Patent Publication Number: US-2023155554-A1

Title: Apparatus and methods for envelope tracking

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 17/247,707, filed Dec. 21, 2020, and titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING,” which claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Patent Application No. 62/959,476, filed Jan. 10, 2020 and titled “APPARATUS AND METHODS FOR ENVELOPE TRACKING,” each of which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the invention relate to electronic systems, and in particular, to power amplifiers for radio frequency (RF) electronics. 
     Description of the Related Technology 
     Power amplifiers are used in RF communication systems to amplify RF signals for transmission via antennas. It is important to manage the power of RF signal transmissions to prolong battery life and/or provide a suitable transmit power level. 
     Examples of RF communication systems with one or more power amplifiers include, but are not limited to, mobile phones, tablets, base stations, network access points, customer-premises equipment (CPE), laptops, and wearable electronics. For example, in wireless devices that communicate using a cellular standard, a wireless local area network (WLAN) standard, and/or any other suitable communication standard, a power amplifier can be used for RF signal amplification. An RF signal can have a frequency in the range of about 30 kHz to 300 GHz, such as in the range of about 410 MHz to about 7.125 GHz for Fifth Generation (5G) cellular communications in Frequency Range 1 (FR1). 
     SUMMARY 
     In certain embodiments, the present disclosure relates to a mobile device. The mobile device includes a transceiver configured to generate a radio frequency signal, a front end circuit including a power amplifier configured to amplify the radio frequency signal and to receive power from a power amplifier supply voltage, and a power management circuit including an envelope tracker configured to control the power amplifier supply voltage based on an envelope signal indicating an envelope of the radio frequency signal. The envelope tracker includes a multi-level supply modulator having an output electrically connected to the power amplifier supply voltage and a plurality of inputs configured to receive a plurality of regulated voltages including a first regulated voltage and a second regulated voltage of different voltage levels. The envelope tracker further includes a control circuit configured to control the multi-level supply modulator to selectively connect one or more of the plurality of regulated voltages to the output, and to control the multi-level supply modulator to provide a gradual transition from selecting the first regulated voltage to selecting the second regulated voltage. 
     In some embodiments, the mobile device further includes a battery operable to provide a battery voltage to the envelope tracker. 
     In various embodiments, the mobile device further includes an antenna configured to radiate a radio frequency wave in response to receiving an amplified radio frequency signal from the power amplifier. 
     In several embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output. According to a number of embodiments, at least a portion of the first plurality of selectable paths have different resistances from one another, and at least a portion of the second plurality of selectable paths have different resistances from one another. In accordance with some embodiments, the gradual transition includes a change from a first state in which all of the first plurality of selectable paths are selected and none of the second plurality of selectable paths are selected to a second state in which all of the second plurality of selectable paths are selected and none of the first plurality of selectable paths are selected. 
     In some embodiments, the control circuit is further configured to operate the multi-level supply modulator in a period of overlap in which both the first regulated voltage is connected to the output and the second regulated voltage is connected to the output. According to various embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output, the period of overlap corresponding to a state in which both at least one of the first plurality of selectable paths is selected and at least one of the second plurality of selectable paths is selected. In accordance with a number of embodiments, each of the first plurality of selectable paths has a different resistance from one another, the at least one of the first plurality of selectable paths corresponding to a highest resistance path of the first plurality of selectable paths. According to several embodiments, each of the second plurality of selectable paths has a different resistance from one another, the at least one of the second plurality of selectable paths corresponding to a highest resistance path of the second plurality of selectable paths. 
     In a number of embodiments, the envelope tracker further includes a supply voltage filter electrically connected between the output of the multi-level supply modulator and the power amplifier supply voltage. 
     In various embodiments, the envelope tracker further includes a multi-level supply DC-to-DC converter configured to receive a battery voltage and to generate the plurality of regulated voltages. According to several embodiments, the envelope tracker further includes a first supply voltage filter electrically connected between the output of the multi-level supply modulator and the power amplifier supply voltage. In accordance with some embodiments, the multi-level supply DC-to-DC converter is further configured to generate a DC voltage, the envelope tracker further including a second supply voltage filter electrically connected between the DC voltage and the power amplifier supply voltage. 
     In several embodiments, the envelope tracker is operable in a plurality of operating modes including an envelope tracking mode in which the envelope tracker controls the power amplifier supply voltage using envelope tracking, and an average power tracking mode in which the envelope tracker controls the power amplifier supply voltage using average power tracking. 
     In certain embodiments, the present disclosure relates to a method of envelope tracking. The method includes amplifying a radio frequency signal using a power amplifier, powering the power amplifier using a power amplifier supply voltage, and controlling the power amplifier supply voltage based on an envelope signal indicating an envelope of the radio frequency signal using an envelope tracker, including providing a plurality of regulated voltages including a first regulated voltage and a second regulated voltage of different voltage levels to a multi-level supply modulator, controlling the power amplifier supply voltage with an output of the multi-level supply modulator, and controlling the multi-level supply modulator to selectively connect one or more of the plurality of regulated voltages to the output and to gradually transition from selecting the first regulated voltage to selecting the second regulated voltage. 
     In a number of embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output. According to several embodiments, at least a portion of the first plurality of selectable paths have different resistances from one another, and at least a portion of the second plurality of selectable paths have different resistances from one another. In accordance with some embodiments, controlling the multi-level supply modulator to gradually transition includes changing the multi-level supply modulator from a first state in which all of the first plurality of selectable paths are selected and none of the second plurality of selectable paths are selected to a second state in which all of the second plurality of selectable paths are selected and none of the first plurality of selectable paths are selected. 
     In various embodiments, the method further includes controlling the multi-level supply modulator to operate in a period of overlap in which both the first regulated voltage is connected to the output and the second regulated voltage is connected to the output. According to a number of embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output, the period of overlap corresponding to a state in which both at least one of the first plurality of selectable paths is selected and at least one of the second plurality of selectable paths is selected. In accordance with several embodiments, each of the first plurality of selectable paths has a different resistance from one another, the at least one of the first plurality of selectable paths that is selected corresponding to a highest resistance path of the first plurality of selectable paths. According to some embodiments, each of the second plurality of selectable paths has a different resistance from one another, the at least one of the second plurality of selectable paths that is selected corresponding to a highest resistance path of the second plurality of selectable paths. 
     In several embodiments, the method further includes controlling the power amplifier supply voltage with the output of the multi-level supply modulator through a supply voltage filter. 
     In a number of embodiments, the method further includes generating the plurality of regulated voltages using a multi-level supply DC-to-DC converter that receives a battery voltage. 
     In various embodiments, the method further includes controlling the envelope tracker to operate in a selected mode chosen from a plurality of operating modes including an envelope tracking mode and an average power tracking mode. 
