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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?

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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.

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  • \$\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

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