CMFB in a cascode differential pair

[deepa]

In the cascode differential pair,the vcmfb is to be connected to either A or B,it seems as though connecting to A is a wise option

 

[ahmad_abdulghany]

Do you state it or ask?

Anyway, i prefer to connect the CMFB to the outer most MOS so that to enhance voltage headroom.

 

[deepa]

i am asking ??

 

[hr_rezaee]

Hi
I think that A is better because M7,8 has more gain than M5,6 .
regards

 

[safwatonline]

i think u have to connect them to A , as if u connect A to a certain DC voltage then it has a certain Vgs which will make a certain Current flow which may not be equal to half Ibias , which means that No CMFB is done , i.e. u have to connect it to A to get the FB action

 

[gingerjiang]

of course you should connect the cmfb to point A
alternatively, you may connect the cmfb to Ibias, i think these two methods are same

 

[seanyang]

No matter you connect to bias or point A, the concept is to control the current if the Vout1 and Vout2 rise(fall) and restoring the output CM level.

Ref: Design of Analog CMOS Integrated Circuit "Razavi"

thx~~

 

[pfd001]

A is good. If possible, you can also connect to the tail current

 

[deepa]

Can u clearly explain why is this an advantage over connecting Vcmfb to B

 

[seeker137]

The cmfb voltage cnt has to be applied, as per the ckt schematic, such a way that you control the currents in the diff pair load so as to correct any cmfb imbalances.
You can do that by applying the cmfb control voltage to pnt A . Hope that helps.

 

[wjxcom]

I do not think that Vcmfb should connects to B. We can look the M7,8 as current source, so the current is desided by M7,8. at the same, M5,6 do not deside the current, so M5,6 will do not deside the voltage of Von and Vop, i.e. M5,6 will not stabilize the common voltage of Von and Vop!!

As pfd001 said, you can also connect to the tail current !

 

[hr_rezaee]

I do not think that Vcmfb should connects to B. We can look the M7,8 as current source, so the current is desided by M7,8. at the same, M5,6 do not deside the current, so M5,6 will do not deside the voltage of Von and Vop, i.e. M5,6 will not stabilize the common voltage of Von and Vop!!

As pfd001 said, you can also connect to the tail current !

Hi
VGS5,6 can control VDS7,8 and then current of M7,8.
But feedback factor is small and we have a weak feedback.
regards

 

[jonashat]

The purpose of the CMFB is to control the current in the main opamp so that your output common mode settles to a desired value.

Ideally, the voltage at B doesn't affect the current in the opamp branches (which is determined by M_7 and M_8). In reality it will have little effect on the current which means a very low loop gain. This will results in a large systematic offset in your output common mode.

Added after 49 seconds:

Like what hr_rezaee said

 

[jeepx5]

For the common mode path, feedbacking to the point A will provide a higher common mode gain. It makes the CMFB more efficient.

 

[jeffsky520]

A: higher gain & larger PM benefits your circuit.

Added after 50 minutes:

But who can tell cmfb control point A or Ibias more better? why?

 

[gingerjiang]

as for feedback control point A or Ibias, i think the stability should be considered, a feedback path which has a proper bandwidth and phase margin is better