  We describe some families of differentiable vector fields with the Hopf
bifurcation at infinity, without assuming the continuous differentiability.
These vector fields have isolated singular points on the plane, and the initial
families are obtained by special perturbations at infinity of a vector field
with some spectral property, for instance the dissipativity. The strong
domination imposed by the spectral condition in the differentiable vector field
is used, and then we do not apply the standard Poincar\'e compactification.
Moreover, the perturbation of planar systems with a global period annulus is
also considered.
