The present invention discloses structures of a planar transformer and a balanced-to-unbalanced transformer. The structure of the planar transformer includes a first planar coil, a second planar coil and a third planar coil. The first planar coil has a first ring structure, a second ring structure, and a connecting section. The first ring structure and the second ring structure are connected by the connecting section. A range of the second planar coil and a range of the first ring structure at least partially overlap. A range of the third planar coil and a range of the second ring structure at least partially overlap. A transformer is constituted by the first planar coil and the second planar coil or by the first planar coil and the third planar coil.
