Adaptive envelope tracking for biasing radio frequency power amplifiers

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.

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's overall efficiency is compromised because of the linear regulator'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.

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.

FIG. 1is 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 PA100comprises a high-speed envelope tracking power supply120and an RF PA102. The envelope tracking power supply120is configured to provide a bias voltage Vcc to the RF PA102and comprises a boost direct current (“DC”) converter101plus an output capacitor105, a capacitive network108, and a controller (not shown). The capacitive network108comprises switches103and104, and capacitor106. The capacitor105balances the voltage ripple in the output voltage of and regulates the response speed of the boost DC converter101. The controller controls the switches103and104as well as regulates the operations (i.e., switching on and off switches of the boost DC converter101) of the boost DC converter101. The controller typically is implemented as circuitry. The switches103and104may be MOSFET switches, silicon CMOS, SOI, or HEMT etc.

The ET PA100includes ports110,111and112. The input DC voltage, which is the system supply voltage Vbatt, is received at the port110. The RF PA102receives an input RF signal RFinat the port111and outputs the output RF signal RFoutat the port112. The output of the envelope tracking power supply120is coupled to the RF PA102. The envelope tracking power supply120provides the DC bias voltage Vcc to bias the RF PA102, which amplifies the input signal RFinto the amplified output signal RFout. The RF PA102is designed to operate efficiently for average power levels when biased at the system supply voltage, i.e., when Vcc=Vbatt. 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 Vbattis the voltage supplied by a battery source for a mobile device.

Within the envelope tracking power supply120, the boost DC converter101is coupled between the input DC voltage Vbattand the RF power amplifier102. The boost DC converter101is configured to boost the input DC voltage Vbattto a higher voltage, which is then used as the DC bias voltage Vcc for biasing the RF PA102. The switching frequency of the boost DC converter101typically 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 converter101increases. Moreover, the capacitive network108is also coupled between the port110and the bias port for the RF power amplifier102. The capacitive network108, when coupled, is configured to provide a boosting voltage in series with the input DC voltage Vbatt, thereby to instantaneously boost the DC bias voltage Vcc. The RF PA102is accordingly biased by voltages at different levels that meet input RF signal's envelope speed and can amplify RF signals at different levels while maintaining the operating efficiency. As such, the ET PA's100operating efficiency is improved.

Because the RF PA102is designed to be efficient using a bias voltage Vcc=Vbattat 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 PA102is increased to be higher than the input voltage Vbatt. In other words, the DC bias voltage Vcc supplied to the RF PA102at the highest power is higher than the system voltage Vbatt(i.e. the battery voltage or a system wide voltage). Conversely, the lowest bias voltage Vcc is the system supply voltage Vbattso there is no need for a voltage step down converter.

When the input RF signal RFinlevel is low, the ET PA100operates at a low power mode, where the envelope tracking power supply120provides the system supply voltage (i.e., the input DC voltage Vbatt) to bias the RF PA102. The capacitive network108is decoupled from the input DC voltage Vbattand the boost DC converter101is regulated to operate at a lower duty cycle D1(e.g., 0%). The DC bias voltage Vcc equals the input DC voltage Vbatt. During the low power mode, the switch104is on and the switch103is off, as shown inFIG. 1. The capacitor106is charged by the DC bias voltage Vcc such that the voltage Vc2across the capacitor106equals to the input DC voltage Vbatt.

When the input RF signal's RFinlevel is high, the envelope power supply100operates at a high power mode, where the envelope tracking power supply120provides a high voltage (e.g., 2Vbatt) to bias the RF PA102. When the input RF signal RFintransitions to the high level, the controller couples the capacitive network108to the input DC voltage Vbattand regulates the boost DC converter101to operate at a higher duty cycle D2(e.g., 50%). Accordingly, the DC bias voltage Vcc is increased and the RF PA102is ensured to amplify the input RF signal RFin. When the input RF signal RFintransitions to the high level, the controller turns off the switch104and turns on the switch103to instantaneously increase the DC bias voltage Vcc such that the DC bias voltage Vcc follows the input RF signal's envelope speed. The DC bias voltage Vcc is instantaneously boosted by the boosting voltage VC2across the capacitor106, because the boosting voltage Vc2is in series with the input DC voltage Vbatt. In the illustrated example, the DC bias voltage Vcc is increased to 2Vbatt, twice the input DC voltage Vbatt. At the same time, the capacitor106supplies a current to the RF PA102. As such, the RF PA102is ensured to continuously amplify the input RF signal RFinand to output an output RF signal RFout. In some cases, the DC bias voltage Vcc may be a little lower than 2Vbattbecause some charges in the capacitor106may be transferred to the capacitor105such that the nodes113and114are at the same electric potential Vcc. The ratio between the capacitors105and106typically is in the range of 1:10 to 1:5.

