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Why does my opamp, configured as a buffer, oscillate?

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hitlgc

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why my op amp oscillate?

Hi all,

I designed a folded cascode op amp with a class-AB output stage to drive a large capacitor. When I configure it as a buffer, it oscillates.

I did the .pz analysis, and find out that the frist pole is +16.3369MHz.

What should I do?

Thank you for you help!
 

Re: why my op amp oscillate?

I don't see any results for ur circuit, but try to check the phase margin of ur OP-AMP, as it may need a sort of compensation
 

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    hitlgc

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Re: why my op amp oscillate?

eng_Semi said:
I don't see any results for ur circuit, but try to check the phase margin of ur OP-AMP, as it may need a sort of compensation
Click to expand...


the phase margin increases at 10MHz, reaches 60 degree at 40MHz, and then fall down.

I add a capacitor between the input and output of the second stage, but it doesn't work.
 

Re: why my op amp oscillate?

A schematic diagram of your circuit incl parts values would be helpful. Then we all can take a close look in order to help you.
 

    H

    hitlgc

    Points: 2
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Re: why my op amp oscillate?

VSWR said:
A schematic diagram of your circuit incl parts values would be helpful. Then we all can take a close look in order to help you.
Click to expand...

I don't know how to display the pictures directly, so I have to upload the files.

I modified the circuit after I obtain this result, the result seems better, but the problem also exists.
 

Re: why my op amp oscillate?

Look in THE ART OF ELECTRONICS on pag 242 until 248. In forum has the book for download ... the circuit when you work with a buffer is very nerly of 180º margin, if you add a RC network on output you elimine the internal frequency compesation of aomp. Then aomp osc... you have to create a dominante pole for eliminate this effect.

leomecma
 

    H

    hitlgc

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Re: why my op amp oscillate?

I think there is some problem with ur circuit regarding phase margin. The phase plot which u have given is not showing dominant pole characteristic rather it is not the actual characteristic which u should get from ur design.u should keep the phase margin more than 60 degree to get the thing stabilize.have u checked ur circuit with out any output stage if yes then tell me what was the gain and phage margin u r getting from there.
 

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    hitlgc

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why my op amp oscillate?

hi all

I modified the compensation capacitor, and now the problem has been saved.
thank you for your help.
 

Re: why my op amp oscillate?

can you tell me how can i find this book?
i looked for this book in the forum,but i couldn't find it.

leomecma said:
Look in THE ART OF ELECTRONICS on pag 242 until 248. In forum has the book for download ... the circuit when you work with a buffer is very nerly of 180º margin, if you add a RC network on output you elimine the internal frequency compesation of aomp. Then aomp osc... you have to create a dominante pole for eliminate this effect.

leomecma
Click to expand...
 

    H

    hitlgc

    Points: 2
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Re: why my op amp oscillate?
 

Re: why my op amp oscillate?

Is the plot of the phase right ?? Does the y-axis represent the phase or the phase margin ??

If it represents the phase, so u have a phase margin about (180 + 60 ), i.e. PM = 240 which is not logic !!!
 

    H

    hitlgc

    Points: 2
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why my op amp oscillate?

For the plot I have given, the phase is not right.
I changed the structure of the class-AB output stage, and modified the compensation capacitor.
Now, it's ok.
 

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