An amplifier device for amplifying an input signal Sx at a device input X to an output signal Sy at a device output Y, for driving a loudspeaker or a comparable reactive load, said amplifier device comprising:
- a self oscillating Class D power amplifier stage (10) with first transfer function KA(s), comprising:
- a comparator (01), comparing the output signal Sy of, via first feedback filter Hosc (06) with local input voltage SKin, said comparator
- a Class D power stage (03), being coupled to and driven by the output of said comparator (01), converting the output voltage of said comparator to a higher voltage level with high current capability
- a second order lowpass output filter (04) comprising an inductor and a capacitor (04), coupled between the power stage (03) and the loudspeaker or comparable load, for removing at least part of high frequency switching noise from the output signal of said Class D power stage;
- a time delay unit (02), said time delay unit being configured to provide a time delay in accordance with one or more of said comparator, Hosc and said power stage;
- a first feedback path from device output Y, coupling output signal Sy, via Hosc (06), back to the comparator (01), said first feedback path providing control over the oscillation frequency and providing at least partly, low frequency feedback in to reduce distortion of signals in the audio frequency band;
- a correction stage, comprising:
- a second feedback path, coupling said output signal Sy from device output Y to a feedback input of said correction stage via H1 (104) to summing node S2, being a virtual ground input of transimpedance amplifier HZ1 (102), for further correcting an error of said output signal (Sy), generated by said amplifier stage, wherein said further correction is performed by pre-distorting the power amplifier input signal SKin of said amplifier stage (10) with an inverse of an isolated and filtered distortion component caused by the power amplifier stage, the output filter, and the loudspeaker or the comparable load, wherein the isolated distortion component is obtained by running the incoming signal Sr through tracking filter KT, (101), and subsequently subtracting the result from said feedback signal from the output of H1 (104) being a scaled version of the output signal Sy.
- said correction stage further comprising:
- a third feedback path, coupling said output signal Sy to an additional feedback input via H2+H3, and through HZ2 of said correction stage, to further enhance said correction of said output signal Sy by said second feedback path, wherein said third feedback path is configured as a higher order addition for enhanced selectivity of the feedback path and for providing additional feedback gain, and wherein said third feedback path is comprised of a network that implements a first transfer function H2 applied to the output signal Sy and a second, different, transfer function H3 applied to the input signal Sx, said H2 and H3 being combined within a single passive network comprising resistors and capacitors, and wherein the respective outputs of H2 and H3 are summed and supplied as a current input to a transimpedance amplifier HZ2, the output of which defines the input signal Sr.