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Ricky Chen Community Member Offline Posts: 49	Common Mode Feedback Contorl Oct 29^th, 2011, 11:31pm Dear friends, I have attached the file containing two CMFB Networks. One controls a fraction of the Bias current while the Other controls the entire bias current. Which one is better ? and why ? -Ricky
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raja.cedt Senior Fellow Offline Posts: 1516 Germany	Re: Common Mode Feedback Contorl Reply #1 - Oct 30^th, 2011, 12:26am hello, there is no diference. Both are same. Thanks, Raj.
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rajkumar palwai

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Re: Common Mode Feedback Contorl
Reply #2 - Oct 30^th, 2011, 1:16am

Hi Ricky,
The CMFB loop gain of the 1st fig is lower, because gm7 would be smaller compared to that of 2nd fig. By varying the M7 size, u can play with the phase margin of the CMFB loop and thus can adjust for the best CM settling. Also u can adjust the CM loop stability.

But, Fig2 will have more cmfb loop gain and the error in the final settled Common mode value will be less.

-rajkumar

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raja.cedt

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Re: Common Mode Feedback Contorl
Reply #3 - Oct 30^th, 2011, 1:30am

hello rajkumar,
you are correct, you are saying with in the first fig you have less current through m7 so less gm compared to 2nd fig, but gm can be varied by current m7 size as well, what i mean to say is to get desired gm no need to put some fixed current correct me if am worng?.

Infact some people say if cmfb doesn't work due to some reason still you get some bias current through fixeed current.

Thanks,
raj.

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yvkrishna

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Re: Common Mode Feedback Contorl
Reply #4 - Oct 30^th, 2011, 2:35am

Hi,

I agree with rajkumar,
we often end up with difficulty in stabilizing the common-mode loop ( though this case looks simple) because not many device parameters �in the signal path are in control to tweak and stabilise this CM loop to get a clean common mode settling response.

Having said that it all depends on the application - how much error OR slowness �we can tolerate in this CM loop.

@Ricky
you can refer Paul Gray's �book for this topic.

Thanks,
yvkrishna

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rajkumar palwai Community Member Offline Posts: 46 Sunnyvale, California	Re: Common Mode Feedback Contorl Reply #5 - Oct 30^th, 2011, 5:51am Hi Raja, Yes, the gm7 can also be reduced by just reducing its size. But then voltage headroom required to be in saturation (while passing the entire tail current) will increase and that may hurt. So people typically put a parallel current source and reduce the current through M7. -rajkumar
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Ricky Chen

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Re: Common Mode Feedback Contorl
Reply #6 - Oct 30^th, 2011, 6:26am

Ya it is related to the transient response (settling time) of the circuit.
That is what I was suspecting.

@ raja.cedt

You can change the size of m7 to have desired gm.
However for swing reason one desires to have current source and M7 to have same overdrive so gm/Id for M7 is fixed by the Diff-Amp ckt design.
So to reduce gm of M7 one is left with no option other than to reduce the current of M7

-Ricky

rajkumar palwai wrote on Oct 30^th, 2011, 1:16am:

Hi Ricky,
The CMFB loop gain of the 1st fig is lower, because gm7 would be smaller compared to that of 2nd fig. By varying the M7 size, u can play with the phase margin of the CMFB loop and thus can adjust for the best CM settling. Also u can adjust the CM loop stability.

But, Fig2 will have more cmfb loop gain and the error in the final settled Common mode value will be less.

-rajkumar

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Common Mode Feedback Contorl Reply #7 - Oct 30^th, 2011, 4:50pm No DC bias at the common mode feedback current point? What's missing here?
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Ricky Chen Community Member Offline Posts: 49	Re: Common Mode Feedback Contorl Reply #8 - Oct 31^st, 2011, 12:50am loose-electron wrote on Oct 30^th, 2011, 4:50pm: No DC bias at the common mode feedback current point? What's missing here? There is DC f/b.. Please Look at the circuit carefully !!!
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rajkumar palwai Community Member Offline Posts: 46 Sunnyvale, California	Re: Common Mode Feedback Contorl Reply #9 - Oct 31^st, 2011, 2:36am Hi Ricky, There is no DC path at the gate of M7. If u simply connect the circuit like that feedback doesnot work. -rajkumar
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Ricky Chen Community Member Offline Posts: 49	Re: Common Mode Feedback Contorl Reply #10 - Oct 31^st, 2011, 8:04am rajkumar palwai wrote on Oct 31^st, 2011, 2:36am: Hi Ricky, There is no DC path at the gate of M7. If u simply connect the circuit like that feedback doesnot work. -rajkumar There is DC path. If you kick one side of the open CAP the other end will also get the same kick. By the way what do you mean by DC path ??
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wave

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Re: Common Mode Feedback Contorl
Reply #11 - Oct 31^st, 2011, 1:42pm

Arg! I'm seeing several incorrect statements today.

Loose E is correct.
You need Resistors in parallel with the Caps for a DC sense point and DC feedback.

Raja - there IS a difference. Without a constant bias, it may never startup. If your DC bias point (broken already) is at a place where Loopgain = 0, you have no CMFB regulation.

Cheesy

Wave

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Common Mode Feedback Contorl Reply #12 - Oct 31^st, 2011, 3:36pm LOL!
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Ricky Chen

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Re: Common Mode Feedback Contorl
Reply #13 - Oct 31^st, 2011, 11:58pm

wave wrote on Oct 31^st, 2011, 1:42pm:

Arg! �I'm seeing several incorrect statements today.

Loose E is correct. �
You need Resistors in parallel with the Caps for a DC sense point and DC feedback.

Raja - there IS a difference. �Without a constant bias, it may never startup. �If your DC bias point (broken already) is at a place where Loopgain = 0, you have no CMFB regulation.

Cheesy

Wave

OK !! So your statement is without resisitor one can NOT sense CM voltage then how things work in SWITCH CAP CMFB circuit.
By the way the above circuits are the snapshot of one state of SWITCH CAP sense circuit

One thing to mention, even if you give constant current source bias CMFB start-up will still exist if you solely consider only the CKT shown above !!

For your information, there is no difference between a resistive divider (R1 & R2) and a CAPCACITIVE Voltage divider (provided you have control on the initial condition of the CAPs which generally happens in CAP-CMFB circuit).
For Res case Vout=Vin[R2/(R1+R2)] & for CAP case Vout =Vin[C1/(C1+C2)]

!!! Cheesy

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Praveen K Junior Member Offline Posts: 25 Bangalore	Re: Common Mode Feedback Contorl Reply #14 - Nov 1^st, 2011, 12:43am Hi wave, why are saying the circuit on right will not start up? won't the current sources M3 and M4 rise X & Y nodes initially and then later get stabilized with M7 in action. k.praveen
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