SYSTEM

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to US Provisional Patent Application Number 62/828,771 filed on April 3, 2019, entitled“CLOSED LOOP EFFICIENCY TRACKING IN DIGITAL ENVELOPE TRACKING SYSTEM,” which is incorporated herein by reference for all purposes.

BACKGROUND

[0002] Envelope tracking is a technique by which the bias or supply voltage (e.g., Vcc) and current of a power amplifier (PA) in a transmit chain is controlled based on the radio frequency (RF) signal envelope of the transmit signal being amplified by the power amplifier. The idea is to operate the power amplifier close to or slightly in compression and to lower the PA supply voltage when the instantaneous signal amplitude is low, thereby boosting the efficiency of the power amplifier and its supply generation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] FIG. 1 is a block diagram of an exemplary transmitter architecture that includes a digital envelope tracking system for a power amplifier, in accordance with various aspects described.

[0004] FIG. 1 A illustrates an exemplary mapping of PA supply voltages to an envelope of an RF transmit signal as performed by the system of FIG. 1 .

[0005] FIG. 2 is a block diagram of an exemplary digital envelope tracking system that includes amplifier efficiency tracking circuitry, in accordance with various aspects described.

[0006] FIG. 3 is plot illustrating PA gain and efficiency contours as a function of supply voltage and RF envelope for an example power amplifier. [0007] FIG. 4 is a block diagram of an exemplary supply voltage circuitry, in accordance with various aspects described.

[0008] FIG. 5 is a flow diagram outlining an exemplary method for performing envelope tracking, in accordance with various aspects described.

[0009] FIG. 6 is a block diagram of an exemplary device configured to perform envelope tracking, in accordance with various aspects described.

DETAILED DESCRIPTION

[00010] Some transmitters that employ envelope tracking techniques generate the supply voltage for the power amplifier using an analog control loop. Within the loop, the power amplifier supply voltage is sensed, compared to a target voltage that tracks the envelope of the signal being amplified, and the difference is used to steer a continuous actuator such as an amplifier to correct the power amplifier supply voltage. This analog- based envelope tracking solution suffers from several problems. For example, the realization of the analog control loop becomes difficult for increasing envelope signal bandwidth while maintaining reasonable system efficiency. Further, the alternating current (AC) signal path used to generate and control the supply voltage to be equal to the target voltage and the direct current (DC) signal path used to determine the target voltage are normally separated into two supply chains, which yields an unattractively large solution area on the printed circuit board (PCB). The analog control loop for supply voltage control is feasible for 2x or possibly 3x carrier aggregation in the cellular context. For higher levels of carrier aggregation in cellular applications and for

WLAN/WiFi applications, the analog control loop solution does not scale.

[00011] Throughout this description, components that are exemplary versions of a same or analogous component are assigned reference characters having the same value for the last two digits while the initial digit(s) of reference characters are assigned based on the FIG. number in which they are first introduced.

[00012] FIG. 1 illustrates an exemplary transmitter architecture 100 that includes a transmitter chain 1 10 and an exemplary envelope tracking system 140. The transmitter chain 1 10 processes a digital baseband transmit signal to generate an RF transmit signal. The RF transmit signal is amplified by a PA to generate an uplink signal that is transmitted by an antenna or cable (not shown). The exemplary transmit chain 1 10 includes transmit digital processing circuitry 120, which operates on a digital baseband transmit signal to convert the signal into amplitude and phase components. The amplitude and phase components are converted into the analog RF transmit signal by transmit analog processing circuitry 130. The transmit digital processing circuitry 120 may also include digital pre-distortion circuitry (DPD) (see FIG. 2) that operates to pre distort the transmit data to account for non-linearities in the analog processing circuitry 130 and the PA.

[00013] The envelope tracking system 140 includes envelope circuitry 150 to generate a target voltage signal, which is used to control a supply voltage circuitry 160 to supply a selected supply voltage to the PA. The envelope circuitry 150 samples the baseband transmit signal to project an envelope of the RF transmit signal that will be amplified by the PA to generate the uplink signal. FIG. 1 A illustrates an exemplary RF transmit signal and a projected envelope that bounds the RF transmit signal. The envelope circuitry 150 determines the envelope of the RF transmit signal and generates the target voltage signal to control the supply voltage circuitry 160 to provide a PA supply voltage that closely matches the envelope.

