# Patent

## Title
Noise reduction for envelope tracking

## Abstract

A direct current (DC)-DC converter, which includes a parallel amplifier, a radio frequency (RF) trap, and a switching supply, is disclosed. The switching supply includes switching circuitry and a first inductive element. The parallel amplifier has a feedback input and a parallel amplifier output. The switching circuitry has a switching circuitry output. The first inductive element is coupled between the switching circuitry output and the feedback input. The RF trap is coupled between the parallel amplifier output and a ground.

## Background
RELATED APPLICATIONS
This application claims the benefit of U.S. provisional patent application No. 61/580,874, filed Dec. 28, 2011, the disclosure of which is incorporated herein by reference in its entirety.


FIELD OF THE DISCLOSURE
The present disclosure relates to direct current (DC)-DC converters and circuits that use DC-DC converters.
BACKGROUND
DC-DC converters often include switching power supplies, which may be based on switching at least one end of an energy storage element, such as an inductor, between a source of DC voltage and a ground. As a result, an output voltage from a DC-DC converter may have a ripple voltage resulting from the switching associated with the energy storage element. Typically, the ripple voltage is undesirable and is minimized as much as sizes and costs permit. Thus, there is a need to minimize ripple voltage using techniques that minimize sizes and costs.
SUMMARY
Embodiments of the present disclosure relate to a direct current (DC)-DC converter, which includes a parallel amplifier, a radio frequency (RF) trap, and a switching supply. The switching supply includes switching circuitry and a first inductive element. The parallel amplifier has a feedback input and a parallel amplifier output. The switching circuitry has a switching circuitry output. The first inductive element is coupled between the switching circuitry output and the feedback input. The RF trap is coupled between the parallel amplifier output and a ground.
In one embodiment of the DC-DC converter, the parallel amplifier partially provides a first power supply output signal via the parallel amplifier output based on a voltage setpoint. The switching supply partially provides the first power supply output signal via the first inductive element. The switching supply may provide power more efficiently than the parallel amplifier. However, the parallel amplifier may provide a voltage of the first power supply output signal more accurately than the switching supply. As such, in one embodiment of the DC-DC converter, the parallel amplifier regulates the voltage of the first power supply output signal based on the voltage setpoint of the first power supply output signal. Further, the switching supply regulates the first power supply output signal to minimize an output current from the parallel amplifier to maximize efficiency. In this regard, the parallel amplifier behaves like a voltage source and the switching supply behaves like a current source.
In one embodiment of the DC-DC converter, the RF trap has a frequency response with an RF notch at an RF notch frequency. The RF trap filters the first power supply output signal based on the frequency response. As such, the RF trap may significantly reduce unwanted noise from the first power supply output signal at the RF notch frequency.
Those skilled in the art will appreciate the scope of the disclosure and realize additional aspects thereof after reading the following detailed description in association with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings incorporated in and forming a part of this specification illustrate several aspects of the disclosure, and together with the description serve to explain the principles of the disclosure.
FIG. 1 shows a direct current (DC)-DC converter according to one embodiment of the present disclosure.
FIG. 2 shows the DC-DC converter according to an alternate embodiment of the DC-DC converter.
FIG. 3 is a graph illustrating a frequency response of a radio frequency (RF) trap illustrated in FIG. 1 according to an additional embodiment of the DC-DC converter.
FIG. 4 shows the DC-DC converter according to another embodiment of the DC-DC converter.
FIG. 5 shows the DC-DC converter according to a further embodiment of the DC-DC converter.
FIG. 6 shows a radio frequency (RF) communications system according to one embodiment of the present disclosure.
FIG. 7 shows the RF communications system according to an alternate embodiment of the RF communications system.
FIG. 8 shows the RF communications system according to an additional embodiment of the RF communications system.
FIG. 9 shows the RF communications system according to another embodiment of the RF communications system.
FIG. 10 shows the DC-DC converter according to one embodiment of the DC-DC converter.
FIG. 11 shows the DC-DC converter according to an alternate embodiment of the DC-DC converter.



