resistors in a logic gate

Discussion in 'Homework Help' started by Voltboy, Jan 10, 2007.

  1. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    im just learning about circuits and when i saw that they use resistors in a logic gate i was confused, why u need resistors if its enough with transistors??
     
  2. Dave

    Retired Moderator

    Nov 17, 2003
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    Can you post up the logic gate where you have seen this? It largely depends on the technique used to implement the logic gate; for example traditional nMOS logic gates (a precursor to CMOS logic) utilise a resistor in the pull-up path. The current path to Vdd would be through the pull-up resistor providing a logic '1' at the output dependant on the nMOS configuration in the pull-down. Unless the resistors you are referring to are for protection purposes.

    A circuit diagram of an example would make an explanation easier.

    Dave
     
  3. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    It seems that you are surprized to find that the inner workings of ic's are not magic. In every case, the functions are done by collections of transistors and/or fet's. Since the semiconductors functions as saturated switches, resistors have to be present to limit current.
     
  5. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    but then how to know how many Ohms to add, using Ohm law?? beacause i dont know how much the current must be limited
     
  6. Dave

    Retired Moderator

    Nov 17, 2003
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    Without a circuit to comment on this question is like asking how long is a piece of string. The current capabilities of a transistor vary considerably and knowing how much the current must be limited by is dependant on, not only the configuration of the transistors in the logic gate, but also the transistors used to implement the gate. For example, current flow in CMOS logic (argueably the most popular implementation - correct me if I'm wrong) tends to only be significant during gate transistions, i.e. in steady-state conditions there is no (read as negligible) current flow from Vdd to ground, and the inputs and outputs tend to drive capacitive loads, hence there is negligible current flow from this perspective. If we are looking at current limiting for the inputs and output to the logic gate (i.e. external to the gate design) then this is a distinctly different question than why they use resistors in logic gates.

    Dave
     
  7. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    Thanks alot:D
     
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