OPAMP Buffer dimension

[sykab]

Hi!

I'm designing an opamp with a differential pair. I added a second stage (common-souce amplifier) and I'm thinking that I need a third stege (source-follower).
The problem his that I have no idea how to dimension the third stage. I know that it should have a low output resistance (but between which values?).
When I test if the opamp output follows the input changes (doing a vin sweep), I realise that the Resistance Voltage can follow the input sweep just when Vi is between 0 and 0,200 V. Then it (VR) keeps its value in 0,2 V.
Outside the opamp I have some elements, as you can see in the image.

Can anyonde help me?
Should the problem be outside or inside the opamp?

 

[jecyhale]

I think it should be the linearity of the OPAMP inside.
Or the linearity of the outside MOS.

 

[ashish_chauhan]

can you please explain what exactly are you trying to do?

The image you have shown is somtihing like a precision current sink... where ur output current is given by Vin/R1.

am I getting it right?

 

[lvlei0309]

this a LDO structure

 

[sykab]

can you please explain what exactly are you trying to do?

The image you have shown is somtihing like a precision current sink... where ur output current is given by Vin/R1.

am I getting it right?

yes, I'm trying to do a current source.
I think that I have solved the problem. I'm using, in the third stage, a source follower, as you can see in the image. I don't know if I have done it right, but it works. I used a PMOS (M1) and a NMOS(M2) to do it. Is it right?

 

[northeast1]

Hi!

I'm designing an opamp with a differential pair. I added a second stage (common-souce amplifier) and I'm thinking that I need a third stege (source-follower).
The problem his that I have no idea how to dimension the third stage. I know that it should have a low output resistance (but between which values?).
When I test if the opamp output follows the input changes (doing a vin sweep), I realise that the Resistance Voltage can follow the input sweep just when Vi is between 0 and 0,200 V. Then it (VR) keeps its value in 0,2 V.
Outside the opamp I have some elements, as you can see in the image.

Can anyonde help me?
Should the problem be outside or inside the opamp?

explain detailly.