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I am building a transimpedance amplifier circuit. Feedback resistor used is 1 mega ohm. A photodiode is connected to the inverting input of the amplifier (LM741C). Using 23 volt supply, current reading is 32 mA. The current across the resistor, measured with a multimeter shows 23.4 uA. (which makes sense, since V=IR). However, the dark current shows a reading of 4 uA (with all other parameters being the same). Is this a suitable value and if not, what is the mistake I am making?

I have already spent a day searching for any links which could guide me and have not found anything useful. Any help would be appreciated. The link I used for the circuit is Fig.2 of the PDF.

http://www.ti.com/lit/ug/tidu535/tidu535.pdf

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  • \$\begingroup\$ Is this to power a dark sucker? Do you mean reverse current? or quiescent current? \$\endgroup\$ – Solar Mike Mar 15 '18 at 7:02
  • \$\begingroup\$ @SolarMike Dark current through a photodiode is the current through the photodiode when there is no light... obviously... \$\endgroup\$ – user253751 Mar 15 '18 at 10:06
  • \$\begingroup\$ @immibis so that is what I was taught as quiescent current ie the low or non conducting state... \$\endgroup\$ – Solar Mike Mar 15 '18 at 10:16
  • \$\begingroup\$ @SolarMike Sure, but in the specific context of a photodetector (not limited to photodiodes). \$\endgroup\$ – user253751 Mar 17 '18 at 6:34
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Your first mistake is using a 741 because it is extremely poor for these types of circuits. The next problem is that you appear to be using a supply of 23 volts for the 741 with the negative supply pin connected to circuit 0 volts. The link above tells you that your inputs cannot be close to either supply rail so if you are using this circuit: -

enter image description here

Then it's not going to work because the input common-mode voltage range does not include the negative power pin of the 741.

what is the mistake I am making?

  • Using a 741 (a dinosaur) when clearly the document you linked shows an OPA320 biased correctly for single supply operation (page one of that document).
  • Running a crappy op-amp from a single power supply rail and expecting things to work when the input is at 0 volts.
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