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OPA855: Phase shift questions

Part Number: OPA855
Other Parts Discussed in Thread: TINA-TI

Hello,

 I currently use the OPA855 as the first stage, and then I want to cascade two or three stages  but I have some difficulties. Question about the phase: Debug the OPA855 closed-loop bandwidth reached 2GHz, does my phase shift from the graph by 120°?Or do I want to measure a specific phase offset between the input and output of a specific experiment test? How do you calculate the phase shift caused by the RF amplifier if I  want to add RF amplification under phase margin? If the first  RF amplifier brings 45° phase shift, can only one stage be cascaded again at most?

Best regards

kailyn

  • Hi Kailyn
    Yes, operating the OPA855 at 2GHz will cause a phase shift of around 120°. And for the cascaded design, if you are closing the loop individually, then you only need to take care of the phase margin of the individual loop for the stability of each stage, if all your stages are stable individually then your entire system will also be stable. But one thing you need to take into account is that the input capacitance of the amplifier from one stage may affect the phase of the previous stage amplifier (though, since you have RF amplifier in the cascaded stage so I assume its input cap value to be very small and hence not affecting the phase much).

    But if your design has a composite loop where you are feeding back the signal from the output of one stage to any of the previous stages then in that case the phase shift from each amplifier will add up and you need to take the sum of all the phase shifts into consideration for the stability analysis.

    Regards

    Anant

  • Hi Kailyn,

    did you already run a TINA-TI simulation?

    Can you show a schematic?

    Kai