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Homework Help: Logic gates using diodes

  1. May 6, 2010 #1
    1. The problem statement, all variables and given/known data

    http://www.play-hookey.com/digital/electronics/dl_gates.html [Broken]
    revered members,
    in the above link, for OR GATE, when either of the diodes are forward biased we get the output voltage as logic HIGH.
    but for AND GATE, when when either of the diodes are forward biased or both forward biased we get the output voltage as logic LOW.
    it has been mentioned that resistor works a pull down resistor in the case of AND GATE. i.e, resistance in AND gate makes the output to go low even if the diodes conduct.
    i dont understand the concepts involved. please help how this works.

    2. Relevant equations

    3. The attempt at a solution
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 6, 2010 #2
    What parts do you specifically not understand? Do you not understand how diodes behave, or are you having trouble applying that behavior to the creation of logic gates?
  4. May 6, 2010 #3
    thanks for the reply.
    in OR GATE if any input is 1 either of the diodes is forward biased and start to conduct and if both the inputs are 1 then both the diodes start to conduct and the current flows thro the load resistance and we get the output high that is logic 1.
    but in AND gate, going by the diagram given in that site, when the both the inputs are zero, both the diodes are forward biased and start to conduct. but how i get logic 0 as the output despite the diodes conducting.
    i presume the load resistance connected to + Vcc plays a part in this, but i dont understand the concept here, that is how this load resistance plays a role in making the output to logic 0 despite the diodes conducting.
    please help sir.
  5. Sep 17, 2012 #4
    HEY someone give me the applications of AND gate and OR gate using diode only
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