# CMOS inverter

#### tuoyo22

Joined Nov 9, 2007
3
Hi everybody just want to know this for my Cadence design

In a CMOS inverter why do we assume that the load Capacitance is INDEPENDENT of Voltage, if we want to calculate for the fall or rise time.

Any Idea will be appreciated
Thanks
T

#### beenthere

Joined Apr 20, 2004
15,819
For all capacitors, the formula is: T = RC. The charge time is not dependent on voltage.

#### tuoyo22

Joined Nov 9, 2007
3
Thank you!

#### Ron H

Joined Apr 14, 2005
7,014
Actually, the typical load for a CMOS gate is more CMOS gates, plus some wiring capacitance. The input capacitance to a CMOS gate is somewhat dependent on voltage, but the calculation would be an absolute nightmare, so assuming that the load is independent of voltage is a good idea.
Simulation takes the voltage dependence into account and does the calculation for you, and is therefore somewhat more accurate.