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Folded cascode OTA - need help!

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cheenu2002

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Hi,
I have tried to design a folded cascode. The schematic and AC response are attached with this mail. I have 2 questions:

1. The PhaseMargin is very poor. Can you help me to understand how to
improve PM . Is it bcoz I havnt connected load cap (Cl)?
2. I am able to easily bias the transistors if I use a pmos current mirror load.
But if I use a current source load, then either qp31,29 or qn14,12 goes into
linear region. I am not able to get how to bias the folded cascode with the
current source load. Is there any technique that I should follow. Can anyone
pls help me.
 

Hi.
Add capacitor at the output.
Make sure all transistors are properly bias.

I did not get what you mean by putting a current load at the output.
You are using the folded cascode for what purpose?
 

Hi,
I am not connecting a current ,oad at the output.
Like for an opamp, you can use a resistive load, current mirror load or a current source load right. So, in my circuit, I have used a cascode current mirror load. If I want to use a current source load, then I am not able to bias. Hope I made my question clear. Pls refer to the attached image.
[/
 

Hi,

Use a current mirror load.
If you used a current source pmos, you have to properly bias your 'biasn1-2 ' and 'biasp1-2' such that your cascode transistors does not go into linear region. Any mismatches will lead these transistor to linear region.

I suggest you use a current mirror load assuming you dont need a differential output.
 

yeah.. you are right. I coudnt bias the cascode transistors. Either the pmos or the nmos cascode goes into linear region. I want to use it for a BGR. Can I use this architecture to design a folded cascode with gain boosting? bcoz when I go for gain boosting, I cant use current mirror right? Do you have any reference on how to generate the bias voltages...
 

You can use this circuit as a reference for your design.
If you have some supply voltage constraints, you can make a wide swing folded cascode.
 

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