Re: fast NPN in LT Spice

On 7/06/2024 4:05 am, legg wrote:

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>

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.

You don't get the choice, unless you are in the position of developing a new transistor for a new market.
Most of us are in the position of finding a transistor which we can buy -  preferably off the shelf - which we can use to do a specific job.
The options tend to be pretty restricted. There are a lot fewer broad-band transistors on the market than there used to be.
The manufacturer's Spice model give us the option of modelling a circuit which might work in some version of Spice, and tweaking the circuit to get the simulation to perform our job tolerably well.
We can't sell the simulation - customers want real circuits that work in real life.
--
Bill Sloman, Sydney
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