High-performance CMOS current mirrors are used in a large variety of circuit as fundamental elements of analog design. As a consequence, several topologies have been proposed over the years to improve their performances. Among these solutions, the active input is mainly used to improve the speed of high-performance current mirrors. In this paper, we develop a complete formalism to evaluate the potential gain that can be expected using this topology. It allows designers to compute the optimal solution for their current source architecture. Then, comparative case studies are given to demonstrate the efficiency of our formalism to estimate capabilities and limits of the active-input current mirror topology.