How to make SC CMFB circuit for a class-A opamp ?

[lilac]

SC CMFB circuit?

I want to make a switched capacitor CMFB for a class-A opamp, who can tell me how to realize it or give me some advice about it. Thanks in advance!

 

[eda4you]

Re: SC CMFB circuit?

This is a very complex thing, also it doesn't seem so at first sight. I suggest you to read chapter 12 of Mayer&Gray's (4th edition) book "Analysis and Design of Analog integrated Circuits".

 

[markty]

SC CMFB circuit?

Can anyone share some CMFB info. Many books, even Gray's, only give simple example.
I am confused with the phase i should use for the feedback phase & sample phase.

 

[lilac]

Re: SC CMFB circuit?

To eda4you,
Thank you for your reply, I will try to find this book and read it. The attachment is the SC CMFB circuit I had seen from some books. Can I use it to the second-stage of the opamp directly? Wait for your answer!

 

[terryssw]

Re: SC CMFB circuit?

You cannot just apply the CMFB to second stage, since the common-mode output of the first stage also need to stablize. There is two methods: One is apply the CMFB to the first stage only, then through the feedback from first to second stage both stage's CM output can stablize; but you must be care of the proper polarity when you applying such feedback (you are going to apply negative, not positive CMFB to the first stage, but it is difficult for normal two-stage amplifier). Another approaches is using two separate CMFB for first and second stage. This is easier to achieved but with larger area and power consumption.

 

[lilac]

Re: SC CMFB circuit?

Thank you for you answer. But as you said, if I adopt the first methold, that is to say, only apply the CMFB to the first stage. So, how can I control the output voltage and make it equal to the input voltage? it is difficult to decide the Vcm, isn't it?
Thanks a lot!

 

[terryssw]

Re: SC CMFB circuit?

You see in your figure you have the input terminal called Vcm? This is your desired output CM voltage. So if you want your output CM level at 1V, just apply 1V at this terminal, and the feedback automatically correct the output CM voltage to approx. 1V. But again, make sure your feedback polarity is correct.

 

[lilac]

Re: SC CMFB circuit?

I see, but if I only apply the CMFB to the first-stage of the opamp, the circuit like following:
Vcm is connected to the output of the first-stage. Vcmfb is connected with the bias of the first-stage. How can I control the output voltage of the second-stage?
Thanks again!

 

[terryssw]

Re: SC CMFB circuit?

Vcm terminal is not connected to the first stage output. It should be connected to a constant voltage source (this set your desired output Vcm voltage), then the CMFB automatically control the output Vcm voltage equals to the value of constant voltage source.

 

[lilac]

Re: SC CMFB circuit?

Thank you very much.
I've decided to apply two CMFB to my circuit. thanks for your reply!

 

[avinash]

Re: SC CMFB circuit?

i think we need a cmfb at both the stages of opamp