Abstract:
An RF PA is designed to operate efficiently for average powers when biased at the system supply voltage, and uses an envelope tracking power supply to boost the bias voltage to maintain good efficiency at higher powers. As a result, for a majority of the time when transmitting average power signals, the RF PA bias voltage is the system-wide supply voltage (e.g. 3.4V in cell phones), which eliminates the need for stepping down voltages. The bias voltage is boosted during the less frequent times when higher power is needed. As a result, only a boost type of DC voltage converter is needed. The efficiency of the RF PA is therefore increased because voltage conversion is required less frequently and only when higher power RF signals are transmitted.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 62/032,444, “High Speed Envelope Tracking Power Amplifier,” filed Aug. 1, 2014; U.S. Provisional Patent Application Ser. No. 62/038,159, “Adaptive Envelope Tracking,” filed Aug. 15, 2014; and U.S. Provisional Patent Application Ser. No, 62/047,237, “High Voltage Power Amplifier Design,” filed Sep. 8, 2014; which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to power amplifiers and, more specifically, to providing adaptive envelope tracking bias voltages for biasing radio frequency (“RF”) power amplifiers. 
     2. Description of the Related Arts 
     RF power amplifiers, for example in cell phones, are used to transmit information in the form of modulated radio frequency electromagnetic waves. Power amplifiers are used in many applications such as WiFi, GPS, and the transmission of voice and data. Voice and data applications may also employ multiple frequency bands. The transmission distance is a function of the RF output power. The further the transmission distance, the higher the required output power, and the more battery power is consumed. 
     Power amplifiers (“PA”) consume most of the battery power in many usage cases, for example when a cell phone constantly transmits data to the nearby cell towers. The existing power supply architecture in cell phones uses the system supply voltage (e.g., the battery voltage) as the maximum bias voltage to the power amplifiers. Under this concept, the PA is designed to operate at peak efficiently for maximum powers when biased at the system supply voltage. However, under this design, RF PAs have overall low efficiency in many applications, such as smartphones, tablets, etc. This is because, when the RF PA is biased at the system supply voltage, the system and RF PA are designed for efficiency only when there is an RF signal of maximum power. However, for most of the time, RF PAs do not operate at full power. The average power for an RF PA typically is 1/2 to 1/7 of its saturated power. Accordingly, a large amount of DC power is wasted when the RF PA operates at these lower powers. 
     To improve the RF PA efficiency at lower power levels, envelope tracking (ET) or average power tracking (APT) techniques are used. Envelope tracking adjusts the bias voltage applied to the PA to increase the PA operating efficiency. In other words, the power supply voltage is adjusted to ensure that the PA is operating at peak efficiency for the power required at each instant of transmission. The envelope is the magnitude of the modulated RF signal. The speed of the envelope variation is typically in the MHz range and increases in wider bandwidth modulation applications. One approach is to use a linear regulator (e.g., LDO) and a buck-boost DC-converter. However, this approach has many disadvantages. The PA&#39;s overall efficiency is compromised because of the linear regulator&#39;s low efficiency. Moreover, when the bandwidth of LTE or other RF signals increases (e.g., reaching 40 MHz or 60 MHz under carrier aggregation), linear regulators typically will have difficulty to meet the signal envelope speed, and degradations in linearity may become unacceptable. 
     Furthermore, PAs must meet linearity requirements at high output power while operating at system supply voltage (e.g., 3.4V in cell phones). Cell phones output high power less frequently than low power, and PAs in cellphones often step down the supply voltage in order to bias the PA at a point that increases the efficiency. However, stepping down the supply voltage induces power loss. The lower the output voltage is, the lower the efficiency of the envelope tracking power supply system. 
     Accordingly, there is a need for PAs to work more efficiently across a range of power conditions. 
