[eispice] Re: behavior test case problems

Hi Jon,

I think you may have forgotten to include the resistor in your analysis. 
Based on my hand-analysis eipsice returns the correct result (I also 
tried using the MathCAD root-solver and got the same result). Of course 
I could have just as easily made an error, if you see one please let me 
know.

Bn = 5e-6 x 2 / 1
Bp = 2e-6 x 2 / 1
Vt = 0.7
Vct = 3.3 - 0.7
R = 100e3

In(Vg) = (Vg > Vt) ? (0.5 x Bn x (Vg-Vt)^2) : 0.0
Ip(Vg) = (3.3 - Vg > Vt) ? (-0.5 x Bp x (Vct-Vg)^2) : 0.0
Ir(Vg) = Vg/R

Need to find the root of:
In(Vg) + Ip(Vg) + Ir(Vg)

Using the world famous quadratic equation (assuming 2.6 > Vg > 0.7):
x = (-b +/- sqrt(b^2 - 4*ac)) / 2a
a = 0.5 x Bn - 0.5 x Bp
b = (0.5 x Bn x -2 x Vt) - (0.5 x Bp x -2 x Vct) + 1/R
c = 0.5 x Bn x Vt^2 - 0.5 x Bp x Vct^2

I get x = 0.107 and -34.6, neither falls into the 2.6 to 0.7 range so 
one on the FETs must be off.

Assuming only the NMOS device is on, (can just set Bp to 0.0), 3.3 > Vg 
 > 2.6:
x = -0.026 and x = -18.574, neither falls in the range.

Assuming only the PMOS device is on, (can just set Bn to 0.0), 0.7 > Vg 
 > 0.0:
x = 55.077 and x = 0.123
0.123 V falls in the range, and is the value that eispice calculates.

The reason that I'm guessing that you may have forgotten to include the 
resistor in your analysis is because if I substitute 1Gig-Ohm in the 
equations above, and in eispice I get 1.436V, which is what you were 
calculating.

Let me know if you think I'm on the right-track or have over-looked 
something.

Thanks for the great discussion! Analysis / discussion like this one is 
a great way to improve eispice!