[00014] In one example, the target voltage signal generated by the envelope circuitry includes voltage domain information that may be a control word or voltage that communicates the desired supply voltage or a selection setting from the plurality of voltage levels to the supply voltage circuitry 160. In addition, the target voltage signal may include time domain information that communicates a time during which the desired supply voltage should be provided to the PA. For example, the target voltage signal may specify voltage domain information Vs2 and time domain information stn to cause the supply voltage circuitry 160 to change the PA supply voltage from Vsi to Vs2 at the switching time stn as shown in FIG. 2. The time domain information may also include a duration of time until a next voltage level and switching time will be

communicated. [00015] In order to reduce noise, the envelope circuitry 150 may determine a switching time that will coincide with a relatively low RF transmit signal (e.g., when the RF transmit signal is crossing the frequency axis as shown in FIG. 2). In one example, “relatively low” means that the RF transmit signal is lower or equal to a predetermined threshold. In an alternate implementation the envelope circuitry 150 may choose a switching time when the instantaneous envelope signal is low, (i.e., when the

instantaneous signal power is low). In this case the next selected voltage is an upper bound of all instantaneous target voltages which occur ^' until another low phase is reached. At the next low phase another voltage is selected and so on. Thus the switching time may be selected based on either a zero crossing of the RF signal or a close to zero condition of the envelope signal.

[00016] The supply voltage circuitry 160 includes voltage generation circuitry 170 and a supply modulator 1 90. The principle of the supply voltage circuitry 1 60 is to replace the analog control loop of prior envelope tracking systems and instead generate the PA supply voltage in a feed-forward manner. The voltage generation circuitry 170 is an analog circuit that generates a regulated output voltage from the battery voltage VBAT. The voltage generation circuitry 170 includes a voltage splitter circuitry 1 80, which may be a switched capacitor network or charge pump, that generates a plurality of output voltages {e.g., Vsi , ..., Vs4) having differing levels from the regulated output voltage. The supply modulator 190 is a switching circuit that connects one of these output voltages to the output of the supply voltage circuitry 160. The output of the supply voltage circuitry 1 60 (i.e., the PA supply voltage) is connected to a supply input (not shown) of the PA.

[00017] It can be seen in FIG. 1 A that the PA supply voltage provided by the supply voltage circuitry 1 60 varies in a stepwise fashion to approximate the envelope. While analog envelope tracking PA power supply solutions may also be able to closely follow the envelope, recall that analog solutions have limited applicability in high frequency applications and present the other drawbacks discussed above. The feed forward digital envelope tracking provides effective envelope tracking in a manner that scales for higher frequencies and presents a small package size. [00018] The voltage supply circuitry 160 is capable of producing any number of voltage levels. To leverage this feature the supply voltage circuitry 160 may receive transmitter operation conditions or parameters that include, for example, a transmit power level, a mode of operation, and or an estimated or measured saturation level of the power amplifier. The supply voltage circuitry 160 may use this information to control or scale the voltage levels that are produced by the supply voltage circuitry 1 60. For example, if the transmit power level is relatively small, the set of voltages for selection may span a smaller range so that the PA supply voltage can more closely follow the envelope or the number of voltages for selection may be reduced to a smaller set. In contrast, if the transmit power level is large, the set of voltages for selection may span a larger range to cover the variation in the envelope. Adaptation of the voltage generation circuitry 170 in response to power amplifier saturation or gain characteristic will be described below.

[00019] It can be seen in FIG. 1 that the there is no control loop in the generation of the PA supply voltage. The output PA supply voltage is delivered from pure voltage sources instead of a regulated stage. Thus the PA supply voltage may be more accurate and less load dependent especially at high frequencies and during fast switching. Fast switching multiplexers or other switches are available to be used for the supply modulator 1 90. This is advantageous because the higher signal bandwidths of modern devices translate into a need for faster switching between the different voltage levels. The fast switching provided by the supply modulator 190 thus fits well to modern, digital dominated technologies. The illustrated supply voltage circuitry 160 separates the analog task of voltage level generation from the control of voltage selection, which is digital.

