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I intend to design a CMOS voltage regulator that takes 2.5V supply and outputs 1.2V. I am starting off with an opamp+pass transistor design.

Will there be a specific advantage to using an NMOS pullup device vs PMOS pulldown for the pass transistor?

Additional details: The regulator supplies this constant voltage but is also supposed to sink a lot of current (~5 mA)

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    \$\begingroup\$ This is an IC design or a discrete components design? \$\endgroup\$ – The Photon Jun 13 '13 at 23:19
  • \$\begingroup\$ Sorry. It is an IC design question. \$\endgroup\$ – BobLobLaw Jun 13 '13 at 23:21
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The primary advantage of an NMOS transistor is that the higher mobility of electrons means that you can use a physically smaller transistor to pass a given amount of current. Of course, the disadvantage is that if you use an NMOS pass transistor then the maximum \$V_{GS}\$ you can provide is 1.3V, but if you use a PMOS transistor the maximum \$V_{GS}\$ is 2.5V. Since you want to sink as well as source current the typical way of doing this is to add a fully complementary buffer (i.e. a big inverter) at the output of the op amp, with a PMOS to 2.5V and an NMOS to ground.

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  • \$\begingroup\$ I do not have to source any current. The output of this regulator will only be switched in when the receiving circuit (high cap load) needs to sink current while expecting a constant voltage for its bias. I am assuming this would mean NMOS would be preferable since using a PMOS would mean no path for the current to sink to. \$\endgroup\$ – BobLobLaw Jun 19 '13 at 19:10
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    \$\begingroup\$ Yes, you certainly want an NMOS. What you are describing would not normally be called a voltage regulator with a pass transistor, it's a constant voltage load. \$\endgroup\$ – Joe Hass Jun 19 '13 at 19:35
  • \$\begingroup\$ Would that be called a constant voltage load even if the load itself is variable? I was under the impression this would be a shunt regulator or just a general voltage regulator that sinks instead of supplying current. I see why the NMOS wouldn't be a 'pass' device as we are not trying to pull the voltage to ground but I am not clear on the nomenclature. \$\endgroup\$ – BobLobLaw Jun 19 '13 at 20:50

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