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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: fast NPN in LT Spice
Date: Fri, 7 Jun 2024 14:03:16 +1000
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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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