DETAILED DESCRIPTION
The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the disclosure and illustrate the best mode of practicing the disclosure. Upon reading the following description in light of the accompanying drawings, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.
FIG. 1 shows a direct current (DC)-DC converter 10 according to one embodiment of the present disclosure. The DC-DC converter 10 includes a switching supply 12, a parallel amplifier 14, and a radio frequency (RF) trap 18. The switching supply 12 includes switching circuitry 16 and a first inductive element L1. The parallel amplifier 14 has a feedback input FBI and a parallel amplifier output PAO. The switching circuitry 16 has a switching circuitry output SCO. The first inductive element L1 is coupled between the switching circuitry output SCO and the feedback input FBI. The RF trap 18 is coupled between the parallel amplifier output PAO and a ground. In one embodiment of the DC-DC converter 10, the parallel amplifier output PAO is directly coupled to the feedback input FBI, as shown.
In one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides a first power supply output signal PS1 via the parallel amplifier output PAO based on a voltage setpoint. The switching supply 12 partially provides the first power supply output signal PS1 via the first inductive element L1. The switching supply 12 may provide power more efficiently than the parallel amplifier 14. However, the parallel amplifier 14 may provide a voltage of the first power supply output signal PS1 more accurately than the switching supply 12. As such, in one embodiment of the DC-DC converter 10, the parallel amplifier 14 regulates the voltage, called a first voltage V1, of the first power supply output signal PS1 based on the voltage setpoint of the first power supply output signal PS1. Further, the switching supply 12 regulates the first power supply output signal PS1 to minimize an output current, called a parallel amplifier output current IP, from the parallel amplifier 14 to maximize efficiency. In this regard, the parallel amplifier 14 behaves like a voltage source and the switching supply 12 behaves like a current source. Additionally, the switching circuitry 16 provides a switching output voltage VS and an inductor current IL to the first inductive element L1 via the switching circuitry output SCO.
In one embodiment of the DC-DC converter 10, the DC-DC converter 10 receives a DC source signal VDC, such that the parallel amplifier 14 partially provides the first power supply output signal PS1 using the DC source signal VDC and the switching supply 12 partially provides the first power supply output signal PS1 using the DC source signal VDC.
FIG. 2 shows the DC-DC converter 10 according to an alternate embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in FIG. 2 is similar to the DC-DC converter 10 illustrated in FIG. 1, except the DC-DC converter 10 illustrated in FIG. 2 further includes power supply control circuitry 20 and the switching supply 12 further includes a filter capacitive element CF. The filter capacitive element CF is coupled between the parallel amplifier output PAO and the ground. As such, the filter capacitive element CF may significantly reduce unwanted noise, ripple, or both from the first power supply output signal PS1. The power supply control circuitry 20 receives the DC source signal VDC and is coupled to the parallel amplifier 14 and the switching circuitry 16.
FIG. 3 is a graph illustrating a frequency response 22 of the RF trap 18 (FIG. 1) illustrated in FIG. 1 according to an additional embodiment of the DC-DC converter 10. The RF trap 18 (FIG. 1) has the frequency response 22 with an RF notch 24 at an RF notch frequency RNF. Therefore, the RF trap 18 (FIG. 1) filters the first power supply output signal PS1 (FIG. 1) based on the frequency response 22. As such, the RF trap 18 (FIG. 1) may significantly reduce unwanted noise, ripple, or both from the first power supply output signal PS1 (FIG. 1) at the RF notch frequency RNF.
In a first embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 10 megahertz. In a second embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 20 megahertz. In a third embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 30 megahertz. In a fourth embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 40 megahertz. In a fifth embodiment of the frequency response 22, the RF notch frequency RNF is equal to about 50 megahertz.
FIG. 4 shows the DC-DC converter 10 according to another embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in FIG. 4 is similar to the DC-DC converter 10 illustrated in FIG. 2, except the DC-DC converter 10 illustrated in FIG. 4 further includes an offset capacitive element CO coupled between the parallel amplifier output PAO and the feedback input FBI. Additionally, the RF trap 18 includes a trap capacitive element CT and a trap inductive element LT coupled in series. The trap capacitive element CT and the trap inductive element LT form a resonant circuit having a resonant frequency. The RF notch frequency RNF (FIG. 3) is based on the resonant frequency. A shape of the frequency response 22 (FIG. 3) near the RF notch frequency RNF (FIG. 3) may be based on a Q factor of the resonant circuit.
The parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO and the offset capacitive element CO based on the voltage setpoint. The offset capacitive element CO allows the first voltage V1 to be higher than a voltage at the parallel amplifier output PAO. As a result, the parallel amplifier 14 may properly regulate the first voltage V1 even if the first voltage V1 is greater than a maximum output voltage from the parallel amplifier 14 at the parallel amplifier output PAO. In the embodiment of the DC-DC converter 10 illustrated in FIG. 4, the filter capacitive element CF is coupled between the parallel amplifier output PAO and the ground through the offset capacitive element CO. In an alternate embodiment of the DC-DC converter 10, the offset capacitive element CO is omitted.
FIG. 5 shows the DC-DC converter 10 according to a further embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in FIG. 5 is similar to the DC-DC converter 10 illustrated in FIG. 4, except in the DC-DC converter 10 illustrated in FIG. 5, the RF trap 18 includes the trap capacitive element CT, the trap inductive element LT, and a trap resistive element RT coupled in series. The trap capacitive element CT, the trap inductive element LT, and the trap resistive element RT form a resonant circuit having a resonant frequency. The RF notch frequency RNF (FIG. 3) is based on the resonant frequency. A shape of the frequency response 22 (FIG. 3) near the RF notch frequency RNF (FIG. 3) may be based on a Q factor of the resonant circuit.
FIG. 6 shows a radio frequency (RF) communications system 26 according to one embodiment of the present disclosure. The RF communications system 26 includes RF transmitter circuitry 28, RF system control circuitry 30, RF front-end circuitry 32, an RF antenna 34, and a DC power source 36. The RF transmitter circuitry 28 includes transmitter control circuitry 38, an RF power amplifier (PA) 40, the DC-DC converter 10, and PA bias circuitry 42. The DC-DC converter 10 functions as an envelope tracking power supply. In an alternate embodiment of the RF communications system 26, the DC power source 36 is external to the RF communications system 26.
In one embodiment of the RF communications system 26, the RF front-end circuitry 32 receives via the RF antenna 34, processes, and forwards an RF receive signal RFR to the RF system control circuitry 30. In one embodiment of the RF communications system 26, the RF receive signal RFR has an RF receive frequency. Further, the RF notch frequency RNF (FIG. 3) is about equal to the RF receive frequency, which may reduce noise, ripple, or both in the receive path from the transmit path or other noise sources. The RF system control circuitry 30 provides a power supply control signal VRMP and a transmitter configuration signal PACS to the transmitter control circuitry 38. The RF system control circuitry 30 provides an RF input signal RFI to the RF PA 40. The DC power source 36 provides a DC source signal VDC to the DC-DC converter 10. In one embodiment of the DC power source 36, the DC power source 36 is a battery. In one embodiment of the power supply control signal VRMP, the power supply control signal VRMP is an envelope power supply control signal. Specifically, the DC power source 36 provides the DC source signal VDC to the parallel amplifier 14 (FIG. 1) and to the switching supply 12 (FIG. 1).
The transmitter control circuitry 38 is coupled to the DC-DC converter 10 and to the PA bias circuitry 42. The DC-DC converter 10 provides the first power supply output signal PS1 to the RF PA 40 based on the power supply control signal VRMP. In this regard, the DC-DC converter 10 is an envelope tracking power supply and the first power supply output signal PS1 is a first envelope power supply signal EPS. The DC source signal VDC provides power to the DC-DC converter 10. As such, the first power supply output signal PS1, which is the first envelope power supply signal EPS, is based on the DC source signal VDC. The power supply control signal VRMP is representative of a voltage setpoint of the first envelope power supply signal EPS. The RF PA 40 receives and amplifies the RF input signal RFI to provide an RF transmit signal RFT using the first envelope power supply signal EPS. The first envelope power supply signal EPS provides power for amplification to the RF PA 40.
In one embodiment of the DC-DC converter 10, the first envelope power supply signal EPS is amplitude modulated to at least partially provide envelope tracking. In one embodiment of the RF PA 40, the RF PA 40 operates with approximately constant gain, called isogain, and with gain compression. In a first embodiment of the gain compression, the gain compression is greater than about one decibel. In a second embodiment of the gain compression, the gain compression is greater than about two decibels. In a third embodiment of the gain compression, the gain compression is equal to about two decibels. In a fourth embodiment of the gain compression, the gain compression is equal to about three decibels. In a fifth embodiment of the gain compression, the gain compression is equal to about four decibels. By operating with higher levels of gain compression, efficiency of the RF PA 40 may be increased, which may help compensate for reduced efficiency in the DC-DC converter 10.
In a first embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is greater than or equal to about 10 megahertz. In a second embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is less than or equal to about 10 megahertz. In a third embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is greater than or equal to about 20 megahertz. In a fourth embodiment of the first envelope power supply signal EPS, a bandwidth of the first envelope power supply signal EPS is less than or equal to about 20 megahertz.
