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Old 22nd January 2013, 11:22 AM   #3781
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Quote:
Originally Posted by guitar89 View Post
my star ground plan is slightly different
Please, take your output ground at the star point.
Are all resistances non inductive ?
If you do not have an oscillo to see what happen exactly:
You can try to add an input filter at 200khz, can only have benefits.
If it does not solve the problem, try to add 20pf in parallel with the feedback resistances.
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Old 22nd January 2013, 11:38 AM   #3782
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Quote:
Originally Posted by Esperado View Post
Please, take your output ground at the star point.
Are all resistances non inductive ?
If you do not have an oscillo to see what happen exactly:
You can try to add an input filter at 200khz, can only have benefits.
If it does not solve the problem, try to add 20pf in parallel with the feedback resistances.
So where should my output ground takes from ? according to my layout ?
What do you mean by inductive resistance ?

I don't quite like input filter since they tends to degrade the input (more parts integrated to modify the input) , but I will give it a try. So it is -3db at 200khz ?

Ohhh yeah, never though of that idea. Anyway, does adding that cap parallel to feedback resistor have any drawbacks ? Will try it tmrw !
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Old 22nd January 2013, 11:57 AM   #3783
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Ohhh yeah, never though of that idea. Anyway, does adding that cap parallel to feedback resistor have any drawbacks ? Will try it tmrw !
I would not try this, since CFB resistors paralleled with cap usually cause immediate oscillations. More you'll gain with correct Cdom.
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Old 22nd January 2013, 12:11 PM   #3784
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I would not try this, since CFB resistors paralleled with cap usually cause immediate oscillations. More you'll gain with correct Cdom.
Ok, get it. So VFB won't be affected by this ? Still couldn't understand why CFB resistor would trigger oscillation, maybe its why I couldn't precisely understand how CFB works
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Old 22nd January 2013, 12:21 PM   #3785
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Originally Posted by Lazy Cat View Post
I would not try this, since CFB resistors paralleled with cap usually cause immediate oscillations. More you'll gain with correct Cdom.
On my amp, at those values (10/20pf), it just dump the little overshoot on square waves. Don't you had some pf on you own boards, that you adjust with a variable cap ?

guitar89: inductive resistances are wired-wound power ones (to be avoided).
Your output grounds has to be taken on each IC board, where the power ground goes-in.
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Old 22nd January 2013, 12:31 PM   #3786
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Originally Posted by guitar89 View Post
I couldn't precisely understand how CFB works
VFB returns the output signal to an inverting device (the differential -input) to be compared with the original signal.
CFB return the output signal to the same input device used for signal. The signal is amplified in an common emitter cofiguration, while the feedback signal is entered in a common base configuration. So, the same input device is used to compare both.
As common base is low impedance, that's why it is called 'Current" feedback.
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Old 22nd January 2013, 12:38 PM   #3787
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Originally Posted by Esperado View Post
Don't you had some pf on you own boards, that you adjust with a variable cap ?
Always Cdom only, never have had any cap on current feedback resistors. Can try it by VSSA at squares loaded with capacitor, to compensate ringing.
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Old 22nd January 2013, 12:49 PM   #3788
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Originally Posted by Lazy Cat View Post
I would not try this, since CFB resistors paralleled with cap usually cause immediate oscillations. More you'll gain with correct Cdom.
Cdom is the worst compensation one can use on a CFB circuit, severly limits bandwith and the vas linearity (also causes imbalanced currents to flow through the two vas transistors and limits PSRR ) its much better using shunt compensation in combination with a little compensation accross the feedback resistor.
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Old 22nd January 2013, 12:54 PM   #3789
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how to implement shunt compensation..???
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Old 22nd January 2013, 01:25 PM   #3790
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Originally Posted by homemodder View Post
Cdom is the worst compensation one can use on a CFB circuit, severly limits bandwith and the vas linearity (also causes imbalanced currents to flow through the two vas transistors and limits PSRR ) its much better using shunt compensation in combination with a little compensation accross the feedback resistor.
It is unconditionaly stable with Cdom=12 pF only, squares response is perfect. If you know more, you have to support your claimes with explanations. Criticism is easy in any form.
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