====== Experimentally finding model parameters ======

Sometimes you need to determine model parameters, based on having no real information.

In this example, we find the parameter "IS" of a diode, based on its forward voltage.

We would like to have a diode...

  D2 (a k) 1n4004

Now you need a model statement ...

  .model 1n4004 d (a whole bunch of parameters)

You could do a web search for a detailed model, but usually you 
can just do some experimenting..   

If you are at the stage where you are asking about this, the 
detail will only confuse you, most likely of the worst kind of 
confusion, where you don't realize it.

Most of the time, the only parameter you need for a diode is 
saturation current, which of course isn't in the data sheet.

ok .. so how do I figure that out??

Well ...  Look at the specs, or measure it.  Let's say we found 
that the forward voltage at 1 amp is .7 volts.  

Use the simulator to build a test circuit ..

  I1 (0 1) dc 1
  D1 (1 0) my_diode

You need a model statement ..
  .model my_diode d (is=something)

but I don't know what the value of something is.

With gnucap .. try this.......

  gnucap> build
  >i1 (0 1) dc 1
  >d1 (1 0) my_diode
  >.model my_diode d is=something
  >                   ******** blank line to exit build mode
  gnucap> probe  op  v(1)
  parameter something has no value
  gnucap> op
  #           v(1)      
   27.        0.83376                    **** using default IS
  gnucap> param something=1u
  gnucap> op
  #           v(1)      
   27.        0.35733   
  gnucap> param something=1n
  gnucap> op
  #           v(1)      
   27.        0.53599   
  gnucap> param something=1p
  gnucap> op
  #           v(1)      
   27.        0.71465   
  gnucap> param something=2p
  gnucap> op
  #           v(1)      
   27.        0.69673   
  gnucap> param something=1.5p
  gnucap> op
  #           v(1)      
   27.        0.70417   
  gnucap> param something=1.8p
  gnucap> op
  #           v(1)      
   27.        0.69945   
  gnucap> param something=1.7p
  gnucap> op
  #           v(1)      
   27.        0.70093   
  gnucap> param something=1.75p
  gnucap> op
  #           v(1)      
   27.        0.70018   
  gnucap> exit


There ..  close enough ,,,  
Now use the value 1.75p for IS.

Here's your model statement:
  .model my_diode d is=1.75p