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HdrChopper

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Re: How to improve phase margine for conventional folded cascode OTA
Reply #30 - Oct 18^th, 2011, 7:15am

raja.cedt wrote on Oct 18^th, 2011, 7:12am:

hello Tosei ,
your Gout is wrong seems...

a. Conductance looking down from output=gds8*gds7/gm8
b.Conducatnce looking up from output=gds5*(gds4+gds2)/gm5
so total Gout is sum of these two.

Thanks,
raj.

Raja,

Look at my Adc expression and you will see the total conductance is the same one you suggest.

Tosei

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Praveen K

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Re: How to improve phase margine for conventional folded cascode OTA
Reply #31 - Oct 18^th, 2011, 11:42pm

Hi Tosei,

can you please explain how you arrive at this Gm expression

>>>GMeq = gm2 * (gm5+gds5) / (gm5+gds2+gds4+gds5)

is it by the small signal model or some intuitive method?

hi Ashutosh,

>>>i have taken half circuit gain only....is that wrong??....how to >>>calculate fully differential gain of fully diff. amplifier??(using spectre)

I'm also learning to do this stability analysis for full diff amp. there are some discussions in this forum itself,

http://www.designers-guide.org/Forum/YaBB.pl?num=1268207804

http://www.designers-guide.org/Forum/YaBB.pl?num=1170321868

The above two talks about using cmdmprobe to do STB, there is something called diffstbprobe which is new.

http://www.cadence.com/Community/forums/t/20208.aspx

regards,
praveen

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raja.cedt

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Germany

Re: How to improve phase margine for conventional folded cascode OTA
Reply #32 - Oct 19^th, 2011, 12:39am

hello praveen,
signal current from gm2 will be dividing at drain of Q4, there you have 4 branches whose coductances are gds4,gds2,gds5 and gm2. so find the total current flowing through gds5 and gm5 (means total current through 5), hence gm2*(gds5+gm5)/(gds5+gm5+gds4+gds2)

Thanks,
raj.

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HdrChopper Community Fellow Offline Posts: 493	Re: How to improve phase margine for conventional folded cascode OTA Reply #33 - Oct 19^th, 2011, 5:25am Hi Praveen, What Raja explained is correct. It is basically the current division in the folding node into the different branches hanging from there. Best Tosei
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Ashutosh_rane

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NIT calicut

Re: How to improve phase margine for conventional folded cascode OTA
Reply #34 - Oct 19^th, 2011, 8:54am

Praveen K wrote on Oct 18^th, 2011, 11:42pm:

Hi Tosei,

can you please explain how you arrive at this Gm expression

>>>GMeq = gm2 * (gm5+gds5) / (gm5+gds2+gds4+gds5)

is it by the small signal model or some intuitive method?

hi Ashutosh,

>>>i have taken half circuit gain only....is that wrong??....how to >>>calculate fully differential gain of fully diff. amplifier??(using spectre)

I'm also learning to do this stability analysis for full diff amp. there are some discussions in this forum itself,

http://www.designers-guide.org/Forum/YaBB.pl?num=1268207804

http://www.designers-guide.org/Forum/YaBB.pl?num=1170321868

The above two talks about using cmdmprobe to do STB, there is something called diffstbprobe which is new.

http://www.cadence.com/Community/forums/t/20208.aspx

regards,
praveen

hello praveen ,
thank for ur reply, ..but
i asked about how to calculate differential gain for a fully differential opamp because when we do ac analysis we get single ended gain . but same procedure done for diff gain it gives gain in "-"dB's (not correct ... Undecided

)...so how to get the differential gain of opamp??is it just addition of two gain on single ended or single ended gain is only differential gain??

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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: How to improve phase margine for conventional folded cascode OTA Reply #35 - Oct 19^th, 2011, 8:59am hello Ashutosh_rane .ac will give gain each and every point, it is up to you what to infer. For a fully diferential amp just find voutp-voutn, or simply add 6 db for single ended gain (any how you know that both nodes swings out of phase, if CMFB is working fine) Thanks, raj.
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Ashutosh_rane Junior Member Offline Posts: 17 NIT calicut	Re: How to improve phase margine for conventional folded cascode OTA Reply #36 - Oct 19^th, 2011, 9:08am thank u raja
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Praveen K Junior Member Offline Posts: 25 Bangalore	Re: How to improve phase margine for conventional folded cascode OTA Reply #37 - Oct 19^th, 2011, 9:30pm Hi raj and tosei, thanks for the explanation on Gm. regards, praveen. K
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weber8722 Community Member Offline Posts: 95	Re: How to improve phase margine for conventional folded cascode OTA Reply #38 - Nov 17^th, 2011, 8:13am For that simple op-amp type PM is normally good. Probably your transistors are too big, i.e. too large caps compared to load cap.
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vivkr

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Posts: 780

Re: How to improve phase margine for conventional folded cascode OTA
Reply #39 - Nov 18^th, 2011, 12:02am

agree fully with this last post here. I've designed folded cascode OTAs countless number of times and never ever had to worry about the PM in the standard version. So you must be doing something weird if you are running into this problem. Probably the folding transistors are way too big and the load cap too small in comparison.

The elimination of the compensation problems is the number one selling point for folded cascode OTAs. So I would advise you to try and fix the problem.

Vivek

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RobG Community Fellow Offline Posts: 570 Bozeman, MT	Re: How to improve phase margine for conventional folded cascode OTA Reply #40 - Nov 21^st, 2011, 5:46pm I'm late to this party so I'm probably in left field, but those PMOS transistors appear to have their bulks tied to the drains, not the source. That would qualify as "something weird."
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