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On Tue, 4 Jun 2024 00:10:32 +1000, Bill Sloman <bill.sloman@ieee.org>The pulse stretching happens between the two BFR92 transistors, which act as a classical emitted-coupled monostable, which stretch the input spike out to about 30nsec
wrote:
<snip>Perhaps its the nonlinearity of the output stage, which is biased off.Gosh, what a hideous mess, in many respects.>
Do tell us why. You do claim to revel in electronic discussion.
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 isNot one that I made. The BFR92a is just the default broad-band transistor, which is why Jeroen Belleman picked it too.
only a guess.
The bfr92a model written into your simulation turns out toIf 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.
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)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.
- oscillate at an unrelated frequency.(19)
- stall.(1)Up the gain.
- give incoherent wild results (2)Change the base-stopper resistance.
http://ve3ute.ca/query/Tf_20ps_vs_bfr92a.zipNo 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.
Just why one model does one thing, while another does something
else might be interesting to figure out.
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