[00020] FIG. 2 shows an overview of a multi-level-supply envelope tracking system that includes amplifier efficiency tracking circuitry 225. In a transmission path a digital baseband transmit signal 201 is input to transmit digital processing circuitry 220 that includes DPD circuitry 221 . The digital predistortion circuitry 221 operates to take the digital transmit data (e.g., I/Q data) and selectively modify such data to account for non- linearities that exist downstream in the transmit signal path. The output predistorted data is labelled in FIG. 2 as pre-PA signal 204. The predistorted transmit data 204 is then input to transmit analog processing circuitry 230 that operates, for example, to convert the predistorted data 204 into an analog form via a digital to analog converter (DAC) and then upconvert the analog data from baseband to radio frequency to form an RF input signal 206 that is input to the power amplifier 208. The power amplifier 208 amplifies the RF input signal 206 to form an amplified RF output signal 210 that is provided to an antenna (not shown) for transmission.

[00021] As highlighted above, the envelope tracking system 200 provides a tracking supply voltage 212 to the PA 208 to improve system efficiency and thereby reduce power consumption. The tracking supply voltage 212 is provided by supply voltage circuitry 260 that receives a level select signal 214 from envelope circuitry 250. The level select signal 214 is a control signal that the supply voltage circuitry 260 employs to select one of a plurality of available supply voltages having differing voltage levels (e.g., Vsi , ... Vs4 of FIG. 1 ). In one example the supply voltage circuitry 260 operates to provide a programmable mapping of supply lines to the generated plurality of differing voltage levels. The supply voltage circuitry 260 is configured to select one of a plurality of supply voltages based on the level select signal 214. The selected supply voltage 218 is then filtered, in one example, using a supply filter circuit 220 to provide a filtered signal. The filtered signal is provide, as the tracking supply voltage 212, to the PA 208.

[00022] The envelope tracking system also includes envelope circuitry 250 that generates the level select signal 214 (e.g., target voltage in FIG. 1 ) that controls the supply voltage circuitry 260 to select a desired supply voltage 218 out of the N differing supply voltages. In one example, the envelope circuitry 250 selects the supply voltage 218 based on several thresholds of baseband signal amplitude as illustrated in FIG. 1 A.

[00023] Recall that the supply voltage circuitry 260 selects the differing voltage levels for the supply voltages from which the supply voltage is selected. In one example, the differing voltage levels are selected based on operating parameters of the transmitter. The selected voltage levels may be adjusted based on a saturation or gain

characteristic of the PA. The supply voltage circuitry 260 is controlled by the level select signal and provides a programmable mapping of supply lines to the differing voltage levels. A DPD circuitry 221 linearizes the system behavior and outputs a pre- PA signal 204 that is input to transmit analog processing circuitry 230. [00024] The pre-PA signal 204 and the level select signal 214 are input to a saturation detector circuitry 257 of the amplifier efficiency tracking circuitry 255. The saturation detector circuitry 257 is configured to measure or estimate the gain compression per supply level based on the pre-PA signal. During operation, the measured level of gain compression for the present PA supply voltage (as indicated by the level select signal), called“saturation level”, is an input to a controller 258, which, in response, generates an adjustment signal 228a and/or 228b. In one example the adjustment signal 228a tunes the thresholds used by the envelope circuitry 250 to generate the level select signal 214. In another example, the adjustment signal 228a adjusts other aspects of the envelop circuitry instead of or in addition to the thresholds, to cause the envelope circuitry 250 to adjust the level select signal 214 based on the saturation level measured by the saturation detector circuitry 257. In another example, the adjustment signal 228b causes the supply voltage circuitry to shift or adjust the differing voltage levels mapped to the respective supply voltages that are present on each supply line.

[00025] FIG. 3 is a graph that illustrates power amplifier gain compression and efficiency contours 300 versus RF envelope (Ain(Vrf)) and PA supply (Vcc). Three rectangles 302, 304 and 306, representing levels 1 , 2 and 3, respectively, overlay the contours 300 and indicate three discrete supply levels that are deemed to be the“best choice” based on particular selection criteria. For example, if a 2dB gain compression is a target, then a best efficiency is achieved by all three supply levels (e.g., 31 -32% efficiency contours are reached). As an example, if level 3 306 is employed for smaller Ain amplitudes and level 2 304 is selected for higher Ain amplitudes, one obtains a lower efficiency of 31 % instead of 32%, and the gain compression becomes 1 dB instead of 2dB.