     In certain embodiments, an envelope tracking system is provided. The envelope tracking system includes a power amplifier configured to amplify a radio frequency signal and to receive power from a power amplifier supply voltage, and an envelope tracker configured to control the power amplifier supply voltage based on an envelope signal indicating an envelope of the radio frequency signal. The envelope tracker includes a multi-level supply modulator having an output electrically connected to the power amplifier supply voltage and a plurality of inputs configured to receive a plurality of regulated voltages including a first regulated voltage and a second regulated voltage of different voltage levels, the envelope tracker further including a control circuit configured to control the multi-level supply modulator to selectively connect one or more of the plurality of regulated voltages to the output, and to control the multi-level supply modulator to provide a gradual transition from selecting the first regulated voltage to selecting the second regulated voltage. 
     In various embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output. According to a number of embodiments, at least a portion of the first plurality of selectable paths have different resistances from one another, and at least a portion of the second plurality of selectable paths have different resistances from one another. In accordance with several embodiments, the gradual transition includes a change from a first state in which all of the first plurality of selectable paths are selected and none of the second plurality of selectable paths are selected to a second state in which all of the second plurality of selectable paths are selected and none of the first plurality of selectable paths are selected. According to some embodiments, the control circuit is further configured to operate the multi-level supply modulator in a period of overlap in which both the first regulated voltage is connected to the output and the second regulated voltage is connected to the output. In accordance with a number of embodiments, the multi-level supply modulator includes a first plurality of selectable paths between the first regulated voltage and the output, and a second plurality of selectable paths between the second regulated voltage and the output, the period of overlap corresponding to a state in which both at least one of the first plurality of selectable paths is selected and at least one of the second plurality of selectable paths is selected. According to several embodiments, each of the first plurality of selectable paths has a different resistance from one another, the at least one of the first plurality of selectable paths corresponding to a highest resistance path of the first plurality of selectable paths. In accordance with some embodiments, each of the second plurality of selectable paths has a different resistance from one another, the at least one of the second plurality of selectable paths corresponding to a highest resistance path of the second plurality of selectable paths. 
     In several embodiments, the envelope tracker further includes a supply voltage filter electrically connected between the output of the multi-level supply modulator and the power amplifier supply voltage. 
     In a number of embodiments, the envelope tracker further includes a multi-level supply DC-to-DC converter configured to receive a battery voltage and to generate the plurality of regulated voltages. According to a number of embodiments, the envelope tracker further includes a first supply voltage filter electrically connected between the output of the multi-level supply modulator and the power amplifier supply voltage. In accordance with several embodiments, the multi-level supply DC-to-DC converter is further configured to generate a DC voltage, the envelope tracker further including a second supply voltage filter electrically connected between the DC voltage and the power amplifier supply voltage. 
     In some embodiments, the envelope tracker is operable in a plurality of operating modes including an envelope tracking mode in which the envelope tracker controls the power amplifier supply voltage using envelope tracking, and an average power tracking mode in which the envelope tracker controls the power amplifier supply voltage using average power tracking. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of one example of a communication network. 
         FIG.  2 A  is a schematic diagram of one example of a communication link using carrier aggregation. 
         FIG.  2 B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG.  2 A . 
         FIG.  2 C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG.  2 A . 
         FIG.  3 A  is a schematic diagram of one embodiment of an envelope tracking system for a power amplifier. 
         FIG.  3 B  is a schematic diagram of another embodiment of an envelope tracking system for a power amplifier. 
         FIG.  4 A  is a schematic diagram of one example of a multi-level supply (MLS) modulator that operates without soft switching. 
         FIG.  4 B  is one example of a timing diagram for the MLS modulator of  FIG.  4 A . 
         FIG.  5 A  is a schematic diagram of one embodiment of an MLS modulator that operates with soft switching. 
         FIG.  5 B  is one example of a timing diagram for the MLS modulator of  FIG.  5 A . 
         FIG.  5 C  is another example of a timing diagram for the MLS modulator of  FIG.  5 A . 
         FIG.  6    is a schematic diagram of one embodiment of a control circuit for an MLS modulator. 
         FIG.  7    is a schematic diagram of another embodiment of a control circuit for an MLS modulator. 
         FIG.  8    is a schematic diagram of another embodiment of a control circuit for an MLS modulator. 
         FIG.  9    is a graph of one example of simulation results for an MLS modulator in accordance with one embodiment. 
         FIG.  10    is a schematic diagram of one embodiment of a mobile device. 
         FIG.  11    is a schematic diagram of one embodiment of a communication system for transmitting RF signals. 
         FIG.  12    is a schematic diagram of one example of a power amplifier system including an envelope tracker. 
         FIG.  13 A  shows a first example of a power amplifier supply voltage versus time. 
         FIG.  13 B  shows a second example of a power amplifier supply voltage versus time. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The following detailed description of certain embodiments presents various descriptions of specific embodiments. However, the innovations described herein can be embodied in a multitude of different ways, for example, as defined and covered by the claims. In this description, reference is made to the drawings where like reference numerals can indicate identical or functionally similar elements. It will be understood that elements illustrated in the figures are not necessarily drawn to scale. Moreover, it will be understood that certain embodiments can include more elements than illustrated in a drawing and/or a subset of the elements illustrated in a drawing. Further, some embodiments can incorporate any suitable combination of features from two or more drawings. 
     The International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) responsible for global issues concerning information and communication technologies, including the shared global use of radio spectrum. 
     The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications standard bodies across the world, such as the Association of Radio Industries and Businesses (ARIB), the Telecommunications Technology Committee (TTC), the China Communications Standards Association (CCSA), the Alliance for Telecommunications Industry Solutions (ATIS), the Telecommunications Technology Association (TTA), the European Telecommunications Standards Institute (ETSI), and the Telecommunications Standards Development Society, India (TSDSI). 
     Working within the scope of the ITU, 3GPP develops and maintains technical specifications for a variety of mobile communication technologies, including, for example, second generation (2G) technology (for instance, Global System for Mobile Communications (GSM) and Enhanced Data Rates for GSM Evolution (EDGE)), third generation (3G) technology (for instance, Universal Mobile Telecommunications System (UMTS) and High Speed Packet Access (HSPA)), and fourth generation (4G) technology (for instance, Long Term Evolution (LTE) and LTE-Advanced). 
     The technical specifications controlled by 3GPP can be expanded and revised by specification releases, which can span multiple years and specify a breadth of new features and evolutions. 
     In one example, 3GPP introduced carrier aggregation (CA) for LTE in Release 10. Although initially introduced with two downlink carriers, 3GPP expanded carrier aggregation in Release 14 to include up to five downlink carriers and up to three uplink carriers. Other examples of new features and evolutions provided by 3GPP releases include, but are not limited to, License Assisted Access (LAA), enhanced LAA (eLAA), Narrowband Internet of things (NB-IOT), Vehicle-to-Everything (V2X), and High Power User Equipment (HPUE). 
     3GPP introduced Phase 1 of fifth generation (5G) technology in Release 15, and plans to introduce Phase 2 of 5G technology in Release  16  (targeted for 2019). Subsequent 3GPP releases will further evolve and expand 5G technology. 5G technology is also referred to herein as 5G New Radio (NR). 
