[time-nuts] Simulation

[J. Forster]

>  On 8/14/2010 10:08 AM, J. Forster wrote:
>> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters
>> or
>> instructions in a production design is a fool.
>>
>> If you are that silly, you must fully specify the selection criteria.
>>
>> -John
> This is, easily said, a wonderful goal, and absolute fantasy.   It's
> optimistic at best to expect someone to anticipate all contingencies.
> It's certainly good practice to specific critical parameters, but it's
> rarely makes economic sense to specify every possible detail.

OK, important uncontrolled parameters.

For example, I'd consider things like hFE; VCEsat; VCBO, fT and others
important, but not the package capacitance in a low frequency transistor.
There are clearly unimportant parameters and essentially irrelevant ones.
That's where experience and good judgement comes in.

If your circuit is not stable with a high fT part, that needs to be tested
or the design fixed.

> As to relying upon unspecified parameters, most datasheets are woefully
> incomplete.  If you are going to use any significant number parts, it's
> unlikely that you'll be able to get everything specified, much less get
> compliance commitments for each parameter.  Few vendors are willing to
> do the testing required to guarantee a substantial number of parameters,
> and the simple reason is no one is willing to pay for it.

If your design is that critical, you may have to do incoming
inspectrion/selection or send the parts to a company that does.

> I've spent quite a bit of time dealing with maintenance of military
> systems that would be long obsolete in any other business.  After
> obsolescence, the number one problem was parts that meet all published
> specs, but had changed performance so much (for better or worse) that
> they no longer functioned in the application.  A common problem is Ft or
> gain, but leakages are often orders of magnitude different. As often as
> not, they were much worse.

Certainly, old Ge power transistors have ICBO issues.

-John

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