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I just had some doubts regarding voltage references, etc after viewing this circuit:

enter image description here

I'm usually used to using ground as a reference and assuming that it's 0V. In this case I'm a bit confused.

Q1: In this scenario, is the source of the bottom MOSFET equal to -Vgs which is equal to -Vds which in turn implies that Vgs = Vds?

Q2: Say I labelled a new point between the MOSFETs as point B. Now could I say that the Vds of the bottom MOSFET is equal to Vb-(-Vgs) or Vb-(-Vds) or would it just be that Vds=Vb?

Q3: Lastly, If I did ground the bottom MOSFET source as usual, wouldn't the following be true $$-V_{GS}=-V_{DS}=0$$ Now, couldn't we just exclude that last equals sign and from that say that Vgs=Vds? I feel like I've just confused myself even more. Never thought about this before

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The source of the bottom FET is the negative reference for both Vgs and Vds. This does not imply that Vgs is equal to Vds.

You can consider the bottom terminal a "Zero Volts/Ground" if you wish. (Actually, you can consider any point in a circuit as "Zero Volts", but in this case using any point other than the bottom terminal as "Zero Volts" will only cause confusion.)

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  • \$\begingroup\$ Ah. Much better. I have no idea why I had it in my brain that -Vgs = -Vds. Thank you so much. \$\endgroup\$ – AlfroJang80 Sep 22 '18 at 23:51

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