I am studying the following circuit and my professor calls it a two-stage amplifier. However I don't understand why as I just see

  • a NMOS differential pair: MN0 and MN1
  • a PMOS active load (current mirror): MP0 and MP1
  • the bias circuit current mirror: MN2 and MN3

Isn't this just a differential pair (one stage)? Where is the second stage (common source)?

enter image description here

  • 5
    \$\begingroup\$ Not an answer, but I, my (RFIC design) thesis advisor, and my former VLSI professor all call this structure a single-stage five-transistor op amp. You may want to check with your professor directly in their office hour; this could be a typo or they could have reasoning that isn't clear to us. \$\endgroup\$
    – nanofarad
    Commented Dec 21, 2021 at 22:23
  • 4
    \$\begingroup\$ This is indeed a 5 transistor op amp (ota), there is only one stage. \$\endgroup\$
    – Mike
    Commented Dec 21, 2021 at 22:55
  • 1
    \$\begingroup\$ And if the prof says it's a two stage amplifier, then when you are in that class it's a two stage amplifier, and maybe you should avoid more classes with that prof... \$\endgroup\$
    – TimWescott
    Commented Dec 22, 2021 at 4:03
  • \$\begingroup\$ It is a differential stage with an active load. Search brings up many sites such as this. \$\endgroup\$
    – Syed
    Commented Dec 22, 2021 at 4:07

2 Answers 2


It is not called a two-stage amplifier in any common usage.

It is a single stage differential amplifier because there is a single node where voltage gain occurs -- at the drain of MN1. The signal there is the difference of the signals at the gate of MN1 (directly), and MN2 (via the PMOS current mirror)


When you calculate the gain and the input impedance of the shown circuit - in particular for unsymmetric operation with Vin1=0 (or Vin2=0) - you can treat the whole device as a common-drain-common gate combination.

Hence, it is not wrong to say that the system acts as a two-stage amplifier.


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