Cheers,
Charles

Jon Choy wrote:
>   Charles,
>
>     You are right about the sign error; but the solution is incorrect. The 
> answer is around 1.4V. I ran this circuit with an Excel spreadsheet and 
> writing down the equations as well as writing a Python program that 
> calculates the DC point using a Newton - Raphson iteration technique it 
> get's a similar answer of about 1.43V.
>
>   Jon
>
>
>   
>> From: Charles Eidsness <charles@xxxxxxxxxxxxxxxxxxxxx>
>> Reply-To: eispice@xxxxxxxxxxxxx
>> To: eispice@xxxxxxxxxxxxx
>> Subject: [eispice] Re: behavior test case problems
>> Date: Tue, 19 Dec 2006 15:46:15 -0500
>>
>> Hi Jon,
>>
>> I don't have a lot of experience modelling semi-conductor devices in
>> this way  so I may be wrong, but shouldn't the current through the PMOS
>> device run in the opposite direction w.r.t. the NMOS device? If it's
>> turned around, i.e.:
>> return -0.5*self.betap*(Vs - Vg - self.vtp)**self.power
>> the simulation runs, but gives in an output of 0.7Vs. Which may not be
>> what you were expecting?
>>
>> Cheers!
>> Charles
>>
>> Jon Choy wrote:
>>     
>>>   Charles,
>>>
>>>     Thanks for the feedback. I did notice the typos after I sent the 
>>>       
>> case on
>>     
>>> to you; but they still failed and now I know why.  No problem about 
>>>       
>> working
>>     
>>> through the bugs. I anticipated this being sort of an alpha user. Work 
>>>       
>> on it
>>     
>>> when you get a chance, I'm sure you've got your day job as well.
>>>
>>>   Jon
>>>
>>>
>>>
>>>       
>>>> From: Charles Eidsness <charles@xxxxxxxxxxxxxxxxxxxxx>
>>>> Reply-To: eispice@xxxxxxxxxxxxx
>>>> To: eispice@xxxxxxxxxxxxx
>>>> Subject: [eispice] Re: behavior test case problems
>>>> Date: Mon, 18 Dec 2006 18:47:03 -0500
>>>>
>>>> Hi Jon,
>>>>
>>>> You're not abusing, you're stretching. :) It was intended to be used in
>>>> that way, but you're the first to try it.
>>>>
>>>> It looks like I have some work to do on error messages. They're way to
>>>> cryptic, especially for the new PyB device. I'll see what I can do.
>>>>
>>>> Your B-Element based model uncovered a bug in the calculon library's
>>>> parser. It requires a space between the '-' sign and constants. I've
>>>> fixed it in my stream and it will be fixed for the next release but for
>>>> now you can just add a space. Also, the B element doesn't recognize the
>>>> 'GND' identifier, it only recognizes 0 as a ground indicator. I can add
>>>> GND as well but I'm hoping that as soon as we get this PyB element
>>>> working the only use for the old B element will be for porting in 
>>>>         
>> Spice3
>>     
>>>> decks which only use 0.
>>>>
>>>> If you change:
>>>> d.pmosx = dev.B('4', GND, 'i',"0.5*2e-6*(v(1,4)-0.7)^2")
>>>> to:
>>>> d.pmosx = dev.B('4', GND, 'i',"0.5*2e-6*(v(1,4) - 0.7)^2")
>>>>
>>>> and:
>>>> d.nmosx = dev.B(4, GND, 'i',"0.5*5e-6*(v(4,GND)-0.7)^2")
>>>> to:
>>>> d.nmosx = dev.B(4, GND, 'i',"0.5*5e-6*(v(4,0) - 0.7)^2")
>>>>
>>>> The simulation runs, until it fails to linearize. I'll have to fiddle
>>>> with the simulator to get it working further than that, which will take
>>>> a little bit longer. I'll post again once I get it working.
>>>>
>>>> The two PyB based models had a couple of typos. The lines:
>>>> def model(self, Vg, Vs)
>>>> should have a 4th argument (because you defined 3 input arguments), you
>>>> can change them to something like:
>>>> def model(self, Vg, Vs, Vp)
>>>> Also, for the second model the if statement references vtp but it 
>>>>         
>> should
>>     
>>>> reference vtn, i.e.:
>>>> if ((Vg - Vs)> self.vtp):
>>>> should be:
>>>> if ((Vg - Vs)> self.vtn):
>>>>
>>>> Once you make those two changes it should run as far as the 'failed to
>>>> linearize' as well.
>>>>
>>>> I'm sure with some useful messages those two typos would have popped
>>>> right out. It's tough to debug when every error message is "TypeError:
>>>> bad argument type for built-in operation". I'm guessing it's really
>>>> referring to some bug in the way I'm passing error messages between C
>>>> and Python, I'll look into it.
>>>>
>>>> Thanks for having patience as we work through the bugs, and I'll get
>>>> back to you on those 'failed to linerise's'
>>>>
>>>> Cheers,
>>>> Charles
>>>>
>>>>
>>>> Jon Choy wrote:
>>>>
>>>>         
>>>>>   I've tried a couple of behavior cases that don't seem to run. I hope 
>>>>>           
>> I
>>     
>>>>> haven't abused the capability. I'm simply seeing if a mos divider
>>>>>
>>>>>           
>>>> circuit
>>>>
>>>>         
>>>>> will work. I have two devices and behavioral nmos and pmos with the
>>>>>
>>>>>           
>>>> gate,
>>>>
>>>>         
>>>>> drain of both the pmos and nmos tied together. I have tried three
>>>>>
>>>>>           
>>>> different
>>>>
>>>>         
>>>>> cases. Let me know what you think.
>>>>>
>>>>> #!/usr/bin/python
>>>>>
>>>>> from eispice import *
>>>>>
>>>>> # -- PyB Defined Model --
>>>>> class pmos_sat(dev.PyB):
>>>>>
>>>>>   def __init__(self, p, n, g,  s, b, kp=2e-6,wp=2,lp=1,power=2.0):
>>>>>     dev.PyB.__init__(self, p, n, Current, v(g), v(s), v(b))
>>>>>     self.kp = kp
>>>>>     self.wp = wp
>>>>>     self.lp = lp
>>>>>     self.vtp= 0.7
>>>>>     self.betap = kp*wp/lp
>>>>>     self.power=power
>>>>>   def model(self, Vg, Vs):
>>>>>     if ((Vs - Vg)> self.vtp):
>>>>>       return 0.5*self.betap*(Vs - Vg - self.vtp)**self.power
>>>>>     else :
>>>>>       return 0
>>>>> class nmos_sat(dev.PyB):
>>>>>
>>>>>   def __init__(self, p, n, g,  s, b, kn=5e-6,wn=2,ln=1,power=2.0):
>>>>>     dev.PyB.__init__(self, p, n, Current, v(g), v(s), v(b))
>>>>>     self.kn = kn
>>>>>     self.wn = wn
>>>>>     self.ln = ln
>>>>>     self.vtn= 0.7
>>>>>     self.betan = kn*wn/ln
>>>>>     self.power=power
>>>>>   def model(self, Vg, Vs):
>>>>>     if ((Vg - Vs)> self.vtp):
>>>>>       return 0.5*self.betan*(Vg - Vs - self.vtn)**self.power
>>>>>     else :
>>>>>       return 0
>>>>>
>>>>> # Now to use the model
>>>>> kp = 2e-6
>>>>> wp = 2
>>>>> lp = 1
>>>>> vtp= 0.7
>>>>> betap = kp*wp/lp
>>>>> power=2
>>>>> kn = 5e-6
>>>>> wn = 2
>>>>> ln = 1
>>>>> vtn= 0.7
>>>>> betan = kn*wn/ln
>>>>>
>>>>> d = Deck('PyB Defined Behavioral MOS divider')
>>>>>
>>>>> d.V0 = dev.V(1, GND, 3.3)
>>>>> #d.pmosx = pmos_sat(4, GND, 4, 1, 1)
>>>>> #d.pmosx = dev.B('4', GND, 'i',"0.5*betap*(v(1,4)-vtp)^2")
>>>>> d.pmosx = dev.B('4', GND, 'i',"0.5*2e-6*(v(1,4)-0.7)^2")
>>>>> #d.nmosx = nmos_sat(4, GND, 4, GND, GND)
>>>>> #d.nmosx = dev.B(4, GND, 'i',"0.5*betan*(v(4,GND)-vtn)^2")
>>>>> d.nmosx = dev.B(4, GND, 'i',"0.5*5e-6*(v(4,GND)-0.7)^2")
>>>>> d.rout = dev.R(4,GND, 100000)
>>>>>
>>>>> d.op()
>>>>>
>>>>> d.plot()
>>>>>
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