The RF front-end circuitry 32 receives, processes, and transmits the RF transmit signal RFT via the RF antenna 34. In one embodiment of the RF transmitter circuitry 28, the transmitter control circuitry 38 configures the RF transmitter circuitry 28 based on the transmitter configuration signal PACS. In one embodiment of the RF communications system 26, the RF communications system 26 operates in a full duplex environment, such that the RF transmit signal RFT and the RF receive signal RFR may be active simultaneously. The RF transmit signal RFT has an RF transmit frequency and the RF receive signal RFR has the RF receive frequency. A difference between the RF transmit frequency and the RF receive frequency is about equal to an RF duplex frequency. In one embodiment of the RF communications system 26, the RF notch frequency RNF (FIG. 3) is about equal to the RF duplex frequency, which may reduce noise in the receive path from the transmit path.
In a first embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 10 megahertz. In a second embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 20 megahertz. In a third embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 30 megahertz. In a fourth embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 40 megahertz. In a fifth embodiment of the RF duplex frequency, the RF duplex frequency is greater than or equal to about 50 megahertz.
The PA bias circuitry 42 provides a PA bias signal PAB to the RF PA 40. In this regard, the PA bias circuitry 42 biases the RF PA 40 via the PA bias signal PAB. In one embodiment of the PA bias circuitry 42, the PA bias circuitry 42 biases the RF PA 40 based on the transmitter configuration signal PACS. In one embodiment of the RF front-end circuitry 32, the RF front-end circuitry 32 includes at least one RF switch, at least one RF amplifier, at least one RF filter, at least one RF duplexer, at least one RF diplexer, at least one RF amplifier, the like, or any combination thereof. In one embodiment of the RF system control circuitry 30, the RF system control circuitry 30 is RF transceiver circuitry, which may include an RF transceiver IC, baseband controller circuitry, the like, or any combination thereof. In one embodiment of the RF transmitter circuitry 28, the DC-DC converter 10 provides the first envelope power supply signal EPS, which has switching ripple. In one embodiment of the RF transmitter circuitry 28, the first envelope power supply signal EPS provides power for amplification and at least partially envelope tracks the RF transmit signal RFT.
FIG. 7 shows the RF communications system 26 according to an alternate embodiment of the RF communications system 26. The RF communications system 26 illustrated in FIG. 7 is similar to the RF communications system 26 illustrated in FIG. 6, except in the RF communications system 26 illustrated in FIG. 7, the RF transmitter circuitry 28 further includes a digital communications interface 44, which is coupled between the transmitter control circuitry 38 and a digital communications bus 46. The digital communications bus 46 is also coupled to the RF system control circuitry 30. As such, the RF system control circuitry 30 provides the power supply control signal VRMP (FIG. 6) and the transmitter configuration signal PACS (FIG. 6) to the transmitter control circuitry 38 via the digital communications bus 46 and the digital communications interface 44.
FIG. 8 shows details of the DC-DC converter 10 illustrated in FIG. 6 according to one embodiment of the DC-DC converter 10. The DC-DC converter 10 includes the power supply control circuitry 20, the parallel amplifier 14, and the switching supply 12. The power supply control circuitry 20 controls the parallel amplifier 14 and the switching supply 12. The parallel amplifier 14 and the switching supply 12 provide the first power supply output signal PS1, such that the parallel amplifier 14 partially provides the first power supply output signal PS1 and the switching supply 12 partially provides the first power supply output signal PS1.
FIG. 9 shows the RF communications system 26 according to another embodiment of the RF communications system 26. The RF communications system 26 illustrated in FIG. 9 is similar to the RF communications system 26 illustrated in FIG. 6, except in the RF communications system 26 illustrated in FIG. 9, the PA bias circuitry 42 (FIG. 6) is omitted and the RF PA 40 includes a driver stage 48 and a final stage 50, which is coupled to the driver stage 48. The DC-DC converter 10 provides the second power supply output signal PS2, which is a second envelope power supply signal, to the driver stage 48 based on the power supply control signal VRMP. Further, the DC-DC converter 10 provides the first power supply output signal PS1, which is the first envelope power supply signal, to the final stage 50 based on the power supply control signal VRMP. The driver stage 48 receives and amplifies the RF input signal RFI to provide a driver stage output signal DSO using the second envelope power supply signal, which provides power for amplification. Similarly, the final stage 50 receives and amplifies the driver stage output signal DSO to provide the RF transmit signal RFT using the first envelope power supply signal, which provides power for amplification.