     SUMMARY 
     In one aspect, an RF PA is designed to operate efficiently for average powers when biased at the system supply voltage, and uses an envelope tracking power supply to boost the bias voltage to maintain good linearity at higher powers. As a result, for a majority of the time when transmitting average power signals, the RF PA bias voltage is the system-wide supply voltage (e.g. 3.4V in cell phones), which eliminates the need for stepping down voltages. The bias voltage is boosted during the less frequent times when higher power is needed. As a result, only a boost type of DC voltage converter is needed. The efficiency of the RF PA is therefore increased because voltage conversion is required less frequently and only when higher power RF signals are transmitted. 
     In one embodiment, an RF PA system includes an envelope tracking power supply that has a voltage conversion architecture, which includes a boost DC converter and a capacitive network. The envelope tracking power supply can increase the bias voltage instantaneously with little power loss as well as providing a steady boosted bias voltage when needed, by switching the capacitive network and by regulating the boost converter. The capacitive network allows the envelope tracking power supply to track the envelope speed of RF signals while using a boost DC converter that operates at a frequency lower than the RF signal. The power loss of the voltage conversion architecture is reduced because there is no step down voltage conversion, so the overall efficiency of the envelope tracking power supply is higher than conventional envelope tracking systems. 
     Other aspects include devices, components, systems, applications, improvements, variations, modifications, methods, processes and other technologies related to the foregoing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the present disclosure can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
         FIG. 1  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to one embodiment. 
         FIG. 2  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to another embodiment. 
         FIGS. 3A and 3B  show operation waveforms of example high-speed envelope tracking radio frequency power amplifier systems. 
         FIG. 4  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to yet another embodiment. 
         FIG. 5  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to yet another embodiment. 
         FIG. 6  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to yet another embodiment. 
         FIG. 7  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system, according to yet another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The Figures FIG.) and the following description relate to embodiments of the present disclosure by way of illustration only. The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings and specification. Moreover, it should be noted that the language used in the specification has been principally selected for adability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter. 
     Reference will now be made in detail to several embodiments of the present disclosure, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the embodiments of the disclosure described herein. 
       FIG. 1  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”)  100 , according to one embodiment. The illustrated ET PA  100  comprises a high-speed envelope tracking power supply  120  and an RF PA  102 . The envelope tracking power supply  120  is configured to provide a bias voltage Vcc to the RF PA  102  and comprises a boost direct current (“DC”) converter  101  plus an output capacitor  105 , a capacitive network  108 , and a controller (not shown). The capacitive network  108  comprises switches  103  and  104 , and capacitor  106 . The capacitor  105  balances the voltage ripple in the output voltage of and regulates the response speed of the boost DC converter  101 . The controller controls the switches  103  and  104  as well as regulates the operations (i.e., switching on and off switches of the boost DC converter  101 ) of the boost DC converter  101 . The controller typically is implemented as circuitry. The switches  103  and  104  may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. 
     The ET PA  100  includes ports  110 ,  111  and  112 . The input DC voltage, which is the system supply voltage V batt , is received at the port  110 . The RF PA  102  receives an input RF signal RF in  at the port  111  and outputs the output RF signal RF out  at the port  112 . The output of the envelope tracking power supply  120  is coupled to the RF PA  102 . The envelope tracking power supply  120  provides the DC bias voltage Vcc to bias the RF PA  102 , which amplifies the input signal RF in  to the amplified output signal RF out . The RF PA  102  is designed to operate efficiently for average power levels when biased at the system supply voltage, i.e., when Vcc=V batt . The average power level typically is 20-30% of the peak power and may be around 200 mW for many mobile devices. In many applications, the system supply voltage V batt  is the voltage supplied by a battery source for a mobile device. 
     Within the envelope tracking power supply  120 , the boost DC converter  101  is coupled between the input DC voltage V batt  and the RF power amplifier  102 . The boost DC converter  101  is configured to boost the input DC voltage V batt  to a higher voltage, which is then used as the DC bias voltage Vcc for biasing the RF PA  102 . The switching frequency of the boost DC converter  101  typically is in the MHz range, for example a couple MHz. The DC bias voltage Vcc increases as the duty cycle D of the boost DC converter  101  increases. Moreover, the capacitive network  108  is also coupled between the port  110  and the bias port for the RF power amplifier  102 . The capacitive network  108 , when coupled, is configured to provide a boosting voltage in series with the input DC voltage V batt , thereby to instantaneously boost the DC bias voltage Vcc. The RF PA  102  is accordingly biased by voltages at different levels that meet input RF signal&#39;s envelope speed and can amplify RF signals at different levels while maintaining the operating efficiency. As such, the ET PA&#39;s  100  operating efficiency is improved. 
