Noise in phototransistors/photodiodes/light
Peter L. Peres email (remove spam text)
DC fed bulbs have been used in cinematography and other applications for
many years. These applications are noise-sensitive and there never were
bulb-related problems. The detector and the amplifier were the culprits in
evey case. The best approximation of an incandescent bulb run within
parameters is a black body radiator of the same temperature ;-) Assume
light is corpuscular and calculate the number of photons that reach your
detector from the solid angle of view. From this work out the shot noise.
I suspect that all other things being equal a LED may be quieter than a
blackbody (on account of bandwidth) and a laser even quieter (for the same
Note that over-volted small bulbs and certain halogen bulbs are not quiet.
This is because the filament is physically boiling (evaporating metal into
the gas filling) and because of strong convection currents in the gas
Once upon a time IR remote receivers were discrete-built and one way was
to feed a parallel LC tank directly from a PIN photodiode. I once made a
test circuit using a ZN414 and a 60 kHz tape bias tank pulled down to 40
kHz. It worked but the ZN414 is hard to come by. You could try a
monolithic AM radio IC using its IF section only. You can achieve gains of
60 dB easily like this and noise will be low even with a single tank.
I think that the extra noise you saw when shining the flashlight into the
detector is due to the changed input impedance of the circuit when the
photodetector is overloaded. This may change the way your filter works and
inject or amplify noise.
hope this helps,
PS: If anyone knows where ZN414s can be had, or their equivalents, please
say so. I have a few ideas that would greatly benefit from this chip. Why
did they stop making it ?!
PS2: My postings are becoming longish. If anyone is bothered by this then
please say so.
See also: www.piclist.com/techref/index.htm?key=noise+phototransistorsphotodiodeslight
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