Amplifier current drive reversal

1. A bridge-tied load (BTL) amplifier system for driving an at least partially reactive load, comprising:

first and second amplifiers configured to receive an input signal and to produce an amplified output signal, each of the first and second amplifiers including an output stage, each of the output stages including:

a pull-up device configured to draw current from a first power rail; and

a pull-down device configured to sink current into a second power rail;

the first and second amplifiers, configured to alternate between a pull-up mode and a pull-down mode, each amplifying substantially half of a full wave output signal; and

digital logic circuitry configured to receive an indication of output current in the load and to select a current drive direction of the first and second amplifiers using the received output current information.

2. The amplifier system of claim 1 , wherein the first and second amplifiers are configured to drive an at least partially reactive load.

3. The amplifier system of claim 1 , wherein the digital logic circuitry is configured to receive the indication of output current in the load, the output current at least partially out of phase with an output voltage in the load.

4. The amplifier system of claim 1 , wherein the pull-up and pull-down devices include n-type devices.

5. The amplifier system of claim 1 , wherein the pull-up device includes a pull-up switch configured to draw current from and hold its output to the first power rail; and

wherein the pull-down device includes a transconductive pull-down device configured to sink a variable current into the second power rail in response to the received input signal.

6. The amplifier system of claim 1 , wherein the pull-up device includes a transconductive pull-up device configured to draw a variable current from the first power rail in response to the received input signal; and

wherein the pull-down device includes a pull-down switch configured to sink current to and hold its output to the second power rail.

7. The amplifier system of claim 1 , wherein the first power rail includes one of a plurality of available power rails;

wherein the second power rail includes one of the plurality of available power rails; and

wherein the digital logic circuitry is configured to receive an indication of a target output voltage in a load and to select the first and second power rails for each of the first and second amplifiers from the plurality of available power rails using the received output voltage information.

8. The amplifier system of claim 7 , wherein the digital logic circuitry is configured to select the first and second power rails for each of the first and second amplifiers using the received target output voltage information to minimize a voltage drop between the selected first and second power rails and the received target output voltage information.

9. A bridge-tied load (BTL) amplifier system, responsive to both load voltage and load current, for driving an at least partially reactive load, comprising:

first and second amplifiers configured to receive an input signal and to produce an amplified output signal, each of the first and second amplifiers including an output stage, each of the output stages including:

a pull-up switch configured to draw current from and hold its output to one of a plurality of available power rails; and

a transconductive pull-down device configured to sink a variable current into one of the plurality of available power rails in response to the input signal;

the first and second amplifiers, configured to alternate between a switched pull-up mode and a variable pull-down mode, each amplifying substantially half of a full wave output signal; and

digital logic circuitry configured to receive an indication of an output current and a target output voltage in the load, to select a current drive direction of the first and second amplifiers using the received output current information, and to select the power rails for each of the first and second amplifiers from the plurality of available power rails using the received output voltage information.

10. The amplifier system of claim 9 , wherein the first and second amplifiers are configured to drive an at least partially reactive load.

11. The amplifier system of claim 9 , wherein the digital logic circuitry is configured to receive the indication of output current and the target output voltage in the load, the output current at least partially out of phase with the target output voltage.

12. The amplifier system of claim 9 , wherein the pull-up and pull-down devices include n-type devices.

13. The amplifier system of claim 9 , wherein the digital logic circuitry is configured to select the power rails for each of the first and second amplifiers using the received output voltage information to minimize a voltage drop between the selected power rails and the received target output voltage information.

14. A method of driving an at least partially reactive load, comprising:

receiving an input signal at first and second amplifiers;

receiving an indication of an output current in the load;

selecting a drive current direction between the first and second amplifiers using the received output current information; and

producing an amplified output signal using the selected drive current direction and the first and second amplifiers, the first and second amplifiers, configured to alternate between a pull-up mode and a pull-down mode, each amplifying half of a full wave output signal.

15. The method of claim 14 , wherein the producing the amplified output signal includes producing an amplified output signal to drive an at least partially reactive load.

16. The method of claim 14 , wherein the receiving the indication of the output current in the load includes receiving an indication of an output current at least partially out of phase with an output voltage in the load.

17. The method of claim 14 , including:

drawing current for each of the first and second amplifiers from a first power rail using pull-up devices and sinking current for each of the first and second amplifiers to a second power rail using pull-down devices.

18. The method of claim 17 , wherein the drawing and sinking the current for the first and second amplifiers includes drawing current using n-type pull-up devices and sinking current using n-type pull-down devices.

19. The method of claim 17 , wherein the drawing the current for the first and second amplifiers includes using a pull-up switch configured to draw current from and hold its output to the first power rail; and

wherein the sinking the current for the first and second amplifiers includes using a transconductive pull-down device configured to sink a variable current into the second power rail in response to the received input signal.

20. The method of claim 17 , wherein the drawing the current for the first and second amplifiers includes using a transconductive pull-up device configured to draw a variable current from the first power rail in response to the received input signal; and

wherein the sinking the current for the first and second amplifiers includes using a pull-down switch configured to sink current into and hold its output to the second power rail.

21. The method of claim 14 , including:

drawing current for each of the first and second amplifiers from one of a plurality of available power rails using pull-up devices and sinking current for each of the first and second amplifiers from one of the plurality of available power rails using pull-down devices.

22. The method of claim 21 , including:

receiving an indication of a target output voltage in a load; and

selecting the power rails for each of the first and second amplifiers from the one of the plurality of available power rails using the received target output voltage information.

23. The method of claim 22 , wherein the selecting the power rails for each of the first and second amplifiers includes to minimize a voltage drop between the selected first and second power rails and the received target output voltage information.