I have an Isocom ISQ2X quad opto-isolator (datasheet) that I will use to drive a logic input. The opto-isolator's simplified model given by the datasheet is pictured below. The transistor base pin is not brought out.
I have two possible configurations, pictured below:
- Config1 where the logic output is taken from the transistor's collector
- Config2 where the logic output is taken from the transistor's emitter
My question is: Aside from the inverted logic, are there any differences between these configurations? For example: does raising the collector voltage (when the transistor in Config2 is activated) have any effect on things like current-transfer-ratio or saturation voltages?
Additional, unnecessary, information:
- If both configuration are equal I will go with Config2 because it is preferential for my application to have a logic low output if the diode driver is accidentally disconnected.
- The logic input belongs to Intersil HIP4081A H-bridge driver which is powered by the +12V rail in the picture (Logic low threshold: 1V max, Logic high threshold: 2.5V min, Logic absolute maximum: VDD+0.3V).
- The diode current will be controlled by a series resistor, probably to around 10 mA.
- I could consider adding a buffering transistor/FET/IC if it is beneficial to my application.
- I have these opto-isolators on hand and would therefore prefer to use them.
- My intuition tells me that there isn't any difference between the configurations, since the phototransistor is effectively just series connected with a resistor and doesn't even "know" which configuration it is in. I ask because two colleagues prefer and recommend Config1, but that might be because they are thinking of Config2 as a standard NPN BJT common-collector circuit where they base pin would need to be raised as the collector is driven higher. Some literature I've looked at present both options without mentioning any differences.
Edit: I guess I was hoping for an explanation involving some sort of a phototransistor model and/or an answer citing some book/literature. As I don't think this question will pickup steam I've marked Olin's answer as accepted since he elaborated a bit on his answer – but I'm still open to more people weighing in on this.