FIG. 10 shows the DC-DC converter 10 according to one embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in FIG. 10 is similar to the DC-DC converter 10 illustrated in FIG. 1, except in the DC-DC converter 10 illustrated in FIG. 10, the switching supply 12 further includes a second inductive element L2. The second inductive element L2 is coupled between the feedback input FBI and the parallel amplifier output PAO. The switching supply 12 partially provides the first power supply output signal PS1 via the first inductive element L1 and the second inductive element L2. Specifically, the switching supply 12 partially provides the first power supply output signal PS1 via a series combination of the first inductive element L1 and the second inductive element L2.
In one embodiment of the switching supply 12, a connection node 52 is provided where the first inductive element L1 and the second inductive element L2 are connected to one another. The connection node 52 provides a second voltage V2 to the parallel amplifier 14 via the feedback input FBI. Further, in one embodiment of the parallel amplifier 14, the parallel amplifier 14 has a limited open loop gain at high frequencies that are above a frequency threshold. At such frequencies, a group delay in the parallel amplifier 14 may normally limit the ability of the parallel amplifier 14 to accurately regulate the first voltage V1 of the first power supply output signal PS1. However, by feeding back the second voltage V2 to the feedback input FBI instead of the first voltage V1, a phase-shift that is developed across the second inductive element L2 at least partially compensates for the limited open loop gain of the parallel amplifier 14 at frequencies that are above the frequency threshold, thereby improving the ability of the parallel amplifier 14 to accurately regulate the first voltage V1. In this regard, in one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO based on the voltage setpoint and feeding back a voltage to the feedback input FBI from the connection node 52 between the first inductive element L1 and the second inductive element L2.
FIG. 11 shows the DC-DC converter 10 according to an alternate embodiment of the DC-DC converter 10. The DC-DC converter 10 illustrated in FIG. 11 is similar to the DC-DC converter 10 illustrated in FIG. 10, except the DC-DC converter 10 illustrated in FIG. 11 further includes the offset capacitive element CO and the switching supply 12 further includes the filter capacitive element CF. The offset capacitive element CO is coupled between the parallel amplifier output PAO and the second inductive element L2. In one embodiment of the DC-DC converter 10, the parallel amplifier 14 partially provides the first power supply output signal PS1 via the parallel amplifier output PAO and the offset capacitive element CO based on the voltage setpoint. The first inductive element L1 and the second inductive element L2 provide a second power supply output signal PS2 via the connection node 52.
The first inductive element L1, the second inductive element L2, and the filter capacitive element CF form a first low-pass filter 54 having a first cutoff frequency. The second inductive element L2 and the filter capacitive element CF form a second low-pass filter 56 having a second cutoff frequency. The second cutoff frequency may be significantly higher than the first cutoff frequency. As such, the first low-pass filter 54 may be used primarily to filter the switching output voltage VS, which is typically a square wave. However, the second low-pass filter 56 may be used to target specific high frequencies, such as certain harmonics of the switching output voltage VS.
In a first embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 10 times greater than the first cutoff frequency. In a second embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 100 times greater than the first cutoff frequency. In a third embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 500 times greater than the first cutoff frequency. In a fourth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is at least 1000 times greater than the first cutoff frequency. In a fifth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is less than 1000 times greater than the first cutoff frequency. In a sixth embodiment of the first low-pass filter 54 and the second low-pass filter 56, the second cutoff frequency is less than 5000 times greater than the first cutoff frequency.
The first inductive element L1 has a first inductance and the second inductive element L2 has a second inductance. In a first embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 10 times greater than a magnitude of the second inductance. In a second embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 100 times greater than a magnitude of the second inductance. In a third embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 500 times greater than a magnitude of the second inductance. In a fourth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is at least 1000 times greater than a magnitude of the second inductance. In a fifth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is less than 1000 times greater than a magnitude of the second inductance. In a sixth embodiment of the first inductive element L1 and the second inductive element L2, a magnitude of the first inductance is less than 5000 times greater than a magnitude of the second inductance.
An analysis of improved ripple cancellation performance of the DC-DC converter 10 illustrated in FIG. 11 is presented. In general, the first power supply output signal PS1 is fed to a load (not shown) having a load resistance RL, such as the RF PA 40 (FIG. 6). The switching output voltage VS has a DC component called a DC voltage VD and a ripple component called an AC voltage VA given by EQ. 1, as shown below.

