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

[jalbers@bsu.edu]

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

 

[John Fields]

On Thu, 25 Sep 2008 12:04:20 -0700 (PDT), "jalbers@bsu.edu"
<jalbers@bsu.edu> wrote:

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.

---
http://www.edn.com/article/CA6378084.html

JF

 

[John Popelish]

jalbers@bsu.edu wrote:

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

Light passing through the silicon produces holes and
electrons somewhat like forward biasing the base emitter
junction does. So light is just a source of base emitter
junction bias current. You can use the base lead to add or
subtract from that light generated photo current. If you
can't imagine a reason to do that, then don't.

But if you want some ideas, consider a transistor used as a
linear amplifier. You bias it into some mid conduction
state so that a small additional signal cap pass through the
transistor and be somewhat linearly amplified. In the case
of a photo transistor, the light signal can be either the
bias generator, or the signal to be amplified. there are
uses for both cases. Think about how a base to emitter
capacitor would affect the response of the transistor to a
pulse of light. Or a capacitor connected in series with a
resistor. Many useful variations are possible.

Also, a gate combines two or more inputs to produce an
output. The light input and the direct base bias can be
looked at as two logic input signals and the collector
current (or voltage) as the gate output. Think light
activated flip flops or electrical enable signals for a
light signal.


I think your idea to open a power transistor case
(especially one in a metal can) is an excellent educational
idea.

--
Regards,

John Popelish

 

[John Popelish]

John Popelish wrote:
(snip)

You bias it into some mid conduction state so that a small
additional signal cap pass through the transistor and be somewhat
linearly amplified.
(snip)

Should read, "You bias it into some mid conduction state so
that a small additional signal can pass through the
transistor and be somewhat linearly amplified."

--
Regards,

John Popelish

 

[John Larkin]

On Thu, 25 Sep 2008 12:04:20 -0700 (PDT), "jalbers@bsu.edu"
<jalbers@bsu.edu> wrote:

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

A resistor from base to emitter, hundreds of k ohms typically, will
reduce sensitivity, reduce leakage current, increase speed, and
increase breakdown voltage. I do that on optocouplers sometimes.

An uncapped power transistor probably won't conduct much photocurrent,
so really wouldn't be a "power" phototransistor. A regular TO-5
transistor would (I'm guessing) work about as well. Phototransistors
are designed to expose a lot of silicon to light, but regular
transistors aren't.

LEDs are photosensitive, too.

If you zener the base-emitter junction of some bipolar transistors,
they can emit a little bit of white light. Fast CMOS logic emits light
when it switches, but not many photons.

John

 

[Rich Grise]

On Thu, 25 Sep 2008 12:04:20 -0700, jalbers@bsu.edu wrote:

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

 

jalbers@bsu.edu wrote:

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.

--
Regards,

John Popelish

 

[whit3rd]

On Sep 27, 9:22 am, jpopel...@rica.net wrote:

jalb...@bsu.edu wrote:
I want to play around with some phototransistors and am curious about
the base lead.  It seems like this lead is treated as taboo .

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.