An apparatus for coding a signal by means of amplitude shift keying comprises a class E amplifier including a switching transistor, to whose gate is supplied a voltage having an operating frequency for operating the class E amplifier. For achieving an amplitude shift keying in the output signal of the class E amplifier, a circuit for switching the operating frequency of the voltage supplied to the gate of the switching transistor, or the resonance frequency of the class E amplifier, between a first value and a second value is provided and in order to switch a deviation degree between the operating frequency and the resonance frequency between a first value and a second value.