     Because the RF PA  102  is designed to be efficient using a bias voltage Vcc=V batt  at the average power level of the input RF signal, when a higher output power is desired, the DC bias voltage Vcc supplied to the RF PA  102  is increased to be higher than the input voltage V batt . In other words, the DC bias voltage Vcc supplied to the RF PA  102  at the highest power is higher than the system voltage V batt  (i.e. the battery voltage or a system wide voltage). Conversely, the lowest bias voltage Vcc is the system supply voltage V batt  so there is no need for a voltage step down converter. 
     When the input RF signal RF in  level is low, the ET PA  100  operates at a low power mode, where the envelope tracking power supply  120  provides the system supply voltage (i.e., the input DC voltage V batt ) to bias the RF PA  102 . The capacitive network  108  is decoupled from the input DC voltage V batt  and the boost DC converter  101  is regulated to operate at a lower duty cycle D 1  (e.g., 0%). The DC bias voltage Vcc equals the input DC voltage V batt . During the low power mode, the switch  104  is on and the switch  103  is off, as shown in  FIG. 1 . The capacitor  106  is charged by the DC bias voltage Vcc such that the voltage V c2  across the capacitor  106  equals to the input DC voltage V batt . 
     When the input RF signal&#39;s RF in  level is high, the envelope power supply  100  operates at a high power mode, where the envelope tracking power supply  120  provides a high voltage (e.g., 2V batt ) to bias the RF PA  102 . When the input RF signal RF in  transitions to the high level, the controller couples the capacitive network  108  to the input DC voltage V batt  and regulates the boost DC converter  101  to operate at a higher duty cycle D 2  (e.g., 50%). Accordingly, the DC bias voltage Vcc is increased and the RF PA  102  is ensured to amplify the input RF signal RF in . When the input RF signal RF in  transitions to the high level, the controller turns off the switch  104  and turns on the switch  103  to instantaneously increase the DC bias voltage Vcc such that the DC bias voltage Vcc follows the input RF signal&#39;s envelope speed. The DC bias voltage Vcc is instantaneously boosted by the boosting voltage V C2  across the capacitor  106 , because the boosting voltage V c2  is in series with the input DC voltage V batt . In the illustrated example, the DC bias voltage Vcc is increased to 2V batt , twice the input DC voltage V batt . At the same time, the capacitor  106  supplies a current to the RF PA  102 . As such, the RF PA  102  is ensured to continuously amplify the input RF signal RF in  and to output an output RF signal RF out . In some cases, the DC bias voltage Vcc may be a little lower than 2V batt  because some charges in the capacitor  106  may be transferred to the capacitor  105  such that the nodes  113  and  114  are at the same electric potential Vcc. The ratio between the capacitors  105  and  106  typically is in the range of 1:10 to 1:5. 
     The controller also increases the duty cycle D of the boost DC converter  101  (e.g., from D 1  to D 2 ) to increase the output voltage of the boost DC converter  101 , when the input RF signal RF in  transitions to the high level. The DC bias voltage Vcc equals to the sum of the voltage V c2  across the capacitor  106  and the input voltage V batt . When the output voltage of the boost DC converter  101  increases to a level that equals to a voltage that is the sum of the input DC voltage V batt  and the voltage V C2  across the capacitor  106 , the boost DC converter  101  replaces the capacitor  106  to provide a current to the RF PA  102 . The controller regulates the boost power converter  101  to operate at the higher duty cycle D 2  when the ET PA operates at the high power mode. As such, the RF PA  102  can work at high power levels continuously. 
