Information about CMFB and its application

[firsttimedesigning]

Can anyone please explain to me what is CMFB all about? Why do people use it?
After spending about 2 hours reading the book, the only thing I get is that due to the mismatches in NMOS and PMOS, CMFB is required...
I am not even sure if that is right...
Please recommend me some good websites that I can go to.
Thanks.

 

[amarendra_p14]

CMFB

see razavi for further details .. it is essential for any op-amp design..

 

[renwl]

CMFB

you should do some simulation. you can find it from simulation.

 

[barath_87]

Re: CMFB

you should do some simulation. you can find it from simulation.

I was thinking that CMFB is used to counteract device mismatches that occurs while fabricating the devices .....

How can you simulate it ....

Pls correct me if I am wrong....

 

[amriths04]

Re: CMFB

try checking the output dc bias point without cmfb and then with cmfb you will understand it's importance. you can fix your output bias using this technique, giving some room of freedom to vary your output impedance.

you should do some simulation. you can find it from simulation.

I was thinking that CMFB is used to counteract device mismatches that occurs while fabricating the devices .....

How can you simulate it ....

Pls correct me if I am wrong....

 

[barath_87]

Re: CMFB

Ok thank you amriths04 now i get the picture...

 

[iamxo]

Re: CMFB

in the fully differential circuits, the differential mode feedback will not fix the node voltage at large impedance node, so we should use common mode feedback to fix the node voltage.

 

[nohanag]

Re: CMFB

*CMFB

In the diff Amp (one stage), the Common Mode level depends on how close Id3 & Id4 to the value of Iss / 2

But in practice, mismatches in the PMOS & NMOS, current mirrors will create a finite error between Id3 & Id4 with Iss/2

The above difficulties arises only in high gain Amp, cuz we use P-type current source and N-type


a first transistor passing a first current in response to a first output voltage of a fully differential amplifier;

a second transistor passing a second current in response to a second output voltage of the fully differential amplifier;

a summation circuit summing the first and second currents to produce a summation current;

a reference current generator generating a reference current;

So as the error increase, the Vocm will be Uncontrollable
and the o/p will finally saturate .....>

-->@ Vdd if Id4 > Id1
-->@GND if Id4 < Id1

For this reason, in a high gain Op-amps cct's we must ensure that the o/p CM Voltage Vocm is controlled to a well defined ref value

we'll make a -ve FB from o/p CM & ensure that o/p will track Vref, so we are sure that o/p is Controlled.

 

[suria3]

Re: CMFB

CMFB is used to control on the output dc crossover voltage when there is process, mismatch variation after fabrication. Let say your output design is targeted to have a dc voltage at 1.2V, thus during fabrication, your design will fall within SS to FF process which will have threshold voltage difference in transistor and let your current to vary. And when dc current tend to vary, definitely your output will not kept at 1.2V. So CMFB circuit helps here by controlling the tail voltage (etc differential amplifier) in order to sink the required current back to make the dc back to 1.2V.

 

[nohanag]

Re: CMFB

see razavi for further details .. it is essential for any op-amp design..

CMFB section in Razavi

 

[gpwu]

Re: CMFB

In an differential opamp, common mode voltage can drift to a voltage that could reduce opamp gain. CMFB functions as bring opamp common mode to desired voltage.

 

[mba7]

CMFB

i am also looking for a documentation on th CMFB is there any place to look in order to understand more

thanx

 

[jnuhope]

CMFB

I'd like to take the question a step further. How fast should we design CMFB? What's the loop bandwidth compares with differential loop bandwidth? Can anyone explain the reason or referred me any papers? Thanks in advance.

 

[atran_zh]

Re: CMFB

I'd like to take the question a step further. How fast should we design CMFB? What's the loop bandwidth compares with differential loop bandwidth? Can anyone explain the reason or referred me any papers? Thanks in advance.

The most important of CMFB design is: CMFB must have enough high Gain otherwise it can cause ringing at the common mode output, and unstable. (Please note that CMFB always need to be well compensated, its phase margin and step response should be found with simultion). The high speed of the CMFB circuit is also desirable to minimize the effects of high-frequency common-mode noise, but it is not the first important of all. Sometime, CMFB can be the most difficult part of the circuit.

My suggestion is the UGBW of the CMFB is equal to the UGBW of the op-amp, if you can get more, it is quite good but the power consumption will increase...

One of the best book write for CMFB( I think) is Analog Integrated Circuit of David John

 

[jnuhope]

Re: CMFB

Thanks so much! That's really helpful.

 

[AMS2007]

Re: CMFB

Pls think about the common mode control.

Some applications use CMFB loop which is passive or active type.
For example, fully differential opamp in PipelineADC, fully differential chare pump in PLL, differential signaling drivers...,

To control common mode level in differential’s signaling system, it need to control the common mode because of interaction with another stage.
(If we do not control common mode..., gain? distortion?, malfunction? ... )

But, in cases of PVT variations (including device variation), it's impossible to control and keep common mode exactly as you want. so we have to use feed-back loop with reference.

Good luck!