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" <[email protected]> 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:[email protected] >>> Oz >>> POB 93167 >>> Southlake, TX 76092 (Near DFW Airport) >>> >>> >>> >>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >>> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