     When the envelope power supply switches to the low power mode from the high power mode, the controller reduces the duty cycle D of the boost DC converter  101  from D 2  to D 1  (e.g., from 50% from 0%) and decouples the switch network  108  from the input DC voltage V batt . When the input RF signal RF in  transitions to the low level, the controller turns off the switch  103  and turns on the switch  104  to decouple the capacitive network  108  from the input port  110 . The DC bias voltage Vcc provided to the RF PA  102  is reduced to V batt . 
     The illustrated ET PA  100  tracks the RF signal&#39;s envelope speed and has high operating efficiency. This is because the DC bias voltage Vcc can be doubled almost instantaneously by switching the capacitor  106  to be in series with the input DC voltage V batt , at a much higher speed than a linear regulator. In addition, the loss is also lower because of the high quality factors of the capacitors  105  and  106 . The DC bias voltage Vcc supplied to the RF PA  102  can be increased at a high speed and a high current is provided to the RF PA  102  by balancing the value of the capacitors  105  and  106 . 
       FIG. 2  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”)  200 , according to another embodiment. The illustrated ET PA  200  comprises a high-speed envelope tracking power supply  220  and an RF PA  102 . The envelope tracking power supply  220  is configured to provide a bias voltage Vcc to the RF PA  102  and comprises a boost direct current (“DC”) converter  101  plus output capacitor  105 , a capacitive network  208 , and a controller (not shown). The capacitive network  208  includes a capacitor and switch ladder, with capacitors  106 ,  206  and  207 . The envelope tracking power supply  220  achieves finer tuning of the DC bias voltage Vcc, compared to the envelope tracking power supply  120  illustrated in  FIG. 1  The switches in the capacitor and switch ladder  208  may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. The capacitors  106 ,  206 , and  207  may have the same or different capacitance. When the capacitors  106 ,  206 , and  207  have the same capacitance, they are charged to have the same voltage. For example, when the ET PA  200  operates at the low power mode, the controller decouples the capacitive network  208  from the input DC voltage V batt . The switches  104 ,  204 , and  205  are turned on and the switches  103 ,  202 , and  203  are turned off. The capacitors  106 ,  206 , and  207  are each charged to a third of the DC voltage, or V batt /3, when they have the same capacitance. 
     When the incoming signals RF in  are high power signals, the ET PA  200  operates at a high power mode. The controller may couple the capacitive network  208  to the input DC voltage V batt . When being coupled to the input DC voltage V batt , the capacitive network  208  may be configured to provide different levels of boosting voltages (e.g., 1/3*V batt , 2/3*V batt , or V batt ). The DC bias voltage Vcc can be increased instantaneously to various levels (e.g., 4/3*V batt , 5/3*V batt , or 2V batt ) to meet different amount of power needed by the RF PA  102 . For example, when the controller configures the capacitive network  208  such that the switches  202 ,  104  are on and the switches  103  and  203  through  205  are off, the voltage V c2  across the capacitor  106  is in series with the voltage V c3  across the capacitor  206 , both of which are in series with the input DC voltage V batt . As a result, the DC bias voltage Vcc equals to V batt +(2/3)*V batt , when the capacitors  106  and  206  have the same capacitance. Other architectures of switch and capacitor ladders can also be used. 
       FIG. 3A  shows operation waveforms of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”), according to one embodiment. The ET PA includes an envelope tracking power supply that includes a boost DC converter of which the switching frequency is 100 KHz. The waveforms  302  and  304  illustrate the DC bias voltage (i.e., Vcc) provided to the RF PA and the output current of the boost DC converter, respectively. Before the time point t 1  at 50 us, the boost DC converter (e.g., the boost DC converter  101 ) operates at a duty cycle of 0%. As illustrated by the waveform  302 , the DC bias voltage supplied to the RF PA is at 3.3V during this time. At the time point 50 us, the DC bias voltage supplied to the RF PA immediately jumps to 5.5V due to the boosting voltage across a capacitor (e.g., the capacitor  106 ) being coupled in series with the input DC voltage. Subsequently, during the time period between t 1  and t 2 , the DC bias voltage drops gradually while the capacitor (e.g., the capacitor  106 ) discharges and provides a current to the PA. At the time point t 2  at 110 us, the DC bias voltage stops decreasing and starts to increase, when the boost DC converter starts to provide the DC bias voltage and the current to the RF PA. 
