# Concerning rds in MOSFET

Why is $r_{o1}$ connected to the ground in the second diagram which is the small signal equivalent (formal solution, hence most likely correct)?:

• There is some confusion regarding the names of the resistors. On the title you say $r_{ds}$, in the text you say $r_{o1}$ and then in the lower figure we can see only $r_{os}$. Besides that, are you sure this $r_{os}$ refers to the MOSFETs? It seems as if it is the substitution of the current sources. – nickagian Jun 21 '17 at 19:24
• @nickagian You're right, it's the output resistance of the current sources :-) – peripatein Jun 21 '17 at 19:27
• @nickagian But what's the difference? I notice that the book states that all these resistances are equal to $V_A/I_D$, which is the standard rds of a MOSFET. – peripatein Jun 21 '17 at 19:30
• In the 2nd schematic I do not see the feedback resistor R2 being properly incorporated into the schematic. The 2nd schematic seems to have no feedback at all. So I don't see how it be the full small signal version of the 1st schematic. – Bimpelrekkie Jun 21 '17 at 19:31
• @Bimpelrekkie It includes instead the input and output feedback resistances, namely R1||R2 and R1+R2 – peripatein Jun 21 '17 at 19:32