Hi Leakage
Bob > On Jun 19, 2016, at 1:23 PM, Attila Kinali <[email protected]> wrote: > > Good evening, > > I have recently been looking into BJT's and their switching properties. > Because a time-to-amplitude converter does similar things as I would like to, > I have been looking in what people do with those. First thing that strikes > me as kind of odd is that almost all designs I have seen use some general > purpose transistor (like 2N2222, 2N3904, BC848...). The only two exceptions > seems to be Guide Technology, who use an UPA806T (RF npn pair) for the > diff-pair current switch[1] in their TAC and a group at Oulu University[2]. > But even Guide Technology uses an 2N3904 for the current source. > > Having put the circuit through Spice, I see that the current through > the tail fluctates violently during the time when the current switches > from one transistor of the pair to the other. The reason for this seems > to be that the f_t of the current source transistor is too low to compensate. > Trying to replace the current source with an RF transitor like BFU520 > that has an f_t of 10GHz helps to dampen these fluctuations by a factor of 2, > but they are still there. > > Why do people use general purpose transistors in these places, even > though RF transistors definitly improve switching behaviour? > > I dimply remember that someone said/wrote once, that RF transistors have > a higher noise. But if I look at the datasheet, the quoted noise figure > for the BFU520 is <1.6dB while the noise figure of the 2N3904 is 2dB best > case. > As I still do not really know how to read single transistor datasheets, > I am pretty sure I missed something fundamental there. > > > Attila Kinali > > [1] "Time Interval Analyzer Having Interpolator With Constant Current > Capacitor Control", US Patent 6'091'671 > > [2] "Wide-Range Time-To-Digital Converter With 1ps Single-Shot Precision", > by Keränen, Määttä, Kostamovaara, 2011 > > -- > Malek's Law: > Any simple idea will be worded in the most complicated way. > _______________________________________________ > 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.
