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

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    \$\begingroup\$ 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. \$\endgroup\$ – nickagian Jun 21 '17 at 19:24
  • \$\begingroup\$ @nickagian You're right, it's the output resistance of the current sources :-) \$\endgroup\$ – peripatein Jun 21 '17 at 19:27
  • \$\begingroup\$ @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. \$\endgroup\$ – peripatein Jun 21 '17 at 19:30
  • \$\begingroup\$ 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. \$\endgroup\$ – Bimpelrekkie Jun 21 '17 at 19:31
  • \$\begingroup\$ @Bimpelrekkie It includes instead the input and output feedback resistances, namely R1||R2 and R1+R2 \$\endgroup\$ – peripatein Jun 21 '17 at 19:32

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