     5G NR supports or plans to support a variety of features, such as communications over millimeter wave spectrum, beamforming capability, high spectral efficiency waveforms, low latency communications, multiple radio numerology, and/or non-orthogonal multiple access (NOMA). Although such RF functionalities offer flexibility to networks and enhance user data rates, supporting such features can pose a number of technical challenges. 
     The teachings herein are applicable to a wide variety of communication systems, including, but not limited to, communication systems using advanced cellular technologies, such as LTE-Advanced, LTE-Advanced Pro, and/or 5G NR. 
       FIG.  1    is a schematic diagram of one example of a communication network  10 . The communication network  10  includes a macro cell base station  1 , a small cell base station  3 , and various examples of user equipment (UE), including a first mobile device  2   a , a wireless-connected car  2   b , a laptop  2   c , a stationary wireless device  2   d , a wireless-connected train  2   e , a second mobile device  2   f , and a third mobile device  2   g.    
     Although specific examples of base stations and user equipment are illustrated in  FIG.  1   , a communication network can include base stations and user equipment of a wide variety of types and/or numbers. 
     For instance, in the example shown, the communication network  10  includes the macro cell base station  1  and the small cell base station  3 . The small cell base station  3  can operate with relatively lower power, shorter range, and/or with fewer concurrent users relative to the macro cell base station  1 . The small cell base station  3  can also be referred to as a femtocell, a picocell, or a microcell. Although the communication network  10  is illustrated as including two base stations, the communication network  10  can be implemented to include more or fewer base stations and/or base stations of other types. 
     Although various examples of user equipment are shown, the teachings herein are applicable to a wide variety of user equipment, including, but not limited to, mobile phones, tablets, laptops, IoT devices, wearable electronics, customer premises equipment (CPE), wireless-connected vehicles, wireless relays, and/or a wide variety of other communication devices. Furthermore, user equipment includes not only currently available communication devices that operate in a cellular network, but also subsequently developed communication devices that will be readily implementable with the inventive systems, processes, methods, and devices as described and claimed herein. 
     The illustrated communication network  10  of  FIG.  1    supports communications using a variety of cellular technologies, including, for example, 4G LTE and 5G NR. In certain implementations, the communication network  10  is further adapted to provide a wireless local area network (WLAN), such as WiFi. Although various examples of communication technologies have been provided, the communication network  10  can be adapted to support a wide variety of communication technologies. 
     Various communication links of the communication network  10  have been depicted in  FIG.  1   . The communication links can be duplexed in a wide variety of ways, including, for example, using frequency-division duplexing (FDD) and/or time-division duplexing (TDD). FDD is a type of radio frequency communications that uses different frequencies for transmitting and receiving signals. FDD can provide a number of advantages, such as high data rates and low latency. In contrast, TDD is a type of radio frequency communications that uses about the same frequency for transmitting and receiving signals, and in which transmit and receive communications are switched in time. TDD can provide a number of advantages, such as efficient use of spectrum and variable allocation of throughput between transmit and receive directions. 
     In certain implementations, user equipment can communicate with a base station using one or more of 4G LTE, 5G NR, and WiFi technologies. In certain implementations, enhanced license assisted access (eLAA) is used to aggregate one or more licensed frequency carriers (for instance, licensed 4G LTE and/or 5G NR frequencies), with one or more unlicensed carriers (for instance, unlicensed WiFi frequencies). 
     As shown in  FIG.  1   , the communication links include not only communication links between UE and base stations, but also UE to UE communications and base station to base station communications. For example, the communication network  10  can be implemented to support self-fronthaul and/or self-backhaul. 
     The communication links can operate over a wide variety of frequencies. In certain implementations, communications are supported using 5G NR technology over one or more frequency bands that are less than 6 Gigahertz (GHz) and/or over one or more frequency bands that are greater than 6 GHz. For example, the communication links can serve Frequency Range 1 (FR1), Frequency Range 2 (FR2), or a combination thereof. In one embodiment, one or more of the mobile devices support a HPUE power class specification. 
     In certain implementations, a base station and/or user equipment communicates using beamforming. For example, beamforming can be used to focus signal strength to overcome path losses, such as high loss associated with communicating over high signal frequencies. In certain embodiments, user equipment, such as one or more mobile phones, communicate using beamforming on millimeter wave frequency bands in the range of 30 GHz to 300 GHz and/or upper centimeter wave frequencies in the range of 6 GHz to 30 GHz, or more particularly, 24 GHz to 30 GHz. 
     Different users of the communication network  10  can share available network resources, such as available frequency spectrum, in a wide variety of ways. 
     In one example, frequency division multiple access (FDMA) is used to divide a frequency band into multiple frequency carriers. Additionally, one or more carriers are allocated to a particular user. Examples of FDMA include, but are not limited to, single carrier FDMA (SC-FDMA) and orthogonal FDMA (OFDMA). OFDMA is a multicarrier technology that subdivides the available bandwidth into multiple mutually orthogonal narrowband subcarriers, which can be separately assigned to different users. 
     Other examples of shared access include, but are not limited to, time division multiple access (TDMA) in which a user is allocated particular time slots for using a frequency resource, code division multiple access (CDMA) in which a frequency resource is shared amongst different users by assigning each user a unique code, space-divisional multiple access (SDMA) in which beamforming is used to provide shared access by spatial division, and non-orthogonal multiple access (NOMA) in which the power domain is used for multiple access. For example, NOMA can be used to serve multiple users at the same frequency, time, and/or code, but with different power levels. 
     Enhanced mobile broadband (eMBB) refers to technology for growing system capacity of LTE networks. For example, eMBB can refer to communications with a peak data rate of at least 10 Gbps and a minimum of 100 Mbps for each user. Ultra-reliable low latency communications (uRLLC) refers to technology for communication with very low latency, for instance, less than 2 milliseconds. uRLLC can be used for mission-critical communications such as for autonomous driving and/or remote surgery applications. Massive machine-type communications (mMTC) refers to low cost and low data rate communications associated with wireless connections to everyday objects, such as those associated with Internet of Things (IoT) applications. 
     The communication network  10  of  FIG.  1    can be used to support a wide variety of advanced communication features, including, but not limited to, eMBB, uRLLC, and/or mMTC. 
       FIG.  2 A  is a schematic diagram of one example of a communication link using carrier aggregation. Carrier aggregation can be used to widen bandwidth of the communication link by supporting communications over multiple frequency carriers, thereby increasing user data rates and enhancing network capacity by utilizing fragmented spectrum allocations. 
     In the illustrated example, the communication link is provided between a base station  21  and a mobile device  22 . As shown in  FIG.  2 A , the communications link includes a downlink channel used for RF communications from the base station  21  to the mobile device  22 , and an uplink channel used for RF communications from the mobile device  22  to the base station  21 . 
     Although  FIG.  2 A  illustrates carrier aggregation in the context of FDD communications, carrier aggregation can also be used for TDD communications. 
     In certain implementations, a communication link can provide asymmetrical data rates for a downlink channel and an uplink channel. For example, a communication link can be used to support a relatively high downlink data rate to enable high speed streaming of multimedia content to a mobile device, while providing a relatively slower data rate for uploading data from the mobile device to the cloud. 
