A complete stability analysis is performed on a planar system of the form (
x) over dot = sigma(Ax) where A is a Hurwitz matrix and a is the saturation
function, Necessary and sufficient conditions for the system to be globall
y asymptotically stable (GAS) or to have a closed trajectory are explicitly
given in terms of the entries of A. These conditions also indicate that th
e system always has a closed trajectory if it is not GAS.