# Equivalent resistance in this mosfet circuit 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.

• 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. – Ariser Sep 24 at 15:02
• 3/2RonC vs 2RonC for Vc>0 – Tony Stewart Sunnyskyguy EE75 Sep 24 at 15:16
• Hint: this isn't really a pi configuration. Two of your FETs are actually in parallel. – The Photon Sep 24 at 16:11

Redrawing the ckt like this may ring some bells.

For example the first ckt: 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
• 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. – Elliot Alderson Sep 24 at 22:47
• Yep. But approximations will help on competitive exam questions like this because no information is given on mosfets and how much '1' is. – Meenie Leis Sep 25 at 2:34