mosfet driving LED

[drkidd22]

I have a mosfet trying to drive an LED. I have 2 different configuration as shown.
Why isn't the current through the resistor the same when connections are interchanged.

 

[ericgibbs]

hi,
Consider the actual Vgs voltage in the two cases.
In the M1 case the full V1 appears across the Gate to Source junction.
In the M2 case Vgs is reduced from V1 by the voltage drop across the Source resistor and LED.
E

 

[drkidd22]

Got it, my mistake was visualizing Vgs from G to GND not actually Vgs.
Thanks.

 

[MikeMl]

M1 is an "inverter", and M2 is a "source-follower" (non-inverting).

Here is a more useful way (DC sweep) of using LTSpice to show what is happening in the source-follower configuration:



This plots I(D1) vs the gate voltage. Note that you would have to drive the gate more positive than 3.3V to get significant current into D1