[00026] Returning to FIG. 2, by measuring or otherwise determining the gain compression on each supply level and generating an adjustment signal 228a that adjusts thresholds within the level select circuit 220 that modifies when the next higher supply level is used, performance in terms of efficiency is improved. Alternatively, via signal 2228b the differing voltage levels mapped to the supply lines can be modified by the supply voltage circuitry 260. In one example, both 228a and 228b are used to adjust the supply voltage 218 based on a measured or estimated efficiency of the power amplifier. Such use of gain compression to tune the envelope circuitry 250 is particularly advantageous under changing operating conditions such as over variation in temperature or antenna mismatch, or other conditions.

[00027] In one example the saturation detector circuitry 257 comprises or accesses storage media storing a table of values comprising PA gain compression and efficiency contour values versus radio frequency envelope amplitude (e.g., pre-PA signal) and PA supply voltage values. Further, in one example the table of value may comprise a plurality of stored tables of value that each reflect a unique set of operating conditions. By evaluating such a table based on various desired performance criteria, the controller 258 may select, via adjustment signals 228 and/or 228b, what tuning should be used to optimize envelope tracking system efficiency.

[00028] In another example the saturation detector circuitry 257 is configured to determine the gain compression characteristic for each of the differing voltage levels based on the level select signal, a pre-power amplifier transmit signal, and an output signal associated with a radio frequency signal output from the PA. For example, an output signal 230 may be taken from the output of the PA 208 via a coupler and fed back to a feedback receiver (FBR) 256 provided to the saturation detector circuitry 257, as illustrated in FIG. 2.

[00029] In another example the saturation detector circuitry 257 is configured to align, with respect to time, the level select signal 214, the pre-PA transmit signal 204 and the output signal 230. Further, the saturation detector circuitry 257 is configured to calculate a power amplifier gain or gain compression characteristic (e.g.,“saturation level”) 224 using the level select signal 214, the time aligned pre-power amplifier transmit signal 204 and the output signal 230. In another example the saturation detector circuitry 257 is configured to group the power amplifier gains or gain compression characteristics for each of the plurality of available supply voltages having differing voltage levels. The saturation detector circuitry 257 is further configured to compute the gain compression characteristic for each of the plurality of available supply voltages having differing voltage levels using the power amplifier gains of each group, respectively. The controller 258 is further configured to utilize such data in tuning either the envelope circuitry 250 or the supply voltage circuitry 260, or both. [00030] FIG. 4 illustrates one example of supply voltage circuitry 460 that can be adjusted based on PA efficiency. However, other ways of generating the differing voltage levels and mapping the plurality of differing voltage levels to supply lines may be employed, and all such variations are contemplated as falling within the scope of the present disclosure. The supply voltage circuitry 460 is configured to provide a regulated output voltage to a power amplifier according to an envelope tracking scheme. The supply voltage circuitry 460 includes a voltage generation circuitry 470 configured to convert a source voltage (e.g., battery voltage) to a regulated output voltage based on a target voltage {e.g., the level select signal 214) that is determined for envelope tracking purposes. The voltage generation circuitry 470 includes in one example an adjustable boost circuitry 473 and a step-down regulator circuitry 477. The adjustable boost circuitry 473 is configured to multiply the source voltage to generate an input voltage having a voltage equal to or greater than the source voltage. The step-down regulator circuitry 477 is configured to regulate the input voltage to generate a regulated output voltage having a voltage that is less than or equal to the input voltage.

[00031] Depending on voltage feedback from a voltage splitter circuitry 480 as compared to the target voltage/level select signal, the adjustable boost circuitry 473 may boost the source voltage by a selectable boost factor to an input voltage that is equal to or slightly greater than the target voltage. The step-down converter 477 regulates the input voltage“down” to the regulated output voltage. The voltage generation circuitry 470 supplies the regulated output voltage to the voltage splitter circuitry 480 which generates at least one derived output voltage from the regulated output voltage. A supply modulator 490 selects one of the derived output voltages based on a target voltage and provides the selected output voltage to a power amplifier (PA).

