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

[Attila Kinali]

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.