I'm having trouble working with MOSFETs. I've never actually designed them into any circuits before, and trying to do so based on what I think I've learned about 'em isn't working.
I'm attaching a schematic, a SPICE list, and a graph of the SPICE run. I would expect the graph to show V(100) dropping virtually to zero shortly after V(2) goes positive, but it ain't happening.
Perhaps the 2N7000 model I have isn't right, though I think I got it from the manufacturer. That's also hinted at by the fact that SPICE (ngspice, actually) thinks the gate is enough of a current sink to drop a comparator's high output from 5V to about 1V unless I introduce a 10MΩ gate resistor. However, the graph doesn't look much different using a generic NMOS model.
What really obvious and simple thing am I not getting here?
Thanks!
(The end result is to use the MOSFET to act as close to a zero-resistance path for a meter, but shut off current when the voltage goes over the meter's rating.)
I'm attaching a schematic, a SPICE list, and a graph of the SPICE run. I would expect the graph to show V(100) dropping virtually to zero shortly after V(2) goes positive, but it ain't happening.
Perhaps the 2N7000 model I have isn't right, though I think I got it from the manufacturer. That's also hinted at by the fact that SPICE (ngspice, actually) thinks the gate is enough of a current sink to drop a comparator's high output from 5V to about 1V unless I introduce a 10MΩ gate resistor. However, the graph doesn't look much different using a generic NMOS model.
What really obvious and simple thing am I not getting here?
Thanks!
(The end result is to use the MOSFET to act as close to a zero-resistance path for a meter, but shut off current when the voltage goes over the meter's rating.)
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