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OPA657: Phase margin calculation of TIA circuit

Part Number: OPA657
Other Parts Discussed in Thread: OPA659, TINA-TI, OPA695


I have studied some papers to understand how I should calculate the phase margin of Amplifiers based on RosenStark method and it was so useful bcoz the common methods failed to calculate the correct PM.

now I want to design a TIA with OPA657 to have the Bessel frequency response.

could you please help me to calculate the phase margin of TIA based on RosenStark method?


  • is it right to calculate the PM of the TIA circuit with plotting Vfb/Vin? But I'm more interested in RosenStark method.

    Cp is the parasitic capacitance.

    Cs is the photodiode capacitance.

  • Hi,

    I don't think you can calculate PM of the TIA circuit with the one you have attached, because the loop is broken at the input side. The simulation does not include the input capacitance of the OPA659, and thus does not give the correct feedback factor for loop gain calculation.

    You could use the below circuit for calculating PM of the TIA circuit you have attached. The PM is approximately 84' for your design. Attached TINA-TI circuit for the same if you want to take a look. OPA659_PM_sim.TSC

    Best Regards,


  • Dear Rohit :)
    I've read in a book that the spice models doesn't include the input capacitances! is it true? bcoz you have also mentioned it :)
    usually I download the spice models from official website.
    also OPA657 spice model has some problems. I can't simulate it with LTspice properly.
  • Hi,

    Usually if the spice model is transistor based, it is likely that the input capacitance is included in the model. The OPA695 spice model should include the input capacitance as it is a transistor based model.

    I am sorry but I am not very familiar with LTSpice. I would recommend you to post your query in WEBENCH modeling forum for importing OPA657 spice model in LTSpice.

    Best Regards,