       FIG. 3B  shows operation waveforms of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”), according to another embodiment. The ET PA includes an envelope tracking power supply that includes a boost DC converter of which the switching frequency is 1 MHz. The waveforms  312  and  314  illustrate the DC bias voltage (i.e., Vcc) provided to the RF PA and the output current of the boost DC converter, respectively. At the time point t 3  at 50 us, the DC bias voltage supplied to the PA immediately jumps to 6.3V from 3.3V due to the boosting voltage across a capacitor (e.g., the capacitor  106 ) being coupled in series with the input DC voltage. Subsequently, during the time period between t 3  and t 4 , the DC bias voltage drops gradually while the capacitor (e.g., the capacitor  106 ) discharges and provides a current to the PA. However, because the boost DC converter operates at a higher switching frequency than 100 KHz, the output voltage stops decreasing at time point t 4  at around 80 us. With the boost DC converter operating at a higher switching frequency, the DC bias voltage supplied to the RF PA is increased faster, and thereby shortens the amount of time that the boost DC converter takes to provide a current to the RF PA. 
       FIG. 4  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”)  400 , according to yet another embodiment. The illustrated ET PA  400  comprises a high-speed envelope tracking power supply  430  and an RF PA  402 . The envelope tracking power supply  430  is configured to provide a bias voltage Vcc to the RF PA  402  and comprises a boost DC converter  401 , a bypass switch  403 , a capacitive network  408 , and a controller (not shown). The capacitive network  408  comprises switches  404  and  405  and a capacitor  406 . The controller controls the switches  403  through  405  as well as regulates the operations (i.e., switching on and off switches) of the boost DC converter  401 . The controller typically is implemented as circuitry. The switches  403  through  405  may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. 
     The ET PA  400  includes ports  410 ,  411  and  412 . The input DC voltage V batt  (e.g., the battery voltage from the phone board, or other system-wide supply voltage) is received at the port  410 . The RF PA  402  receives an input RF signal RF in  at the port  411  and outputs the output RF signal RF out  at the port  412 . The bypass switch  403  is coupled between the port  410  and the RF power amplifier  402 . The bypass switch  403  is on when the envelope power supply  400  operates in the low-power mode. The output of the envelope tracking power supply  430  is coupled to the RF PA  402 . The envelope tracking power supply  430  provide the DC bias voltage Vcc to bias the RF PA  402 , which amplifies the input signal RF in  to the amplified output signal RF out . The RF PA  402  is configured to operate at a low power range with a high operating efficiency without the need to reduce the voltage below the input DC voltage V batt  (e.g., 3.4V). 
     Within the envelope tracking power supply  430 , the boost DC converter  401  is coupled between the port  410  and the RF power amplifier  402 . The capacitive network  408  is also coupled between the port  410  and the RF PA  402 . The boost DC converter  401  and the capacitive network  408  are configured to provide a DC bias voltage Vcc to bias the RF power amplifier  402 , which amplifies the input signal RF in  to the amplified output signal RF out . The capacitive network  408 , when coupled, is configured to provide a boosting voltage in series with the input DC voltage V batt , thereby to instantaneously boost the DC bias voltage Vcc. The capacitor  406  and the switches  404  and  405  are configured such that the capacitor  406  can be coupled to be in series with the input DC voltage V batt  via regulating the on and off of the switches  404  and  405 . The RF PA  402  is designed to operate efficiently at the average power level of the input RF signals RF in  when biased at the input voltage V batt . 
