why do we need high CMFB loop gain ?

[yixiusky]

site:www.edaboard.com 26db

Normally when we design common mode feedback for amplifier, we wish the CMFB loop gain is high.

The higher CMFB loop gain, the better performance . I would like to ask the reason of that why we need high CMFB loop gain.

Thank you very much !

 

[PaloAlto]

Re: CMFB loop gain

It just depends on how much precision you need for your common mode. If you can accept 20% errors in your cm then you don't need so high gain.

The higher the loop gain, the smaller the common mode value errors.

 

[yixiusky]

Re: CMFB loop gain

thank you very much ^^ this is good qualitative explanation.

Is there any book or reference can explain this in the quantitative way ? I mean some formulars ^^

thank you very much ^^

 

[PaloAlto]

Re: CMFB loop gain

I think that you have to look at the application of your amp. Imagine that what-ever-there-is after your amp (your load) has a terrible CMRR and it can not accept cm variation larger than a certain value, for instance 100mV.

Imagine that your common mode is going to be set at 1V. The error coming from all sources cannot be larger than 100mV. You generate the 1V reference with a bandgap that is already introducing a 5% error (50mV). That means that your cmfb loop cannot introduce an error larger than 50mV too. So you need at least 26dB.

 

[yixiusky]

Re: CMFB loop gain

thank u very much ^^ i got it ^^

 

[lscutewolf]

Re: CMFB loop gain

How do you calculate the number 26dB?thanks

I think that you have to look at the application of your amp. Imagine that what-ever-there-is after your amp (your load) has a terrible CMRR and it can not accept cm variation larger than a certain value, for instance 100mV.

Imagine that your common mode is going to be set at 1V. The error coming from all sources cannot be larger than 100mV. You generate the 1V reference with a bandgap that is already introducing a 5% error (50mV). That means that your cmfb loop cannot introduce an error larger than 50mV too. So you need at least 26dB.