Capacitor in parallel with negative feedback resistor

Tyimo · 16th April 2011, 07:15 PM

Hi!

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

Greets:
Tyimo

DF96 · 16th April 2011, 07:21 PM

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

Tyimo · 07:53 PM

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

jcx · 16th April 2011, 08:17 PM

figuring out where to put the corner frequency is a little more complex

Nyquist, Bode worked out the theory ~70 years ago

DF96 · 16th April 2011, 08:33 PM

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?

16th April 2011, 07:15 PM	#1
Tyimo diyAudio Member Join Date: Nov 2002 Location: Hungary	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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16th April 2011, 07:21 PM	#2
DF96 diyAudio Member Join Date: May 2007	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

16th April 2011, 08:17 PM	#4
jcx diyAudio Member Join Date: Feb 2003 Location: ..	figuring out where to put the corner frequency is a little more complex Nyquist, Bode worked out the theory ~70 years ago

16th April 2011, 08:33 PM	#5
DF96 diyAudio Member Join Date: May 2007	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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