> 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:o...@ozindfw.net > Oz > POB 93167 > Southlake, TX 76092 (Near DFW Airport) > > > > > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
