logic gates - nmos

Discussion in 'General Electronics Chat' started by alexmath, Dec 5, 2014.

  1. alexmath

    Thread Starter New Member

    May 2, 2014
    Hey! Easy question for you guys. Why do we need both nmos and pmos transistor to build a logic gate? Why can't we just use only nmos transistors like bipolar logic gates?
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    You can if you use an open collector output, but the classic totem pole output won't work very well because of the high gate to source voltage.
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Because you would have to use resistors instead, and that wastes power and space.
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    NMOS for pulldown with resistors for pullup is called RTL (Resistor Transistor Logic). It was one of the first commonly available IC logic families. Hogged huge amounts of power.

    NMOS pullup requires a Gate voltage higher than VDD, so is not practical.

    PMOS pullup means that the transistor is turned-on by pulling the Gate to VSS, so coupled with NMOS for pulldown is the wonderful combination.