VS=VD+VA.  EQ. 1:

Further, the inductor current IL has a DC current ID and an AC current IA given by EQ. 2, as shown below.

IL=ID+IA.  EQ. 2:

The DC-DC converter 10 regulates the DC voltage VD to be about equal to the voltage setpoint. The first inductive element L1 and the second inductive element L2 appear approximately as short circuits to the DC component. Further, the filter capacitive element CF appears approximately as an open circuit to the DC component. Therefore, the DC voltage VD is approximately applied to the load resistance RL, as intended. As a result, the DC current ID is based on the DC voltage VD and the load resistance RL, as shown in EQ. 3 below.

ID=VD/RL.  EQ. 3:

Most of the ripple components of the switching output voltage VS is filtered out from the first voltage V1 by the first low-pass filter 54 and the second low-pass filter 56. As a result, most of the AC voltage VA is across the series combination of the first inductive element L1 and the second inductive element L2. The first inductive element L1 has a first inductance I1 and the second inductive element L2 has a second inductance I2. Therefore, the AC current IA is based on the AC voltage VA, the first inductance I1 and the second inductance I2, where s=j2πf, j=√−1, and f=frequency, as shown in EQ. 4 below.

IA=VA/[s(I1+I2)].  EQ. 4:

Much of what remains of the ripple component is cancelled out from the first voltage V1 by the parallel amplifier 14. However, to the extent that the parallel amplifier 14 cannot completely cancel out the remains of the ripple component, the first voltage V1 has a first residual ripple voltage VR1 and the second voltage V2 has a second residual ripple voltage VR2. Two approaches to ripple cancellation will be compared against one another. In the first approach, the DC-DC converter 10 is the DC-DC converter 10 illustrated in FIG. 10, such that the second voltage V2 is fed to the feedback input FBI, as shown. In this regard, the second residual ripple voltage VR2 drives the parallel amplifier 14 to provide a ripple cancellation current, which is the parallel amplifier output current IP. In the second approach, the DC-DC converter 10 is similar to the DC-DC converter 10 illustrated in FIG. 10, except the first voltage V1 is fed to the feedback input FBI instead of the second voltage V2, such that the first residual ripple voltage VR1 drives the parallel amplifier 14 to provide the ripple cancellation current, which is the parallel amplifier output current IP.
In the following analysis, the parallel amplifier 14 has a DC open loop gain GO and an open loop bandwidth factor T. As a result, the parallel amplifier 14 has a gain G, as shown in EQ. 5 below.

G=GO/(1+sT).  EQ. 5:

As a result, at frequencies significantly below an open loop bandwidth of the parallel amplifier 14, the open loop bandwidth factor T is small compared to one, such that the gain G approaches the DC open loop gain GO. Conversely, at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the open loop bandwidth factor T is large compared to one, such that the gain G approaches GO/sT.
In the first approach, described above wherein the second residual ripple voltage VR2 drives the parallel amplifier 14 and at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the parallel amplifier output current IP is based on the second residual ripple voltage VR2, as shown in EQ. 6 below.

