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

[jimlux]

On 6/19/16 10:23 AM, 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.
>
>



I'll bet the noise contribution is trickier to figure out. There's 
current noise and voltage noise, for one thing.  A few months back, we 
were designing (and building) a low noise amplifier for 5-30 MHz. 
There's a whole trade between microwave parts (specified in terms of NF 
with 50 ohm source, typically down to the "DC" frequency of 50MHz) and 
Op-Amps (which have noise voltage and current plots in the data sheet) 
and discrete devices of one sort or another (which have scant noise 
information).

There's also huge differences among specific mfrs and lots for the same 
JEDEC number (e.g. 2n2222 or 2n3904) and noise, since to use the JEDEC 
number, you just have to meet the requirement.. after that, you could be 
orders of magnitude better or just squeak by.  There's a very low 
leakage JFET (that I can't remember the number) where there's one 
company that makes "really, really good" ones, and the rest are "meet 
the data sheet". Nuclear instrumentation amplifier builders want only 
the "special" ones.

There's plenty of low noise amplifier designs out there and the 
descriptions typically assert "we selected this device because it had 
good noise properties", but then, typically, do not explain why that one 
worked, and the other half dozen that are superficially similar (on the 
data sheet or in terms of internal construction) didn't.   I suspect 
that in some cases, it's what they had in stock, they built it, it 
worked, and then there was no more to be done.

There's also the "other requirements" aspect: maybe you need really low 
noise, but you also need strong signal handling at the same time, so 
you're running your device with a ton of drain/collector current so it 
doesn't saturate.  So you had to pick a device that would take that. 
Or, you needed a device that could tolerate 30V, because you're running 
off a 24V supply.

The SPICE model probably is the "nominal device"...

Ultimately, you have to get some sample devices and measure them (no 
trivial matter in itself).