Re: Driving a transistor base from a voltage divider

Jon Danniken wrote:

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

What I am trying to figure out is the appropriate values of R1 and R2,
given a desired Base current and source voltage/source current
available (I know the source voltage, and how much current I can pull
from it).

If I eliminate the "bottom" (R2) resistor in the voltage divider and
only use one resistor, I can easily determine the current that flows
through that resistor (E/R), and therefore the current that flows
through the base ((E-0.7)/R), which is just computing the value of a
Base resistor.

Right now I am getting hung up on what effect the "bottom" resistor
has on
the base current. I know that without the transistor, I can figure
out the voltage at the center point as a ratio of the resistances, and
the current through both resistors, but what current is available from
the center point to drive a transistor base?

The closest I have come is by calculating a "resistance" for the
transistor Base/Emitter junction by dividing 0.7V by the desired base
current, and treating the transistor as a resistance in parallel with
R1, but I'm not very confident with that solution.

BTW, here is an ascii of what I have:

---+
|
|
\
R1 / C/
\ | /
| |/
|-----|
| B|\
\ | \
R2 / E\
\
|
|
gnd

Thanks for any help,

Jon

If the current through R1 + R2 is much larger than the required base
current then the junction will set the base voltage. That will allow
you to set the collector current anywhere between zero and maximum.

However for a switch the transistor would be hard on or hard off. In
which case a single resistor will limit base current to a safe level,
at the point where its switched hard on.

--
Best Regards:
Baron.
.

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