Yes, I think you meant collector current proportional to emitter current? We did discusss all this back in 2015 under the thread "a very silly circuit". About 25 years ago I think Stephen Woodward posted a design idea in ED magazine using the effect to create a tiny negative bias for opamp offset nulling in a positive rail only environment.
piglet
PV optocouplers are great for genereating a quiet floating voltage, but a transistor is cheaper.
I guess a regular, transistor type optocoupler, will generate a floating voltage too. Something else to try.
It will, but you have to accept an efficiency on the order of CTR/beta, i.e. something below 0.1%.
Cheers
Phil Hobbs
One could parallel the c-b and b-e junctions. Might help a little.
OK for some tiny bias thing, probably not for running motors.
This would be another curiosity measurement. You never know if such a thing might be useful some day.
Cheaper, yes, but not durable (the base breakdown makes dirt move on the transistor surface, kills the base leakage). The passivation of a transistor assumes B-E voltages an order of magnitude lower than breakdown, and in the opposite direction.
You'll hate the current transfer ratio of that 'PV optocoupler', but the light source has much better aging characteristics than the transistor does.
A phototransistor doesn't damage itself. It biases its own b-e junction forward about 0.6 volts.
The official PV couplers make, typically, 8 volts and 30 uA. They are just a string of photodiodes in series. They usually make more volts and uA than specified. Maybe that's to account for degrdation, but more likely it's because LEDs have kept getting better.
Here are some measurements I just made:
Seems bulk of the goodness comes from the C-B junction and paralleling E+C makes hardly any difference.
piglet
I made some tests and I think you are correct, lower beta gives higher output.
piglet
The photocurrent is driving the diode forward conduction curve, 60 mV per decade of current. Solar cells short themselves out.
That's cool. I bet we'd see more voltage without the 1 meg resistors.
One might pulse the LED hard at some low duty cycle and hang a cap on the output, to get more voltage.
There must be a use for this! Opamp nulling has been suggested.
I might have to design a multi-channel super-precise DC power source. This might fit in.
That's expected; geometrically, the E is just a tiny dot, and an opaque one (heavy doping, nearly a conductor) and the collector is virtually the entire volume of the semiconductor die.
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