IP=G*VR2 ̃(GO*VR2)/sT.  EQ. 6:

In the second approach described above, when the first residual ripple voltage VR1 drives the parallel amplifier 14 and at frequencies significantly above the open loop bandwidth of the parallel amplifier 14, the parallel amplifier output current IP is based on the first residual ripple voltage VR1, as shown in EQ. 7 below.

IP=G*VR1(GO*VR1)/sT.  EQ. 7:

However, a difference between the first residual ripple voltage VR1 and the second residual ripple voltage VR2 is based on the AC current IA and the second inductance I2, as shown in EQ. 8 and EQ. 9 below.

(VR2−VR1)=(s)(IA)(I2),  EQ. 8: or

VR2=(s)(IA)(I2)+VR1.  EQ. 9:

Substituting EQ. 9 into EQ. 6 provides EQ. 10 and EQ. 11, as shown below.

IP ̃(GO)(VR1)/sT+(GO)(s)(IA)(I2)/sT,  EQ. 10: or

IP ̃(GO)(VR1)/sT+(GO)(IA)(I2)/T.  EQ. 11:

EQ. 11 is representative of the first approach and EQ. 7 is representative of the second approach. As a reminder, in the first approach, the second residual ripple voltage VR2 drives the parallel amplifier 14 and in the second approach, the first residual ripple voltage VR1 drives the parallel amplifier 14. In both equations, a smaller first residual ripple voltage VR1 represents better ripple cancellation performance. For comparison purposes, both approaches are assumed to provide the same magnitude of parallel amplifier output current IP. However, in the second approach, the parallel amplifier output current IP is phase-shifted from the first residual ripple voltage VR1 by about 90 degrees. As such, the parallel amplifier output current IP is phase-shifted from the ripple current it is trying to cancel by about 90 degrees, thereby degrading ripple cancellation performance. However, in the first approach, according to EQ. 11, the parallel amplifier output current IP has two terms, namely the (GO)(VR1)/sT term and the (GO)(IA)(I2)/T term. The (GO)(VR1)/sT term has the same phase-alignment shortcoming as in the second approach. But the (GO)(IA)(I2)/T term phase-aligns the parallel amplifier output current IP with the ripple current it is trying to cancel. Overall, the phase-alignment in the first approach is improved over the second approach. Additionally, to the extent that the (GO)(VR1)/sT term is smaller than the (GO)(IA)(I2)/T term, the first residual ripple voltage VR1 is reduced, thereby improving ripple cancellation. In this regard, if the (GO)(IA)(I2)/T term is equal to the (GO)(VR1)/sT term in EQ. 7, then in the (GO)(VR1)/sT term in EQ. 11, the first residual ripple voltage VR1 is equal to about zero, such that the first approach is greatly improved over the second approach.
Some of the circuitry previously described may use discrete circuitry, integrated circuitry, programmable circuitry, non-volatile circuitry, volatile circuitry, software executing instructions on computing hardware, firmware executing instructions on computing hardware, the like, or any combination thereof. The computing hardware may include mainframes, micro-processors, micro-controllers, DSPs, the like, or any combination thereof.
None of the embodiments of the present disclosure are intended to limit the scope of any other embodiment of the present disclosure. Any or all of any embodiment of the present disclosure may be combined with any or all of any other embodiment of the present disclosure to create new embodiments of the present disclosure.
Those skilled in the art will recognize improvements and modifications to the embodiments of the present disclosure. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.

## Claims

1. Circuitry comprising:
a parallel amplifier having a feedback input and a parallel amplifier output;
a radio frequency (RF) trap coupled between the parallel amplifier output and a ground;
a switching supply comprising:
switching circuitry having a switching circuitry output; and
a first inductive element coupled between the switching circuitry output and the feedback input; and

a direct current (DC) power source adapted to provide a DC source signal to the parallel amplifier and the switching supply.

2. The circuitry of claim 1 wherein the parallel amplifier output is directly coupled to the feedback input.

3. The circuitry of claim 1 wherein the RF trap comprises a trap capacitive element and a trap inductive element coupled in series.

4. The circuitry of claim 1 wherein the RF trap comprises a trap capacitive element, a trap inductive element, and a trap resistive element coupled in series.

