Fully differential folded cascode amplifier

[Junus2012]

Hello Friends

I am designing fully differential folded cascode amplifier with rail to rail input stage which consist of nmos and pmos differential input stage based on the circuit shown by the attachment,

I am wondering whether i need to design two CMFB one for the upper side and the other for the down side, or one is enough at either sides.

Thank you in advance

 

[frankrose]

Re: Fully differential folded amp with rail to rail input stage

One CMFB should be enough to control for example the NMOS biasing transistors, and the PMOS transistors can take current from a simple current mirror then. This is usually the way, to tune a current to an other to get proper output common mode voltage, in the above described case I tuned the NMOS currents to the PMOS currents.

 

[Junus2012]

Thank you frankrose for your help

I just want to ask you about the transistor you are referring in your explanation, do you mean to say the transistor at the load not for the differential tail,

 

[frankrose]

There aren't only one solution to implement the CMFB, but the common is maybe that they tune one portion of the circuit, others are constant current biased.
In a rail2rail amplifier probably not the best choice to tune a differential pair's tail current, you are feeling the point, because under or above a voltage level one of the tail current sources will be starved, load transistors however are always in saturation.
If you want to implement a tail current controlled CMFB for a rail2rail amplifier that is a challenge, and probably doesn't make sense too much.