Capacitor in parallel with negative feedback resistor - diyAudio
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Old 16th April 2011, 08:15 PM   #1
Tyimo is offline Tyimo  Hungary
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Default Capacitor in parallel with negative feedback resistor

Hi!

Is there any rule how to use it and how to calculate this capacitor's value?

Greets:
Tyimo
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Old 16th April 2011, 08:21 PM   #2
DF96 is offline DF96  England
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This capacitor usually has two functions:
1. set the closed-loop HF roll-off
2. add some phase advance to aid loop stability

The first one is easy to calculate, as it is a normal CR response (near enough). The second depends on several other things, including stray capacitances so difficult to calculate.

Some people use a third option:
3. adjust to get a 'good' square wave response
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Old 16th April 2011, 08:48 PM   #3
Tyimo is offline Tyimo  Hungary
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Thanks!
Could you give me the equation of the closed-loop HF roll-off?

Quote:
The first one is easy to calculate, as it is a normal CR response (near enough).
Do you mean this?: 1/(C*R*2PI) = )=-3dB rolloff point

Last edited by Tyimo; 16th April 2011 at 08:53 PM.
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Old 16th April 2011, 09:17 PM   #4
jcx is offline jcx  United States
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figuring out where to put the corner frequency is a little more complex

Nyquist, Bode worked out the theory ~70 years ago
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Old 16th April 2011, 09:33 PM   #5
DF96 is offline DF96  England
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Yes, the equation is easy - deciding what to put in it is the hard part! I assume the OP is not familiar with loop stability theory?
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