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Nandish Mehta

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Problem with phase response of folded cascode OTA
Feb 28^th, 2011, 8:27am

Hello friends,

Can any one help me understand the phase response of my folded cascode OTA.
I am not able to understand following:
1) Why I am getting bumps in phase response at 700 MHz?
2) Is this common? (I have seen one such phase response in Ken's paper on stability). However I don't have proper understanding why does it occur.
3) Can I correct for such response? If so how can I correct it?

Marker M3 is the phase at UGB. Marker M0 shows PM at that 46 Mhz. I use this OTA to work at integrator in close loop. During the period of clock non-overlap the OTA shows ringing with frequency of 46 MHz.

Please advise me.

Regards
Nandish

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freq_response.png

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vp1953

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Re: Problem with phase response of folded cascode OTA
Reply #1 - Feb 28^th, 2011, 3:17pm

Hi Nandish,

My quick observations are that there might be a cluster of poles and zeros around that bump (unless of course there is nothing wrong with the simulation). Looking at the gain, phase plots, it looks like there is a pole a little before the bump, 2 zeros right before the bump and 2 poles at or after the bump; and there is another zero at the very end of the phase plot where the phase starts going up again

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aaron_do Senior Fellow Offline Posts: 1398	Re: Problem with phase response of folded cascode OTA Reply #2 - Feb 28^th, 2011, 4:50pm Hi, apart from the above comments, are you looking at the loop gain or opamp's open loop gain? You should be looking at the loop gain to determine stability. cheers, Aaron
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Nandish Mehta

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Re: Problem with phase response of folded cascode OTA
Reply #3 - Feb 28^th, 2011, 7:59pm

Hi Aaron,

I am looking for closed loop gain. My problem is that the OTA when put in closed loop like an integrator, it starts ringing.

That is why I want to understand the phase response of the OTA.

Surely in this case measuring phase margin will not work.

Can any body on the community suggest me how to check for the stability of the OTA?

Your inputs will be highly appreciated.

Regards
Nandish

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philcorb

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Re: Problem with phase response of folded cascode OTA
Reply #4 - Mar 1^st, 2011, 2:11am

At the ringing frequency it looks like you have approximately 110 degrees phase shift. The ota is being used as an integrator, adding another 90 degrees due to the capacitor in the feedback path. That gives roughly 180 degrees of phase shift, hence ringing/oscilation at this frequency. It appears that perhaps the feedback network is not being included when you break the loop.

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Nandish Mehta

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Re: Problem with phase response of folded cascode OTA
Reply #5 - Mar 1^st, 2011, 3:14am

Hi Philcorb,

Thanks a ton. I did'nt think about extra 90 degree from feedback ... I have now some thing to try and fix ...
thanks ...
while getting the bode plots i have not included the feed-back loop. As my integrator is switched capacitor based i don't know how to get the magnitude and phase response of it while it is operating in feed back. Thats why I was working only with open loop and trying to add good phase margin so that in feed back it works properly.

thanks again for your post.
regards
nandish

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