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Topic: phototransistor sensor (Read 6775 times) previous topic - next topic


Let me rephrase things - the difference in voltage gain between common emitter and common collector configurations is not applicable in this case.  The base is not accessible so we can't really tell what the input voltage with respect to ground might be in either configuration.  What really matters is the base current which is directly controlled by the illumination (and thus also the base to emitter voltage).  The collector to emitter current should be the same for a given illumination whether there is a resistor from 5V to the collector or from the emitter to ground.


Try it and see.

I tried both configurations (5V-res-PT-gnd & 5V-PT-res-gnd) with a Vishay TEPT5600 phototransistor and can see no difference in sensitivity between the two.  Where one would produce an ADC value of "n", the other would produce approximately the inverse (1023 - n) within the limits of my ability to control the illumination (AC dimmer switch controlling overhead lighting).


What value rssistor? With low values you will see little difference. In my experiments I have noticed a difference. Especially into the analogue input with 100K.


I used 100K ohms as I was interested in low light levels.

Another way to look at this:  In a common collector NPN circuit, the voltage gain is essentially 1 because as the input voltage at the base (referenced to GND) increases, collector to emitter current through the transistor increases causing the voltage at the emitter to also increase (referenced to GND).  This limits the increase in base-to-emitter voltage resulting in reduced overall voltage gain (ratio of input to output) through the circuit.

But for a phototransistor, the input is light (not referenced to GND) so the common collector analysis doesn't really apply.  Light injects current into the base which the transistor's gain turns into a larger collector to emitter current.  If you want to think in terms of voltage, light induces a base to emitter voltage - this is the input to the circuit.  Whether you have a resistor from the emitter to GND or from 5V to the collector, the input is still Vbe, ie, the input floats with respect to GND.

Having written this, I realize I was ignoring the effect of the base current so I'll concede that there could be a very small difference in sensitivity between the two configurations but it should be in favor of "common collector"!

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