Amplifier circuit

1. An audio amplifier circuit, comprising:

an input, for receiving an input signal;

an amplifier for receiving a modified input signal and outputting an amplified signal; and

a feedback loop comprising digital circuitry for feeding back the amplified signal and combining a feedback signal with the input signal to generate the modified input signal;

wherein said feedback loop comprises:

an analog to digital converter;

a digital low-pass filter for filtering the digital signal received from the analog to digital converter;

a digital comparator for comparing the filtered digital signal with a predetermined threshold value and outputting a comparison signal, wherein said predetermined threshold value represents a target DC offset value; and

digital integrator for integrating the comparison signal;

wherein the feedback signal is based on the output of the integrator.

2. An audio amplifier circuit as claimed in claim 1 , wherein the feedback loop comprises an adaptive gain stage, for receiving the comparison signal and outputting to the integrator.

3. An audio amplifier circuit as claimed in claim 1 , further comprising latching means for latching the feedback signal.

4. An audio amplifier circuit as claimed in claim 1 , wherein the feedback loop operates in one of a first mode and a second mode.

5. An audio amplifier circuit as claimed in claim 4 , wherein the first mode is a power-up mode when the amplifier circuit is initially powered on.

6. An audio amplifier circuit as claimed in claim 4 , wherein the second mode is a playback mode.

7. An audio amplifier circuit as claimed in claim 4 , further comprising:

a preamplifier for receiving the modified input signal and outputting a preamplified signal to the power amplifier, in accordance with a volume signal; and

means for determining when the volume signal changes,

wherein the feedback loop operates in the first mode when the volume signal changes.

8. An audio amplifier circuit as claimed in claim 4 , wherein the feedback loop operates at a first frequency in the first mode and at a second frequency in the second mode.

9. An audio amplifier circuit as claimed in claim 8 , wherein the low-pass filter is clocked at said first frequency in the first mode and at said second frequency in the second mode.

10. An audio amplifier circuit as claimed in claim 8 , wherein the first frequency is higher than the second frequency.

11. An audio amplifier circuit as claimed in claim 4 , wherein the feedback loop is powered on in the first mode and powered down in the second mode.

12. An audio amplifier circuit as claimed in claim 4 , wherein the feedback loop is connected to the amplifier circuit in the first mode and disconnected from the amplifier circuit in the second mode.

13. An audio amplifier circuit as claimed in claim 4 wherein the feedback loop comprises an adaptive gain stage, for receiving the comparison signal and outputting to the integrator, wherein the adaptive gain stage is configured to apply a first gain in the first mode, and a second gain in the second mode.

14. An audio amplifier circuit as claimed in claim 1 , wherein the feedback loop comprises logic circuitry configured to perform a binary search to generate said feedback signal.

15. An audio amplifier circuit as claimed in claim 14 , wherein the feedback loop comprises an adaptive gain stage, for receiving the comparison signal and outputting to the integrator and wherein in a first mode the feedback loop is configured to operate with said adaptive gain stage to generate the feedback signal, and wherein in a second mode the feedback loop is configured to operate with said logic circuitry to generate said feedback signal.

16. An integrated circuit, comprising:

an audio amplifier circuit as claimed in claim 1 .

17. An audio system, comprising an integrated circuit as claimed in claim 16 .

18. An audio system as claimed in claim 17 , wherein the audio system is a portable device.

19. An audio system as claimed in claim 17 , wherein the audio system is a mains-powered device.

20. An audio system as claimed in claim 17 , wherein the audio system is an in-car, in-train, or in-plane entertainment system.

21. An audio system as claimed in claim 17 , wherein the input digital signal is representative of ambient noise for use in a noise-cancellation process.

22. A video system, comprising an integrated circuit as claimed in claim 16 .

23. A video system as claimed in claim 22 , wherein the video system is a portable device.

24. A video system as claimed in claim 22 , wherein the video system is a mains-powered device.

25. A video system as claimed in claim 22 , wherein the video system is an in-car, in-train, or in-plane entertainment system.

26. An audio amplifier circuit as claimed in claim 1 wherein the feedback loop comprises a digital-to-analog converter for receiving the output of the integrator and outputting the feedback signal.

27. An audio amplifier circuit comprising:

a plurality of inputs, for receiving a plurality of input signals;

a plurality of amplifiers, for receiving a corresponding plurality of modified input signals and outputting a corresponding plurality of amplified signals; and

a feedback block, multiplexed across the plurality of amplifiers, for receiving each of the plurality of amplified signals and combining a corresponding one of a plurality of feedback signals with a corresponding one of the plurality of input signals to generate the plurality of modified input signals, the feedback block comprising:

an analog to digital converter configured to convert a corresponding one of the plurality of amplified signals to a digital signal;

a digital low-pass filter for filtering said digital signal produced by the analog to digital converter;

a comparator for comparing the filtered signal with a predetermined threshold value and outputting a comparison signal, wherein said predetermined threshold value represents a target DC offset value; and

digital integrator for integrating the comparison signal and outputting a signal as the basis for a corresponding one of the plurality of feedback signals.

28. An audio amplifier circuit as described in claim 27 , further comprising a plurality of latches for latching each of the corresponding feedback signals.

29. A method of amplifying an audio signal, comprising:

receiving an input signal;

receiving a modified input signal and outputting an amplified signal; and

feeding back the amplified signal via a digital feedback loop and combining a feedback signal with the input signal to generate the modified input signal, wherein the step of feeding back comprises:

converting the amplified signal into a digital signal;

digitally filtering the digital signal;

comparing the filtered digital signal with a predetermined threshold value and outputting a comparison signal, wherein the predetermined threshold value represents a target DC offset level;

integrating the comparison signal; and

using the integrated signal to provide the feedback signal.

30. A method as claimed in claim 29 , wherein the step of feeding back comprises:

converting the amplified signal into a digital signal;

integrating the digital signal;

reducing the word length of the digital signal using a sigma-delta modulator; and

converting the integrated signal into an analog signal to output the feedback signal.

31. A method of amplifying a plurality of signals, comprising:

receiving a plurality of input signals;

receiving a corresponding plurality of modified input signals and outputting a corresponding plurality of amplified signals; and

feeding back each of the plurality of amplified signals and combining a corresponding one of a plurality of feedback signals with a corresponding one of the plurality of input signals to generate the plurality of modified input signals,

wherein said step of feeding back comprises:

converting a corresponding one of the plurality of amplified signals into a digital signal;

digitally filtering the digital signal;

comparing the filtered digital signal with a predetermined threshold value and outputting a comparison signal, wherein the predetermined threshold value represents a target DC offset level; and

integrating the comparison signal and outputting a corresponding one of the plurality of feedback signals.

32. An audio amplifier circuit as claimed in claim 31 , wherein the feedback loop operates in one of a first mode and second mode.

33. An audio amplifier circuit as claimed in claim 32 , wherein the feedback loop operates at a first clock frequency in the first mode and at a second clock frequency in the second mode.