Re: fast NPN in LT Spice

On Tue, 4 Jun 2024 00:10:32 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:
<snip>

Gosh, what a hideous mess, in many respects.

>
Do tell us why. You do claim to revel in electronic discussion.

Perhaps its the nonlinearity of the output stage, which is biased off.

So it's a switch, but the slow speed of the output, if engaged,
results in a stretched pulse.

The assertion that spice parameter Tf is related to spec sheet Ft is
only a guess.

The bfr92a model written into your simulation turns out to
be part of a more complete model published as a die-within-a-package.
There's little difference in performance when substituted into the
simulation.

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)
- act roughly like the original simulation.(217)
- oscillate at an unrelated frequency.(19)
- stall.(1)
- give incoherent wild results (2)

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.

RL