Disclosed is an apparatus for encoding a signal by means of amplitude shift keying. Said apparatus comprises a class E amplifier with a switching transistor, to the gate of which a voltage having a triggering frequency is applied to trigger the class E amplifier. In order to achieve amplitude shift keying in the output signal of the class E amplifier, a circuit for switching the triggering frequency of the voltage (FT) that is applied to the gate of the switching transistor or the resonant frequency (FR) of the class E amplifier between a first value and a second value is provided so as to switch a degree of variation between the triggering frequency and the resonant frequency between a first value and a second value.