‌‌‌My setup is 2.1V on the gate. A 330 Ohm load between Vdd and drain. And Source to ground.

I stepped Vdd from 0V to 9V in small increments and measured the current through Vdd to ground. Got currents in the 2mA range and saturated around 1.8V.

I'm trying to compare current against the Voltage drop across JUST the MOSFET but there's voltage drop across the 330 Ohm resistor too. How do I isolate just the Vds drop without measuring the drop across the resistor and PLEASE give an example calculation. I've already captured the current through the system at each increment AND the voltage on VDD (0-9V). I know that once I get to saturation voltage the voltage drop across the MOSFET will be minimal. Just trying to understand (also trying to understand Q-point but I suppose that's another question).


  • 2
    \$\begingroup\$ do you undersand Ohm's law? \$\endgroup\$
    – jsotola
    Nov 12, 2019 at 21:32
  • \$\begingroup\$ I do but I don't know the resistivity of the MOSFET except during saturation zone? So how would I calculate drop without knowing how much dropped across the 330 Ohm resistor? Edit: I think I understand Ohm's law I should say. \$\endgroup\$ Nov 12, 2019 at 21:59
  • \$\begingroup\$ Oh man. I have the current. Sorry. Answered my own question. I don't need total resistance because I measured current. Apparently 2 sleepless nights diminishes brainpower. \$\endgroup\$ Nov 12, 2019 at 22:15

1 Answer 1


Just realized I measured the current. Just answering the question for anyone suffering from too little sleep and missing the obvious...

I measured the current and can calculate voltage drop across the 330 Ohm resistor with current.

e.g. 330Ohm x 2mA = 0.66V

Then I can subtract that drop with Vdd and that's the drop across the MOSFET.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

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