     When the input RF signal RF in  is at a low level, the ET PA  400  operates at the low power mode, where the envelope tracking power supply  430  provides a low voltage (i.e., the input DC voltage V batt ) to bias the RF PA  102 . The capacitive network  408  is decoupled from the input DC voltage V batt  and the bypass switch  403  is on. The capacitor  406  is charged by the input DC voltage V batt . During the low power mode, the switch  404  is off and the switch  405  is on. In addition, the controller regulates the boost converter  401  to operate at a lower duty cycle D 1  (e.g., 0%). The voltage drop across the boost DC converter  401  is minimized because the switch  403  is on. 
     When the input RF signal RF in  level is high, the envelope power supply  400  operates at a high power mode, where the envelope tracking power supply  430  provides a high voltage (e.g., 2Vbatt) to bias the RF PA  402 . When the input RF signal RF in , transitions to the high level, the controller couples the capacitive network  408  to the input DC voltage V batt , turns off the bypass switch  403 , and regulates the boost DC converter  401  to operate at a high duty cycle D 2  (e.g., 50%). Accordingly, the DC bias voltage Vcc is increased and the RF PA  402  is ensured to amplify the input RF signal RF in . When the input RF signal RF in , transitions to the high level, the controller turns off the switches  403  and  405  and turns on the switch  404  thereby to instantaneously increase the DC bias voltage Vcc such that the DC bias voltage Vcc follows the input RF signal&#39;s envelope speed. The DC bias voltage Vcc is instantaneously boosted by the boosting voltage V c1  across the capacitor  406 , because the boosting voltage V c1  is in series with the input DC voltage V batt . In the illustrated example, the DC bias voltage Vcc is increased to 2V batt , twice the input DC voltage V batt , and the RF PA  402  saturation power is quadrupled. As such, the RF PA  402  operates linearly and amplifies input RF signal RF in , at high levels. 
     When the capacitor  406  is first coupled in series with the input DC voltage V batt , the capacitor  406  is discharged and provides a current to the RF PA  402 . The voltage V c1  across the capacitor  406  decreases at a higher rate with smaller capacitance. Because a lower than desired DC bias voltage Vcc can cause the distortion in the output RF signal RF out , the controller increases the duty cycle of the boost DC converter  401  thereby to increase the output voltage of the boost DC converter  401  to stabilize the DC bias voltage Vcc provided to the RF PA  402 . When the input RF signal RF in , transitions to the low level, the ET PA  400  returns back to the low power mode. The envelope tracking power supply  430  reduces the DC bias voltage provided to the RF PA  402  by decoupling the capacitive network  408  from the input DC voltage V batt  and turning on the bypass switch  403 . The DC bias voltage Vcc can be decreased to the low level (e.g., V batt ) instantaneously by turning on the bypass switch  403 . The controller turns off the switch  404  and subsequently turns on the bypass switch  403  to bias the RF PA  402  with the input DC voltage V batt . Subsequently, the controller turns on the switch  405  to charge the capacitor  406 . The controller further reduces the duty cycle of the boost DC converter  101  from D 2  to D 1  (e.g., from 50% from 0%). 
       FIG. 5  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system (“ET PA”)  500 , according to yet another embodiment. The illustrated ET PA  500  comprises a high-speed envelope tracking power supply  530  and an RF PA  402 . The envelope tracking power supply  530  is configured to provide a bias voltage Vcc to the RF PA  402  and comprises a boost DC converter  401 , a bypass switch  403 , a capacitive network  508 , and a controller (not shown). The capacitive network  508  includes a capacitor and a switch ladder. The envelope tracking power supply  530  achieves finer tuning of the DC bias voltage Vcc, compared to the envelope tracking power supply  430  illustrated in  FIG. 4 . The switches in the capacitor and switch ladder may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. The capacitors  406 ,  506 , and  507  may have the same or different capacitance. As an example, when the capacitors  406 ,  506 , and  507  have the same capacitance, they are charged to have the same voltage. For example, when the ET PA  500  operates at the low power mode, the controller decouples the capacitive network  508  from the input DC voltage V batt . The switches  405 ,  504 , and  505  are turned on and the switches  404 ,  502 , and  503  are turned off. The capacitors  406 ,  506 , and  507  are each charged to a third of the DC voltage, 1/3*V batt . 
