When do we use a small signal model for electronic components? $(3)$

$2)$ What is the function of a rectifier circuit? $(3)$

$3)$ Why was MOSFET and not the BJT the precursor of the digital era? $(3)$

$4)$ What gives switching power supplies their high efficiency? $(3)$

$5)$ Why do we have an open circuit voltage terminal as part of the MOSFET small signal model? $(3)$