For this circuit, a PMOS input stage folded cascode amplifier as a gain booster, I have two questions. The PMOSs (Mppp1 to Mppp4) in the cascode banches are in strong-inversion saturation while the NMOSs (Mnnn1 to Mnnn4) are in weak inversion saturation (Vgs < Vth, but Vds > Vdssat). However, I don't manage to put the Mnnn1 to Mnnn4 in strong inversion saturation. Is this a huge problem when one side of the cascode is in weak inversion saturation and the other part in strong inversion saturation (Mppp1 to Mppp4)? The functionality seems fine to me. The figures are shown below.
The full schematic in LTspice file can be download from this link: https://drive.google.com/file/d/1Zr2CRn1vcVBtQ7_So4opipGoO-pey7_6/view?usp=sharing
One more question, since the previous schematic is done with an ideal CMFB, the actual circuit should have a non-ideal CMFB. My implementation is seen below.
Here, the Mppp9 and Mppp10 should be sized such that both the PMOSs are in deep triode. However, with this implementation in the tail part of the PMOS differential pair, the Mppp9 and Mppp10 are still in saturation whereas the Mppp6 is not in saturation. The other transistors are not in saturation, unfortunately. Hence, my question is: is it possible to implement the CMFB circuit in the tail part of the PMOS differential pair like what I did? Or is it better to use a different CMFB circuit before biasing?
Full schematic with non-ideal CMFB: https://drive.google.com/file/d/1x25jEk6ODmBD6e5xGWV-aiqN1jmcVJHP/view?usp=sharing
The model is given in this link for LTspice file: https://drive.google.com/file/d/1bwkuksGx4w6FvInAr4KF1ndcx49Y7NXU/view?usp=sharing
With kind regards, Kwok