     In the illustrated example, the base station  21  and the mobile device  22  communicate via carrier aggregation, which can be used to selectively increase bandwidth of the communication link. Carrier aggregation includes contiguous aggregation, in which contiguous carriers within the same operating frequency band are aggregated. Carrier aggregation can also be non-contiguous, and can include carriers separated in frequency within a common band or in different bands. 
     In the example shown in  FIG.  2 A , the uplink channel includes three aggregated component carriers f UL1 , f UL2 , and f UL3 . Additionally, the downlink channel includes five aggregated component carriers f DL1 , f DL2 , f DL3 , f DL4 , and f DL5 . Although one example of component carrier aggregation is shown, more or fewer carriers can be aggregated for uplink and/or downlink. Moreover, a number of aggregated carriers can be varied over time to achieve desired uplink and downlink data rates. 
     For example, a number of aggregated carriers for uplink and/or downlink communications with respect to a particular mobile device can change over time. For example, the number of aggregated carriers can change as the device moves through the communication network and/or as network usage changes over time. 
       FIG.  2 B  illustrates various examples of uplink carrier aggregation for the communication link of  FIG.  2 A .  FIG.  2 B  includes a first carrier aggregation scenario  31 , a second carrier aggregation scenario  32 , and a third carrier aggregation scenario  33 , which schematically depict three types of carrier aggregation. 
     The carrier aggregation scenarios  31 - 33  illustrate different spectrum allocations for a first component carrier f UL1 , a second component carrier f UL2 , and a third component carrier f UL3 . Although  FIG.  2 B  is illustrated in the context of aggregating three component carriers, carrier aggregation can be used to aggregate more or fewer carriers. Moreover, although illustrated in the context of uplink, the aggregation scenarios are also applicable to downlink. 
     The first carrier aggregation scenario  31  illustrates intra-band contiguous carrier aggregation, in which component carriers that are adjacent in frequency and in a common frequency band are aggregated. For example, the first carrier aggregation scenario  31  depicts aggregation of component carriers f UL1 , f UL2 , and f UL3  that are contiguous and located within a first frequency band BAND 1 . 
     With continuing reference to  FIG.  2 B , the second carrier aggregation scenario  32  illustrates intra-band non-continuous carrier aggregation, in which two or more components carriers that are non-adjacent in frequency and within a common frequency band are aggregated. For example, the second carrier aggregation scenario  32  depicts aggregation of component carriers f UL1 , f UL2 , and f UL3  that are non-contiguous, but located within a first frequency band BAND 1 . 
     The third carrier aggregation scenario  33  illustrates inter-band non-contiguous carrier aggregation, in which component carriers that are non-adjacent in frequency and in multiple frequency bands are aggregated. For example, the third carrier aggregation scenario  33  depicts aggregation of component carriers f UL1  and f UL2  of a first frequency band BAND 1  with component carrier f UL3 of a second frequency band BAND 2 . 
       FIG.  2 C  illustrates various examples of downlink carrier aggregation for the communication link of  FIG.  2 A . The examples depict various carrier aggregation scenarios  34 - 38  for different spectrum allocations of a first component carrier f DL1 , a second component carrier f DL2 , a third component carrier f DL3 , a fourth component carrier f DL4 , and a fifth component carrier f DL5 . Although  FIG.  2 C  is illustrated in the context of aggregating five component carriers, carrier aggregation can be used to aggregate more or fewer carriers. Moreover, although illustrated in the context of downlink, the aggregation scenarios are also applicable to uplink. 
     The first carrier aggregation scenario  34  depicts aggregation of component carriers that are contiguous and located within the same frequency band. Additionally, the second carrier aggregation scenario  35  and the third carrier aggregation scenario  36  illustrates two examples of aggregation that are non-contiguous, but located within the same frequency band. Furthermore, the fourth carrier aggregation scenario  37  and the fifth carrier aggregation scenario  38  illustrates two examples of aggregation in which component carriers that are non-adjacent in frequency and in multiple frequency bands are aggregated. As a number of aggregated component carriers increases, a complexity of possible carrier aggregation scenarios also increases. 
     With reference to  FIGS.  2 A- 2 C , the individual component carriers used in carrier aggregation can be of a variety of frequencies, including, for example, frequency carriers in the same band or in multiple bands. Additionally, carrier aggregation is applicable to implementations in which the individual component carriers are of about the same bandwidth as well as to implementations in which the individual component carriers have different bandwidths. 
     Certain communication networks allocate a particular user device with a primary component carrier (PCC) or anchor carrier for uplink and a PCC for downlink. Additionally, when the mobile device communicates using a single frequency carrier for uplink or downlink, the user device communicates using the PCC. To enhance bandwidth for uplink communications, the uplink PCC can be aggregated with one or more uplink secondary component carriers (SCCs). Additionally, to enhance bandwidth for downlink communications, the downlink PCC can be aggregated with one or more downlink SCCs. 
     In certain implementations, a communication network provides a network cell for each component carrier. Additionally, a primary cell can operate using a PCC, while a secondary cell can operate using a SCC. The primary and second cells may have different coverage areas, for instance, due to differences in frequencies of carriers and/or network environment. 
     License assisted access (LAA) refers to downlink carrier aggregation in which a licensed frequency carrier associated with a mobile operator is aggregated with a frequency carrier in unlicensed spectrum, such as WiFi. LAA employs a downlink PCC in the licensed spectrum that carries control and signaling information associated with the communication link, while unlicensed spectrum is aggregated for wider downlink bandwidth when available. LAA can operate with dynamic adjustment of secondary carriers to avoid WiFi users and/or to coexist with WiFi users. Enhanced license assisted access (eLAA) refers to an evolution of LAA that aggregates licensed and unlicensed spectrum for both downlink and uplink. 
     Examples of MLS Envelope Tracking Systems with Soft Switching 
     Envelope tracking (ET) is a technique that can be used to increase power added efficiency (PAE) of a power amplifier by efficiently controlling a voltage level of a power amplifier supply voltage in relation to an envelope of a radio frequency (RF) signal amplified by the power amplifier. Thus, when the envelope of the RF signal increases, the voltage supplied to the power amplifier can be increased. Likewise, when the envelope of the RF signal decreases, the voltage supplied to the power amplifier can be decreased to reduce power consumption. 
     Envelope tracking systems for power amplifiers are provided herein. In certain embodiments, an envelope tracker is provided for a power amplifier that amplifies an RF signal. The envelope tracker includes a multi-level switching circuit having an output that provides an output current that changes in relation to an envelope signal indicating an envelope of the RF signal when the envelope tracker is operating in an envelope tracking mode. The multi-level switching circuit includes a multi-level supply (MLS) modulator that receives multiple regulated voltages of different voltage levels, and an MLS control circuit that controls the selection of the MLS modulator over time based on the envelope signal. When transitioning the MLS modulator from selection of one regulated voltage level to another regulated voltage level, the MLS control circuit provides a soft transition to gradually switch the regulated voltage levels. 
