I need to modify this circuit to serve as an operational amplifier with low output resistance enter image description here

Because the operational amplifier that I'm familiar with uses this differential pair: enter image description here

I thought about making my former circuit single-ended so we can use a common-drain amplifier and achieve low output resistance, maybe get rid of the degenerative \$R_{ss}\$ if we wish for higher -not as linearized- gain, then it was turn to think about if we really need to have that current mirror at the top with the reference being diode-connected, for a long time I thought we only did this for current mirrors to avoid the reference transistor from entering the linear region (I found out that when we do this, the small-signal circuit also changes and \$g_{mp}\$ shows up in the transfer function). Thus, I'm reluctant about whether I should include ditching \$V_{b}\$ and making \$M_{3}\$ diode-connected in my steps of transforming the original circuit to an operational amplifier.

  • \$\begingroup\$ Have you looked into MOSFET op-amp implementations at all? \$\endgroup\$
    – Andy aka
    Feb 15, 2021 at 10:24
  • \$\begingroup\$ We study a specific implementation from Razavi's book, it's here: docdroid.net/l1p7qvm/ec-lecture-ix-pdf#page=3 \$\endgroup\$
    – Essam
    Feb 15, 2021 at 10:31
  • 2
    \$\begingroup\$ A proper opamp is much more complex with a proper output stage for example. Like Andyaka I suggest that you study real opamp designs to see what those look like and try to understand why in those designs, things are done as they are. Also, as a beginner, designing a "proper" opamp is quite hard. My prediction is that you will end up with an instable and unusable opamp. So first study what others do and understand why. \$\endgroup\$ Feb 15, 2021 at 10:41
  • \$\begingroup\$ Well, you only need the whole buffer and output part… the differential input is only about 1/4 of the stuff needed for a whole opamp. Good luck with that \$\endgroup\$ Feb 15, 2021 at 12:35


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