How is the output impedance of a closed loop opamp derived?
Draw the small signal equivalent of the circuit as shown below:



simulate this circuit &ndash; Schematic created using CircuitLab
Write KVL as follows:
$$-ABv_x + i_xZ_o = v_x$$
$$Z_{oCL} = \frac{v_x}{i_o} = \frac{Z_o}{1+AB}$$