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urian

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CMFB of OTA
Oct 30^th, 2012, 6:39am

Hi,there.
Recently I am designing a OTA used in a front-end S/H for pipeline ADC. The OTA circuit is shown below, and I dont know how exactly the CMFB is working. The VDD is 3V and CM voltage of input and output are both 1.4v, while using SC CMFB. When simulating, I find that the output CM voltage is affected by other transistors parameters especially the M3 and M4 even with CMFB is working! In other words, the output CM voltage is hard to adjust to the desired value 1.4v, and is susceptible by other transistors with CMFB. I dont know why the CM voltage can not be controlled by CMFB. The DC gain of differential is about 60 dB and the first stage has a gain about 2, so I think the CMFB loop gain is large enough to force deep feedback.

Regards
urian

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ota.jpg

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AnalogDE Senior Member Offline Posts: 137	Re: CMFB of OTA Reply #1 - Oct 30^th, 2012, 5:06pm What does your CMFB circuit look like?
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urian Junior Member Offline Posts: 30	Re: CMFB of OTA Reply #2 - Oct 30^th, 2012, 9:23pm AnalogDE wrote on Oct 30^th, 2012, 5:06pm: What does your CMFB circuit look like? Hi,AnalogDE, the CMFB is a traditional SC CMFB.
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Dan Clement Community Member Offline Posts: 95 Salt Lake City, Utah, USA	Re: CMFB of OTA Reply #3 - Oct 31^st, 2012, 4:26am Shouldn't m3 and m4 be pmos?
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urian Junior Member Offline Posts: 30	Re: CMFB of OTA Reply #4 - Oct 31^st, 2012, 7:34am Dan Clement wrote on Oct 31^st, 2012, 4:26am: Shouldn't m3 and m4 be pmos? Hi,It's NMOS for sure.
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Dan Clement Community Member Offline Posts: 95 Salt Lake City, Utah, USA	Re: CMFB of OTA Reply #5 - Oct 31^st, 2012, 7:40pm I've never seen this topology before. From what I see I don't think there is a chance for this circuit to act as an amplifier. Can you explain how it is supposed to work? It doesn't make sense to me.
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urian Junior Member Offline Posts: 30	Re: CMFB of OTA Reply #6 - Oct 31^st, 2012, 9:11pm Hi,it's from the JSSC paper " A 10b 20M 35mW Pipeline A/D Converter" by Cho,Mar,1995
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Dan Clement

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Salt Lake City, Utah, USA

Re: CMFB of OTA
Reply #7 - Nov 1^st, 2012, 5:40am

I will have to look into that paper.

After a good night of sleep I can now see it as an amp. The gain of the first stage will be quite low.

The signal swing of the first stage will be severely limited by the cmfb as you can't pull the drains of the input transistors any higher than the cmfb node minusvavthreshold voltage.

You are probably having a startup issue where the input transistors are being clamped by the cmfb source followers or you are having a swing issue and losing gain.

You will need to level shift the cmfb to a high enough voltage to avoid this.

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urian Junior Member Offline Posts: 30	Re: CMFB of OTA Reply #8 - Nov 1^st, 2012, 9:26pm Yes, the gain of first stage is low, between 1-2, the drain of input transistor is around 800mv with the bias, but the output will change largely with SF transistors parameters.
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Lex Senior Member Offline Posts: 201 Eindhoven, Holland	Re: CMFB of OTA Reply #9 - Nov 5^th, 2012, 4:25am Are the problems gone with a higher common mode voltage? Then I guess you know the answer to your problems.
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boe Community Fellow Offline Posts: 615	Re: CMFB of OTA Reply #10 - Nov 5^th, 2012, 4:30am Lex wrote on Nov 5^th, 2012, 4:25am: Are the problems gone with a higher common mode voltage? Then I guess you know the answer to your problems. I"d try lower (input) common mode voltage... - B O E
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urian

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Re: CMFB of OTA
Reply #11 - Nov 5^th, 2012, 7:38am

Lex wrote on Nov 5^th, 2012, 4:25am:

Are the problems gone with a higher common mode voltage? Then I guess you know the answer to your problems.

Do you mean the output CM voltage? If so, I am afraid I cant afford this as the CM voltage is determined by BIAS and it tends to be smaller. Actually, I find that with this cmfb, the DC OP is very important to guarantee that the CMFB loop has enough gain.

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boe

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Re: CMFB of OTA
Reply #12 - Nov 5^th, 2012, 11:08am

urian wrote on Nov 5^th, 2012, 7:38am:

...
Do you mean the output CM voltage? If so, I am afraid I cant afford this as the CM voltage is determined by BIAS and it tends to be smaller.

I believe Lex meant changing it to test the circuit behavior.
Quote:

Actually, I find that with this cmfb, the DC OP is very important ...

As I'd expect given the topology.
Quote:

... to guarantee that the CMFB loop has enough gain.

So what is your question?

- B O E

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Lex Senior Member Offline Posts: 201 Eindhoven, Holland	Re: CMFB of OTA Reply #13 - Nov 6^th, 2012, 12:03am boe wrote on Nov 5^th, 2012, 4:30am: Lex wrote on Nov 5^th, 2012, 4:25am: Are the problems gone with a higher common mode voltage? Then I guess you know the answer to your problems. I"d try lower (input) common mode voltage... - B O E I think we're on the same page.
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urian Junior Member Offline Posts: 30	Re: CMFB of OTA Reply #14 - Nov 6^th, 2012, 2:19am Sorry I made a mistake.When I re-check the circuit, I find that it is because the DC OP of output CM voltage will change largely with SF W/L, and this will put great pressure on CMFB.
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