An amplifier for modulating the amplitude of an RF signal, the amplifier including: a plurality of amplifier circuits, each circuit being connected to a first power source, each circuit including a charge storage device and an output across which a potential difference supplied by the first power source can be applied; a switching arrangement for switching connections between the first power source, the charge storage device, and the output in each amplifier circuit, wherein each circuit includes a first switched configuration in which the charge storage device is charged by the first power source and a second switched configuration in which the charge storage device, once charged, will apply an additional potential difference across the output. The amplifier is configured to vary the amplitude of the RF signal in proportion to the sum of the potential differences applied across the output in each amplifier circuit.