Transformer circuitry comprising: a transformer having a primary coil and a secondary coil, the primary coil having first and second primary terminals and the secondary coil having first and second secondary terminals, and a secondary coil driver configured to drive a secondary voltage signal V2 across the secondary terminals which has a target relationship with a primary voltage signal V1 driven across the primary terminals by a primary coil driver so that an inductance value measured between the primary terminals is governed by the target relationship.