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Electronics => Beginners => Topic started by: sean0118 on November 16, 2015, 12:44:06 pm

Title: Modeling Depletion Mode MOSFETS using LTspice?
Post by: sean0118 on November 16, 2015, 12:44:06 pm

Hi everyone,

I have made component models in LTspice before, but this one has me stuck. What parameter in the model makes it operate as a depletion mode MOSFET? (as in, not enhanced mode)


I have seen it suggested online that a negative values for Vth is what's needed (for an NPN), but this doesn't make any sense to me. There should be no threshold voltage, NPN depletion mode MOSFETs conduct with the base at 0V...  :o

Any ideas?  ;)

Title: Re: Modeling Depletion Mode MOSFETS using LTspice?
Post by: c4757p on November 16, 2015, 01:07:25 pm

No, that's still accurate. They still have a threshold voltage - N-channel depletion MOSFETs do indeed conduct with the gate at 0V, but stop conducting at a certain negative voltage. That's the threshold voltage. If you look at the I_D(V_GS) curve of a depletion-mode MOSFET, it's just shifted to the left from that of an enhancement-mode MOSFET.

Title: Re: Modeling Depletion Mode MOSFETS using LTspice?
Post by: sean0118 on November 16, 2015, 01:22:34 pm

Quote from: c4757p on November 16, 2015, 01:07:25 pm

...curve of a depletion-mode MOSFET, it's just shifted to the left from that of an enhancement-mode MOSFET.

Ah, of course! Thanks I was somehow thinking of it all backwards, makes sense now.  (I was thinking for a Vth of -3V in LTspice it would conduct even more at -4V, not the case obviously  ;) )

Title: Re: Modeling Depletion Mode MOSFETS using LTspice?
Post by: Jay_Diddy_B on November 16, 2015, 01:24:54 pm

To the OP,

What kind of circuit are you modelling? switching behaviour or small-signal?

Can you provide the part number of the part that you want to model?

Regards,

Jay_Diddy_B