[time-nuts] Switching transistors, current sources, nonidealties and noise

Bob Camp kb8tq at n1k.org
Sun Jun 19 15:23:16 EDT 2016


Hi

Leakage

Bob

> On Jun 19, 2016, at 1:23 PM, Attila Kinali <attila at kinali.ch> 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.
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