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OPA855: TIA with bootstarp stability issue

Part Number: OPA855
Other Parts Discussed in Thread: OPA695,

Tool/software:

Hello,

I am currently designing a TIA with a bootstrap circuit using the OPA855. Additionally, I am incorporating the OPA695 to achieve a higher gain. The target bandwidth is 100 MHz, with the maximum possible gain. The photodiode capacitance varies up to 120 pF. I have encountered a few questions during the analysis and simulation:

1. When I remove the feedback capacitor (1 pF), the bandwidth seems fine, but the final output (Vo2) appears unstable.

However, when I change the OPA695 supply to a single +5V, the bandwidth decreases slightly, and the output becomes stable. Could you help clarify why this is happening? The test was performed with a 30 MHz input signal.

2. I attempted a stability analysis by finding the phase margin at gain = 0. Could you please review this and let me know if I carried out the analysis correctly?

3. How can I calculate the output noise for this configuration?

Thank you
OPA855_Bootstrap.TSC

  • Hi Mojtaba,

    • One issue we found in your circuit is that U1 will not be stable as OPA855 is not unity gain stable (>= 7 V/V stable).
    • Also why is the R5 and R10 resistors needed?
    • I didnt understand how you break the loop, but usually we break the feedback loop and analyze stability, and I think for the TIA the stability can be analyzed separately by ignoring U1 and U3.

    • I got the above circuit unstable for U2 configuration with 1pF cap, but the circuit is stable at 150fF and below.
    • Also we have developed an online calculator tool for designing TIA circuit. You can find the link here: Analog circuit design studio. You can insert all the required values and design the circuit. You will also receive suggestions on the circuit and suggested devices too. 
    • You can see the noise analysis of the circuit in the noise tab also.

    OPA855 TIA Stability.TSC

    Thanks 

    Aditya Gosavi

  • Hi Aditya,
    Thank you for your prompt reply.