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Homework Help: Boolean and truth table for circuit

  1. Feb 17, 2013 #1
    I attached the problem statement.

    The problems I have done in the past always involved gates (AND, OR, etc...)
    Now this involves transistors, I honestly don't know how to go about doing this.

    One of the hints for the problem is to assume the ideal transistors are operating in cutoff or saturation.

    I don't understand how this circuit can be turned into a boolean expression.

    Attached Files:

  2. jcsd
  3. Feb 17, 2013 #2


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    Staff: Mentor

    If A and B are low, what is the output?

    If either A or B are high, what is the output?

    Makes sense?
  4. Feb 17, 2013 #3
    If A and B are low then the output is 0.
    If A and B are high then the output is 1.

    If A or B are high, the output is 1.

    But what do the transistors do for this case?
  5. Feb 17, 2013 #4


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    Staff: Mentor

    How did you work out these answers? Do you understand how transistor switches operate? (Not exactly a rhetorical question, though it may appear so.)

    The transistors are the voltage-controlled switches that perform the gating operation. Without them, you don't have logic gates.
  6. Feb 21, 2013 #5
    Perhaps the best way to look at the logic is 'what path will the current take when either transistor is enabled. Remember when no current is flowing, the potentials across either side are the resistor are the same. Hope that provide a clue.
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