A broad area laser diode (BALD) with external cavity feedback is experimentally investigated and analyzed by using ray transfer matrices. In the experiment, a grating or a plane mirror that is placed at the image plane of the output facet of the BALD is used as an external cavity mirror. By tilting the grating or plane mirror, the certain transverse mode is selected and other transverse modes oscillating in the cavity are limited. With this setup, a single-lobed nearly diffraction-limited laser beam with an output power of 230 mW and a spectral line width of 0.6 nm (300 GHz) is obtained with a grating, while a laser beam with an output power of 320 mW and a spectral line width of 1.5 nm is obtained with plane mirror.