The controller also increases the duty cycle D of the boost DC converter101(e.g., from D1to D2) to increase the output voltage of the boost DC converter101, when the input RF signal RFintransitions to the high level. The DC bias voltage Vcc equals to the sum of the voltage Vc2across the capacitor106and the input voltage Vbatt. When the output voltage of the boost DC converter101increases to a level that equals to a voltage that is the sum of the input DC voltage Vbattand the voltage VC2across the capacitor106, the boost DC converter101replaces the capacitor106to provide a current to the RF PA102. The controller regulates the boost power converter101to operate at the higher duty cycle D2when the ET PA operates at the high power mode. As such, the RF PA102can 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 converter101from D2to D1(e.g., from 50% from 0%) and decouples the switch network108from the input DC voltage Vbatt. When the input RF signal RFintransitions to the low level, the controller turns off the switch103and turns on the switch104to decouple the capacitive network108from the input port110. The DC bias voltage Vcc provided to the RF PA102is reduced to Vbatt.

The illustrated ET PA100tracks the RF signal's envelope speed and has high operating efficiency. This is because the DC bias voltage Vcc can be doubled almost instantaneously by switching the capacitor106to be in series with the input DC voltage Vbatt, at a much higher speed than a linear regulator. In addition, the loss is also lower because of the high quality factors of the capacitors105and106. The DC bias voltage Vcc supplied to the RF PA102can be increased at a high speed and a high current is provided to the RF PA102by balancing the value of the capacitors105and106.

FIG. 2is 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 PA200comprises a high-speed envelope tracking power supply220and an RF PA102. The envelope tracking power supply220is configured to provide a bias voltage Vcc to the RF PA102and comprises a boost direct current (“DC”) converter101plus output capacitor105, a capacitive network208, and a controller (not shown). The capacitive network208includes a capacitor and switch ladder, with capacitors106,206and207. The envelope tracking power supply220achieves finer tuning of the DC bias voltage Vcc, compared to the envelope tracking power supply120illustrated inFIG. 1The switches in the capacitor and switch ladder208may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. The capacitors106,206, and207may have the same or different capacitance. When the capacitors106,206, and207have the same capacitance, they are charged to have the same voltage. For example, when the ET PA200operates at the low power mode, the controller decouples the capacitive network208from the input DC voltage Vbatt. The switches104,204, and205are turned on and the switches103,202, and203are turned off. The capacitors106,206, and207are each charged to a third of the DC voltage, or Vbatt/3, when they have the same capacitance.

When the incoming signals RFinare high power signals, the ET PA200operates at a high power mode. The controller may couple the capacitive network208to the input DC voltage Vbatt. When being coupled to the input DC voltage Vbatt, the capacitive network208may be configured to provide different levels of boosting voltages (e.g., 1/3*Vbatt, 2/3*Vbatt, or Vbatt). The DC bias voltage Vcc can be increased instantaneously to various levels (e.g., 4/3*Vbatt, 5/3*Vbatt, or 2Vbatt) to meet different amount of power needed by the RF PA102. For example, when the controller configures the capacitive network208such that the switches202,104are on and the switches103and203through205are off, the voltage Vc2across the capacitor106is in series with the voltage Vc3across the capacitor206, both of which are in series with the input DC voltage Vbatt. As a result, the DC bias voltage Vcc equals to Vbatt+(2/3)*Vbatt, when the capacitors106and206have the same capacitance. Other architectures of switch and capacitor ladders can also be used.

FIG. 3Ashows 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 waveforms302and304illustrate 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 t1at 50 us, the boost DC converter (e.g., the boost DC converter101) operates at a duty cycle of 0%. As illustrated by the waveform302, 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 capacitor106) being coupled in series with the input DC voltage. Subsequently, during the time period between t1and t2, the DC bias voltage drops gradually while the capacitor (e.g., the capacitor106) discharges and provides a current to the PA. At the time point t2at 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. 3Bshows 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 waveforms312and314illustrate 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 t3at 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 capacitor106) being coupled in series with the input DC voltage. Subsequently, during the time period between t3and t4, the DC bias voltage drops gradually while the capacitor (e.g., the capacitor106) 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 t4at 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. 4is 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 PA400comprises a high-speed envelope tracking power supply430and an RF PA402. The envelope tracking power supply430is configured to provide a bias voltage Vcc to the RF PA402and comprises a boost DC converter401, a bypass switch403, a capacitive network408, and a controller (not shown). The capacitive network408comprises switches404and405and a capacitor406. The controller controls the switches403through405as well as regulates the operations (i.e., switching on and off switches) of the boost DC converter401. The controller typically is implemented as circuitry. The switches403through405may be MOSFET switches, silicon CMOS, SOI, or HEMT etc.

