At 09:42 PM 10/12/2005 -0700, you wrote:

>   from this datasheet
>
>http://www.fairchildsemi.com/ds/2N/2N3904.pdf
>
>   what is the saturation voltage for Vce and Vbe. It
>looks like the saturation for Vce is 0.2V?? While Vbe
>is 0.85V. I am confused on this part(saturation).

Okay, what the specifications you are referring to means
is that if you *force* a current through the base of
1.0mA, and adjust the load such that the collector
current is 10mA, then take voltage measurements,
you'll be guaranteed to find* that both:

1. Collector to emitter voltage is 0.2V or less
2. Base to emitter voltage is between 650mV and 850mV

* Note the test conditions- 25=B0C, and the test is done
with 300usec pulses of < 2% duty cycle. The most important consequence
of those oddball conditions is that the junction temperature is going to
be pretty close to 25=B0C.

 From those numbers you can calculate/infer how much
base current you must supply to be guaranteed to
switch a load in the 10mA - 50mA range, and beyond if
want to risk extrapolating.

I think maybe your problem is that you don't yet
understand which are dependent and which are the
independent variables. In the case of hFE, the collector current
is an independent variable. In the case of Vce(sat), the collector
current is a dependent variable.

P.S. The numbers on that data sheet are your guarantee of what the
transistor will do at those particular conditions. It's a bit unlikely
you'll actually be using it at those exact conditions. From knowledge of
how a transistor works you can infer to a fair degree (always allow some
safety margin) how it will behave under other conditions. The 'typical'
curves can help with this (but remember that *you* must guarantee operati=
on
at worst-case figures and conditions if you want a reliable design).

To this end, see this more detailed data sheet from On semi:

http://www.onsemi.com/pub/Collateral/2N3903-D.PDF

You may find the figures 15-18 interesting. The *typical* Vce at 10mA
with Ic/Ib =3D 10 is only 100mV @ Tj=3D 25=B0C , half the minimum guarant=
eed,
but you can see that it's typically just starting to lift off the
100mV 'floor'. If you have Ic/Ib =3D10 and Ic =3D 3-5mA, you can be prett=
y
sure that Vce will be well under 200mV.

You can also see that hFE typically peaks at around 6-10mA IC, and has
a strong temperature dependence (fig. 15).

Do NOT use the typical curves directly for design-- use the guaranteed
numbers. The typical curves are for insight into behavior between and
beyond the guaranteed points.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rew=
ard"
speff@interlog.com             Info for manufacturers: http://www.trexon.=
com
Embedded software/hardware/analog  Info for designers:  http://www.speff.=
com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZs=
peff



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