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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.design
Subject: Re: fast NPN in LT Spice
Date: Thu, 06 Jun 2024 14:05:24 -0400
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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