# Calculate collector current of phototransistor

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1. Feb 10, 2017

### yoamocuy

Hello,

I'm wondering if it is possible to calculate current the collector current of a phototransistor that is paired with an IR Led? I have an IR Led that is 13.6 cm away from the phototransistor and has a radiant intensity of between 65 mw/sr and 125 mw/sr. The phototransistor that I'm using is SFH 3015 FA. It's photocurrent could range anywhere from 160 uA to 800 uA at an irradiance of 0.1 mW/cm^2 and VCE=5V. The phototransistor also has a radiant sensitive area of 0.4 mm^2. I would have a resistor of 1.6kohm from emitter to ground.

I believe that irradiance at the phototransistor can be calculated based on the radiant intensity of the Led but I'm not sure if it would just be a simple radiant intensity/distance^2 or if I need to consider the radiant sensitive area as well? Also, the irradiance that I calculate at the phototransistor will not be 0.1 mW/cm^2. Can I assume a linear relationship between current and irradiance? When I have a range of values for the photocurrent, are those values typically the same as collector current? I would assume that I need to multiply that value by hfe of the phototransistor to get collector current but if that's the case it seems that the phototransistor datasheet is fairly useless since no typical hfe is given. Lastly, does my resistor have any effect on the collector current? Ultimately I will be in the active region of the DC load line so can I assume that the resistance has a negligible effect?

2. Feb 11, 2017

### Baluncore

If the sensitive area remains the same then you can keep things proportional to r2.
To calculate a collector current factor, divide the "sensitive area" by the "surface area of a sphere" having a radius of r. That will handle the r2 for you.

Probably yes.

I believe so.

You do not have to multiply by hfe. Any hfe proportional effect has been included in the data.

The effect will not be negligible, which is why an op-amp is used with the I to V resistor in the feedback circuit. That clamps the collector voltage.
See; https://en.wikipedia.org/wiki/Transimpedance_amplifier