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Transimpedance Photodiode Amplifier Design w/ OPA847

Prodigy 10 points

Replies: 1

Views: 2695

Hello there,

I am currently a student in college, who has been working on building a transimpedance photodiode amplifier. Not long ago, while I was looking for a design, I came across the transimpedance amplifier design in the OPA847 technical documents (page 11). While it was outside the range I was aiming for, it was something that I wanted to attempt and test myself by designing it. At that moment, I did not have the exact parts to use the circuit, so I substituted some of the parts for components that I had available. While the parts I used differed from the schematics, the end results was not something that I expected. For me, the photodiode was not working at all and according to the oscilloscope, the circuit fluctuated so often that it did not have a stable point. I know that I shouldn't have expected the circuit to replicate what occurred in the technical documents. But I would assume that the circuit should have worked, albeit with an absurd amount of noise and at a different frequency or at least the photodiode to work.

The parts that I used that were the same: OPA847, two 12k resistors, one 100pF capacitor, one .1uF capacitor.

The parts that I used that were different: BPW24R (3.8pF, reverse-biased at -5V) instead of a 1pF photodiode and .25pF capacitance for CF instead of .18pF capacitance.

-Edit: I was informed by a few mentors that the flaw to my design is likely linked to the fact that I was using a breadboard instead of a PCB. Would this be a contributing factor to the result?

Here are some pictures of my design and the resultant frequency -

  • Hello Andrew,

    Yes, breadboards contain a lot of parasitic capacitance which will cause the amplifier to become unstable and oscillate.

    Your best bet is to build PCB with your circuit in place and solder the OPA847 to the board. There are companies like PCB Express that offer free software tools for board layout and will also build and ship your board to you very quickly for a low cost. There are some limitations but overall they are a good resource for board prototyping and they can handle 2- to 6-layer designs.

    There are also some unpopulated EVMs that would work for the OPA847  foootprint. The DEM-OPA-SO-1B and DEM-OPA-SOT-1B would work here.

    I would also recommend the reading following articles for designing transimpedance applications:

    • Transimpedance Considerations for High-Speed Operational Amplifiers - sboa122
    • Compensate Transimpedance Amplifiers Intuitively - sboa055

    Regards,
    Luke LaPointe
    High Speed Amplifiers 

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