Driving a transistor base from a voltage divider

[BobW]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
newsOidnaic6Nxt4nzUnZ2dnUVZ_jidnZ2d@web-ster.com...

You can't really control the base current. It draws whatever is required
to support the emitter current that is flowing.

Absolutely, positively, wrong.

Wrong? Unless the transistor is in saturation then it is absolutely,
positively, correct.

Now, if the transistor is in its active region and if you drive the base
with a current source then you've got control of its base current. However,
this is not a common situation.

Bob
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[whit3rd]

On Apr 11, 11:57 am, "Jon Danniken" <jondanSPAMni...@yaSPAMhoo.com>
wrote:

Hi, I am trying to determine the operating specs for operating a transistor
base (as a switch) from a voltage divider.

Your voltage divider and the input signal define a 'load line', and
you
can graph the I/V characteristic of this combination.

The transistor base (with emitter grounded) also has an I/V
characteristic,
which has some perhaps non-negligible temperature dependence.
Graph that.

Overlay the graphs. Where they intersect, is the base current so high
that the transistor will burn up? Is it so low that the transistor
will not
switch on?

Since the input is a 'signal', it has both high and low excursions;
look
at the low excursion and graph the load line for that case. Overlay
the graphs, and determine if the base is reverse biased beyond the
spec sheet limits (usually about -5V). Is the base current low enough
to really switch the transistor OFF in this condition?

The answers to these questions, if satisfactory, will tell you that
the
divider is going to switch the transistor. The exact values of the
components can influence ANY of these multiple tests, there is
no single right combination.

You can eliminate R1 values that, ignoring R2, don't deliver enough
current.
You can eliminate R2 values that would dissipate excessive heat
at transistor turn-on voltages.

 

[Jon Kirwan]

On Sun, 12 Apr 2009 08:26:40 -0700, "Jon Danniken"
<jondanSPAMniken@yaSPAMhoo.com> wrote:

"Tim Wescott" wrote:
Saturation happens when the B-C junction starts forward biasing, and that
depends on the collector current, indeed it does. Hold the C-E potential
to 5V and the transistor won't saturate, no matter how much current you
pour into the base. There will be some Really Bad Things happening to the
transistor for a very short while, but it won't saturate.

To do this really right you figure out the current gain when the
transistor is in saturation (it'll be on the data sheet, and it's usually
way less than the best value of current gain), then put in more than
enough base current to provide your desired collector current with that
current gain.

Okay, gotcha, thanks Tim. So if I am reading you correctly, I should using
the minimum Beta for when the transistor is at maximum rated Ic (indeed
given by the specs, and definitely less than the best Beta), and using that
Beta, figure out the Base current for my desired Ic to be switched? I hope
I got that right.
snip

Various transistors have various saturation betas, but many datasheets
will use a beta factor of 10 as their assumed "saturation case" partly
because it is a commonly used value (well recognized) and the factor
of 10 works most places. You can go to the datasheet itself to check,
though.

For example, if you look for a spec on the collector-emitter voltage
(labeled as Vcesat, often), you will see that they often specify it
with an Ic that is 10 times the Ib. That's a clue that you may want
to use that value. If you see it specified with an IC that is 5 times
the Ib, you should take that as a clue to use 5, not 10. For most
small signal BJTs I've looked at (not so many, really), 10 is the
common factor used. But if you examine, for example, the datasheet
from OnSemi for the 2N3055, you will see two entries -- one for a beta
of 10 (Ic=4A, Ib=0.4A) and a beta of 3 (Ic=10A, Ib=3.3A). This should
be a clue to think a little more about it.

Beta is a slippery thing and none of this means you must use 10 (or 5)
for a saturation beta. Actually, depending on what you can tolerate
for Vce in your application, you might actually want to use a larger
number for the beta because the Vce you actually get may be quite
fine. So now go over to the "Collector Saturation Region" curves, if
you can find them included in the datasheet. For example, in the
2N2222 sheet from OnSemi I'm looking at right now, there are four
curves for a fixed Ic -- 1ma, 10mA, 150mA, and 500mA. The y-axis will
be Vce and the x-axis will be a log-scale of Ib. I find that at a
Vce=0.2V, which might be acceptable to me, that at an Ic=10ma, Ib=55uA
-- that's a beta of about 180!! So using a beta of 10 might be
overkill. Of course, the curve is for the "typical case" so you need
to keep that in mind, too. So here, I might choose a beta of 30
instead of 10 or 180. Just to be safe, yet not require quite so much
base current.

This can an important thing to think about when a microcontroller is
driving things. The pins may be only able to sink or source some
particular level of milliamps with any guarantee of the output voltage
being in a well-defined range. And you may be forced into using two
BJTs to get the desired Vce when switched on if you assumed a beta of
10 as an absolute rule, when in fact you would find out you can get
away with one BJT if you only had looked at an actual curve.

By the way, on that curve of Ic=10mA, a beta of 10 suggests Vce=30mV.
So you can see that using a smaller beta assumption means achieving a
really low (well, low for most of us normal humans) Vce. But
typically only a difference of 170mW between a beta of 180 and 10. You
may not need that kind of extreme difference in Vce.

The datasheet has a lot of stuff and you should spend a little time
familiarizing yourself. Gradually, it will make increasing sense what
to look for and where to look for it.

Also, different manufacturers make different things even when they use
the same number. The 2N2222 comes in Vceo of 40V and 60V, but from
different manufacturers. Different processes and topologies, I guess.
(I'm a hobbyist, not a professional, so I don't need to know exactly
why.) So it helps, in getting a "feel" for the 2N2222 to examine a
variety of datasheets to see where they seem alike and where they seem
different enough to notice.

Jon