# MOSFET analysis

I'm trying to understand how this circuit works:

The solution says that the source and drain exchange roles depending on the value of $V_X$.

For case 1:

$$0 < V_X < 1$$

$$V_{TH} = 0.7V$$

the following equations are: $$V_{GS} = 1.9 - V_X$$ $$V_{DS} = 1 - V_X$$ $$V_{OD} = 1.2 - V_X$$

For this case, I'm assuming the drain and sourced switched roles. So the source is at $V_X$. What I don't understand for this case is this equation: $$V_{DD} = 1.2 - V_X$$

For the next case:

It doesn't specify but I assume when $$V_X > 1$$ $$V_{TH} = 0.7V$$

the following equations are: $$V_{GS} = 1.9 - 1$$ $$V_{DS} = Vx - 1$$ $$V_{OD} = 0.9 - 0.7$$

Again I do not understand this equation: $$V_{DD} = 0.9 - 0.7$$

Also, how do you determine if the source and drain switch roles?

EDIT: Made a mistake, it wasn't $V_{DD}$ it was $V_{0D} = V_{GS} - V_{TH}$. So my only question is how do you know if the source and drain switch roles?

• I hate to admit it, but I'm still hating these mosfet symbols you illustrate here. A discussion happened on EE.SE about 3 yrs back. See: electronics.stackexchange.com/questions/69311/… (Not an answer, just an annoyance I'm still struggling over with mosfet symbols.) – jonk Sep 19 '16 at 3:53
• Are you talking about the picture? Lol, I just print screened that from the book. – user367640 Sep 20 '16 at 0:01
• I know you did. I just hate those short-hand versions with the arrow pointing differently than you'd expect with the body diode showing. Instead, they show the current flow. Annoying. – jonk Sep 20 '16 at 0:18