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OPAMP Buffer dimension

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sykab

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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.

80_1210250801.jpg


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

jecyhale

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

    sykab

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ashish_chauhan

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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?
 

    sykab

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lvlei0309

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this a LDO structure
 

    sykab

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sykab

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ashish_chauhan said:
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?

95_1210330781.jpg
 

northeast1

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sykab said:
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.

80_1210250801.jpg


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

explain detailly.
 

    sykab

    Points: 2
    Helpful Answer Positive Rating
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