Re: gEDA-user: always build a prototype!

[al davis <ad151@xxxxxxxxxxxxxxxx>]

On Thursday 09 August 2007, Bob Paddock wrote:
> What do any of the circuit simulators give for this
> configuration:
>
> With the emitter of a 2N3904 or 2N2222 hooked to +12V through
> a 1k resistors, and the base grounded, what is the voltage
> measured from the base to the collector?  The collector is
> open.
>
> Any (NG)Spice/GNUCap etc. users out there that can try it?

It doesn't simulate correctly.

gnucap> build
>q1 1 0 2 n
>re 2 3 1k
>vb 3 0 dc 12
>.model n npn
>
gnucap> print op v(nodes) vbe(q1) vce(q1)
gnucap> op
#           v(1)       v(2)       v(3)       vbe(q1)    vce(q1)   
 27.        3.0568     12.        12.       -12.       -8.9432    
gnucap> 


The BE junction is reverse biased, so it will act like a zener 
in the real circuit, about 5 volts.  The simulation model does 
not include this effect, so Vbe simulates as -12.

The collector voltage gets messed up by "gmin", which adds 
strays to avoid a singular matrix.  I am not sure what it would 
be on a real circuit.  It would be between 0 and Ve.  What 
gnucap says depends on settings like gmin.

If you build it, and measure it, the meter will influence the 
reading.

Let's add the meter resistance.....

gnucap> build
>rmeter 1 0 10meg
>
gnucap> op
#           v(1)       v(2)       v(3)       vbe(q1)    vce(q1)   
 27.        119.99u    12.        12.       -12.       -12.       
gnucap> 

Gnucap and Spice use the same model, so the result should be the 
same.  It doesn't model BE breakdown.

Models make assumptions about how they will be used.  If you go 
outside the valid region, you get bad results.




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