and gate trans

Discussion in 'General Electronics Chat' started by duxbuz, Jun 11, 2014.

  1. duxbuz

    Thread Starter Member

    Feb 23, 2014
    133
    0
    Hi

    It states here:
    http://en.wikipedia.org/wiki/Transistor_count#Logic_functions

    6 transistors for an AND gate.

    I thought it might be just 2 transistors, as in lots of examples.

    I think I read about this somewhere a while back, but I cannot remember.I thought someone may just know the reason/answer to why its not just 2 transistors to perform the simple AND logic.

    Is it something to do with active low or something?

    Thanks
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,778
    932
    Simply stated it is a matter of size. Transistors are easy to make on the silicon substrate and occupy very little space. Resistors require very large amounts of silicon 'real estate' and are therefore used as little as possible. Transistors are made to function as resistors and diodes of all sorts in order to reduce the size of the etched circuitry. The number of transistors required is a reflection of this process.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,523
    1,247
    The two-transistor AND schematic shown here:

    http://en.wikipedia.org/wiki/AND_gate

    is not a real circuit. It demonstrates the concept, but lacks several features required for a general purpose logic gate. Among other things, a real gate has a totem-pole output stage that can both source and sink current instead of a simple resistor. This gives a much lower output impedance and consistent speed performance with varying numbers of load connections.

    ak
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Back when I was a kid in the pre TTL days there existed RTL: Resistor Transistor Logic. Simple gates were actually made up similar to that example for the AND gate, except that particular AND gate was never used as the circuit is horrible!

    I know, designed it then I built it. It was one of the first lessons from the school of hard knocks.

    To get any sort of drive you need to keep Rout small. But when A=lo and B=hi there is still a path for B to drive the output, albeit without any current gain so you get a resistive divider minus a diode drop out.

    That's still greater then the "almost zero" one would expect.

    Yahoo up "and gate schematic" and look at the images returned. Some are actually correct, and none of them has fewer then 4 devices (CMOS NAND), and the TTL AND gate shown needs no fewer than 6 transistors, 6 resistors, and 4 diodes.
     
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