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mosfet driving LED

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drkidd22

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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.
 

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ericgibbs

Well-Known Member
Most Helpful Member
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
A01.gif
 

MikeMl

Well-Known Member
Most Helpful Member
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:

D11.png

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
 

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