# (LTSpice)How to find the interception of the tangent line ??

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#### Beckett

##### Newbie level 1
Extract threshold voltage (Vt) of n- and p-type FETs. Use TT (nominal) model file. Hint: While supply voltage to the transistor’s drain/source is around 10mV (note that supply can also be at its full DC value), run a DC simulation over gate- source/source-gate voltage and find the derivative of the drain/source current and find the interception of the tangent line to the drain/source current curve with x-axis (when Vgs or Vsg holds value for Id=Is=0). You may also consider extracting gm curve for the explanation of your result, if relevant. This is just a guide you may use your own technique.
Any method you use, the Vt must be the direct result of simulation meaning that you’re not allowed to do any post-processing in (a). You may use .measure to achieve the goal of (a). You may use data visualization tool of your choice solely to plot your data.
This is the question, and I don't know how to use .meas command to do a tangent line...

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#### erikl

##### Super Moderator
Staff member
Something like this should work in HSPICE:

Code PHP - [expand]1
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.MEAS DC gm_max MAX d(Id(M1)) FROM 0V TO 1V
.MEAS DC vg1 AT Id(d(Id(M1)))='gm_max'
.MEAS DC id1 AT Vg(Id(M1))='vg1'
.MEAS DC b PARAM='id1-gm_max*vg1'
.MEAS DC vg0 PARAM='-b/gm_max'

Don't know the corresponding syntax for LTSpice.

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