This paper proposes a fixed-time differentiator running in parallel with a feedback linearisation-based controller, which allows quadrotors to track a given trajectory. The fixed-time differentiator estimates outputs' derivatives in a predefined convergence time, largely compensating for initial condition problems and solving the delay problem. Combined with a state reconstruction step, a whole observer-estimator-controller scheme can be constructed for quadrotors to solve the trajectory tracking problem. Also, an Linear Matrix Inequality (LMI) optimisation-based algorithm to tune parameters of the differentiator is developed here. The high performance of the proposed model is illustrated by numerical simulations.