[00032] In one example, the adjustable boost circuitry 473 includes a capacitor-based adjustable charge pump that multiplies a charge level of the source voltage by the boost factor to generate the input voltage. In other examples, other charge boosting circuits may be used to embody the adjustable boost circuitry 473. In one example, the step- down regulator circuitry 477 includes DC/DC buck converter as shown in FIG. 4. Other step-down regulator circuitries can be used. [00033] Control circuitry 472 is configured to determine the target voltage and select a boost factor (X1 , X1 .5, and X2 are illustrated, different boost factors may be used) for the adjustable boost circuitry 473 based on the target voltage and the source voltage. The control circuitry 472 also generates control signals (e.g., charge and discharge control signals) for the step-down regulator circuitry 477 based on a difference between the target voltage and a voltage feedback signal from the voltage splitter circuitry 280.

[00034] When the target voltage is less than or equal to the source voltage, the control circuitry 472 is configured to operate the adjustable boost pump 473 in a bypass mode (shown in dashed lines) such that the source voltage is input directly to the step- down regulator circuitry 477 as the input voltage. In one example, the step-down regulator circuitry 477 includes a DC/DC buck converter having a single inductor with a maximum current rating approximately equal to a current that is drawn by the PA when a maximum regulated output voltage is provided to the PA.

[00035] In one example, the control circuitry 472 determines a ratio control signal that defines a ratio between the regulated output voltage and the derived output voltages. The control circuitry 472 determines the ratio based on transmitter operation parameters that can be used to determine a range of desired derived output voltages or indicate that the transmitter is in a power saving mode. The control circuitry 472 can use the ratio to scale the maximum derived output voltage (as well as the other derived output voltages) to be less than the regulated output voltage by some factor. The control circuitry 472 provides the ratio control circuitry to the voltage splitter circuitry 480 to control the voltage level of the various derived output voltages as a proportion of the regulated output voltage. The voltage level shift adjustment signal 228b (FIG. 2) may cause the control circuitry 472 to change the ratio control signal or other signals generated by the control circuitry to shift or adjust the differing voltage levels (e.g., derived output voltages).

[00036] Following are several flow diagrams outlining example methods. In this description and the appended claims, use of the term“determine” with reference to some entity (e.g., parameter, variable, and so on) in describing a method step or function is to be construed broadly. For example,“determine” is to be construed to encompass, for example, receiving and parsing a communication that encodes the entity or a value of an entity. “Determine” should be construed to encompass accessing and reading memory (e.g., lookup table, register, device memory, remote memory, and so on) that stores the entity or value for the entity. “Determine” should be construed to encompass computing or deriving the entity or value of the entity based on other quantities or entities. “Determine” should be construed to encompass any manner of deducing or identifying an entity or value of the entity.

[00037] As used herein, the term identify when used with reference to some entity or value of an entity is to be construed broadly as encompassing any manner of determining the entity or value of the entity. For example, the term identify is to be construed to encompass, for example, receiving and parsing a communication that encodes the entity or a value of the entity. The term identify should be construed to encompass accessing and reading memory (e.g., device queue, lookup table, register, device memory, remote memory, and so on) that stores the entity or value for the entity.

[00038] As used herein, the term select when used with reference to some entity or value of an entity is to be construed broadly as encompassing any manner of determining the entity or value of the entity from amongst a plurality or range of possible choices. For example, the term select is to be construed to encompass accessing and reading memory (e.g., lookup table, register, device memory, remote memory, and so on) that stores the entities or values for the entity and returning one entity or entity value from amongst those stored. The term select is to be construed as applying one or more constraints or rules to an input set of parameters to determine an appropriate entity or entity value. The term select is to be construed as broadly encompassing any manner of choosing an entity based on one or more parameters or conditions.

