I simulated two circuits as shown below.

  1. The body terminal is shorted to source terminal and the resulting source voltage is 5V.

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  1. The body terminal is shorted to ground and Vs = 1V (Vg - Vth). I can't understand the reason behind this.

enter image description here

Also in the second case, why should Vov be zero. Why can't Vs be equal to Vg?


  • \$\begingroup\$ If the source is floating, the only current can come from the drain-source leakage and the body diode leakage. \$\endgroup\$ – τεκ Jan 25 '18 at 5:17
  • \$\begingroup\$ Yes, the body current is almost zero. So is the drain current. Since the source is floating the current could be 1 femtoampere and the source would still be at 1V. \$\endgroup\$ – τεκ Jan 25 '18 at 5:22
  • \$\begingroup\$ Why can't Vs be equal to Vg? Why should Vs and Vg be equal? what makes you think they should? I mean, they're not connected in any way. \$\endgroup\$ – Bimpelrekkie Jan 25 '18 at 6:48

Q: What is the voltage at node x?


simulate this circuit – Schematic created using CircuitLab

A: depends on the channel leakage of the MOSFET and the leakage current of the diode.

  • 1
    \$\begingroup\$ Although this does explain what happens to the source voltage, this isn't exactly the same situation as OP is asking. If you put 3 V DC at the gate using a Vdc then it would be identical. Then at x you'd get Vs = 3 V - Vt due to the source-bulk diode's leakage current. \$\endgroup\$ – Bimpelrekkie Jan 25 '18 at 6:46

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