An amplifier for modulating the amplitude of an RF signal, the amplifier comprising a plurality of amplifier circuits, each circuit being connected to a first power source, each circuit having 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 has 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 being 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.