Apparatus and methods for power amplifier bias circuits

1. A power amplifier system comprising:

a power amplifier stage configured to receive a bias voltage that biases the power amplifier stage;

a current source configured to generate a reference current;

a plurality of reference bipolar transistors;

a selection circuit configured to select one or more selected reference bipolar transistors from the plurality of reference bipolar transistors, the one or more selected reference bipolar transistors having a current therethrough that changes in relation to the bias voltage; and

a transimpedance amplifier configured to control the bias voltage based on an error current corresponding to a difference between the reference current and the current through the one or more selected reference bipolar transistors.

2. The power amplifier system of claim 1 wherein the transimpedance amplifier provides the bias voltage to the power amplifier stage via an isolation circuit.

3. The power amplifier system of claim 2 wherein the isolation circuit includes at least one of an inductor or a resistor.

4. The power amplifier system of claim 1 wherein the transimpedance amplifier and the reference current source are configured to receive an enable signal and to selectively turn on or off the power amplifier stage based on a state of the enable signal.

5. The power amplifier system of claim 1 wherein the power amplifier stage includes a bipolar power amplifier transistor including a base that receives the bias voltage.

6. The power amplifier system of claim 5 further comprising an array of transistor elements, the bipolar power amplifier transistor implemented by a first plurality of transistor elements of the array, the plurality of reference bipolar transistors implemented by a second plurality of transistor elements of the array.

7. The power amplifier system of claim 1 further comprising a control circuit configured to control the reference current to compensate for temperature variation.

8. The power amplifier system of claim 1 wherein the one or more selected reference bipolar transistors includes at least two selected reference bipolar transistors, the selection circuit configured to electrically connect the at least two selected reference bipolar transistors in parallel with one another.

9. The power amplifier system of claim 1 further comprising a power amplifier configured to amplify a radio frequency (RF) signal, the power amplifier including the power amplifier stage.

10. The power amplifier system of claim 9 wherein the power amplifier stage operates as an output stage of the power amplifier.

11. The power amplifier system of claim 1 wherein the transimpedance amplifier is configured to operate with negative feedback such that the current through the one or more selected reference bipolar transistors is substantially equal to the reference current.

12. A method of biasing a power amplifier, the method comprising:

biasing a power amplifier stage using a bias voltage;

generating a reference current using a current source;

selecting one or more selected reference bipolar transistors from a plurality of reference bipolar transistors, the one or more selected reference bipolar transistors having a current therethrough that changes in relation to the bias voltage;

generating an error current based on a difference between the reference current and the current through the one or more selected reference bipolar transistors; and

controlling the bias voltage based on amplifying the error current using a transimpedance amplifier.

13. The method of claim 12 wherein controlling the bias voltage includes using negative feedback to control the current through the one or more selected reference bipolar transistors to substantially equal the reference current.

14. The method of claim 12 further comprising controlling the reference current to compensate for temperature variation using a control circuit.

15. The method of claim 12 further comprising amplifying a radio frequency signal using a power amplifier that includes the power amplifier stage.

16. The method of claim 12 wherein selecting the one or more selected reference bipolar transistors includes selecting at least two selected reference bipolar transistors, and connecting the at least two selected reference bipolar transistors in parallel with one another.

17. A bias circuit for biasing a power amplifier stage, the bias circuit comprising:

a current source configured to generate a reference current;

a plurality of reference bipolar transistors;

a selection circuit configured to select one or more selected reference bipolar transistors from the plurality of reference bipolar transistors, the one or more selected reference bipolar transistors having a current therethrough that changes in relation to a power amplifier stage bias voltage; and

a transimpedance amplifier configured to control the power amplifier stage bias voltage based on an error current corresponding to a difference between the reference current and the current through the one or more selected reference bipolar transistors.

18. The bias circuit of claim 17 further comprising a control circuit configured to control the reference current to compensate for temperature variation.

19. The bias circuit of claim 17 wherein the one or more selected reference bipolar transistors includes at least two selected reference bipolar transistors, the selection circuit configured to electrically connect the at least two selected reference bipolar transistors in parallel with one another.

20. The bias circuit of claim 17 wherein the transimpedance amplifier is configured to operate with negative feedback such that the current through the one or more selected reference bipolar transistors is substantially equal to the reference current.