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Given that the irradiance and collector current for a (random) photo-transistor is linearly correlated like below.

Irradiance vs collector current

Then I am wondering why the output voltage graph of the transimpedence amplifier from the application notes from Sharp is not completely linear with irradiance.

Amplifier circuit using an operational amplifier

W30
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  • That's a log-log plot, so straight lines don't necessarily mean a linear relationship. – Hearth Dec 02 '22 at 15:40
  • Usually phototransistors are used as switches. If you need much more than a binary light/dark measurement, consider a photodiode. If you want the high gain of the phototransistor with linearity, that's usually an avalanche photodiode. – user1850479 Dec 02 '22 at 15:43

3 Answers3

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The first plot is log-log. While your 2nd plot doesn't have a scale, but it's probably linear. I can imagine that they don't want to just write that the response will be entirely linear, it never is. There will be a limited portion that will be very linear, but that depends on your photodiode and amplifier design.

Designalog
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Why should it be linear given that for a regular transistor (BJT) the ratio of collector current to base current isn't linear. If you look carefully at the first graph you can see it isn't truly linear. There are no indications of scale on the 2nd graph so it's pointless trying to make comparisons.

Andy aka
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That circuit uses a photo transistor instead of a resistive element.

enter image description here Source: https://www.researchgate.net/profile/Sungwoo-Choi-7/publication/224092508/figure/fig7/AS:372479232102418@1465817377786/One-dimensional-model-of-phototransistor.png

The model of the photo transistor is shown above, you can see that there is a small amount of resistance in parallel with the diode that activates the transistor. This resistance would make the circuit not as simple as the first autograph shown I'm assuming that in the ordergraph shown. They modeled it with the first order equation and left out some of the parasitic effects

Voltage Spike
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