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    How to fix the closed loop -3dB Bandwidth when the closed loop gain changes

    Hi,
    I am designing an amplifier in inverting configuration with variable gain.
    But when switching the gain of by changing the value of the feedback resistor, the CL 3dB bandwidth also will change since the GBW is fixed. So how could I fixed the CL bandwidth?? acceptable variation will be +/- 20% over corners.
    Thank you in advance!

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    Re: How to fix the closed loop -3dB Bandwidth when the closed loop gain changes

    One possible solution is to use a current-feedback amplifier (CFA) instead of a classical opamp.



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    Re: How to fix the closed loop -3dB Bandwidth when the closed loop gain changes

    Thanks for your answers!
    Other than this, is there any simple way to do it? how about switching different size of caps in parallel with fb res to limit the bandwidth?



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    Re: How to fix the closed loop -3dB Bandwidth when the closed loop gain changes

    Other than this, is there any simple way to do it? how about switching different size of caps in parallel with fb res to limit the bandwidth?
    Yes, if the amplifier has sufficient high GBW.



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    Re: How to fix the closed loop -3dB Bandwidth when the closed loop gain changes

    There are only a few active filters that lend themselves to variable gain and fixed bandwidth. Usually one must choose a feedforward , or wide band flat amp with variable gain independant of the filter as any negative feedback of the filter that controls gain will also control bandwidth.

    This is one exception.

    Frequency response is within 0.25 dB of flat between 30 Hz and 15 kHz. The gain can be adjusted between unity and +15 dB.
    A best design is easily achieved with good test specs™
    A better question deserves a better answer. ™
    ... so include all your acceptance criteria ( values, % tolerance) and assumptions in your question or any design.

    ... Tony Stewart EE since 1975
    - slightly north of Toronto



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