An RF transformer for supplying power as part of a tank circuit, comprising: a primary side, having at least one main winding and at least one shorting winding, the at least one main winding being configured to receive an RF input; a secondary side, having a first winding inductively coupled to the at least one main winding of the primary side and a second winding inductively coupled to the at least one shorting winding of the primary side; and a switching arrangement, adjustable between a first state in which the at least one shorting winding of the primary side is shorted and a second state in which the at least one shorting winding of the primary side is not shorted, such that the resonant frequency of the tank circuit is changed by adjusting between the first and second states.