     By implementing the envelope tracker in this manner, reduced overshoot is provided. Moreover, lower quantization noise, relaxed output supply voltage filter constraints, and/or reduced power amplifier output noise can be achieved. Furthermore, the envelope tracker can be smoothly transitioned from an envelope tracking mode to another mode (for instance, an average power tracking (APT) mode or a fixed voltage mode) with little to no overshoot or ringing. 
     In certain implementations, the MLS modulator includes multiple paths associated with different resistance levels between the output of the MLS modulator and a particular one of the regulated voltages. Additionally, the soft transition is provided by changing selection amongst the paths of different resistances to provide a gradual transition or soft switching. In certain implementations, the MLS modulator operates with periods of overlap in which two different regulated voltages are both connected to the output of the MLS modulator through resistive paths. 
     Thus, in certain implementations, the MLS control circuit processes the envelope signal to open or close particular switches of the MLS modulator to thereby provide a soft transition when the MLS modulator is transitioned from selection of one regulated voltage level to another regulated voltage level. 
       FIG.  3 A  is a schematic diagram of one embodiment of an envelope tracking system  80  for a power amplifier  51 . The envelope tracking system  80  includes a multi-level switching circuit  68  that provides an MLS output current I MLS_OUT  to a supply voltage filter  53 , which in turn provides a power amplifier supply voltage V CC_PA  to the power amplifier  51 . 
     As shown in  FIG.  3 A , the power amplifier  51  amplifies an RF input signal R IN  to generate an RF output signal R OUT . Although depicted as including two stages, the power amplifier  51  can include more or fewer stages. 
     In the illustrated embodiment, the multi-level switching circuit  68  is powered by a battery voltage V BATT . Additionally, the multi-level switching circuit  68  receives an envelope signal ENV, which changes in relation to an envelope of the RF input signal RF IN . The multi-level switching circuit  68  processes the envelope signal ENV to generate the output current I MLS_OUT . The multi-level switching circuit  68  includes an MLS DC-to-DC converter  71 , an MLS modulator  72 , and a control circuit  74 . 
     As shown in  FIG.  3 A , the MLS DC-to-DC converter  71  receives a battery voltage V BATT  and generates multiple regulated voltages, corresponding to a first regulated voltage V MLS1 , a second regulated voltage V MLS2 , and a third regulated voltage V MLS3 , in this example. Although an example with the MLS DC-to-DC converter  71  generating three regulated voltages is shown, the MLS DC-to-DC converter  71  can generate more or fewer regulated voltages. The MLS DC-to-DC converter  71  can be implemented in a wide variety of ways, including, but not limited to, as a buck-boost converter operable to generate one or more buck voltages below the battery voltage V BATT  and/or one or more boost voltages above the battery voltage V BATT . 
     With continuing reference to  FIG.  3 A , the control circuit  74  processes an envelope signal ENV to generate control signals for the MLS modulator  72 . When operating in an envelope tracking mode, the MLS modulator  72  processes the control signals and the MLS regulated voltages to generate the output current I MLS_OUT  to change in relation to or track the envelope signal ENV. In certain implementations, the MLS modulator  72  includes a bank of switches that are selectively activated using the control signals from the control circuit  74 . In certain implementations, the multi-level switching circuit  68  is a multi-mode circuit that can provide not only the envelope tracking mode, but other mode(s) as well, such as an APT mode and/or fixed voltage mode. 
     When transitioning from selection of one of the MLS regulated voltages to another, the control circuit  74  provides a soft or gradual transition in accordance with the teachings herein. By providing a soft transition, a number of advantages are provided, including, but not limited to, less ringing of the output current I MLS_OUT , reduced quantization noise of the multi-level switching circuit  68 , and/or relaxed design constraints of the supply voltage filter  53 . 
       FIG.  3 B  is a schematic diagram of another embodiment of an envelope tracking system  150  for a power amplifier  51 . The envelope tracking system  150  includes a multi-level switching circuit  138  that provides an MLS output current I MLS_OUT  to a first supply voltage filter  53  and a DC voltage V DC  to a second supply voltage filter  54 . Additionally, the outputs of the first supply voltage filter  53  and the second supply voltage filter  54  are combined to generate a power amplifier supply voltage V CC_PA  to the power amplifier  51 . 
     By implementing the envelope tracking system  150  in this manner, enhanced efficiency can be achieved. 
     As shown in  FIG.  3 B , the multi-level switching circuit  138  an MLS DC-to-DC converter  141 , an MLS modulator  72 , and a control circuit  74 . In comparison to the MLS DC-to-DC converter  71  of  FIG.  3 A , the MLS DC-to-DC converter  141  generates not only the first regulated voltage V MLS1 , the second regulated voltage V MLS2 , and the third regulated voltage V MLS3 , but also the DC voltage V DC  that is provided to the second supply voltage filter  54 . 
     Thus, in the illustrated embodiment the regulated DC voltage V DC  is adjusted by the output current I MLS_OUT  to generate a power amplifier supply voltage V CC_PA  for the power amplifier  51 . 
       FIG.  4 A  is a schematic diagram of one example of an MLS modulator  220  that operates without soft switching. The MLS modulator  220  generates an output current at an output MLS_OUT. The output current is provided to a supply voltage filter  211 , which in turn generates a power amplifier supply voltage V CC_PA  for a power amplifier  212 . As shown in  FIG.  4 A , the power amplifier  212  amplifies an RF input signal RF_IN to generate an RF output signal RF_OUT. 
     The MLS modulator  220  includes switches  201 , . . .  205 ,  206  are connected between the output MLS_OUT and regulated DC voltages DC_ 1 , . . . DC_ 5 , DC_ 6 , respectively. Circuitry for generating the regulated DC voltages DC_ 1 , . . . DC_ 5 , DC_ 6  is not shown in  FIG.  4 A . Although depicted as including three switches, the MLS modulator  220  can include more or fewer switches as indicated by the ellipses. 
     The MLS modulator  220  is controlled by an MLS control circuit  210  that generates switch controls signals Sw 1 , . . . Sw 5 , Sw 6  for controlling switches  201 , . . .  205 ,  206 , respectively. The MLS control circuit  210  changes selection of the switches over time based on an envelope signal ENVELOPE. 
       FIG.  4 B  is one example of a timing diagram for the MLS modulator  220  of  FIG.  4 A . As shown in  FIG.  4 B , when transitioning from selection of regulated DC voltage DC_ 6  to regulated DC voltage DC_ 5 , the MLS control circuit  210  provides a period of non-overlap in which both the switch  205  and the switch  206  are opened. Opening the switches in this manner prevents shoot through current from the regulated DC voltage DC_ 6  to the regulated DC voltage DC_ 5  (which are at different voltage potentials), but also causes current ringing and/or noise. 
       FIG.  5 A  is a schematic diagram of one embodiment of an MLS modulator  230  that operates with soft switching. 
     As shown in  FIG.  5 A , the MLS modulator  230  includes multiple electrical paths between an output MLS_OUT and a corresponding regulated voltage. Additionally, each of the electrical paths is associated with different resistance to facilitate an MLS control circuit to control the MLS modulator  230  to provide a gradual transition from selecting one of regulated DC voltage to another regulated DC voltage. 
