"input" signal corresponding feedback network
The input signal is Vbias and the output signal is the voltage at the drain of Q2. The gain Aol is simply A since Q1 acts as a source follower when driven by the amplifier.

The goal is to stabilize the drain voltage of Q2 to increase the output resistance of the cascode. The technique is called "gain boosting", because the increased output resistance results in a higher gain when this circuit is used as part of an amplifier.

In order to determine the output resistance a two port model can be used as shown below, here the amplifier A is missing, but it could be taken into account by using gm A instead of simply gm. Also the current sensing path is assumed to be ideal which introduces a slight error. To fix this an additional resistor ro2
would be required in at the feedback input of the lower two-port.



Without modification the result is:
$$
R_{out} = \frac{v_x}{i_x} = ro(1 + g_m R_F)
$$