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Design >> Analog Design >> Tow stage Op-Amp common mode feedback
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Message started by Majid on Jun 23^rd, 2018, 1:39am

Title: Tow stage Op-Amp common mode feedback
Post by Majid on Jun 23^rd, 2018, 1:39am

Hi
I have designed a two stage Op-Amp. Should I use two common mode feedback for Op-Amp or one common mode feedback is enough ??

Title: Re: Tow stage Op-Amp common mode feedback
Post by R.kumar on Jun 26^th, 2018, 3:26am

You need to have a common mode feedback for any fully differential (differential input and differential output) high gain stage. Common mode feedback is used to set the output common mode at a sweet spot. So if both the stages have differential outputs then you need two independent common mode feedback.

Title: Re: Tow stage Op-Amp common mode feedback
Post by Jacki on Jun 27^th, 2018, 6:58am

I think you are talking about the general fully-differential miller compensated two-stage opamp. You can either use two separate CMFB loop or you can use only one CMFB loop. If you use one CMFB loop, make sure the input of your CMFB is connected to the output of the two-stage opamp, and check your feedback is stable.

Title: Re: Tow stage Op-Amp common mode feedback
Post by Nyquist on Jul 10^th, 2018, 4:53pm

Hi,

I agree with Jacki. We can have two CMFBs, one for each stage or we can have have a single CMFB, that takes in output of the second stage and feeds back to the mirrors of the first stage.

Title: Re: Tow stage Op-Amp common mode feedback
Post by vroy_92 on Aug 12^th, 2018, 3:10am

No you don't need two common mode feedback circuits. For example, consider the attached image. The block at the bottom of the circuit is your common mode feedback amplifier. It senses the CM of the output of the second stage and tunes the current of the first stage.

The common mode of the o/p of first stage cannot be fixed as it would still not fix your problem, and you will need to have another CMFB for your output stage. CMFB amplifiers are mighty hard to stabilise, and I never liked going for two of them.

Also the second stage typically has a much larger current than the first stage. For reasonable voltage swings at the output, your devices have to be sized such that they have a small vdsat, which means that the gm of the current source load of your second stage will be high. This will make it hard to stabilize your CMFB loop. [also you lose the advantage of having the Miller pole that can help you stabilise the CMFB loop]

Title: Re: Tow stage Op-Amp common mode feedback
Post by dog1 on Aug 21^st, 2018, 3:49am

@Vray, I might misunderstand you, but are you saying the figure you attached is not good and we should go for only one CMFB for the first stage?

I disagree with you in that case. In the figure attached, due to feedback, the CM of the output of 2nd stage is fixed, thus the CM of the output of the first stage is fixed by default. Of course, by design, it should be made sure that this CM ensures that all the transistors of the 1st stage are in saturation.

Title: Re: Tow stage Op-Amp common mode feedback
Post by Mos on Sep 23^rd, 2018, 9:40am

I think if I try to change the current in stage1 to change the common mode of stage2. Is not that a problem because stage2 common-mode is weakly dependent on stage1 common mode? Is not that whole idea of common-mode feedback?
Trying to figure out if I am missing something here

Title: Re: Tow stage Op-Amp common mode feedback
Post by vroy_92 on Sep 29^th, 2018, 2:09pm

dog1 wrote on Aug 21^st, 2018, 3:49am:

I apologize.. there is a typo and this will work.