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Don't touch that base lead! (Photo Transistor)

I want to play around with some phototransistors and am curious about
the base lead. It seems like this lead is treated as taboo . Some
sources say it's just there but you won't have to or don't need to use
it. Other sources say that it is there so that you can bias the
transistor but don't go any further.

What effect does applying a bias current have on the function of the
phototransistor? How is the characteristic curve different from a
normal transistor when it is exposed to different levels of light?
Saturation, active region, cutoff ...

Another experiment that I wanted to try is to cut the top off of a
power transistor to make a homemade "power" phototransistor or combine
it with a LED to make a homemade "power" optocoupler.

Any help would be greatly appreciated. Thanks
 
P

Paul E. Schoen

Jan 1, 1970
0
I want to play around with some phototransistors and am curious about
the base lead. It seems like this lead is treated as taboo . Some
sources say it's just there but you won't have to or don't need to use
it. Other sources say that it is there so that you can bias the
transistor but don't go any further.

What effect does applying a bias current have on the function of the
phototransistor? How is the characteristic curve different from a
normal transistor when it is exposed to different levels of light?
Saturation, active region, cutoff ...

Another experiment that I wanted to try is to cut the top off of a
power transistor to make a homemade "power" phototransistor or combine
it with a LED to make a homemade "power" optocoupler.

Any help would be greatly appreciated. Thanks

Just be careful when you cut open the power transistor, as some of them
have beryllia ceramic substrates and the dust can be hazardous. I had
thought it was extremely toxic, but the following MSDS indicates that it
is not too bad: http://www.brushwellman.com/EHS/MSDS/Z01.pdf

Paul
 
R

Rich Grise

Jan 1, 1970
0
I want to play around with some phototransistors and am curious about
the base lead. It seems like this lead is treated as taboo . Some
sources say it's just there but you won't have to or don't need to use
it. Other sources say that it is there so that you can bias the
transistor but don't go any further.

What effect does applying a bias current have on the function of the
phototransistor? How is the characteristic curve different from a
normal transistor when it is exposed to different levels of light?
Saturation, active region, cutoff ...

Another experiment that I wanted to try is to cut the top off of a
power transistor to make a homemade "power" phototransistor or combine
it with a LED to make a homemade "power" optocoupler.

Any help would be greatly appreciated. Thanks

This could be an awesome opportunity to do some experiments, and actually
find out! All you need is a DC supply, some resistors and pots, and a
meter or two. And maybe some switches. ;-)

Then you'll _really_ learn what the base does!

I once hand-drew the transfer function of a 4N126 or something, and was
astonished to discover how linear it was. ;-)

Have Fun!
Rich
 
I want to play around with some phototransistors and am curious about
the base lead. It seems like this lead is treated as taboo . Some
sources say it's just there but you won't have to or don't need to use
it. Other sources say that it is there so that you can bias the
transistor but don't go any further.
(snip)

Another thought. If you need real speed, you can tie the collector
and emitter together and use them as the cathode of a photo diode and
use the base as the anode. Or tie either the collector or emitter to
the base to use only one of the junctions. I suspect the base emitter
junction generates most of the photo current.

You get no gain from the transistor, only the actual photo current
out, but you can either hold the voltage across this "photodiode" very
near to zero with an opamp to greatly reduce the effect of junction
capacitance on the output, or reverse bias this "diode" to greatly
increase the speed that the photo generated charges are swept through
the device and amplify the photo current externally.

The rise and fall times go from microseconds (or 10s of microseconds)
to 10s of nanoseconds, allowing much faster signals to be coupled
through the device.
 
W

whit3rd

Jan 1, 1970
0
Another thought.  If you need real speed, you can tie the collector
and emitter together and use them as the cathode of a photo diode

Two other tricks to speed up a phototransistor are to use a small
load resistor (the output drops from volts to millivolts, but
gain is cheap; use a comparator), and to connect as a cascode
with a second (low-Miller-capacitance) transistor. In both cases,
the low impedance presented to the transistor keeps the
collector dV/dt low, which is important because of the significant
collector/base capacitance of a phototransistor. The base
area is the light-collecting limit, so phototransistors have
broad base areas, which means high capacitance.
 
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