We present a numerical and experimental study of a confocal unstable ring resonator that performs similarly to a self-filtering unstable resonator. The resonator presents two apertures: one of them is the output coupler and the other provides spatial filtering. A simple rule for the choice of the aperture size enables us to expand the output beam diameter without increasing the magnification or the total length. The resonator is tested in a TEA CO2 laser in which the output beam diameter, the output energy, and the effective reflectivity are measured. The energy ratio of the forward and reverse waves scales approximately as the square of the magnification. The results show good agreement with the theoretical values.