Im currently a year 1 electrical engineering student. May I ask, for a diode-connected PMOS (gate connected to drain), why is |Vsd| = |Vgd| ?

  • \$\begingroup\$ Are asking why you take the absolute value or why they are equal to each other? Please explain what you don't understand the equation that you have provided? ALSO please quote where you saw this equation because it is likely that the location where you got this equation will also provide you some information about the equation. \$\endgroup\$ – KingDuken Apr 13 '18 at 15:02
  • \$\begingroup\$ The equation is not correct. If the gate and drain are shorted together, then \$V_{gd}\$ is zero. But \$V_{gs}\$ will be approximately equal to \$V_{th}\$ (if there's non-zero current through the channel). \$\endgroup\$ – The Photon Apr 13 '18 at 16:14

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The relation you have asked for in the question is wrong.

In the diode connected PMOS, fig.B, Gate and Drain are shorted. Hence \$V_G = V_D\$.

And therefore \$V_{GD} = 0 \$ and \$V_{GS} = V_{DS}\$.

M2 will always be in saturation as \$ V_{DS} > V_{GS} - V_{Th} \$ always.


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