The ET PA400includes ports410,411and412. The input DC voltage Vbatt(e.g., the battery voltage from the phone board, or other system-wide supply voltage) is received at the port410. The RF PA402receives an input RF signal RFinat the port411and outputs the output RF signal RFoutat the port412. The bypass switch403is coupled between the port410and the RF power amplifier402. The bypass switch403is on when the envelope power supply400operates in the low-power mode. The output of the envelope tracking power supply430is coupled to the RF PA402. The envelope tracking power supply430provide the DC bias voltage Vcc to bias the RF PA402, which amplifies the input signal RFinto the amplified output signal RFout. The RF PA402is 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 Vbatt(e.g., 3.4V).

Within the envelope tracking power supply430, the boost DC converter401is coupled between the port410and the RF power amplifier402. The capacitive network408is also coupled between the port410and the RF PA402. The boost DC converter401and the capacitive network408are configured to provide a DC bias voltage Vcc to bias the RF power amplifier402, which amplifies the input signal RFinto the amplified output signal RFout. The capacitive network408, when coupled, is configured to provide a boosting voltage in series with the input DC voltage Vbatt, thereby to instantaneously boost the DC bias voltage Vcc. The capacitor406and the switches404and405are configured such that the capacitor406can be coupled to be in series with the input DC voltage Vbattvia regulating the on and off of the switches404and405. The RF PA402is designed to operate efficiently at the average power level of the input RF signals RFinwhen biased at the input voltage Vbatt.

When the input RF signal RFinis at a low level, the ET PA400operates at the low power mode, where the envelope tracking power supply430provides a low voltage (i.e., the input DC voltage Vbatt) to bias the RF PA102. The capacitive network408is decoupled from the input DC voltage Vbattand the bypass switch403is on. The capacitor406is charged by the input DC voltage Vbatt. During the low power mode, the switch404is off and the switch405is on. In addition, the controller regulates the boost converter401to operate at a lower duty cycle D1(e.g., 0%). The voltage drop across the boost DC converter401is minimized because the switch403is on.

When the input RF signal RFinlevel is high, the envelope power supply400operates at a high power mode, where the envelope tracking power supply430provides a high voltage (e.g., 2Vbatt) to bias the RF PA402. When the input RF signal RFin, transitions to the high level, the controller couples the capacitive network408to the input DC voltage Vbatt, turns off the bypass switch403, and regulates the boost DC converter401to operate at a high duty cycle D2(e.g., 50%). Accordingly, the DC bias voltage Vcc is increased and the RF PA402is ensured to amplify the input RF signal RFin. When the input RF signal RFin, transitions to the high level, the controller turns off the switches403and405and turns on the switch404thereby to instantaneously increase the DC bias voltage Vcc such that the DC bias voltage Vcc follows the input RF signal's envelope speed. The DC bias voltage Vcc is instantaneously boosted by the boosting voltage Vc1across the capacitor406, because the boosting voltage Vc1is in series with the input DC voltage Vbatt. In the illustrated example, the DC bias voltage Vcc is increased to 2Vbatt, twice the input DC voltage Vbatt, and the RF PA402saturation power is quadrupled. As such, the RF PA402operates linearly and amplifies input RF signal RFin, at high levels.

When the capacitor406is first coupled in series with the input DC voltage Vbatt, the capacitor406is discharged and provides a current to the RF PA402. The voltage Vc1across the capacitor406decreases 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 RFout, the controller increases the duty cycle of the boost DC converter401thereby to increase the output voltage of the boost DC converter401to stabilize the DC bias voltage Vcc provided to the RF PA402. When the input RF signal RFin, transitions to the low level, the ET PA400returns back to the low power mode. The envelope tracking power supply430reduces the DC bias voltage provided to the RF PA402by decoupling the capacitive network408from the input DC voltage Vbattand turning on the bypass switch403. The DC bias voltage Vcc can be decreased to the low level (e.g., Vbatt) instantaneously by turning on the bypass switch403. The controller turns off the switch404and subsequently turns on the bypass switch403to bias the RF PA402with the input DC voltage Vbatt. Subsequently, the controller turns on the switch405to charge the capacitor406. The controller further reduces the duty cycle of the boost DC converter101from D2to D1(e.g., from 50% from 0%).

