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I'm designing a transimpedance amplifier but I'm having trouble getting higher gain. I started with Rf being 16.5 kOhms but then I realized I will need higher gain, so I simply switched it out for 50 kOhm, 100 kOhm, and 1 Mohm, but after the 50 kOhm I stopped getting higher gain.

I basically want to take measurements from a photodiode, and I didn't want to mess around with AC so there is Cf to ensure stability and filter any oscillations. So the main question is is there any limiting factor for the opamp that I'm not seeing?

enter image description here

Notes:

  • Vcc = single supply 3.3V
  • Opamp = rail-to-rail MCP617
  • Photodiode = sfh 206 k
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    \$\begingroup\$ Gain at what frequency? What is the application? That's quite a big capacitor there. What type is the capacitor (CF1)? \$\endgroup\$
    – Tim Williams
    Commented Aug 30, 2023 at 16:55
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    \$\begingroup\$ It looks like you've got a dual op amp. Why not use the second op amp as another gain stage? \$\endgroup\$
    – Adam Haun
    Commented Aug 30, 2023 at 17:01
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    \$\begingroup\$ There's a good reason why op-amp symbols were invented = clarity in your circuit. How did you measure the circuit gain? Please link a data sheet for the op-amp and optical device. What is your required 3 dB frequency? \$\endgroup\$
    – Andy aka
    Commented Aug 30, 2023 at 17:13
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    \$\begingroup\$ Please give more information. What is the output voltage with zero light input and with the intended light input with the different feedback resistors. \$\endgroup\$
    – Kevin White
    Commented Aug 30, 2023 at 17:13
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    \$\begingroup\$ In such a system I would always take at least 2 measurements, one without IR LED and one with active LED, and process the difference. This removes all sort of side effects like temperature drift and OpAmp DC offset error. And it allows higher gain in an AC coupled second stage. \$\endgroup\$
    – Jens
    Commented Aug 30, 2023 at 21:03

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Cf is normally pico-Farads. Assume leakage current through this massive MLCC cap is what is limiting the gain. Replace it with a C0G-class ceramic cap (even 100pF) and the gain will likely improve drastically.

A good analysis of the TIA is Digikey's Article about them.

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    \$\begingroup\$ Great point. Here are some measurements of MLCC, from them the series resistance of that huge cap could be as low as a few tens of kiloohms. silabs.com/documents/public/application-notes/… \$\endgroup\$
    – user1850479
    Commented Sep 1, 2023 at 17:14
  • \$\begingroup\$ Thank you so much I had no idea about this, I will try replacing it :) \$\endgroup\$
    – OnHai
    Commented Sep 2, 2023 at 13:58

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