A transformer-based balun circuit is disclosed herein. The balun can be implemented using a spiral transformer, where primary and secondary transformer windings can be inductively coupled and can be implemented on the same metal layer (or different metal layers, e.g. vertically adjacent metal layers). The balun can further include a compensation capacitor and a digital frequency tuning circuit. The compensation capacitor can be introduced at one of the differential terminals to reduce or suppress the amplitude and phase imbalance. The digital frequency tuning circuit can be a switchable bank of capacitors, which allows for tuning the frequency of operation of the transformer-based balun.