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Hello! Can you tell me the benefit of having the differential amplifier connected the way shown in the image. Also what's the purpose of devices in green circle and what's the benefit of placing it there?

Thank you!

  • 3
    \$\begingroup\$ You need to give an attribution to the source of the diagrams if they are not your own. It's site policy. It can also help provide context for the diagrams. \$\endgroup\$
    – Transistor
    Oct 28, 2020 at 12:17
  • \$\begingroup\$ Don't place the same picture (with and without your red/green markings) twice as then I start to look for differences, which there aren't any! Only the picture with the markings is enough. \$\endgroup\$ Oct 28, 2020 at 13:34
  • 1
    \$\begingroup\$ The part marked with red is the differential NMOS input pair with biasing, it is the opamp's input stage. If you do not "see" this then you need to step back and study the differential pair. The MOSFETs in the green area are the "cascodes" of the folded cascode structure. You should be familiar with the cascode already, if not, get studying! \$\endgroup\$ Oct 28, 2020 at 13:39
  • \$\begingroup\$ I know it is the input pair, but what's the benefit of having to bias it that way? I know hat cascodes are again what advantage do they provide by placing it there? \$\endgroup\$
    – realner
    Oct 28, 2020 at 13:49
  • \$\begingroup\$ Without folding the cascode, would you need a higher supply rail to get the required signal swing? \$\endgroup\$ Oct 28, 2020 at 17:13

1 Answer 1


The green FETS, 4 of them, allow rail_to_rail operation of the four nodes,

while (this is important)

preventing high voltage stresses on the rail_referenced FETs.

  • \$\begingroup\$ Which rail-referenced fets are you referring to? \$\endgroup\$
    – realner
    Oct 29, 2020 at 15:43

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