Sigma-delta modulator and a class D amplifier

1 . A sigma-delta modulator, comprising:

a sigma-delta circuit configured to provide a non-limiting output; and

a stabilizer configured to:

receive the non-limiting output of the sigma-delta circuit; and

transmit a stabilized loop signal; wherein:

the sigma-delta circuit and stabilizer are arranged as a sigma-delta loop;

the sigma-delta circuit is configured to receive the stabilized loop signal from the stabilizer for stabilizing the sigma-delta circuit;

the stabilizer comprises:

a saturator arranged to restrict the non-limiting output of the sigma-delta circuit to a predetermined limit;

a subtractor configured to extract an overload signal that is determined by a difference in magnitude between the non-limiting output of the sigma-delta circuit and the predetermined limit;

a first overload adder configured to add the overload signal from the subtractor with the predetermined limit to form a combined feedback signal; and

the combined feedback signal is the stabilized loop signal transmitted by the stabilizer and received by the sigma-delta circuit.

2 . The sigma-delta modulator of claim 1 , the sigma-delta circuit comprising:

a quantizer arranged to provide the non-limiting output.

3 . The sigma-delta modulator of claim 2 , the sigma-delta circuit comprising:

a first loop filter arranged to filter a predetermined range and provide a filtered sigma-delta signal to a non-limiting quantizer.

4 . The sigma-delta modulator of claim 1 , wherein when output signal falls below the predetermined limit, the overload signal is zero.

5 . The sigma-delta modulator of claim 4 , wherein the sigma-delta modulator has an external input signal, and the sigma-delta circuit comprises:

a first sigma-delta adder configured to add the combined feedback signal to an external input of the sigma-delta modulator thereby providing a combined input signal for the sigma-delta circuit.

6 . The sigma-delta modulator of claim 1 , the predetermined limit comprising:

approximately −3 dB.

7 . The sigma-delta modulator of claim 6 , further comprising:

a plurality of nested feedback loops, wherein each nested feedback loop is configured to receive the overload signal, wherein the overload signal is added into all feedback signals thereby ensuring stability in all loops.

8 . The sigma-delta modulator of claim 7 wherein the plurality of nested feedback loops comprises:

a first nested feedback loop comprising the first overload adder and a first sigma-delta adder.

9 . The sigma-delta modulator of claim 8 wherein the plurality of nested feedback loops comprises:

a second nested feedback loop comprising: a second overload adder and a second sigma-delta adder.

10 . The sigma-delta modulator of claim 9 wherein the plurality of nested feedback loops comprises:

a third nested feedback loop comprising: a third overload adder and a third sigma-delta adder.

11 . The sigma-delta modulator of claim 8 wherein the first nested feedback loop is arranged as an innermost of the plurality of nested feedback loops.

12 . A class D amplifier comprising;

a sigma-delta modulator, comprising:

a sigma-delta circuit arranged to provide a non-limiting output;

a stabilizer arranged to receive the non-limiting output from the sigma-delta circuit and transmit a stabilised loop signal; wherein:

the sigma-delta circuit and stabilizer are arranged as a sigma-delta loop;

the sigma-delta circuit receives the stabilized loop signal from the stabilizer for stabilizing the sigma-delta circuit;

the stabilizer comprises:

a saturator arranged to restrict the non-limiting output of the sigma-delta circuit to a predetermined limit;

a subtractor configured to extract an overload signal that is determined by a difference in magnitude between an output signal of the sigma-delta circuit over the predetermined limit;

a first overload adder configured to add the overload signal from the subtractor with the predetermine limit to form a combined feedback signal; and

the combined feedback signal is the stabilized loop signal transmitted by the stabilizer and received by the sigma-delta circuit.

13 . The class D amplifier of claim 12 , wherein:

the saturator limits the class D amplifier to a predetermined maximum modulation index for preventing overload within the class D amplifier.

14 . The class D amplifier of claim 13 is an audio amplifier.

15 . The class D amplifier of claim 14 is being implemented in wireless headphone applications.

16 . A method of stabilizing a sigma-delta modulator comprising:

providing a non-limiting output using a sigma-delta circuit;

receiving the non-limiting output at a stabilizer;

transmitting a stabilized loop signal from the stabilizer;

receiving the stabilized loop signal at the sigma-delta circuit; and

using the stabilized loop signal to stabilize the sigma-delta circuit for stabilizing the sigma-delta modulator, wherein:

the stabilizer comprises:

a saturator arranged to restrict the non-limiting output of the sigma-delta circuit to a predetermined limit;

a subtractor configured to extract an overload signal that is determined by a difference in magnitude between an output signal of the sigma-delta circuit over the predetermined limit;

a first overload adder configured to add the overload signal from the subtractor with the predetermine limit to form a combined feedback signal; and

the combined feedback signal is the stabilized loop signal transmitted by the stabilizer and received by the sigma-delta circuit.

17 . The method of claim 16 , wherein the sigma-delta modulator is part of a class D amplifier.