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.


  • \$\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

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.