Re: fast NPN in LT Spice

On Thu, 6 Jun 2024 23:18:12 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:

On 6/06/2024 1:46 pm, legg wrote:

<snip>

 
If all the models with Tf<20ps are evaluated, you get unpredictable
results. Note that the bfr92a model doesn't actually meet this
limitation, but other similar models do (~bfr93). There are roughly
270 of them.
 
Each will either:
 
- fail to engage with the slow output detector.(31)

>
If they aren't fast enough to get triggered as emitter-coupled
monostable   by a pulse that is 400psec wide at hallf-maximium voltage
of 100mV.
>

- act roughly like the original simulation.(217)
- oscillate at an unrelated frequency.(19)

>
Broad-band transistors will oscillate without a base-stopper of adequate
resistance - I tended to end up with resistors between 22R and 33R.
There are better solutions, but in the work I did it wasn't worth the
trouble of finding them.
>

- stall.(1)

>
Up the gain.
>

- give incoherent wild results (2)

>
Change the base-stopper resistance.
>

http://ve3ute.ca/query/Tf_20ps_vs_bfr92a.zip
 
Just why one model does one thing, while another does something
else might be interesting to figure out.

>
No model is perfect. If you model something and it seems to work, it's
worth putting together a real circuit (which takes longer) and seeing
what it actually does.
>
If it doesn't work, or doesn't work all that well. fiddling with the
model may point the way to something that might work better.
>
--
Bill Sloman, Sydney

It's difficult to isolate a model parameter or product of parameters
 that predict the performance demonstrated in this simulation.

Oscillators seem to have ( ~ mostly) high Rbb, which is
device-specific. . . . but also lower ( 1/100) IKR, which you'd
think was irrelevant.

Bf, Tr, Tf and capacitances don't stand out.

I suppose you'd need to look at the numbers separating astable from
monostable operation. You wouldn't want an unspecified component
characteric to dominate basic circuit function.

RL