An electronic device may include wireless circuitry having a transformer adjustable between first, second, and third modes. The transformer may have first, second, third, and fourth inductors. The third inductor may be magnetically coupled to the first and second inductors with equal coupling constants. The fourth inductor may be magnetically coupled to the first and second inductors with inverse coupling constants. First and second adjustable capacitors coupled to the third and fourth inductors may receive control signals that place the transformer into a selected one of the first, second, or third modes. In the first mode the transformer exhibits a passband that overlaps first and second bands. In the second mode, the transformer passes signals in the second band while filtering interference in the first band. In the third mode, the transformer passes signals in the first band while filtering interference in the second band.