Electronics > Beginners
Convert transistor switching time to frequency
gkmaia:
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?
David Hess:
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.
T3sl4co1l:
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
gkmaia:
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.html
But 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.
nwaelder:
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.
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