A mode-switching transformer comprising a first line in common mode and a second line in differential mode, each line comprising two sections in series respectively coupled with one of the two sections of the other line and all sections having the same lengths, the common mode line being connected in series with a capacitor, to lower the central frequency of the transformer passband, the λ/4 lengths of the sections being chosen to correspond to a central frequency greater than the central frequency desired for the transformer.