Problem with output in an inverter

[sridhara]

doubt in simple inverter

in an simple inverter.........
when i gave Vdd=1.8V for source of nmos and source of pmos connected to ground.......(interchanging the power supplies)......
i kept W/L of pmos as 3 and W/L of nmos as 1 and
when i simulated in hspice
i found the output as DC at 0.9V for a square wave input........

wat might be the reason for this.....

 

[ambreesh]

Re: doubt in simple inverter

Your NMOS and PMOS are bothe off.
And they basically act as huge resistances. The voltage gets divided depending on the off resistances of the devices.

 

[sridhara]

Re: doubt in simple inverter

both are not off....pmos will be in saturation since Vg(pmos)=0V(input being a square wave..amplitude varies between 1.8 and 0) and Vs(pmos)=0V...
Vgs=0 which is less than Vt...

 

[Old Nick]

Re: doubt in simple inverter

both are not off....pmos will be in saturation since Vg(pmos)=0V(input being a square wave..amplitude varies between 1.8 and 0) and Vs(pmos)=0V...
Vgs=0 which is less than Vt...

Vt for a PMOS is -ve

pMOS is off at Vgs = 0V
pMOS is on when Vgs is more -ve than Vth

Vgs > Vth - pMOS is off because Vgs is more +ve than Vth