[00039] In one example, a method comprises, at a high level, of ascertaining a gain compression characteristic of the envelope tracking system per supply voltage level, and using the ascertained gain compression characteristic to tune one or more circuit parameters of the envelope tracking system, so as to optimize efficiency. In one example, level select thresholds are tuned in response to the ascertained gain compression characteristics, and in another example, one or more of the available voltage supply levels are tuned in response to the ascertained gain compression characteristics. [00040] FIG. 5 illustrates an exemplary method 500 for generating a tracking supply voltage for a power amplifier. The method 500 may be performed, for example by the transmitter architecture of FIGs. 2 and/or 4. The method includes, at 510, generating a plurality of supply voltages having a corresponding plurality of differing voltage levels. The method includes, at 520, receiving a transmit signal indicative of a baseband transmit signal. The method includes, at 530, generating a level select signal based on the transmit signal. The method includes, at 540, in response to the level select signal, outputting a selected supply voltage of the plurality of supply voltages as the tracking supply voltage. The method includes, at 550 characteristic for each of the plurality of differing voltage levels. The method includes, at 560, generating an adjustment signal based on the gain compression characteristics. The method includes, at 570, adjusting the tracking supply voltage based on the adjustment signal.

[00041] While the methods are illustrated and described above as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or examples of the disclosure herein. Also, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. In some examples, the methods illustrated above may be implemented in a computer readable medium using instructions stored in a memory. Many other examples and variations are possible within the scope of the claimed disclosure.

[00042] Example 1 is an envelope tracking system configured to generate a tracking supply voltage for a power amplifier. The system includes envelope circuitry, supply voltage circuitry, and amplifier efficiency tracking circuitry. The envelope circuitry is configured to receive a transmit signal indicative of a baseband transmit signal and generate a level select signal based on the transmit signal. The supply voltage circuitry is configured to generate a plurality of supply voltages having a corresponding plurality of differing voltage levels and in response to the level select signal, output a selected supply voltage of the plurality of supply voltages as the tracking supply voltage. The amplifier efficiency tracking circuitry includes a saturation detector circuit configured to determine a gain compression characteristic for each of the plurality of differing voltage levels and a controller configured to receive the gain compression characteristics and generate an adjustment signal. The envelope tracking system adjusts the tracking supply voltage based on the adjustment signal.

[00043] Example 2 includes the subject matter of example 1 , including or omitting optional elements, wherein: the envelope circuitry generates the level select signal based on a plurality of transmit signal amplitude thresholds mapped to respective voltages of the plurality of differing voltage levels; and the controller provides the adjustment signal to the envelope circuitry to cause the envelope circuitry to adjust one or more of the plurality of transmit signal amplitude thresholds.

[00044] Example 3 includes the subject matter of example 1 , including or omitting optional elements, wherein: the controller is configured to provide the adjustment signal to the supply voltage circuitry; and the supply voltage circuitry is configured to select the plurality of differing voltage levels based on the adjustment signal.

[00045] Example 4 includes the subject matter of any one of examples 1 -3, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic by accessing storage media storing a table of power amplifier gain compression and efficiency contour values as a function of radio frequency envelope amplitude and tracking supply voltage.

[00046] Example 5 includes the subject matter of example 4, including or omitting optional elements, wherein the storage media stores a plurality of tables of values that each reflect a unique set of operation conditions.

[00047] Example 6 includes the subject matter of any one of examples 1 -3, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic for each of the differing voltage levels based on the level select signal, a pre-power amplifier transmit signal, and an output signal indicative of a radio frequency signal output from the power amplifier.

[00048] Example 7 includes the subject matter of example 6, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic for each of the differing voltage levels by: aligning, with respect to time, the level select signal, the pre-power amplifier transmit signal and the output signal; calculating a power amplifier gain using the time aligned pre-power amplifier transmit signal and the output signal; grouping the power amplifier gains for each of the plurality of differing voltage levels into one or more groups; and computing a gain compression characteristic for each of the plurality of differing voltage levels using the power amplifier gains of each group, respectively.

[00049] Example 8 is a method for generating a tracking supply voltage for a power amplifier, including: generating a plurality of supply voltages having a corresponding plurality of differing voltage levels; receiving a transmit signal indicative of a baseband transmit signal; generating a level select signal based on the transmit signal; in response to the level select signal, outputting a selected supply voltage of the plurality of supply voltages as the tracking supply voltage; determining a gain compression characteristic for each of the plurality of differing voltage levels; generating an adjustment signal based on the gain compression characteristics; and adjusting the tracking supply voltage based on the adjustment signal.

