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Problem with output in an inverter

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sridhara

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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

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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

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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

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Re: doubt in simple inverter

sridhara said:
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
 

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