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Design >> Analog Design >> about cmfb
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Message started by mists on May 14^th, 2007, 7:21am

Title: about cmfb
Post by mists on May 14^th, 2007, 7:21am

as full-differential n-input fold-cascade amplifier in the picture, we can add the cmfb in the a, b or c point, what is the difference of the three method? which one is better

Title: Re: about cmfb
Post by SATurn on May 18^th, 2007, 3:21pm

Hi,

The main differences among these three approaches, I think, are different gain and phase margin for the CMFB loop. For example the loop gain for (a) and (c) is almost the same (assuming that the current in Ma is two times Mc), however, there is one more non-dominant pole in (a). In case (c), the gain is less and so it is simpler to make the loop stable...

SATurn

Title: Re: about cmfb
Post by loose-electron on May 21^st, 2007, 11:28am

Hm, as with a lot of things in this world there are multiple ways to do the same thing.

That said, getting everything at proper common mode operating point early in a multiple stage system is generally considered good practice.

That said, location C, is somewhat downstream.

The most common place to add this would be location B. This does not disturb your common mode current below the ddifferential pair and will change the gain of the stage slightly (you are changing the impedance of the diff pair load a bit with control added at node B, but that should be slight) and location A tends to be controlled by a common current bias system.

In analysis all would work, in practice I would tend to use B, develop a multiple use location bias system to control A (all the differential pairs in a system) and would avoid C just due to the fact that you want to introduce CMFB early in the system.

Title: Re: about cmfb
Post by mists on May 23^rd, 2007, 5:56am

but somebody say the cmfb in point a and b will affect input transistor pair'gm a little.