The Designer's Guide Community Forum - pole zero doublet in regulator

raja.cedt Community Fellow Offline Posts: 525	pole zero doublet in regulator Jan 23^rd, 2010, 1:03am hi, I am designing a simple regulator with nmos input opamp and pmos pass transistor o/p stage. I am getting a pole zero doublet near 100hz.I am attaching both ckt as well as bode plot,so please let me know your thoughts. Thanks, Rajasekhar.
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raja.cedt Community Fellow Offline Posts: 525	Re: pole zero doublet in regulator Reply #1 - Jan 23^rd, 2010, 1:07am here is the plot.
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thechopper Community Fellow Offline Posts: 424	Re: pole zero doublet in regulator Reply #2 - Jan 25^th, 2010, 4:19pm Hi Rajasekhar, What is your regulated voltage level? Is your pass transistor working in triode mode? One possibility is the capacitance difference between the two branches of the differential pair helping create such doublet. Best Tosei
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raja.cedt

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Re: pole zero doublet in regulator
Reply #3 - Jan 25^th, 2010, 8:40pm

hi tosei,
tx for your reply. This regulator has designed for 1.2 with .8 reference voltage. You mean the cap at the drains of the diff pair transistors? i didn't understand how they will give,in fact i did many regulators of this sort i didn't see any doublet. What i am thinking is at the first stage o/p i have kept rc seris compensation network, parallel to that pass transistor Cgs is there, i feel this may be the culprit. any how please explain this.

Thanks,
rajasekhar.

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vivkr Community Fellow Offline Posts: 711	Re: pole zero doublet in regulator Reply #4 - Jan 25^th, 2010, 11:01pm you've got quite a lot of RC branches in your schematic. Perhaps it would help people answer your question if you specify what these R and C values are. And maybe it would help if we could see the ratio of transistor sizes.
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raja.cedt Community Fellow Offline Posts: 525	Re: pole zero doublet in regulator Reply #5 - Jan 27^th, 2010, 5:56am hi vivkr, sorry for that confusing schematic, the RC what i am talking is at the o/p of the 1st stage. Thanks, Rajasekhar.
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yvkrishna Community Member Offline Posts: 42	Re: pole zero doublet in regulator Reply #6 - Jan 30^th, 2010, 11:05pm hi raja, i too feel that your RC n/w at the o/p of first stage is causing this. what are the values used here? btw how would that series RC branch compensate your loop ? regards, vamshi
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raja.cedt Community Fellow Offline Posts: 525	Re: pole zero doublet in regulator Reply #7 - Jan 31^st, 2010, 1:30am hi vamshi, i have introduced zero at the 2nd pole to compensate the loop..i am not getting what you are asking? Thank, Rajasekhar.
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Berti Community Fellow Offline Posts: 337 Switzerland	Re: pole zero doublet in regulator Reply #8 - Feb 4^th, 2010, 12:13am Rajasekhar, Quote: am getting a pole zero doublet near 100hz The doublet is at 100Mhz not 100hz ???
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raja.cedt Community Fellow Offline Posts: 525	Re: pole zero doublet in regulator Reply #9 - Feb 5^th, 2010, 11:03pm hi, how it won't be at around 100hz,there only i saw some dip in the phase plot. Thanks, rajasekhar.
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Berti Community Fellow Offline Posts: 337 Switzerland	Re: pole zero doublet in regulator Reply #10 - Feb 8^th, 2010, 3:44am Rajasekhar, 100hz is just a very low frequency. Therefore without studying you schematic I cannot see how such a low pole should come from the amplifier as Vamishi suggests. I think it has to be related with the large load capacitance ...?... Cheers
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thechopper

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Re: pole zero doublet in regulator
Reply #11 - Feb 10^th, 2010, 6:32pm

raja.cedt wrote on Jan 25^th, 2010, 8:40pm:

hi tosei,
tx for your reply. This regulator has designed for 1.2 with .8 reference voltage. You mean the cap at the drains of the diff pair transistors? i didn't understand how they will give,in fact i did many regulators of this sort i didn't see any doublet. What i am thinking is at the first stage o/p i have kept rc seris compensation network, parallel to that pass transistor Cgs is there, i feel this may be the culprit. any how please explain this.

Thanks,
rajasekhar.

Hi Rajasekhar,

Take a look at this one yourself asked about...

http://www.designers-guide.org/Forum/YaBB.pl?num=1235622483

Best
Tosei

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