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OPA858: OPA858

deepthi kankanampati
Prodigy 80 points
Part Number: OPA858
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Title: Stability of OPA858 in Transimpedance Amplifier (TIA) Configuration

Question:

I am working with the OPA858 op-amp and trying to use it in a Transimpedance Amplifier (TIA) configuration. From the datasheet, I see that the op-amp is decompensated with a gain bandwidth product requiring a minimum gain of >7 V/V for stable operation in a voltage amplifier configuration.

In the voltage amplifier case, the gain can be stabilized by setting the resistor values such that R2=7R1R_2 = 7R_1R2​=7R1​. However, in the TIA configuration, there is only a feedback resistor RfR_fRf​, and the input is a current source with infinite impedance. Given this, my concern is:

  • Since there is no explicit gain-setting resistor like R1R_1R1​ and R2R_2R2​ in a standard amplifier circuit, how do we ensure that the OPA858 remains stable in a TIA configuration?
  • Does the feedback resistor RfR_fRf​ alone determine the stability, and if so, how do we calculate a stable value?
  • The datasheet mentions TIA applications, but doesn't the infinite input impedance from the current source make the op-amp unstable?
  • Can I use the OPA858 as a TIA without additional compensation, or do I need to add external compensation components?
  • 0
    TI__Intellectual 2065 points

    Hi Deepthi,

    Thank you for your questions!

    1. You are right that there is no explicit gain-setting resistor like in a standard amplifier circuit, so it is very important to ensure that the OpAmp remains stable in a TIA configuration. To stabilize the TIA we need a feedback capacitor Cf across Rf or feedback resistor, such that the high frequency gain of the TIA follows (1+Cin/Cf) > 7, where Cin = Ccm + Cdm + Cpd, (Ccm+Cdm = OpAmp common and differential mode caps, Cpd = photodiode junction cap). To learn more about TIA stability: https://www.ti.com/lit/an/sboa122/sboa122.pdf?ts=1739946375532&ref_url=https%253A%252F%252Fwww.google.com%252F

    2. In a TIA circuit both Rf and Cf determines the stability of the circuit. So the op-amp's inverting input cap and the photodiode cap creates a pole in the feedback loop (Or a zero in 1/beta curve), which makes the circuit unstable. To cancel this pole, we add Cf across Rf which will create a zero in the beta curve (or pole in 1/beta  curve). Please refer the link to understand more about stability of opamps: https://www.ti.com/video/4080235259001. But Rf also determines the bandwidth and noise of the TIA, eg. larger Rf leads to lower bandwidth with leads to lower noise.

      To find the Cf value you can run a simulation in TINA-TI and check the stability of the circuit. If you want a starting point for Cf, please take a look at the TIA circuit design tool: Analog circuit design studio, which will recommend a Cf (and also best op-amps) for the required circuit conditions. Also if your circuit is not following the minimum stable requirement, a warning will pop up in the bottom right window.

    3. The infinite input impedance of the current source does not make the op-amp unstable. In a TIA, the feedback resistor Rf provides negative feedback, ensuring the op-amp maintains a virtual ground at the inverting input. Stability is primarily affected by the interaction of the op-amp’s input capacitance, photodiode capacitance, and Rf, which together create a pole in the loop gain. To ensure stability, a feedback capacitor Cf is added, following the condition (1+Cin/Cf) > 7. Proper compensation with Cf prevents oscillations and ensures stable operation.

    4. The OPA858 typically requires a feedback capacitor Cf for a stable TIA operation, for the reasons mentioned above.

    Please let me know if you have any more queries.

    Regards,

    Aditya Gosavi