tombro

T

tombro

New Member
I am trying to get started with LTSpice but I cannot get an nmos mosfet to fully switch on.
It appears that the nmos has an internal impedance of about 5K ohms since the max drain current I get is approx 3mA.
By raising drain load resistor, the switch on voltage at the drain reduces , being about 5 volts with approx 5K ohms load resistor.
I have changed all voltages up and down and reversed them all but fail to get any high cuurnet in the drain.
Could the nmos in LTSpice simulator be a very high internal reesistance type.
Any suggestions would be gratefuuly appreciated.
 
What model are you using?

The default is a generic depletion mode MOSFET which is normally on and is turned off when the gate voltage falls below the source.
 

Post your .asc file (or .zip file if you are incorporating other subcircuits)
 
I apply a negative voltage to the gate but there does not seem to be any conduction at al (or any change)

* E:\SPICE\Draft3.asc
M1 P001 N003 0 0 NMOS
R2 N001 P001 12R
R1 N003 0 10K
V1 N001 0 12V
V2 N004 0 PULSE(0 4 50m 1m 1m 48m 100m 10)
S1 N002 P002 N004 0 SWITCH
V3 N002 0 -2V Rser=0.01
R3 N003 P002 100R
.model NMOS NMOS
.model PMOS PMOS
.lib C:\Program Files\LTC\LTspiceIV\lib\cmp\standard.mos
.tran 2s
.MODEL SWITCH VSWITCH(Ron=0.01)
.backanno
.end

This is my netlist, hope it means something, ie is it OK.
Thanks for your help
broonie
 
How about attaching the file?

I tried pasting it into a text file and saving it as a .asc but LT Spice wouldn't read it.
 
Here is the Id vs Vg of the default NMOS device. I know that LTSpice is used to simulate IC designs, but offhand I don't know how to pass the channel Width and Length parameters. The default model must be tiny. Any "real" FET in the LTSpice library has Amps of current with Vg > 3*Vt.
 

Attachments

  • D87.png
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Last edited:
Is that the default model?

If so I was wrong earlier when I said it's depletion, that graph is clearly enhancement.
 
Thank you for your help.
I think the low ID current agrees with your Id vs Vgs suggesting I am using the default NMOS(evev though I ddon't know enough about it.)
I think I need a MODEL statement to get moving, so if I can find the Open Forum I will move there.
Thanks again and I apologise if I have taken up your valuable time by using the wrong site forum.
Tombro
 

Place the default NMOS device as you have already done. Right mouse click on the NMOS instance. Select one of the standard devices that are already in the LTSpice library. If the one you want to use is not in the list, then find the Spice Model, and follow the instructions posted many places on how to hook up the new model to the existing NMOS symbol.
 
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