Current amplifier with Mosfet and Op-Amp: Output resistance
The output resistance is increased beyond the observed value of (ro2+R1||R2) because of the negative feedback effect.

It is known that negative feedback does increase input and output resistance for such a circuit with a factor of (1-loop gain).

For the circuit under discussion the loop gain LG=-(gmRE)=-gm(R1||R2).

Hence , we have (with RE=R1||R2)

r,out= RE+ro2(1+gmRE)=RE+ro2+ro2gmRE.

Feedback model for identificatioin of the loop gan:
This simple model results from (and fulfills) the BJT equations:

Ic=Vbe*gm  ,

Ic~Ie  ,

Vout=-gmRc/(1+gmRE)  .

As can be seen: The loop gain is LG=-gmRe .







simulate this circuit &ndash; Schematic created using CircuitLab

This model is based on the known equations for a BJT.
It can be applied for the FET as well by replacing Vbe by Vgs.