Hi Jason,

I believe that Vbe(sat) refers to the base-emitter voltage with the
transistor in saturation (the base-collector junction is forward-biased in
addition to the base-emitter junction). This is higher than the turn-on
voltage. For example, at 25 deg C, 150mA collector current, Vbe(sat) is
given as between -0.75 and -0.95V in the Diodes, Inc. MMBT4403 datasheet.
However, if you look at figure 3, typical base-emitter turn-on voltage, and
follow the line, extrapolating off the right-hand side of the graph, it
looks like the typical turn-on voltage for 150mA would be about 0.77V, not
0.85V as suggested by the previous range. That's because figure 3 is
specifically for a non-saturated state (Vce=3D5V)

One thing to consider is how the rest of your circuit is connected. There
isn't just one Vbe value where a BJT turns on - it will go from conducting
nanoamps to conducting milliamps over a range of maybe 0.2V, and what
happens to the rest of your circuit could cause considerable variation in
the end result. For example, if you have a 100k resistor from the collector
to GND, the emitter to 5V, and there is a digital circuit looking at the
collector, then the variation in the digital circuit threshold could add
0.1 to 0.2V variation to your effective Vbe threshold.

So, all together, I'd say the sources of variation are:
1) Temperature (predictable and you have considered this)
2) Manufacturing variation (I would suggest +/-15% instead of +/-10%)
3) Collector resistor tolerance
4) Threshold tolerance of the next stage
5) Any effect of base current variation on the circuit which drives Vbe
(temperature and manufacturing variation, as well as whether you saturate
the transistor and if so, how hard will all affect the amount of base
current you have for a given collector current - if the source of Vbe is
not low impedance, this could affect it)

Sean
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