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    Stability issue of the closed loop feedback in fully differential ampliifer

    Hello all

    I am trying to simulate the slew rate of my fully differential amplifier, I am using feedback resistors of 100 M Ohm to avoid loading the amplifier, the problem I am getting an oscillated output, while indeed I have good phase margin in my amplifier by simulating it from small signal analyses.

    How can I use more precise way to simulate the slew rate of the OTA without having feedback effect on the circuit


    Thanks

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    Re: Stability issue of the closed loop feedback in fully differential ampliifer

    Maybe those feedback resistors are too high. Combined with the parasitic capacitance you might have built an oscillator. Or perhaps a problem with input bias current. Just try smaller resistors and see what happens.
    Last edited by barry; 6th December 2019 at 20:34.


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    Re: Stability issue of the closed loop feedback in fully differential ampliifer

    Whats's the purpose of feedback resistor in your simulation circuit? If it's DC bias, the feedback path should be filtered in a way that it doesn't ruin phase margin.


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    Re: Stability issue of the closed loop feedback in fully differential ampliifer

    Quote Originally Posted by FvM View Post
    Whats's the purpose of feedback resistor in your simulation circuit? If it's DC bias, the feedback path should be filtered in a way that it doesn't ruin phase margin.
    Dear FvM, Dear barry

    The purpose of the feedback resistors, because I want to set the gain to unity for testing the slew rate of the fully differential amplifier, if for single, ended there will no need and the setup is easily done by using a buffer with output shorted to vin-, this connection we can not use it for fully differential counterpart.

    Now as Barry and also Behzad explained, a big resistor feedback combined with the input capacitance of the differential amplifier creat poles that disturb the differential path. In the same time I cant use small resistor values for high output impedance OTA,

    Then how can I test the slew rate for this case ?



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    Re: Stability issue of the closed loop feedback in fully differential ampliifer

    No one suggested using a small resistor, but 100M is almost no resistor at all. I suggested trying a smaller resistor (1M, 100K). Did you do that?



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    Re: Stability issue of the closed loop feedback in fully differential ampliifer

    Dear Barry,

    Also the same bad oscillation I have

    I would like to learn from you how to simulate the SR from only capacitive feedback loop ? will it possible?



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