     The illustrated envelope power supply  500  is similar to the envelope power supply  400  illustrated in  FIG. 4 , and thus the details of the ports  410 - 412 , the bypass switch  403 , the boost DC converter  401 , and the RF PA  402  are omitted for the sake of brevity. When the incoming signals RF in , are low power signals, the ET PA  500  operates at the low power mode, the capacitive network  508  is decoupled from biasing the RF PA  402 . The bypass switch  403  is on and the RF PA  402  is biased by the input DC voltage V batt . When the incoming signals RF in  transitions into high power signals, the ET PA  500  transitions to operate at a high power mode. The controller turns off the bypass switch  403  and couples the capacitive network  508  to the input DC voltage V batt . When coupled to the input DC voltage V batt , the capacitive network  508  may be configured to provide different levels of boosting voltage (e.g., 1/3*V batt 2/3*V batt , or V batt ). The DC bias voltage V cc  can be increased instantaneously to various levels (e.g., 4/3*V batt , 5/3*V batt , or 2*V batt ) to meet different amount of power needed by the RF PA  402  to maintain linear operation. For example, when the controller configures the capacitive network  508  such that the switches  502  and  405  are on, and the switches  404  and  503  through  505  are off, the voltage V c2  across the capacitors  406  is coupled in series with the voltage V c3  across the capacitor  506 , both of which are coupled in series with the input DC voltage V batt . As a result, the DC bias voltage V cc  equals to V batt +(2/3)*V batt , when the capacitors  406  and  506  have the same capacitance. Other architectures of switch and capacitor ladders can also be used. 
     When the ET PA  500  reverts back to the low power mode, the controller decouples the capacitive network  508  and turns on the bypass switch  403 . The controller turns off the switches  404 ,  502  and  503  and subsequently turns on the bypass switch  403  to couple the RF PA  402  to be biased by the input DC voltage V batt . Subsequently, the controller turns on the switches  405 ,  504  and  505  to charge the capacitors  406 ,  506 , and  507 . 
       FIG. 6  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system  600 , according to yet another embodiment. The illustrated ET PA  600  comprises a high-speed envelope tracking power supply  630  and an RF PA  402 . The envelope tracking power supply  630  is configured to provide a bias voltage Vcc to the RF PA  402  and comprises a boost DC converter  401 , a bypass switch  403 , a capacitive network  608 , and a controller (not shown). The capacitive network  608  comprises switches  601  through  605  and capacitors  606  and  607 . The controller controls the switches  601  through  605  as well as regulates the operations (i.e., switching on and off switches of the boost DC converter  401 ) of the boost DC converter  401 . The controller typically is implemented as circuitry. The switches  601  through  605  may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. 
     The illustrated envelope power supply  600  is similar to the envelope power supply  400  illustrated in  FIG. 4 , and thus the details of the ports  410 - 412 , the bypass switch  403 , the boost DC converter  401 , and the RF PA  402  are omitted for the sake of brevity. The envelope tracking power supply  630  may provide different levels of DC bias voltage such that the RF PA  402  can maintain operation linearity and efficiency for RF signals at different power levels. Within the envelope tracking power supply  630 , the capacitive network  608  is coupled between the port  410  and the RF PA  402 . The capacitors  606  and  607  and the switches  601  through  605  are configured such that the capacitor  606  or the capacitor  606  along with the capacitor  607  can be coupled to be in series with the input DC voltage V batt  via regulating the on and off of the switches  601  through  605 . 
     When the input RF signal RF in , level is low, the envelope power supply  600  operates at the low power mode, where envelope tracking power supply  630  provides a low voltage (i.e., the input DC voltage V batt ) for biasing the RF PA  402 . The capacitive network  608  is decoupled from the input DC voltage V batt  and the bypass switch  403  is on. During the low power mode, the switches  601  and  604  are off and the switches  602 ,  603  and  605  are on. Both capacitors  606  and  607  are charged by the input DC voltage V batt . In addition, the controller regulates the boost converter to operate at a lower duty cycle D 1  (e.g., 0%). 