5. The circuitry of claim 1 further comprising an offset capacitive element coupled between the parallel amplifier output and the feedback input, such that the parallel amplifier is adapted to partially provide a first power supply output signal via the parallel amplifier output and the offset capacitive element based on a voltage setpoint.

6. The circuitry of claim 1 wherein the switching supply further comprises a filter capacitive element coupled between the parallel amplifier output and the ground.

7. The circuitry of claim 1 wherein:
the parallel amplifier is adapted to partially provide a first power supply output signal via the parallel amplifier output based on a voltage setpoint; and
the switching supply is adapted to partially provide the first power supply output signal via the first inductive element.

8. The circuitry of claim 7 wherein the RF trap has a frequency response with an RF notch at an RF notch frequency and is adapted to filter the first power supply output signal based on the frequency response.

9. The circuitry of claim 8 wherein the RF notch frequency is equal to about 30 megahertz.

10. The circuitry of claim 8 wherein the RF notch frequency is about equal to an RF receive frequency.

11. The circuitry of claim 8 further comprising an RF power amplifier (PA), wherein:
the first power supply output signal is a first envelope power supply signal; and
the RF PA is adapted to receive and amplify an RF input signal to provide an RF transmit signal using the first envelope power supply signal.

12. The circuitry of claim 11 wherein the RF notch frequency is about equal to an RF duplex frequency, which is about equal to a difference between an RF transmit frequency and an RF receive frequency.

13. The circuitry of claim 12 wherein the RF duplex frequency is greater than or equal to about 30 megahertz.

14. The circuitry of claim 11 wherein the first envelope power supply signal provides power for amplification to the RF PA.

15. The circuitry of claim 11 wherein a bandwidth of the first envelope power supply signal is greater than or equal to about 20 megahertz.

16. The circuitry of claim 11 further comprising the RF PA.

17. The circuitry of claim 7 wherein the voltage setpoint is based on a power supply control signal.

18. The circuitry of claim 7 wherein:
the parallel amplifier is further adapted to regulate a voltage of the first power supply output signal based on the voltage setpoint; and
the switching supply is further adapted to regulate the first power supply output signal to minimize an output current from the parallel amplifier.

19. The circuitry of claim 7 wherein:
the parallel amplifier is further adapted to partially provide the first power supply output signal using the DC source signal; and
the switching supply is further adapted to partially provide the first power supply output signal using the DC source signal.

20. The circuitry of claim 19 wherein the DC power source is a battery.

21. The circuitry of claim 19 further comprising the DC power source.

22. The circuitry of claim 1 wherein the switching supply further comprises a second inductive element coupled between the feedback input and the parallel amplifier output.

23. The circuitry of claim 22 wherein:
the parallel amplifier is adapted to partially provide a first power supply output signal via the parallel amplifier output based on a voltage setpoint; and
the switching supply is adapted to partially provide the first power supply output signal via the first inductive element and the second inductive element.

24. The circuitry of claim 23 wherein a phase-shift across the second inductive element at least partially compensates for limited open loop gain of the parallel amplifier at frequencies above a frequency threshold.

25. The circuitry of claim 23 wherein:
the first inductive element and the second inductive element are connected to one another at a connection node; and
the first inductive element and the second inductive element are adapted to provide a second power supply output signal via the connection node.

26. The circuitry of claim 22 wherein the first inductive element has a first inductance and the second inductive element has a second inductance, such that a magnitude of the first inductance is at least 10 times greater than a magnitude of the second inductance.

27. The circuitry of claim 22 further comprising an offset capacitive element coupled between the parallel amplifier output and the second inductive element, such that the parallel amplifier is adapted to partially provide a first power supply output signal via the parallel amplifier output and the offset capacitive element based on a voltage setpoint.

28. A method comprising:
providing a DC source signal to a parallel amplifier and a switching supply circuitry;
partially providing a first power supply output signal via a first inductive element inside the switching supply circuitry;
partially providing the first power supply output signal via a parallel amplifier output based on a voltage setpoint;
regulating a voltage of the first power supply output signal received at a feedback input of the parallel amplifier; and
filtering the first power supply output signal based on a frequency response of a radio frequency (RF) trap.