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Diode OR Gate - the role of resistor

  1. Feb 24, 2007 #1
    Hi! What would happen if the resistor in the attached diode OR gate removed (connection with the ground would be preserved)?

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    Last edited: Feb 24, 2007
  2. jcsd
  3. Feb 24, 2007 #2


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    Staff: Mentor

    It ensures that the output is low when both of the inputs are low. Without the resistor, the output voltage would be undefined in that case.
  4. Feb 24, 2007 #3
    What do you mean by the word "low".. like when the inputs are of zero potential, when they're grounded? In that case the output would be also grounded and therefore defined... Or am I mistaken?
  5. Feb 24, 2007 #4


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    In digital logic, a logic zero or low would occur when the input is somewhere in the range of 0 - 2V. Its probably less than 2V, that varies depending on technology being used. For TTL devices, I believe it 0V-0.8V for a logic zero.

    The output is always grounded via the resistor i.e. the output is voltage drop across the resistor and not taken directly from ground.
  6. Feb 25, 2007 #5
    ok, thanks a lot. Another problem occurs when I am about to calculate the current in the output - should I take the output branch and the resistor-ground branch as connected in parallel?
  7. Feb 25, 2007 #6


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    Yup. The current through the resistor is not the same current going to the load connected to the output. The voltage however is the same.
  8. Feb 25, 2007 #7
    Ok, that makes it clear. Thanks
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