to prevent the feedback currents from modulating the drive-current.
Spectral analyzer shows incredibly linear and even response in spectral distribution up to 100 MHz region. Increasing the amplitude of the base signal enlarge the amplitude of all spectral lines in a linear manner, there's no step amplitude response between first few and damping of the others, like it should be. So the 1 k resistor is doing it properly, allowing the drive-supply current (which by definition should be constant) to be modulated by FB current in a very frequency-linear manner. So nothing to blame to resistor, it's doing its job just perfectly.

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This is the main source of error in the circuit and by fitting high (very high) impedance current-sources specs are terrific. I will not show my housekeeping here....But I'am quite sure you catch the direction.
Achieving high impedance current supply by cascoded CCS is not a problem, we also had CCS in the SSA sch long time ago although more or less because of their constant part of equation. OK, I have 14 V potential gap, very constant and clear TL431 power supplies and two twin parts space on PCB for each C-CCS, so all supporting role actors are there. Terrific specs are motivation, sound results end goal, will do the homework.
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Old 2nd April 2012, 08:52 AM   #2319
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So the 1 k resistor is doing it properly, allowing the drive-supply current (which by definition should be constant) to be modulated by FB current in a very frequency-linear manner.
It's not...Higher impedance M-Ohm would make it so...ideal CCS better option. difference is like bouncing a ball on a soft wooden floor and on solid concrete. One rattles the building and is felt everywhere. Both floors bounces the ball one just bounces the ball better..

Last edited by MiiB; 2nd April 2012 at 09:00 AM.
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Old 2nd April 2012, 09:34 AM   #2320
Salas is offline Salas  Greece
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Two LM329 a red LED and a BF245A G&S shorted topped with BF245C cascoding it, would beat TL431 with one hand tied at the back on high Z, noise, and PPM grounds. Provides power on indication too.
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