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Op-amps how could this possibly be a comparator?

  1. May 27, 2013 #1

    Femme_physics

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    I'm asked which type of op-amps these are

    [Broken]


    I think it's clear the SECOND one is a comparator, but the FIRST one is an inverter IMO. It has a feedback loop. If it has a feedback loop, it can't possibly be a comparator! Am I right or what?

    The solution manual my teacher made, had them both written as "comparators" :confused:
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. May 27, 2013 #2

    NascentOxygen

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    Staff: Mentor

    No, sorry, you're wrong. :frown:

    Look closer, there is no negative feedback, no feedback of any description, in fact.

    There are inverting comparators, and there are non-inverting comparators.
     
  4. May 27, 2013 #3

    Femme_physics

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    where R1 is connected- that's not feedback?
     
  5. May 27, 2013 #4

    NascentOxygen

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    R1 just goes to earth. Feedback requires some of the output to feed back to the input (to the (-) input for negative feedback).

    (If you ever see positive feedback (i.e., back to the + input) then that makes a comparator.)
     
  6. May 27, 2013 #5

    Femme_physics

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    Duly noted, thanks for this explanation :-)
     
  7. May 27, 2013 #6

    Femme_physics

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    I assume that whenver there is a LED after the comparator, only if V+ > V- , then the LED will light up. Is that a correct assumption to make? Also in my specific case?
     
  8. May 27, 2013 #7

    NascentOxygen

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    The output that lights the LED is dependent on where the LED's other pin is connected. :smile:
    If the LED is connected between output and ground then it will light up when the output goes high.

    In your case, the LED is connected between OP-AMP output and +Vcc, meaning that when the output is high, there will be not much voltage across the LED.

    So .....
     
  9. May 28, 2013 #8

    Femme_physics

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    That clears it :) Thank you
     
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