Re: [Ngspice-users] sim debug

[Robert L. <Rob...@t-...>]

Hello Nathan,

  your R1 has a part value of
    10M
  which is 10 milli Ohm,
  you certainly wanted
    10MEG

Regards,
  Robert


Nathan Stewart <the...@gm...> writes:

> I'm trying to simulate a rather simple circuit that's well hashed out on
> various forums. My spice simulation however isn't behaving as expected. I
> reduced it to one of the simpler forms here:
> (It's a polarity protection circuit)
>
> The models I pulled from internet sources:
>
> .model 2N3904    NPN(Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259 Ise=
> 6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 Cjc=3.638p Mjc=.3085
> Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf
> =2 Rb=10)
>
> .model 2N3906    PNP(Is=1.41f Xti=3 Eg=1.11 Vaf=18.7 Bf=180.7 Ne=1.5 Ise=0 Ikf=
> 80m Xtb=1.5 Br=4.977 Nc=2 Isc=0 Ikr=0 Rc=2.5 Cjc=9.728p Mjc=.5776 Vjc=.75 Fc=.5
> Cje=8.063p Mje=.3677 Vje=.75 Tr=33.42n Tf=179.3p Itf=.4 Vtf=4 Xtf=6 Rb=10)
>
> .MODEL 1N4148   D(IS = 4.352E-9 N = 1.906 BV = 110 IBV = 0.0001 RS = 0.6458 CJO
> = 7.048E-13 VJ = 0.869 M = 0.03 FC = 0.5 TT = 3.48E-9)
>
> *==============  Begin SPICE netlist of main design ============
> R1 q1base vbat 10M
> Q1 Vout q1base vbat 2n3906
> R3 0 Vout 6.8k
> Vbat vbat 0 DC 9
> D1 1 0 1n4148
> R2 1 q1base 3.3k
> .end
>
> When I run the following simulation:
> dc vbat -9 9 0.1
> plot v(vout) v(q1base)
>
> It shows Vout tracking Vbat minus a diode drop (when it should be blocking) up
> until Vbat = -0.65v. After Vbat goes positive - Vout is zero, as in single
> precision roundoff zero. Also V(q1base) tracks Vbat exactly, when the diode
> should be clamping it to within a diode drop of ground.
>
> Were the circuit functioning in other than intended, I'd maybe expect to see a
> diode drop influence showing up somewhere in the positive output, as well as
> maybe the transistor vsat - so I think I'm doing something wrong in either  my
> simulation or models.
>
> The circuit behaved exactly the same when the diode was replaced by an NPN
> turning on the PNP, so my investigation is focused on the 2n3904 model at the
> moment. Can anyone shed some light on this behavior I'm seeing?
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