In certain aspects, an apparatus includes a first amplifier having a first output and a second output, and a transformer. The transformer includes a first switchable inductor coupled between the first output and the second output, a first capacitor coupled in parallel with the first switchable inductor, a second switchable inductor magnetically coupled to the first switchable inductor, a second capacitor coupled in parallel with the second switchable inductor, a third switchable inductor magnetically coupled to the first switchable inductor, and a third capacitor coupled in parallel with the third switchable inductor.