I am building a transimpedance amplifier that is connected to a photodiode. (I want to measure the light from a laser).

I build one using the opamp LM324N. But it is not working as a TIA amplifier. I checked the photodiode against a resistor, to check whether the diode is faulty. But the diode is working(when i cover the diode, the photo current is changing). When I connect it in TIA amplifier, it has no effect for photocurrent. That is when I cover the photodiode with my hands, there is no change in the value of voltage. Also, the voltage value is around 500mV.

enter image description here

I tried different values of the feedback resistor, but there is not much effect on the gain. Can someone tell me what I'm doing wrong?

Please spare me if the language is a laymans one. I am a newbie and don't have much knowledge in analog electronics

  • \$\begingroup\$ Once you fixed the backwards photodiode, what was the output voltage you get? \$\endgroup\$ Aug 3, 2023 at 14:15

1 Answer 1


Can someone tell me what I'm doing wrong?

For a TIA with a power rail that includes 0 volts, the photodiode MUST BE in the opposite direction to which you have it. See my addition in red: -

enter image description here

Also, the 1 kΩ feedback resistor will not produce much TIA gain; try 100 kΩ or above.

  • \$\begingroup\$ Hi Andy, I tried that too. But no effect on the photocurrent. I meant to say, when I cover the diode with my hands, it is not showing any effect on the voltage output \$\endgroup\$
    – Teena
    Aug 3, 2023 at 14:03
  • 1
    \$\begingroup\$ @Teena you might try increasing the 1 kohm feedback resistor to maybe 1 Mohm. \$\endgroup\$
    – Andy aka
    Aug 3, 2023 at 15:09

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