Below negative rail CMIR OPAMP

[Dinamovist]

Hi Gentlemen,

I'm looking to design for the first time an OPAMP or OTA that can go little (100-200mV) below the negative rail that is gnda in my case. I want to avoid adding a charge pump.
I know the input pair and the mirror suppose to be in active region for a nice high performance OPAMP but that's the input data I have for my design.
My first thought to solve it was :is to have the common source of the input pair, really close to the VSS level thus, very close to active (saturated) region.
My question is where do I need to look first PMOS input vs Bipolar, I rather use MOS but the technology allows me to use both.
I don't think a complimentary P/N input would help because the intention is to operate it only close to the negative rail .
Should I look into folded cascode or telescopic ?

Thank you so much,
Fabio

 

[andre_luis]

Hard to understand exactly what is the problem...

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[Dinamovist]

Hard to understand exactly what is the problem...

+++

Andre,

Not every configuration will work well with inputs below the negative rail. The usual OPAMP has the CMIR > VTHP+VTHN+VOV to have all the transistors in active region