I was recently reminded that BJTs can be used in reverse active mode.

Can somebody enlighten me whether and to what extent the same is true for optocouplers? Do the same limitations apply to reverse active mode as for BJTs?

Edit 1:

Of course I only mean reverse polarity on the "transistor side" of the optocoupler, not on the "LED side"...

  • \$\begingroup\$ I'm not 100% sure (as have never actually used a BJT in reverse active mode) but I would expect everything here to apply: electronics.stackexchange.com/questions/29756/… since you are just swapping collector and emitter. \$\endgroup\$ – stefandz May 16 '16 at 14:48

The photo transistors used in most optocouplers have a very large collector-base area, so that the light from the LED will have the most possible effect on that junction. Operating the transistor in the inverse mode (emitter positive, collector negative) should work slightly, but the reversed biased junction is now the emitter-base junction which has a much smaller area, thus not nearly as sensitive to light. I'm having trouble seeing what benefit you might expect from this mode; perhaps it could be faster switching (but there are better ways to do that); perhaps lower saturation Voltage.


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