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Designing an amp with mfed response

Jon Bean1
Expert 1120 points

Would it be possible to use an off the shelf op amp and design it to have a response that had minimal overshoot and rise time and maximised bandwidth,for a step input. Could this be tailored using external components or is the response largely dictated by the internal structure of the amplifier. Thanks Jon 

  • 0
    TI__Guru 62085 points

    Hi Jon,

    Small signal rise time (tr) is equal to 0.35/fc where Fc is the bandwidth of the op amp.  So op amps with higher gain bandwidths (GBW) will have faster small signal rise time.  The overshoot and ringing are directly related to the stability of the op amp circuit configuration which can be optimized to be under-damped, critically damped, over-damped, etc.

    What are your goals? 

  • 0 in reply to Collin Wells
    Expert 1120 points

    Hi Colin

    I need to design some amplifiers with a gain of 2 and 5. The bandwidth needs to be 200MHz. But I really need the step response to be as good as I can get it. So if I input a square wave in I need it to be recreated faithfully without overshoot. You see I am designing a signal digitizer so I need the time domain response to b very good. 

    Thanks

    Jon

  • 0 in reply to Jon Bean1
    Guru 140200 points
    Hi Jon,

    there are OPAmps which have good step respsonse. But usully the step respsonse will vary depending on the signal amplitude and on the load resistance that has to be driven.

    Kai
  • 0 in reply to kai klaas69
    TI__Guru 62085 points

    Thanks Kai!

    Jon,

    I'm going to move your post over to the high-speed amplifiers forum.  Recreating a 200MHz square wave will take an opamp with probably near 1GHz GBW as the frequency content of the 200MHz square wave is much higher than 200MHz.   

    Have you considered using a comparator?  They do a much better job than amplifiers at creating square wave output signals.

  • 0 in reply to Collin Wells
    TI__Genius 14196 points
    Hi Jon,

    A comparator may be a better solution. However, to also answer your original question, you can tailor amplifiers with external components. The short answer is that Bessel and Multiple Feedback Filters are the best at square waves. For your information, here is an application note on this design process www.ti.com/.../sloa049b.pdf.

    Best regards,

    Sean
  • 0 in reply to Sean Cashin
    Expert 1120 points

    Thanks for the replies. I need to use an amplifier as I am capturing not just square waves but other waves too. Really I was just trying to get a feel for how easy it would be to capture a square wave with an op amp.