Exercise 6.2 A voltage source circuit including an opamp

To build a stabilized power supply we use the circuit below to regulate $V_{\mathit{DD}}$ down to $v_{\mathit{OUT}}$. We assume $M_1$ works in saturation and that it has infinite output resistance. The opamp can be assumed to be ideal ($r_i\to \infty$, $r_o=0$, $a_v\to \infty$). Derive an expression for the (generated) voltage $v_{\mathit{OUT}}$ as a function of the various circuit parameters.