XOR made from 4 Transistor NANDS not working

Discussion in 'Digital Circuit Design' started by Octain, Jul 11, 2019.

  1. Octain

    Thread Starter New Member

    Feb 12, 2019
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    Hello,
    for a school project we need to build logic gates but all made from NANDs made with NPN Transistors.You can see our NAND circuit that we have to use as a base for all the other logic gates in the attachments. The XOR needs to have 2 Inputs (simple switches) with LEDs that indicate wheter its on or off and 1 LED that shows the output. We have to use the basic NAND like it is shown in the attachment. xor_from_4nands.gif NAND_circuitbasic.png Now my friends made the following plan which should be the basic NAND combined like shown in the picture above but it obviously doesn´t work, for example the lowest Transistor to the right is burning everytime because there is no transistor to the base. XOR_fail.jpg

    What do we have to change that the XOR works as intended? Thank you for your Answers.
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    In the drawing of the single NAND gate there is a 220Ω resistor and an LED in series with each input. You could omit the LED if you wish but the resistor is vital.
     
  3. LesJones

    Well-Known Member

    Jan 8, 2017
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    I think this should be in the homework help section. In that section we do not provide the answer to the question but we give hints and some times ask a question to get you thinking about some aspect of the problem. So I will ask have you thought about the clamping effect of the base emitter junctions and it's implication on directly connecting the bases of multiple transistors together ?

    Les.
     
  4. Octain

    Thread Starter New Member

    Feb 12, 2019
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    The two LEDs (they are also in the XOR) in series with each input are necessary because they need to indicate the staus 1 or 0 of the switch (teacher said we have to do it that way). They are no part of the other 3 NANDs of the XOR
     
  5. Octain

    Thread Starter New Member

    Feb 12, 2019
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    Thank you for your answer. I am not experienced with circuits so i haven´t thought about this yet (i only thought about a resistor problem).
     
  6. iimagine

    Senior Member

    Dec 20, 2010
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    The circuit is missing a collector resistor.
     
  7. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    So it is.
     
  8. LesJones

    Well-Known Member

    Jan 8, 2017
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    Re past #6 & #7. I think the TS may have noticed this from his sentence "Transistor to the right is burning everytime because there is no transistor to the base." I think he meant "no resistor to the base" (Rather than transistor.)

    Les.
     
  9. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Except that posts #6, #7 are referring to a missing collector resistor, not a base resistor.
     
  10. LesJones

    Well-Known Member

    Jan 8, 2017
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    It would be the same resistor. (It would prevent the supply voltage being connected directly across the base emitter junction of the lower right hand transistor.
    NOTE to TS. Giving components identifying numbers makes it easier to describe which component we are talking about.

    Les.
     
  11. Octain

    Thread Starter New Member

    Feb 12, 2019
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    oh yeah thank you i meant that that was my misstake
     
  12. Octain

    Thread Starter New Member

    Feb 12, 2019
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    this would be smarter, yes
     
  13. Bordodynov

    Well-Known Member

    May 20, 2015
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    See
    2019-07-12_10-11-13.png
     
    eetech00 and absf like this.
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