Understanding Window Comparators

Discussion in 'Homework Help' started by SkiBum326, Jul 15, 2014.

  1. SkiBum326

    Thread Starter Member

    May 16, 2014
    33
    0
    Hi Everyone,

    I have a question about window comparators, which are discussed on page 862 of Electronic Principles.

    I attached an image of the window comparator in question. According to the book, when vin is between 3 and 4 V, the output transistor of each comparator is cut off, so the output is pulled up to +5v? I can't imagine why this would be the case. Wouldn't each comparator just go into saturation?

    Thanks in advance for any help,

    Austin
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Let as assume that Vin = 3.5V. So for the lower comparator we have:
    Vp = 3.5V and Vn = 3V and since (Vp - Vn) > 0 the LM339 output transistor is cut off.
    Now for the upper comparator we have:
    Vp = 4V and Vn = 3.5V.
    And again (Vp - Vn) > 0. And that means when the Vp > Vn, the output will go towards the Vcc line due to the pull up resistor because the output transistor of each comparator is cut off.
    http://forum.allaboutcircuits.com/showthread.php?p=736584#post736584
     
    SkiBum326 likes this.
  3. SkiBum326

    Thread Starter Member

    May 16, 2014
    33
    0
    Jony130,

    Thanks..that makes so much more sense now.

    Austin
     
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