     When the input RF signals RF in , level is at a medium or high level, the envelope power supply  400  operates at a medium or high power mode, where the RF PA  402  is biased by a medium or high voltage (e.g., 2V batt  or 3V batt ). When the input RF signal RF in  transitions to the high level, the controller couples the capacitive network  608  to the input DC voltage V batt , turns off the bypass switch  403 , and regulates the boost DC converter  601  to operate at a higher duty cycle D (e.g., 50% or 67%). Accordingly, the DC bias voltage Vcc is increased to different levels and the RF PA  402  is ensured to amplify the input RF signal RF in  at different levels. As such, the ET PA  600  is ensured to track the signal envelope of the input RF signal RF in . When being coupled to the input DC voltage V batt , the capacitive network  608  may be configured to provide different levels of boosting voltages (e.g., V batt , or 2V batt ). The DC bias voltage V cc  can be increased instantaneously to various levels (e.g., 2V batt  or 3V batt ) to meet different amount of power needed by the RF PA  402  to maintain linear operation. 
     As one example, when the RF input signal RF in  transitions into the medium level, the controller turns off the switches  603  and  605  and subsequently turns off the bypass switch  403 . Switch  604  was off and remains off. The controller subsequently turns on the switch  601  to couple the capacitor  606  to be in series with the input DC voltage V batt . As such, the DC bias voltage Vcc is instantaneously boosted by the voltage across the capacitor  606 . 
     As another example, when the RF input signal RF in , transitions into a high level, to further boost the voltage, the controller turns off the switch  602  and subsequently turns on the switch  604 . As such, the capacitor  607  is coupled in series with the capacitor  606 , both of which are coupled in series with the input DC voltage V batt . In both cases, the capacitor  606  or the capacitors  606  and  607  are discharged by supplying a current to the RF PA  402 . The controller may regulate the boost DC converter  401  by increasing its duty cycle to stabilize the DC bias voltage Vcc. 
     When the RF input signal RF in , transitions back from a higher level (e.g., the high level, or the medium level) to a lower level (e.g., the medium level, or the low level), the controller regulates the switches in a sequence reverse to the sequence as described above. For example, to lower the DC bias voltage Vcc to V batt  from 3V batt , the controller turns off the switch  604  and subsequently turns on the switches  403 ,  602 ,  603 , and  605 . 
     The ET PA  600  can be adapted to include an envelope tracking power that can vary the output voltage in finer steps, such as the example illustrated in  FIG. 7 .  FIG. 7  is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system  700 , according to yet another embodiment. The illustrated ET PA  700  comprises a high-speed envelope tracking power supply  730  and an RF PA  402 . The envelope tracking power supply  730  is configured to provide a bias voltage Vcc to the RF PA  402  and comprises a boost DC converter  401 , a bypass switch  403 , a capacitive network  708 , and a controller (not shown). The capacitive network  708  includes switch and capacitor ladders  701  and  702 . Similar to the envelope power supply  600  illustrated in  FIG. 6 , the envelope power supply  700  may boost the DC bias voltage to different levels (e.g., 2V batt  and 3V batt ). In addition, the envelope tracking power supply  730  can provide finer steps of voltage boosting. When being coupled to the input DC voltage V batt , the capacitive network  708  may be configured to provide different levels of boosting voltages (e.g., 1/3*V batt , 2/3*V batt , V batt , 4/3*V batt , 5/3*V batt , or 2V batt ). The DC bias voltage V cc  can be increased instantaneously to various levels (e.g., 4/3*V batt , 5/3*V batt , 2V batt , 7/3*V batt , 8/3*V batt , or 3V batt ) to meet different amount of power needed by the RF PA  402  to maintain linear operation. 
     Upon reading this disclosure, those of skill in the art will appreciate still additional alternative designs for providing adaptive envelope tracking bias voltages to radio frequency power amplifiers. Thus, while particular embodiments and applications of the present disclosure have been illustrated and described, it is to be understood that the disclosure is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present disclosure disclosed herein without departing from the spirit and scope of the disclosure.