why is that? help~~~

Discussion in 'Homework Help' started by katherine1117, Dec 13, 2009.

  1. katherine1117

    Thread Starter New Member

    Oct 28, 2009
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    why is a NAND GATE faster than a AND gate and NOR gate? thx
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    I believe your answer can be obtained by taking a look at any manufacturer's datasheet for an AND gate and a NAND gate. Make sure that you compare like technologies. CMOS would be a good technology to use in your investigation.

    Most of them have a typical gate circuit included in their datasheets.

    hgmjr
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    Here are a couple of example datasheets from TI.

    AND gate

    NAND gate

    Take a look at the typical circuit for the 7400 and the 7408 in the linked-to datasheets from TI. The reason for the difference in speed is tied to the propagation from input to output. See if you can spot why it takes longer for a signal change on the input to manifest itself on the output of these two different gates.

    hgmjr
     
  4. katherine1117

    Thread Starter New Member

    Oct 28, 2009
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    i'm still not quite sure, can you explain it further? thx very much
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    214
    In the attachment, you will notice that the AND gate has an additional inverter stage in its typical circuit that is absent in the NAND gate.

    hgmjr
     
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