RF amplifier having a transition shaping filter

1. A radio frequency (RF) power amplifier system, comprising:

an RF amplifier;

a digital-to-RF modulator to generate an RF input signal for the RF amplifier based on at least one modulator control signal;

a voltage control unit configured to provide a variable supply voltage to the RF amplifier in response to at least one voltage control signal, the variable supply voltage being selected from a plurality of discrete voltage levels;

a controller to provide the at least one modulator control signal to the digital-to-RF modulator and the at least one voltage control signal to the voltage control unit based, at least in part, on data to be amplified by the RF amplifier; and

a transition shaping filter coupled between the RF amplifier and the voltage control unit, said transition shaping filter to filter a variable supply voltage to be applied to the RF amplifier so as to provide shaping and/or bandwidth limitation of the voltage transitions between discrete voltage levels,

wherein the transition shaping filter is configured to reduce frequency content at an output of the RF amplifier by shaping the voltage transitions between discrete voltage levels of the variable supply voltage provided to the RF amplifier.

2. The system of claim 1 wherein the transition shaping filter has a low-pass filter response characteristic.

3. The system of claim 1 wherein the transition shaping filter has a filter response characteristic selected to limit high frequency content of a variable supply voltage provided to the RF amplifier.

4. The system of claim 1 wherein the transition shaping filter has a filter response characteristic selected to: (a) limit high frequency content; (b) provide shaping for the voltage transitions between levels; and (c) provide damping for the transitions of a variable supply voltage provided to be provided to the RF amplifier.

5. The system of claim 1 wherein the transition shaping filter comprises one or more passive components.

6. The system of claim 1 , wherein the frequency content is resultant from transitioning the variable supply voltage among the discrete voltages by the voltage control unit.

7. The system of claim 1 , wherein the transition shaping filter is configured to filter frequency content that cannot be compensated by a drive waveform of the RF amplifier.

8. The system of claim 7 , wherein the transition shaping filter is configured to shape frequency content of the variable supply voltage by passing frequency content that can be compensated by the drive waveform of the RF amplifier.

9. The system of claim 1 , wherein the system is implemented in an integrated circuit.

10. The system of claim 1 , wherein the system is employed in a wireless communications device, the wireless communications device comprising one of: a mobile telecommunications device, a wireless router, a wireless access point, a telecommunications infrastructure device and a computing device.

11. In an apparatus comprising a non-transitory computer readable medium having instructions stored thereon that, when executed by a computer, perform a method for operating an RF transmitter having a digital-to-radio frequency (RF) modulator driving a power amplifier, the method comprising:

obtaining transmit data to be transmitted from the RF transmitter;

providing input information for the digital-to-RF modulator based, at least in part, on the transmit data, the input information to control an amplitude and a phase of an RF output signal of the digital-to-RF modulator;

selecting a supply voltage for the power amplifier based, at least in part, on the transmit data, the supply voltage being selected from a plurality of discrete voltage levels, wherein selecting a supply voltage includes making a decision about supply voltage level for the RF power amplifier based, at least in part, on a window of data samples representing data to be transmitted by the RF transmitter; and

filtering the variable supply voltage to be applied to the RF amplifier with a transition shaping filter so as to provide shaping and/or bandwidth limitation of the voltage transitions between discrete voltage levels,

wherein the filtering comprises reducing, by the transition shaping filter, frequency content at an output of the RF amplifier by shaping the voltage transitions between discrete voltage levels of the variable supply voltage provided to the RF amplifier.

12. The method of claim 11 , wherein the transition shaping filter has a low-pass filter response characteristic.

13. The method of claim 11 , further comprising:

selecting a filter response characteristic of the transition shaping filter to limit high frequency content of a variable supply voltage provided to the RF amplifier.

14. The method of claim 11 , further comprising:

selecting a filter response characteristic of the transition shaping filter to: (a) limit high frequency content; (b) provide shaping for the voltage transitions between levels; and (c) provide damping for the transitions of a variable supply voltage provided to be provided to the RF amplifier.

15. The method of claim 11 , wherein the transition shaping filter comprises one or more passive components.

16. The method of claim 11 , wherein the frequency content is resultant from transitioning the variable supply voltage among the discrete voltages by the voltage control unit.

17. The method of claim 11 , further comprising:

shaping, by the transition shaping filter, frequency content of the variable supply voltage by filtering frequency content that cannot be compensated by a drive waveform of the RF amplifier.

18. The method of claim 17 , further comprising:

shaping, by the transition shaping filter, frequency content of the variable supply voltage by passing frequency content that can be compensated by the drive waveform of the RF amplifier.