[time-nuts] Simulation

[J. Forster]

I've seen audio range power amps that will oscillate on a part of a cycle
because an output device with a higher fT was installed. Older vintage
parts with the same type JEDEC number never did that.

-John

============


> Hi
>
> It's a very rare thing to see jelly bean parts screened for RF parameters.
> Much more common to catch and fix an issue at the board level. Pretty rare
> to see discrete RF anymore anyway.
>
> Bob
>
>
>
> On Aug 14, 2010, at 3:56 PM, "J. Forster" <jfor at quik.com> wrote:
>
>>> 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
>>
>> ===========
>>>
>>> --
>>> mailto:oz at ozindfw.net
>>> Oz
>>> POB 93167
>>> Southlake, TX 76092 (Near DFW Airport)
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>
>>
>>
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>