A study of exchange bias phenomenon in ferrimagnetic /ferromagnetic FeGd/ FeSn bilayers is presented. The amorphous FeSn and FeGd alloys have been grown by co-evaporation. Specific growth conditions allow to induce an uniaxial anisotropy in both alloys in a parallel direction. After saturation of the bilayers under a positive field, the hysteresis loop of one of the layer is shifted towards a positive field HE. The sign of the exchange bias field HE is shown to be due to the antiferromagnetic coupling between the net magnetizations of both alloys. The field HE is studied as a function of the thickness of each layer and of the temperature. Using ac-susceptibility measurements and polarized neutron reflectometry, it is shown that the reversal of magnetization of the bilayers is dominated by the presence of a domain wall at the interface. This exchange bias system is shown to act as a potential well for the magnetic domain wall. Within this assumption and thanks to a precise magnetic characterization of each alloy, the evolution of HE with the thickness of the layers is well reproduced using simple one-dimensional analytical models for the domain wall or a more elaborate numerical approach. PACS. 75.60.Ch Domain walls and domain structure-75.70.-i Magnetic properties of thin films, surfaces, and interfaces-75.25.+z Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source X-ray scattering, etc.)