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Original question:

Unwanted oscillations in a Transimpedance Amplifier (TIA) using a OPA637

OPA637: Unwanted oscillations in a Transimpedance Amplifier (TIA) using a OPA637

Ehsanur Rahman
Prodigy 40 points
Part Number: OPA637

Hi

I am using an OPA 637 in transimpedence amplifier configuration. According to the datasheet, this opamp is stable for gain>5. I am using a feedback resistor of 300K. The photodiode capacitance Ci is 10pF. However, I am getting oscillation sometimes around 11 Mhz like sinewaves and sometimes around 60kHz like squarewave. I  found that including a feedback capacitor Cf would decrease the noise gain. But in my configuration, the input capacitance of the photodiode is providing a high-frequency noise gain of 6 (1+Ci/Cf ) if I used a capacitor of 2pF at the feedback. Still, I am getting oscillation.

Can anyone help me to stabilize the op-amp?

 Can anyone help me to remove oscillation?

My schematic is attached

  • 0
    TI__Mastermind 23050 points
    Hi Ehsanur,

    You typically have to use a feedback capacitor because the capacitance of the photodiode causes the output to increase indefinitely over frequency and this degrades your phase margin. You also have to factor in the input capacitance of the amplifier itself, as this influences your effective capacitance at the inverting terminal and for the OPA637 roughly contributes an additional 15pF of capacitance. Even without the photodiode the amplifier would be unstable! You have to be careful here because you need the feedback capacitor to counteract the pole introduced in the feedback network, but since the OPA637 is not unity gain stable you can't roll it off too early. I would try using a 4.2pF capacitor in your feedback instead. If you are bread-boarding this circuit keep in mind that you have about 2pF of parasitic capacitance between your rows and this may also impact your circuit
  • 0 in reply to Zak Kaye
    TI__Genius 11135 points
    Ehsanur

    We haven't heard back from you so we assume this resolved your issue. If not, post a reply below, or create a new thread if this one has timed-out.

    Thanks
    Dennis