4 input And gate CMOS

Discussion in 'Homework Help' started by jstrike21, Feb 10, 2010.

  1. jstrike21

    Thread Starter Member

    Sep 24, 2009
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    This question compares two different ways to implement a four-input CMOS AND gate.
    (a) A four-input AND gate can be constructed by three two-input CMOS AND gates. The outputs of two two-input AND gates are connected to the inputs of the third two-input AND gate. Draw the corresponding circuit diagram. How many transistors (both PMOS and NMOS) are required?
    (b) Show how you can use only 2-input CMOS NAND and NOR gates to build the four-input AND gate. How many transistors (both PMOS and NMOS) are required? (Hint: Use DeMorgan's theorem.)

    I have completed part A.
    Im having trouble visualizing how I can do B
     
  2. jstrike21

    Thread Starter Member

    Sep 24, 2009
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    wouldnt this way be worse? I would think it would use the inverse of 3 nand gates. I dont know why they mention nor gates
     
  3. jstrike21

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    Sep 24, 2009
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    anyone have any ideas?
     
  4. Ron H

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    Apr 14, 2005
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    By DeMorgan's theorem:

    \overline{\overline{A \cdot B} +\overline{C \cdot D}}=A \cdot B \cdot C \cdot D

    It's definitely cheaper (by transistor count) than 3 AND gates.
     
  5. jstrike21

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    Sep 24, 2009
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    thanks that really helped me out
     
  6. Ron H

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    So tell us how many transistors it requires, and how many you saved compared to the 3 AND gates.:)
     
  7. jstrike21

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    Sep 24, 2009
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    6 pmos and 6 nmos for a total of 12 for the nand nor circuit
    9 pmos and 9 nmos for a total of 18 for the and circuit
     
  8. Ron H

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    Excellent! For extra credit, can you do it with 10 transistors?:D
     
  9. jstrike21

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    Sep 24, 2009
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    your really pushing it now:D
     
  10. Ron H

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    It's simple. If you can design a 2 input NAND, you can design a 4 input NAND. Then you just need an inverter to make in an AND gate.
     
  11. dor

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    Feb 20, 2009
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    something like this :)
    [​IMG]
     
  12. Ron H

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    Yep. Very much like that.:D
     
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