     For example, with respect to the electrical paths between the output MLS OUT and the regulated DC voltage DC_ 1 , the MLS modulator  230  includes a first path through switch  201   a , a second path through switch  201   b  and resistor  221   b , a third path through switch  201   c  and resistors  221   c   1  and  221   c   2 , and an nth path through switch  201   n  and resistors  221   n   1 ,  221   n   2 , . . .  221 nm. Each of the n paths has different resistances, and n can be any desired value desired for the MLS modulator  230 . 
     As shown in  FIG.  5 A , the switches  201   a ,  201   b ,  201   c , . . .  201   n  are controlled by switch control signals Sw 1 _ a , Sw 1 _ b , Sw 1 _ c , Sw 1 _ n , respectively. Thus, each of the switches  201   a ,  201   b ,  201   c , . . .  201   n  are separately controlled by an MLS control circuit to provide flexibility in gradually changing the resistance between the output MLS_OUT and the regulated voltage DC_ 1 . 
     With continuing reference to  FIG.  5 A , with respect to the electrical paths between the output MLS_OUT and the regulated DC voltage DC_ 5 , the MLS modulator  220  includes a first path through switch  205   a , a second path through switch  205   b  and resistor  225   b , a third path through switch  205   c  and resistors  225   c   1  and  225   c   2 , and an nth path through switch  205   n  and resistors  225   n   1 ,  225   n   2 , . . .  225 nm. The switches  205   a ,  205   b ,  205   c , . . .  205   n  are controlled by switch control signals Sw 5 _a, Sw 5 _b, Sw 5 _c, . . . Sw 5 _n, respectively. 
     Furthermore, with respect to the electrical paths between the output MLS_OUT and the regulated DC voltage DC_ 6 , the MLS modulator  220  includes a first path through switch  206   a , a second path through switch  206   b  and resistor  226   b , a third path through switch  206   c  and resistors  226   c   1  and  226   c   2 , and an nth path through switch  206   n  and resistors  226   n   1 ,  22562 , . . .  226 nm. The switches  205   a ,  205   b ,  205   c , . . .  205   n  are controlled by switch control signals Sw 6 _ a , Sw 6 _ b , Sw 6 _ c , Sw 6 _ n , respectively. 
     Although the MLS modulator  230  depicts an example in which each bank of switches between a given regulated voltage and the output MLS_OUT has the same configuration, the teachings herein are also applicable to implementations in which the switch banks vary from one regulated voltage to another. Thus, the switch banks can be asymmetric. 
       FIG.  5 B  is one example of a timing diagram for the MLS modulator  230  of  FIG.  5 A . As shown in  FIG.  5 B , timing for the switch control signals Sw 6 _ a , Sw 6 _ b , Sw 6 _ c , Sw 6 _ n  is depicted. Additionally, the MLS modulator  230  begins in a first state in which the output MLS_OUT is disconnected from the regulated voltage DC_ 6 , and is gradually transitioned (by way of modulating resistance) to a second state in which all the electrical paths between the regulated voltage DC_ 6  and the output MLS_OUT are turned on. Thereafter, the electrical paths are deactivated in sequence to return the MLS modulator  230  to the first state. 
     Although one example of timing is depicted, the MLS modulator  230  can be controlled in a wide variety of ways. 
       FIG.  5 C  is another example of a timing diagram for the MLS modulator  230  of  FIG.  5 A . The timing diagram depicts timing of switch control signal Sw 6 _ n  and switch control signal Sw 5 _ n.    
     As shown in  FIG.  5 C , when transitioning from selection of the regulated voltage DC_ 6  to the regulated voltage DC_ 5 , the MLS modulator  230  operates with a period of overlap in which both the regulated voltage DC_ 6  to the regulated voltage DC_ 5  are connected to the output MLS OUT by way of resistive paths since both the switch  206   n  and the switch  205   n  are turned on. 
       FIG.  6    is a schematic diagram of one embodiment of a control circuit  320  for an MLS modulator. The control circuit  320  includes delay elements  301 ,  302 ,  303 ,  304 ,  305 , . . .  306  and logical AND gates  316   a ,  316   b , . . .  316   n.    
     The first delay element receives an input signal SW 6 _in, and the logical AND gates  316   a ,  316   b , . . .  316   n  perform AND operations of delayed versions of the input signal SW 6 _in to generate switch control signals Sw 6 _ a , Sw 6 _ b , . . . Sw 6 _ n , respectively. 
     Although one example of a control circuit is shown, the teachings herein are applicable to MLS control circuitry implemented in a wide variety of ways. 
       FIG.  7    is a schematic diagram of another embodiment of a control circuit  340  for an MLS modulator. The control circuit  340  includes a shift register  331 , a switch matrix  332 , and logical AND gates  316   a ,  316   b , . . .  316   n.    
     The shift register  331  processes a clock signal CLK and an input signal SW 6 _in to generate multiple delayed versions of the input signal SW 6 _in that are provided to the switch matrix  332 . Additionally, the switch matrix  332  is configurable to provide desired delayed versions of the input signal SW 6 _in (with or without inversions) to each of the logical AND gates  316   a ,  316   b , . . .  316   n.    
     Thus, enhanced flexibility for generating the switch control signals Sw 6 _ a , Sw 6 _ b , Sw 6 _ c , Sw 6 _ n  is provided. 
       FIG.  8    is a schematic diagram of another embodiment of a control circuit  350  for an MLS modulator. The control circuit  350  includes logical AND gates  316   a ,  316   b , . . .  316   n  for generating the switch control signals Sw 6 _ a , Sw 6 _ b , Sw 6 _ c , Sw 6 _ n , respectively. As shown in  FIG.  8   , the logical AND gates  316   a ,  316   b , . . .  316   n  are associated with different resistances to provide delay between the switch control signals Sw 6 _ a , Sw 6 _ b , Sw 6 _ c , Sw 6 _ n.    
       FIG.  9    is a graph of one example of simulation results for an MLS modulator in accordance with one embodiment. The simulation depicts an example of a power amplifier supply voltage generated by an MLS modulator in response to an envelope signal. 
     Although one example of simulation results are depicted, simulation results can vary based on a number of factors, including, but not limited to, simulation parameters, circuit models, simulation tools, circuit topology, and/or a wide variety of other factors. 
       FIG.  10    is a schematic diagram of one embodiment of a mobile device  800 . The mobile device  800  includes a baseband system  801 , a transceiver  802 , a front end system  803 , antennas  804 , a power management system  805 , a memory  806 , a user interface  807 , and a battery  808 . 
     The mobile device  800  can be used communicate using a wide variety of communications technologies, including, but not limited to, 2G, 3G, 4G (including LTE, LTE-Advanced, and LTE-Advanced Pro), 5G NR, WLAN (for instance, WiFi), WPAN (for instance, Bluetooth and ZigBee), WMAN (for instance, WiMax), and/or GPS technologies. 
