I understand how the logic levels of an NMOS work with a pull-up resistor. 1. What would happen though if there was no resistor attached? (What would the logic levels look like and why?) 2. What would happen if there was a pull down resistor instead?


N channel mosfets when used in a typical switching configuration connect their output to ground when turned on and leave their output disconnected when turned off.

So with a pull down the output will be permanently stuck low. With no pull at all the output will switch between ground and floating. In the absense of any external influences a floating output will generally stay where it was last set so again the output will be stuck at ground.

Note that there is a trick involving a N channel mosfet with special geometry that can be (and usually was) used as a pullup instead of a resistor.

  • \$\begingroup\$ Specifically a depletion-mode NMOSFET. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 26 '15 at 2:13
  • \$\begingroup\$ Not necessarily - an NMOS with a long channel and gate connected to gate works well enough \$\endgroup\$ – jp314 Nov 26 '15 at 2:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.