Class-D transconductance amplifier

1. A circuit for driving an audio loudspeaker, comprising:

an amplifier;

an input resistor connected between an audio source and an input of the amplifier;

a feedback network connected between an output of the amplifier and an input of the amplifier;

a capacitor connected at a first terminal to a first terminal of the audio loudspeaker and a at a second terminal to a second terminal of the audio loudspeaker;

a power switch adapted to provide drive current to the audio loudspeaker and connected between the output of the amplifier and the first terminal of the capacitor;

a feedback resistor connected between the second terminal of the capacitor and the input of the amplifier;

a sense resistor connected at a first terminal to the second terminal of the capacitor; and

an inductor connected between a second terminal of the sense resistor and ground,

where the capacitor (C) and the inductor (L) form a passive LC reconstruction filter having at least a portion of the passive LC reconstruction filter situated in a feedback loop of the amplifier and the circuit is adapted to operate in a current feedback mode as a voltage-controlled current source.

2. The circuit of claim 1 where the amplifier further comprises an operational amplifier.

3. The circuit of claim 2 further comprising:

a triangular waveform generator adapted to generate a triangular waveform; and

a comparator adapted to compare an output of the operational amplifier with the triangular waveform and drive the power switch based upon the comparison.

4. The circuit of claim 1 where the portion of the passive LC reconstruction filter situated in the feedback loop of the amplifier includes the capacitor and the capacitor is further adapted to provide feedback of an output signal at the audio loudspeaker to the input of the amplifier without phase lag.

5. The circuit of claim 1 further comprising a pair of inductors adapted to decouple the circuit from the audio loudspeaker, where a first inductor of the pair of inductors is connected between the first terminal of the capacitor and the first terminal of the audio loudspeaker and a second inductor of the pair of inductors is connected between the second terminal of the capacitor and the second terminal of the audio loudspeaker.

6. A circuit for driving an audio loudspeaker, comprising:

a non-inverting amplifier adapted to receive an audio signal from an audio source at a positive input terminal;

a feedback network connected between an output of the non-inverting amplifier and a negative input of the non-inverting amplifier;

a capacitor connected at a first terminal to a first terminal of the audio loudspeaker and a at a second terminal to a second terminal of the audio loudspeaker;

a power switch adapted to provide drive current to the audio loudspeaker and connected between the output of the non-inverting amplifier and the first terminal of the capacitor;

a feedback resistor connected between the second terminal of the capacitor and the negative input of the non-inverting, amplifier;

a sense resistor connected at a first terminal to the second terminal of the capacitor; and

an inductor connected between a second terminal of the sense resistor and ground,

where the capacitor (C) and the inductor (L) form a passive LC reconstruction filter having at least a portion of the passive LC reconstruction filter situated in a feedback loop of the amplifier and the circuit is adapted to operate in a current feedback mode as a voltage-controlled current source.

7. The circuit of claim 6 where the portion of the passive LC reconstruction filter situated in the feedback loop of the amplifier includes the capacitor and the capacitor is further adapted to provide feedback of an output signal at the audio loudspeaker to the input of the amplifier without phase lag.

8. The circuit of claim 6 further comprising a pair of inductors adapted to decouple the circuit from the audio loudspeaker, where a first inductor of the pair of inductors is connected between the first terminal of the capacitor and the first terminal of the audio loudspeaker and a second inductor of the pair of inductors is connected between the second terminal of the capacitor and the second terminal of the audio loudspeaker.

9. A method for driving an audio loudspeaker, comprising:

operating a class-D audio amplifier in a current feedback mode as a voltage-controlled current source;

decoupling the audio loudspeaker from the class-D amplifier using a pair of inductors, with a first inductor of the pair of inductors connected in series between a first terminal of a capacitor and a first terminal of the audio loudspeaker and second inductor of the pair of inductors connected in series between a second terminal of the capacitor and a second terminal of the audio loudspeaker;

reconstructing an audio signal at the audio loudspeaker using a passive class-D reconstruction filter, where at least a portion of the passive class-D reconstruction filter is located within a feedback loop to an input of the class-D audio amplifier; and

driving the audio loudspeaker with the reconstructed audio signal.

10. The method of claim 9 , where the passive class-D reconstruction filter further comprising:

connecting the capacitor across terminals of the audio loudspeaker; and

providing feedback to the input of the class-D audio amplifier via a connection to a terminal of the capacitor.

11. The method of claim 9 where the class-D audio amplifier includes an error amplifier, and providing the feedback to the input of the class-D audio amplifier further comprises providing the feedback to an input of the error amplifier.

12. The method of claim 9 where the class-D audio amplifier includes a non-inverting amplifier, and providing the feedback to the input of the class-D audio amplifier further comprises providing the feedback to an input of the non-inverting amplifier.

13. The method of claim 9 where providing the feedback to the input of the non-inverting amplifier further comprises providing the feedback to a negative input of the non-inverting amplifier.

14. The method of claim 9 where the class-D audio amplifier includes a triangular waveform generator, an error amplifier, and a comparator, and providing the feedback to the input of the class-D audio amplifier includes providing the feedback to an input of the error amplifier, and further comprising:

generating a triangular waveform using the triangular waveform generator;

comparing an output of the error amplifier with the triangular waveform using the comparator; and

driving a power switch with an output of the comparator.

15. A class-D audio amplifier, comprising:

a power switch adapted to provide drive current to an audio loudspeaker;

a passive inductor/capacitor (LC) reconstruction filter adapted to situate at least a portion of the passive LC reconstruction filter in a feedback loop to an error amplifier;

where the class-D audio amplifier operates in a current feedback mode as a voltage-controlled current source; and

a pair of inductors adapted to decouple the power switch and the passive LC reconstruction filter from the audio loudspeaker.

16. The class-D audio amplifier of claim 15 where the passive LC reconstruction filter further comprises a capacitor and an inductor and, in being adapted to situate at least a portion of the passive LC reconstruction filter in the feedback loop, the passive LC reconstruction filter is further adapted to situate the capacitor in the feedback loop.

17. The class-D audio amplifier of claim 15 where the capacitor is further adapted to provide feedback of an output signal at the audio loudspeaker to an input of the error amplifier without phase lag.