Class D amplifier circuitry

Class D amplifier circuitry 400 includes input buffer circuitry 130, analog modulator circuitry and quantizer circuitry 460. The input buffer circuitry receives a first digital input signal VINP, VINN modulated according to a first modulation scheme, e.g. BD2, in which the digital input signal can take a first plurality N of discrete signal levels. The analog modulator circuitry generates an analog modulated signal based on an analog output signal VAP, VAN of the input buffer circuitry. The quantizer circuitry generates an output signal VOP, VON based on the analog modulated signal. The output signal is modulated according to a second, multi-level, modulation scheme, e.g. BD3, in which the output signal can take a second plurality M of discrete signal levels, the second plurality M being greater than the first plurality N. The first digital input signal may be a pulse width modulated (PWM) signal generated by a digital signal generator 120 and may be a differential signal pair. In one arrangement (Fig.6), the number of discrete levels of the digital PWM input signal is variable. The class-D amplifier may be used in portable devices such as a laptop or mobile phone where low quiescent power consumption and reduced circuit complexity, cost and area are important.