Photo-resistor based optocouplers vs photo-transistor based optocouplers

[kraal]

Hello,

Does anybody have examples of (pedal) diagrams using optocouplers where the light-sensitive receiver is a photo-transistor (instead of photo-resistor), and if yes could you please share them ? If few/no circuit use photo-transistors is there a specific reason ?

I'm also interested in documentation about when it's best to use photo-transistor vs photo-resistor and about how replacing the later with the former would impact a circuit (maybe it's a 101 question).

Thank you a lot in advance for your help.

[R.G.]

As a general guideline, use LED-photoresistors where you want smooth variation in resistance at the working end, and use phototransistor versions where you want it to switch on and off.

Here is the first layer of fine detail. bipolar output optocouplers have NPN bipolar transistors as outputs. While it is technically possible to use a bipolar as a variable resistance, there are a lot of special requirements on it. In a opto, you have the variations in LED efficiency and variations in the photo-gain of the NPN to worry about, making it almost impossible to reliably get the tiny partial operation resistance of the NPN to work repeatably.
Some few optos have MOSFET outputs. These can switch AC signals well, but suffer from some of the same balancing-in-the-middle problems if you're trying to get variable resistance.

So use photoresistors where you want varying resistance, and transistor versions where you want switches.

[merlinb]

As a general guideline, use LED-photoresistors where you want smooth variation in resistance at the working end, and use phototransistor versi

I've experimented with using BJTs as variable elements in audio, and always run up against the problem of DC shift/thump, due to the base current injection. Do photo-transistors have the same problem or, like, does the base current spontaneously come from the collector current? 

[kraal]

As a general guideline, use LED-photoresistors where you want smooth variation in resistance at the working end, and use phototransistor versions where you want it to switch on and off.

Here is the first layer of fine detail. bipolar output optocouplers have NPN bipolar transistors as outputs. While it is technically possible to use a bipolar as a variable resistance, there are a lot of special requirements on it. In a opto, you have the variations in LED efficiency and variations in the photo-gain of the NPN to worry about, making it almost impossible to reliably get the tiny partial operation resistance of the NPN to work repeatably.
Some few optos have MOSFET outputs. These can switch AC signals well, but suffer from some of the same balancing-in-the-middle problems if you're trying to get variable resistance.

So use photoresistors where you want varying resistance, and transistor versions where you want switches.

Thank you a lot for your clear and instructive reply.

Best regards