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6.9. CMOS

6.9.1. Importance and Features of CMOS Logic Circuits

CMOS is most important logic circuit due to the following.

(i) An enhancement mode p-channel acts as pull up and n-channel enhancement mode MOSFET act as a logic driver (pull down) When n-channel is ON, the p-channel is OFF and when n-channel is OFF, the p-channel is ON. It means the D.C. (steady state) current dissipation between the supply ends is very small as the series resistance is always very high. Current will flow only during the transitions from 0 to 1 or 1 to 0. Power will dissipate only during the transitions from 0 to 1 or 1 to 0. This feature makes it possible to fabricate large or very large or very-very large-scale integrated circuits (LSI or VLSI or VVLSI) using the CMOS pairs.

(ii) Since at an instant one of the MOSFET in the pair is ON and has low resistance, the switching speed at charging and discharging is rapid and turn ON delay and turn OFF delay are nearly same.


  

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