Altermagnets are substances with a momentum-dependent spin splitting of electron bands due to a specific crystal structure that is invariant under time reversal only in combination with rotations and reflections. The developed phenomenological approach allows to obtain a spectrum of electron bands in altermagnets corresponding to antiferromagnets with the same symmetry. We consider antiferromagnetic, weak ferromagnetic, and ferrimagnetic structures. The calculated Berry curvature of electron bands in the alterrmarnet with zero spontaneous magnetisation occurs an odd function of momentum whereas it is even in a weak ferromagnetic state. The Berry curvature in ferrimagnetic state is equal to zero.