The tunneling electroresistance (TER) for ferroelectric tunnel junctions (FTJs) with BaTiO₃ and PbTiO₃ barriers is calculated by combining the microscopic electronic structure of the barrier material with a macroscopic model for the electrostatic potential, which is caused by the ferroelectric polarization. The TER ratio is investigated in dependence on the intrinsic polarization, the chemical potential, and the screening properties of the electrodes. A change in sign in the TER ratio is obtained for both barrier materials in dependence on the chemical potential. The inverse imaginary Fermi velocity describes the microscopic origin of the effect; it qualitatively reflects the variation and the sign reversal of the TER. The quantity of the imaginary Fermi velocity allows to obtain detailed information on the transport properties of FTJs by analyzing the complex band structure of the barrier material.