Re: fast NPN in LT Spice

On Fri, 7 Jun 2024 14:03:16 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:

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

That's sort of what I'm talking about. You choose a part, you choose a
model, you choose a parameter that allows a model to simulate
performance more accurately. You choose a circuit configuration
with component values.

From the larger spreadsheet, you see models for 'similar devices
varying widely in performance in a particular circuit configuration.

So what is it, in the model, or in the circuit configuration, that
allows such a wide variation in performance. What parameter is a
true predictor? What weakness in the circuit is the wild card?

Looking for understanding or beneficial increase in knowledge here.
Not pushing or pulling the benefits or disadvantages of modeling
or breadboarding - already have pretty fixed opinions about that.

RL