Op amps as comparators - lab report help

Discussion in 'Homework Help' started by biggles65, Jan 17, 2009.

  1. biggles65

    Thread Starter New Member

    Jan 17, 2009
    1
    0
    Hi, I'm sorry I can't follow the recommended structure for this, as it's not a problem as such, more of a 'what should I have got?'

    I have to write up a lab report in which we were using Op-Amps as summing amplifiers, comparators and integrators. We used an open loop 741 as the comparator, referenced to ground. A sine wave input signal was passed through the non-inverting input, and the output resembled a square wave. Now, we were meant to increase the frequency of the input signal to see what would happen to the output, but we didn't get this far in the experiment. I know a little about slew rate limiting, and I'm assuming this is where the answer lies. Am I right in saying that at a certain frequency the output would resemble a triangle wave, as there is not enough time for the output to saturate before the input crosses the reference again?

    Thank you for your help.
    Stefan
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes, you are right. Beyond a frequency the output will start to look like a triangular wave. You can find this frequency with the following formula:

    fb=(slew rate)/(2*pi*Vm)

    where

    slew rate is in V/sec
    fb=frequency in Hz
    Vm=peak output voltage

    This frequency determines the full power bandwidth (FPBW) of the op amp.

    As you increase the frequency more the output you approximate more a triangular wave. Above a frequency the output will be a triangular wave and its slope will equal the slew rate. What is more, note that as the frequency is increased above fb the output peak value of the voltage decreases.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    A lousy old 741 opamp runs into slew-rate-limiting at about 9kHz.
    A better opamp (that costs the same) can go up to 100kHz.
     
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