How can I use a LM741 as an Inverting pulse amp?

Discussion in 'Homework Help' started by slavano, May 10, 2016.

  1. slavano

    Thread Starter New Member

    May 10, 2016

    I am trying to set up a circuit using a LM741CN op amp to take in an 20mv negative pulse and amplify it to be a 2.5v positive pulse so that I can have it be read by a digital pin on an arduino Due. The pulse frequency is expected to range from 100 000HZ-800 000HZ. How should I set up my circuit??? The most I have been able to do is increase the pulse, without inverting it, to 6.4v using diagrams online.

    Thank you in advance
  2. crutschow


    Mar 14, 2008
    A 741 is too slow to amplify such a high frequency pulse.
    You need a faster amp. Where do you buy your electronic parts?
    Does the pulse go for 0V to 20mV negative?
  3. slavano

    Thread Starter New Member

    May 10, 2016
    I would still like to try it because our initial results of for the frequency was inconclusive.

    our pulse ranges from -15mV to 35mV from a baseline of 0v.

    we bought this part from digikey.
  4. Papabravo


    Feb 24, 2006
    What you don't know is that the 741 has problems with frequencies above 10 kHz. It is bandwidth and slew rate limited; Don't waste your time with this ancient relic; do yourself a favor and bin those parts.
  5. dannyf

    Well-Known Member

    Sep 13, 2015
    your only chance to do that is with a fake ua741. a real ua741 would not be able to do it.
  6. #12


    Nov 30, 2010
    You can't turn a mule into a race horse, but you can try if you want to. The problem is, it will never win a race (provide a useful output).
  7. crutschow


    Mar 14, 2008
    If you don't want to take our advice then you're on your own. :rolleyes:
  8. hp1729

    Well-Known Member

    Nov 23, 2015
    It is a question of available gain at what frequency. You can get that gain at about 10 Hz, but at 800 KHz it drops to a gain of 1.
    Yes, slew rate just isn't there.

    (edited to add ...)
    I can't find an op amp that will get you there in one stage.
    Last edited: May 10, 2016
  9. slavano

    Thread Starter New Member

    May 10, 2016
    Thanks for the help guys.
  10. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    You are looking to create an amp with a gain bandwidth product of 10^8.

    Taint gonna get that in no 741.

    One would be lucky to get it using two cascaded amps.
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
    A gain of 125 (42 dB) and bandwidth of 800 kHz is a gain-bandwidth product of 100 million. That's a lot.

    The 741 will not work for this application. The open loop gain is way to low, but you could overcome that with several amplification stages in series. The problem is that to recreate an 800 kHz pulse you need a very fast output stage risetime, and the 741 just plain isn't that fast.

    First, determine the minimum slew rate you can tolerate in the output pulse shape. Based on that' select an opamp. A standard opamp gain stage needs at least 20 dB of negative feedback *at the highest frequency of interest* to stabilize the stage gain, frequency response, and output impedance. You might have to have two or three gain stages in series to achieve this. For example, if you have three gain stages in series, each needs to make 14 dB of forward gain at 2.4 MHz (800 kHz plus the third harmonic to improve risetime). With some gain headroom for negative feedback, that means each opamp part must have an open loop gain of at least 34 dB. Still a lot. At 1 MHz, an NE5534 (a much faster and quieter opamp) has an open loop gain of around 26 dB, still not enough. Consider a current-feedback opamp or high-speed comparator.

  12. Wendy


    Mar 24, 2008