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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

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    \$\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
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    \$\begingroup\$ This is indeed a 5 transistor op amp (ota), there is only one stage. \$\endgroup\$
    – Mike
    Commented Dec 21, 2021 at 22:55
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    \$\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

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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)

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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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