piclist 2000\06\04\233529a >
Thread: Noise in phototransistors/photodiodes/light
face picon face BY : Sean Breheny email (remove spam text)

Hi Richard,

Thanks for the response. Please see my answers below:

At 05:09 PM 6/4/00 -0600, Richard Ottosen wrote:
>What is your pulse rate and what is the receiver bandwidth? Of course,
>the narrower the bandwidth the better for noise.

The pulse rate can be whatever I want(I was trying 2kHz because that was
high enough to get away from the 1/f noise of op-amps and low enough so
that op-amps still have significant gain). The bandwidth need only be about

>I have no references at hand but I suspect that a phototransistor is
>noisier than the combination of a photodiode and a low noise transistor.
>This would be because of the difficulty of optimizing both photo-gain
>and noise in one device in the case of the phototransistor.
>You may want to try making your own photodiode/transistor device for
>comparison purposes. I think you would want to choose a transistor that
>has good noise characteristics at very low currents. A 2N5089 is a part
>that comes to mind. I expect that there are better ones.

That makes lots of sense. When I was able to get to 100 feet, I used a
photodiode combined with a transimpedance amp made from half of a LMC6482,
which has a noise of about 30nV/root Hz around 1kHz. I was actually using a
600Hz pulse freq, so it would probably be a bit easier at 2kHz. I wonder if
it is feasible to do better with a discrete transistor?

>Again for comparison purposes, maybe try an LED light source to simulate
>the background light. Maybe a whole bunch of white LED's ???

Well, yes, you may be correct that an LED would have less noise (no
filament vibrations, etc.), but my question wasn't very clear. What I meant
was: is there some fundamental noise which is inherent in ALL noncoherent
(or perhaps even coherent, for that matter) light sources, simply due to
the fact that the light is emitted as discrete photons which come out at
random times? I think there is, and that it is a similar effect (possibly
different probability distribution) to shot noise in an electric current
(noise which exists in an electric current because it consists of discrete

>-- Rich


| Sean Breheny
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