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Design >> Analog Design >> how to choose Vcm level of ota
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Message started by vhdl00 on Jan 25^th, 2006, 10:56pm

Title: how to choose Vcm level of ota
Post by vhdl00 on Jan 25^th, 2006, 10:56pm

Hi all,
I am trying to do dc analysis of open loop OTA. at the beginning I was doing dc differential sweep to make sure all the device working in saturation region, after I has been told to use CMFB to set up the operation point, I am kind of confused: how to choose Vcm , do we have to choose it randomly(such as in the middle of the rail to rail)? or we have to do dc common mode sweep to find it ?

Thanks.

Title: Re: how to choose Vcm level of ota
Post by Cri Azzolini on Jan 27^th, 2006, 12:46am

Hi Vhdl00,

the output common-mode voltage of your OTA depends on many factors: topology, output swing, dc-current, power supply voltage, ...
For a telescopic OTA an output CM of half rail-to-rail voltage is the most diffused. A second wise choise may be this one: for a 5 stacked devices opamp (i.e. telescopic) an output CM of 3/5 of rail-to-rail voltage may also be appropriate.
Bye,
Cri

Title: Re: how to choose Vcm level of ota
Post by Paul on Jan 27^th, 2006, 2:04pm

Hi,

as Cri says, it would be good to specify the OTA topology you are using. Output common-mode around Vdd/2 is indeed the most common variant as most OTAs have a more or less symmetrical output stage between Vdd and ground.

Don't forget that if you keep your OTA in an open-loop configuration, the CMFB will take care of setting the output common mode, but it won't influence the input common-mode. Setting the input common-mode to a different level than what it will be in closed-loop configuration may yield somewhat different results.

For the analysis of differential circuits, please refer to the following paper on this web site:
http://www.designers-guide.org/Analysis/diff.pdf

Paul

Title: Re: how to choose Vcm level of ota
Post by vhdl00 on Jan 27^th, 2006, 6:01pm

Hi Cri and Paul,

I am using a two stage ota.
As Paul mentioned, seting up the input common mode might give a different Voc than the one(vcm) we choosed in our CMFB, will that cause trouble ? or the difference(vcm and voc)has to be pretty small?

One more thing, to determine the Vcm, what do you guys do? let's say the goal is just get max output swing, do we have to do a dc common mode sweep? in open loop or with cmfb?

Title: Re: how to choose Vcm level of ota
Post by Paul on Jan 28^th, 2006, 12:31pm

Hi,

your answer is somewhat confusing. I understand that Vcm is the desired output CM voltage and Voc is the resulting CM output voltage, right? The choice of your input CM should not affect the operation of the CMFB, thus not influence Voc. What I was trying to say is that the choice of the input CM may affect the region of operation of the first stage, depending on how your CMFB is implemented. In that case, your differential gain and bandwidth may be different.

To maximize your output swing, you should analyze the output stage of your circuit. Usually topological considerations give quite a good estimate for the ideal output CM. In simulation, you can do a DC sweep on your desired output voltage while plotting the open-loop DC gain or the large signal swing.

Please correct me if I got something wrong in your post
Paul

Title: Re: how to choose Vcm level of ota
Post by vhdl00 on Jan 28^th, 2006, 5:24pm

Hi Paul,

what i mean is Vcm is the desired output CM voltage, Voc is the CM output voltage without cmfb which is set by input common mode, my question is if we have to make sure Vcm and Voc pretty close.

My thought is just to get max output swing, first we may do a common mode input dc sweep to find the middle point of the output swing without cmfb, which we have to choose it as our desired output CM voltage in our cmfb loop.

thanks

Title: Re: how to choose Vcm level of ota
Post by Paul on Jan 29^th, 2006, 12:36pm

Hi,

Steve_IC wrote on Jan 28^th, 2006, 5:24pm:

what i mean is Vcm is the desired output CM voltage, Voc is the CM output voltage without cmfb which is set by input common mode, my question is if we have to make sure Vcm and Voc pretty close.

As you will define Vcm so that the output swing is maximized, you definitely should have Voc close to Vcm... I still don't see exactly the point, because once you implement the CMFB, there will be no direct relationship between the input CM and the output CM, unless you drive some devices of the CMFB out of saturation. I assume in this statement that your amplifier input goes to a MOS gate, of course.

Quote:

My thought is just to get max output swing, first we may do a common mode input dc sweep to find the middle point of the output swing without cmfb, which we have to choose it as our desired output CM voltage in our cmfb loop.

I would suggest to first implement the CMFB, then determine the input CM so that your differential pair is in a correct region of operation. Once this is done, sweep Vcm in simulation to determine the optimum output CM for your specifications (gain, swing, ...). This corresponds to the real mode of operation of your amplifier, while the input CM sweep is not so realistic. It may drive out of saturation some devices which are not directly related to the output swing of your two-stage amp.

Paul