logic gates - nmos

Thread Starter

alexmath

Joined May 2, 2014
17
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?
 

ronv

Joined Nov 12, 2008
3,770
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.
 

MikeML

Joined Oct 2, 2009
5,444
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.
 
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