     The transceiver  802  generates RF signals for transmission and processes incoming RF signals received from the antennas  804 . It will be understood that various functionalities associated with the transmission and receiving of RF signals can be achieved by one or more components that are collectively represented in  FIG.  10    as the transceiver  802 . In one example, separate components (for instance, separate circuits or dies) can be provided for handling certain types of RF signals. 
     The front end system  803  aids is conditioning signals transmitted to and/or received from the antennas  804 . In the illustrated embodiment, the front end system  803  includes antenna tuning circuitry  810 , power amplifiers (PAs)  811 , low noise amplifiers (LNAs)  812 , filters  813 , switches  814 , and signal splitting/combining circuitry  815 . However, other implementations are possible. 
     For example, the front end system  803  can provide a number of functionalities, including, but not limited to, amplifying signals for transmission, amplifying received signals, filtering signals, switching between different bands, switching between different power modes, switching between transmission and receiving modes, duplexing of signals, multiplexing of signals (for instance, diplexing or triplexing), or some combination thereof. 
     In certain implementations, the mobile device  800  supports carrier aggregation, thereby providing flexibility to increase peak data rates. Carrier aggregation can be used for both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), and may be used to aggregate a plurality of carriers or channels. Carrier aggregation includes contiguous aggregation, in which contiguous carriers within the same operating frequency band are aggregated. Carrier aggregation can also be non-contiguous, and can include carriers separated in frequency within a common band or in different bands. 
     The antennas  804  can include antennas used for a wide variety of types of communications. For example, the antennas  804  can include antennas for transmitting and/or receiving signals associated with a wide variety of frequencies and communications standards. 
     In certain implementations, the antennas  804  support MIMO communications and/or switched diversity communications. For example, MIMO communications use multiple antennas for communicating multiple data streams over a single radio frequency channel. MIMO communications benefit from higher signal to noise ratio, improved coding, and/or reduced signal interference due to spatial multiplexing differences of the radio environment. Switched diversity refers to communications in which a particular antenna is selected for operation at a particular time. For example, a switch can be used to select a particular antenna from a group of antennas based on a variety of factors, such as an observed bit error rate and/or a signal strength indicator. 
     The mobile device  800  can operate with beamforming in certain implementations. For example, the front end system  803  can include amplifiers having controllable gain and phase shifters having controllable phase to provide beam formation and directivity for transmission and/or reception of signals using the antennas  804 . For example, in the context of signal transmission, the amplitude and phases of the transmit signals provided to the antennas  804  are controlled such that radiated signals from the antennas  804  combine using constructive and destructive interference to generate an aggregate transmit signal exhibiting beam-like qualities with more signal strength propagating in a given direction. In the context of signal reception, the amplitude and phases are controlled such that more signal energy is received when the signal is arriving to the antennas  804  from a particular direction. In certain implementations, the antennas  804  include one or more arrays of antenna elements to enhance beamforming. 
     The baseband system  801  is coupled to the user interface  807  to facilitate processing of various user input and output (I/O), such as voice and data. The baseband system  801  provides the transceiver  802  with digital representations of transmit signals, which the transceiver  802  processes to generate RF signals for transmission. The baseband system  801  also processes digital representations of received signals provided by the transceiver  802 . As shown in  FIG.  10   , the baseband system  801  is coupled to the memory  806  of facilitate operation of the mobile device  800 . 
     The memory  806  can be used for a wide variety of purposes, such as storing data and/or instructions to facilitate the operation of the mobile device  800  and/or to provide storage of user information. 
     The power management system  805  provides a number of power management functions of the mobile device  800 . In certain implementations, the power management system  805  includes a PA supply control circuit that controls the supply voltages of the power amplifiers  811 . For example, the power management system  805  can be configured to change the supply voltage(s) provided to one or more of the power amplifiers  811  to improve efficiency, such as power added efficiency (PAE). 
     As shown in  FIG.  10   , the power management system  805  receives a battery voltage from the battery  808 . The battery  808  can be any suitable battery for use in the mobile device  800 , including, for example, a lithium-ion battery. 
       FIG.  11    is a schematic diagram of one embodiment of a communication system  1130  for transmitting RF signals. The communication system  1130  includes a battery  1101 , an envelope tracker  1102 , a baseband processor  1107 , a signal delay circuit  1108 , a digital pre-distortion (DPD) circuit  1109 , an I/Q modulator  1110 , an observation receiver  1111 , an intermodulation detection circuit  1112 , a power amplifier  1113 , a directional coupler  1114 , a duplexing and switching circuit  1115 , an antenna  1116 , an envelope delay circuit  1121 , a coordinate rotation digital computation (CORDIC) circuit  1122 , a shaping circuit  1123 , a digital-to-analog converter  1124 , and a reconstruction filter  1125 . 
     The communication system  1130  of  FIG.  11    illustrates one example of an RF system operating with a power amplifier supply voltage controlled using envelope tracking. However, envelope tracking systems can be implemented in a wide variety of ways. 
     The baseband processor  1107  operates to generate an I signal and a Q signal, which correspond to signal components of a sinusoidal wave or signal of a desired amplitude, frequency, and phase. For example, the I signal can be used to represent an in-phase component of the sinusoidal wave and the Q signal can be used to represent a quadrature-phase component of the sinusoidal wave, which can be an equivalent representation of the sinusoidal wave. In certain implementations, the I and Q signals are provided to the I/Q modulator  1110  in a digital format. The baseband processor  1107  can be any suitable processor configured to process a baseband signal. For instance, the baseband processor  1107  can include a digital signal processor, a microprocessor, a programmable core, or any combination thereof. 
     The signal delay circuit  1108  provides adjustable delay to the I and Q signals to aid in controlling relative alignment between the envelope signal and the RF signal RF IN . The amount of delay provided by the signal delay circuit  1108  is controlled based on amount of intermodulation detected by the intermodulation detection circuit  1112 . 
     The DPD circuit  1109  operates to provide digital shaping to the delayed I and Q signals from the signal delay circuit  1108  to generate digitally pre-distorted I and Q signals. In the illustrated embodiment, the pre-distortion provided by the DPD circuit  1109  is controlled based on amount of intermodulation detected by the intermodulation detection circuit  1112 . The DPD circuit  1109  serves to reduce a distortion of the power amplifier  1113  and/or to increase the efficiency of the power amplifier  1113 . 
     The I/Q modulator  1110  receives the digitally pre-distorted I and Q signals, which are processed to generate an RF signal RF IN . For example, the I/Q modulator  1110  can include DACs configured to convert the digitally pre-distorted I and Q signals into an analog format, mixers for upconverting the analog I and Q signals to radio frequency, and a signal combiner for combining the upconverted I and Q signals into an RF signal suitable for amplification by the power amplifier  1113 . In certain implementations, the I/Q modulator  1110  can include one or more filters configured to filter frequency content of signals processed therein. 
     The envelope delay circuit  1121  delays the I and Q signals from the baseband processor  1107 . Additionally, the CORDIC circuit  1122  processes the delayed I and Q signals to generate a digital envelope signal representing an envelope of the RF signal RF IN . Although  FIG.  11    illustrates an implementation using the CORDIC circuit  1122 , an envelope signal can be obtained in other ways. 
