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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 19 18:39:07 EDT 2016

On Sunday, June 19, 2016 07:23:09 PM Attila Kinali 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
Smaller Early effect at operating voltage for 2N3904 than lower voltage high 
frequency transistor and hence lower current modulation?
When measuring the delay of a 2 stage synchroniser,the delay range of interest 
is 1-2 synchroniser clock periods and nonlinearities within the first clock 
period are of little interest as long as the effect is repeatable and has 
settled out within 1 clock period of the transition, the effect is merely an 
offset and thus of little consequence.

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