Fair enough. My question is: "Don't you think the schematic in the ap note is inherently wrong?" and, while you are there, "What would be the best fix?".
I guess It is all about finding the correct interpretation for the datasheet and understanding how phototransistors work. The datasheet defines a minimum and a typical on state collector current, 0.7 and 3 mA respectively. It does not state anywhere that the current will be limited in any way. What's the meaning of this value? Because the datasheet also states the absolute maximum power dissipation is 100mW, I thougth the on state collector current was a suggestion of how strongly the device should be driven in normal conditions without exceeding the maximum power dissipation which, at the stated Vce(sat) of 0.8V means 125mA.
I've also tried with the more expensive but better documented VISHAY BPW96C (
http://www.farnell.com/datasheets/2047104.pdf): the prototype works just the same (but I didn't expect this since the two devices are designed for different wavelengths).
The speed of the bursts is not an issue for the speed of the phototransistor but it is a problem if you want to measure the collector current with a regular multimeter
The shortest signal is over 50µs: it should be ok since the datasheet reports a typical rise of 15µs (the vishay seems to be much better in this regard, with a rise time of only 2µs).
I have very limited control on how the IR source works and there is definitely no way to keep it on long enough to take a measurement with a multimeter. I could use an IR LED to stimulate the phototransistor, but I'm not sure if the IR intensity would be the same.
Going back to the original questions, I think the resistor connected between the emitter and ground should be connected between emitter and the base of the transistor: in this case I think the resistor value should be (Vcc-Vcesat-Vbe)/Icesat. Does it sound reasonable?