     The shaping circuit  1123  operates to shape the digital envelope signal to enhance the performance of the communication system  1130 . In certain implementations, the shaping circuit  1123  includes a shaping table that maps each level of the digital envelope signal to a corresponding shaped envelope signal level. Envelope shaping can aid in controlling linearity, distortion, and/or efficiency of the power amplifier  1113 . 
     In the illustrated embodiment, the shaped envelope signal is a digital signal that is converted by the DAC  1124  to an analog envelope signal. Additionally, the analog envelope signal is filtered by the reconstruction filter  1125  to generate an envelope signal suitable for use by the envelope tracker  1102 . In certain implementations, the reconstruction filter  1125  includes a low pass filter. 
     With continuing reference to  FIG.  11   , the envelope tracker  1102  receives the envelope signal from the reconstruction filter  1125  and a battery voltage V BATT  from the battery  1101 , and uses the envelope signal to generate a power amplifier supply voltage V CC_PA  for the power amplifier  1113  that changes in relation to the envelope of the RF signal RF IN . The power amplifier  1113  receives the RF signal RF IN  from the I/Q modulator  1110 , and provides an amplified RF signal R OUT  to the antenna  1116  through the duplexing and switching circuit  1115 , in this example. 
     The directional coupler  1114  is positioned between the output of the power amplifier  1113  and the input of the duplexing and switching circuit  1115 , thereby allowing a measurement of output power of the power amplifier  1113  that does not include insertion loss of the duplexing and switching circuit  1115 . The sensed output signal from the directional coupler  1114  is provided to the observation receiver  1111 , which can include mixers for down converting I and Q signal components of the sensed output signal, and DACs for generating I and Q observation signals from the downconverted signals. 
     The intermodulation detection circuit  1112  determines an intermodulation product between the I and Q observation signals and the I and Q signals from the baseband processor  1107 . Additionally, the intermodulation detection circuit  1112  controls the pre-distortion provided by the DPD circuit  1109  and/or a delay of the signal delay circuit  1108  to control relative alignment between the envelope signal and the RF signal RF IN . In certain implementations, the intermodulation detection circuit  1112  also serves to control shaping provided by the shaping circuit  1123 . 
     By including a feedback path from the output of the power amplifier  1113  and baseband, the I and Q signals can be dynamically adjusted to optimize the operation of the communication system  1130 . For example, configuring the communication system  1130  in this manner can aid in providing power control, compensating for transmitter impairments, and/or in performing DPD. 
     Although illustrated as a single stage, the power amplifier  1113  can include one or more stages. Furthermore, the teachings herein are applicable to communication systems including multiple power amplifiers. In such implementations, separate envelope trackers can be provided for different power amplifiers and/or one or more shared envelope trackers can be used. 
       FIG.  12    is a schematic diagram of one example of a power amplifier system  1140  including an envelope tracker  1102 . The illustrated power amplifier system  1140  further includes an inductor  1127 , an output impedance matching circuit  1131 , and a power amplifier  1132 . The illustrated envelope tracker  1102  receives a battery voltage V BATT  and an envelope of the RF signal and generates a power amplifier supply voltage V CC_PA  for the power amplifier  1132 . 
     The illustrated power amplifier  1132  includes a bipolar transistor  1129  having an emitter, a base, and a collector. As shown in  FIG.  12   , the emitter of the bipolar transistor  1129  is electrically connected to a power low supply voltage V 1 , which can be, for example, a ground supply. Additionally, an RF signal (RF IN ) is provided to the base of the bipolar transistor  1129 , and the bipolar transistor  1129  amplifies the RF signal to generate an amplified RF signal at the collector. The bipolar transistor  1129  can be any suitable device. In one implementation, the bipolar transistor  1129  is a heterojunction bipolar transistor (HBT). 
     The output impedance matching circuit  1131  serves to terminate the output of the power amplifier  1132 , which can aid in increasing power transfer and/or reducing reflections of the amplified RF signal generated by the power amplifier  1132 . In certain implementations, the output impedance matching circuit  1131  further operates to provide harmonic termination and/or to control a load line impedance of the power amplifier  1132 . 
     The inductor  1127  can be included to provide the power amplifier  1132  with the power amplifier supply voltage V CC_PA  generated by the envelope tracker  1102  while choking or blocking high frequency RF signal components. The inductor  1127  can include a first end electrically connected to the envelope tracker  1102 , and a second end electrically connected to the collector of the bipolar transistor  1129 . In certain implementations, the inductor  1127  operates in combination with the impedance matching circuit  1131  to provide output matching. 
     Although  FIG.  12    illustrates one implementation of the power amplifier  1132 , skilled artisans will appreciate that the teachings described herein can be applied to a variety of power amplifier structures, such as multi-stage power amplifiers and power amplifiers employing other transistor structures. For example, in some implementations the bipolar transistor  1129  can be omitted in favor of employing a field-effect transistor (FET), such as a silicon FET, a gallium arsenide (GaAs) high electron mobility transistor (HEMT), or a laterally diffused metal oxide semiconductor (LDMOS) transistor. Additionally, the power amplifier  1132  can be adapted to include additional circuitry, such as biasing circuitry. 
       FIGS.  13 A and  13 B  show two examples of power amplifier supply voltage versus time. 
     In  FIG.  13 A , a graph  1147  illustrates one example of the voltage of an RF signal  1141  and a power amplifier supply voltage  1143  versus time. The RF signal  1141  has an envelope  1142 . 
     It can be important that the power amplifier supply voltage  1143  of a power amplifier has a voltage greater than that of the RF signal  1141 . For example, powering a power amplifier using a power amplifier supply voltage that has a magnitude less than that of the RF signal can clip the RF signal, thereby creating signal distortion and/or other problems. Thus, it can be important the power amplifier supply voltage  1143  be greater than that of the envelope  1142 . However, it can be desirable to reduce a difference in voltage between the power amplifier supply voltage  1143  and the envelope  1142  of the RF signal  1141 , as the area between the power amplifier supply voltage  1143  and the envelope  1142  can represent lost energy, which can reduce battery life and increase heat generated in a wireless device. 
     In  FIG.  13 B , a graph  1148  illustrates another example of the voltage of an RF signal  1141  and a power amplifier supply voltage  1144  versus time. In contrast to the power amplifier supply voltage  1143  of  FIG.  13 A , the power amplifier supply voltage  1144  of  FIG.  13 B  changes in relation to the envelope  1142  of the RF signal  1141 . The area between the power amplifier supply voltage  1144  and the envelope  1142  in  FIG.  13 B  is less than the area between the power amplifier supply voltage  1143  and the envelope  1142  in  FIG.  13 A , and thus the graph  1148  of  FIG.  13 B  can be associated with a power amplifier system having greater energy efficiency. 
     Conclusion 
     Some of the embodiments described above have provided examples in connection with mobile devices. However, the principles and advantages of the embodiments can be used for any other systems or apparatus that have needs for envelope tracking. 
     Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Likewise, the word “connected”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
     Moreover, conditional language used herein, such as, among others, “may,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. 
     The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. 
     The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
     While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.