[00050] Example 9 includes the subject matter of example 8, including or omitting optional elements, including generating the level select signal based on a plurality of transmit signal amplitude thresholds mapped to respective voltages of the plurality of differing voltage levels; and adjusting one or more of the plurality of transmit signal amplitude thresholds based on the adjustment signal.

[00051] Example 10 includes the subject matter of example 8, including or omitting optional elements, including selecting the plurality of differing voltage levels based on the adjustment signal.

[00052] Examplel 1 includes the subject matter of any one of examples 8-10, including or omitting optional elements, including determining the gain compression characteristic by accessing storage media storing a table of power amplifier gain compression and efficiency contour values as a function of radio frequency envelope amplitude and tracking supply voltage.

[00053] Example 12 includes the subject matter of example 1 1 , including or omitting optional elements, including selecting one of a plurality of stored tables based on a present operation condition.

[00054] Example 13 includes the subject matter of any one of examples 8-10, including or omitting optional elements, including determining the gain compression characteristic for each of the differing voltage levels based on the level select signal, a pre-power amplifier transmit signal, and an output signal indicative of a radio frequency signal output from the power amplifier. [00055] Example 14 includes the subject matter of example 13, including or omitting optional elements, including determining the gain compression characteristic for each of the differing voltage levels by: aligning, with respect to time, the level select signal, the pre-power amplifier transmit signal and the output signal; calculating a power amplifier gain using the time aligned pre-power amplifier transmit signal and the output signal; grouping the power amplifier gains for each of the plurality of differing voltage levels into one or more groups; and computing a gain compression characteristic for each of the plurality of differing voltage levels using the power amplifier gains of each group, respectively.

[00056] Example 15 is an amplifier efficiency tracking circuitry for envelope tracking system configured to generate a tracking supply voltage for a power amplifier including a saturation detector circuitry and a controller. The saturation detector circuit is configured to determine a gain compression characteristic for each of a plurality of differing voltage levels. The controller is configured to receive the gain compression characteristics; generate an adjustment signal; and provide the adjustment signal to the envelope tracking system. The envelope tracking system adjusts the tracking supply voltage based on the adjustment signal.

[00057] Example 16 includes the subject matter of example 15, including or omitting optional elements, wherein the adjustment signal is configured to cause the envelope tracking system to adjust one or more of a plurality of transmit signal amplitude thresholds used to select the tracking supply voltage from a plurality of supply voltages having differing voltage levels.

[00058] Example 17 includes the subject matter of example 15, including or omitting optional elements, wherein the adjustment signal is configured to cause the envelope tracking system to adjust voltage levels of differing voltage levels from which the envelope tracking system selects the tracking supply voltage.

[00059] Example 18 includes the subject matter of any one of examples 15-17, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic by accessing storage media storing a table of power amplifier gain compression and efficiency contour values as a function of radio frequency envelope amplitude and tracking supply voltage. [00060] Example 19 includes the subject matter of example 18, including or omitting optional elements, wherein the storage media stores a plurality of tables of values that each reflect a unique set of operation conditions.

[00061] Example 20 includes the subject matter of any one of examples 15-17, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic for each of the differing voltage levels based on a level select signal, a pre-power amplifier transmit signal, and an output signal indicative of a radio frequency signal output from the power amplifier.

[00062] Example 21 includes the subject matter of example 20, including or omitting optional elements, wherein the saturation detector circuit is configured to determine the gain compression characteristic for each of the differing voltage levels by: aligning, with respect to time, the level select signal, the pre-power amplifier transmit signal and the output signal; calculating a power amplifier gain using the time aligned pre-power amplifier transmit signal and the output signal; grouping the power amplifier gains for each of the plurality of differing voltage levels into one or more groups; and computing a gain compression characteristic for each of the plurality of differing voltage levels using the power amplifier gains of each group, respectively.

[00063] The term“couple” is used throughout the specification. The term may cover connections, communications, or signal paths that enable a functional relationship consistent with the description of the present disclosure. For example, if device A generates a signal to control device B to perform an action, in a first example device A is coupled to device B, or in a second example device A is coupled to device B through intervening component C if intervening component C does not substantially alter the functional relationship between device A and device B such that device B is controlled by device A via the control signal generated by device A.

[00064] It is well understood that the use of personally identifiable information should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users.

In particular, personally identifiable information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users.