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schematic

simulate this circuit – Schematic created using CircuitLab

I have 3 NMOS are connected as shown above. I need to calculate equivalent resistance as seen by the capacitor so that I am able to calculate time constant for the above two cases.

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    \$\begingroup\$ Welcome to EE.SE. What exactly is your question? A well formed question ends with a question mark and can't be answered with yes or no. \$\endgroup\$ – Ariser Sep 24 at 15:02
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    \$\begingroup\$ 3/2RonC vs 2RonC for Vc>0 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 24 at 15:16
  • \$\begingroup\$ Hint: this isn't really a pi configuration. Two of your FETs are actually in parallel. \$\endgroup\$ – The Photon Sep 24 at 16:11
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Redrawing the ckt like this may ring some bells.

For example the first ckt:

schematic

simulate this circuit – Schematic created using CircuitLab

  • If the mosfet is on, substitute it by its on-resistance \$R_{on}\$
  • If the mosfet is off, substitute it by open ckt.
  • Find the net resistance that comes across A and B in parallel to capacitor
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  • \$\begingroup\$ The "on resistance" is a strong function of the \$V_{GS}\$ and the transistor characteristics. I fear that assuming that there is a single, fixed value for \$R_{ON}\$ will lead to significant errors. Just the fact that you described the MOSFET states as "on" and "off" suggests a gross oversimplification of reality. \$\endgroup\$ – Elliot Alderson Sep 24 at 22:47
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    \$\begingroup\$ Yep. But approximations will help on competitive exam questions like this because no information is given on mosfets and how much '1' is. \$\endgroup\$ – Meenie Leis Sep 25 at 2:34

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