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Why are both an NMOS and PMOS cascode used for folded cascode OTAs

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diarmuid

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Hi All,

I have a folded cascode OTA with PMOS input diff pair. Just
like the attached excepte with CMFB from "voutn" to "vbp2".

Pretty standard architecture. I can see how M4A increases output
impedance but am a little confused as to exactly how M2A increases output
impedance.

Is this NMOS cascode present to increase output impedance? If so, could you
please illustrate with an example.

Cheers,

Diarmuid
 

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erikl

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I can see how M4A increases output impedance but am a little confused as to exactly how M2A increases output impedance.
Same as M4A - both are common base stages, isn't it?

Is this NMOS cascode present to increase output impedance?
I think so.

... with CMFB from "voutn" to "vbp2".
A fully differential OTA needs a real CMFB (controlled by both outputs and a reference), I think.
 
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