How can I refer a Tension to another tension?

[Bode82]

Hi I'm in question about a tension refer to another tension for a mos device:example I've a Pmos with the source connect to Vdd (1V) so its Vsource refer to Vdd it's Vsource =0 (I need of any tension of my mos refer to alimentation Vdd)
The drain is connect to a tension refer to ground Vx=0.3V(remember that Vdd=1v),my question is what is the tension of drain Vd refer to Vdd?

It's Vx-vdd or vdd-vx or other solution?

 

[Bode82]

Please... I need of an answer because I can't continue my project of opamp

 

[VVV]

You should always choose the ground as the reference point, unless it is more meaningful to do otherwise.

In your case, if you refer all voltage to ground, the drain voltage is simply Vx=0.3V

It seems you want to refer the drain voltage to the Vdd. In that case, the drain voltage is Vx-Vdd. You always subtract the "reference" from the actual voltage. As you can see, the drain voltage is NEGATIVE with respect to Vdd, which is normal. And it is correct, because the P-channel MOSFET requires a drain voltage that is negative with respect to its source.

I said that in some cases it is more meaningful to have another reference, instead of ground. Consider your example, but assume that Vdd is 250V and Vd=125V. You need to know if the MOSFET can take it. So it is more meaningful to measure the voltage with respect to Vdd, because that gives you the drain-source voltage, which you can compare to the datasheet and see if the MOSFET will be OK. Otherwise, the two voltages have little meaning with regards to the NOSFET's ability to sustain the voltage.

 

[Bode82]

Thanks for your answer VVV.I must refer any tension to Vdd because I'm using matlab files that must have any tension refer to Vdd.