Convert transistor switching time to frequency - Page 1
Forum section: electronics / beginners

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 13, 2019, 02:42:53 am
content: I've got a vintage transistor datasheet that shows on and off times for the switching in microseconds.But I would like to convert that to MHZ so I can compare to my other transistors.If I were to sum the time it takes to turn on, store and off it would be around 2uS and that is pretty much 0.5Mhz which is a bit weird.If there a way to define frequency base on on/off switching time?

author: David Hess
author_rank: Super Contributor
timestamp: May 13, 2019, 03:47:23 am
content: They are not really comparable. Switching times assume saturated operation and current-gain-bandwidth product assumes linear operation. Add to this that anti-saturation will remove storage time and there are actually three different sets of specifications.Other characteristics like output capacitance sometimes take on more importance in switching applications.Incidentally, you might think that gold-doped RF transistors would perform well as gold-doped fast saturated switches but my experience is that they do not. There must be some fundamental difference in construction.Now if you have a switching application where you know frequency and duty cycle, the switching characteristics tell you everything you need to know.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2019, 01:43:04 pm
content: Broadly speaking, I would guess low MHz, maybe fractional MHz.But that's just a guess. David's spot on, and trying to make any better of a guess really isn't meaningful.Why, what do you need fT for? Or Fsw, or whatever frequency it is you're after?Tim

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 13, 2019, 07:59:22 pm
content: Basically I harvested heaps of N1F4M, N1L3M, AA1F4M, AA1F4N transistors from an old stereo system.https://pdf1.alldatasheet.com/datasheet-pdf/view/112804/NEC/AN1F4M.htmlBut their datasheets do not mention frequency.And usually when I need to mach a transistor for replacement one of the things I check is the frequency.Without knowing what frequency these harvested transistors are I can't really use them.Unless I learn how to define their frequency based on the datasheet... I just do not have the knowledge to do that. But I could learn if there is a way to.

author: nwaelder
author_rank: Newbie
timestamp: May 13, 2019, 08:47:03 pm
content: The relation for one dominant lag is:f(-3db) = 0.35/Tr where Tr is the rise time measured between 10% and 90% of the step input.Lookup risetime-bandwidth relation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2019, 11:17:09 pm
content: Ah...They'll probably replace TIP31/32 or 2N3055 sorts of things. Within voltage, current and power ratings of course.I'd just throw 'em out, they're pretty weak spec. Or, perhaps only usefulbecausethey're so bad? (You can make the mistake of putting in a too-good transistor in a weak circuit, and it oscillates like mad!)Tim

author: bson
author_rank: Unknown
timestamp: May 14, 2019, 01:59:44 am
content: One thing worth paying attention to is the actual test circuit used for measurement. Does it look like how you plan to use the part? Can you infer something for your particular uses from it? If not, or very different, the specs it's used to measure will be meaningless...

author: David Hess
author_rank: Super Contributor
timestamp: May 14, 2019, 02:32:01 am
content: Quote from: nwaelder on May 13, 2019, 08:47:03 pmThat works for a linear system and Ft (1) assumes that as it is a small signal parameter but even without other factors, switching is a large signal behavior and the switching specifications reflect this. There is a relationship but it includes too many other factors to reliably use hfe alone.Quote from: gkmaia on May 13, 2019, 07:59:22 pmThat is true even of some modern datasheets like the MJE371/MJE521. The BD135 to BD140 series have no specified Ft from ST or Fairchild but the Savant (?) datasheets do specify it. Can the Savant specifications be trusted?QuoteAnother good thing to check is capacitance but that is not much help here. Looking at the datasheet you linked, I can see why there is no gain-bandwidth product specification. The AN1F4M is a biased transistor with internal series and shunt resistors. It is not really suited for linear applications so switching times are all that matter.QuoteFor a lot of applications the current-gain-bandwidth product is largely irrelevant. In principle it is not difficult to measure with slow transistors with just a function generator and oscilloscope.(1) Actually fTbut I am lazy.