FIG. 5is 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 PA500comprises a high-speed envelope tracking power supply530and an RF PA402. The envelope tracking power supply530is configured to provide a bias voltage Vcc to the RF PA402and comprises a boost DC converter401, a bypass switch403, a capacitive network508, and a controller (not shown). The capacitive network508includes a capacitor and a switch ladder. The envelope tracking power supply530achieves finer tuning of the DC bias voltage Vcc, compared to the envelope tracking power supply430illustrated inFIG. 4. The switches in the capacitor and switch ladder may be MOSFET switches, silicon CMOS, SOI, or HEMT etc. The capacitors406,506, and507may have the same or different capacitance. As an example, when the capacitors406,506, and507have the same capacitance, they are charged to have the same voltage. For example, when the ET PA500operates at the low power mode, the controller decouples the capacitive network508from the input DC voltage Vbatt. The switches405,504, and505are turned on and the switches404,502, and503are turned off. The capacitors406,506, and507are each charged to a third of the DC voltage, 1/3*Vbatt.

The illustrated envelope power supply500is similar to the envelope power supply400illustrated inFIG. 4, and thus the details of the ports410-412, the bypass switch403, the boost DC converter401, and the RF PA402are omitted for the sake of brevity. When the incoming signals RFin, are low power signals, the ET PA500operates at the low power mode, the capacitive network508is decoupled from biasing the RF PA402. The bypass switch403is on and the RF PA402is biased by the input DC voltage Vbatt. When the incoming signals RFintransitions into high power signals, the ET PA500transitions to operate at a high power mode. The controller turns off the bypass switch403and couples the capacitive network508to the input DC voltage Vbatt. When coupled to the input DC voltage Vbatt, the capacitive network508may be configured to provide different levels of boosting voltage (e.g., 1/3*Vbatt2/3*Vbatt, or Vbatt). The DC bias voltage Vcccan be increased instantaneously to various levels (e.g., 4/3*Vbatt, 5/3*Vbatt, or 2*Vbatt) to meet different amount of power needed by the RF PA402to maintain linear operation. For example, when the controller configures the capacitive network508such that the switches502and405are on, and the switches404and503through505are off, the voltage Vc2across the capacitors406is coupled in series with the voltage Vc3across the capacitor506, both of which are coupled in series with the input DC voltage Vbatt. As a result, the DC bias voltage Vccequals to Vbatt+(2/3)*Vbatt, when the capacitors406and506have the same capacitance. Other architectures of switch and capacitor ladders can also be used.

When the ET PA500reverts back to the low power mode, the controller decouples the capacitive network508and turns on the bypass switch403. The controller turns off the switches404,502and503and subsequently turns on the bypass switch403to couple the RF PA402to be biased by the input DC voltage Vbatt. Subsequently, the controller turns on the switches405,504and505to charge the capacitors406,506, and507.

FIG. 6is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system600, according to yet another embodiment. The illustrated ET PA600comprises a high-speed envelope tracking power supply630and an RF PA402. The envelope tracking power supply630is configured to provide a bias voltage Vcc to the RF PA402and comprises a boost DC converter401, a bypass switch403, a capacitive network608, and a controller (not shown). The capacitive network608comprises switches601through605and capacitors606and607. The controller controls the switches601through605as well as regulates the operations (i.e., switching on and off switches of the boost DC converter401) of the boost DC converter401. The controller typically is implemented as circuitry. The switches601through605may be MOSFET switches, silicon CMOS, SOI, or HEMT etc.

The illustrated envelope power supply600is similar to the envelope power supply400illustrated inFIG. 4, and thus the details of the ports410-412, the bypass switch403, the boost DC converter401, and the RF PA402are omitted for the sake of brevity. The envelope tracking power supply630may provide different levels of DC bias voltage such that the RF PA402can maintain operation linearity and efficiency for RF signals at different power levels. Within the envelope tracking power supply630, the capacitive network608is coupled between the port410and the RF PA402. The capacitors606and607and the switches601through605are configured such that the capacitor606or the capacitor606along with the capacitor607can be coupled to be in series with the input DC voltage Vbattvia regulating the on and off of the switches601through605.

When the input RF signal RFin, level is low, the envelope power supply600operates at the low power mode, where envelope tracking power supply630provides a low voltage (i.e., the input DC voltage Vbatt) for biasing the RF PA402. The capacitive network608is decoupled from the input DC voltage Vbattand the bypass switch403is on. During the low power mode, the switches601and604are off and the switches602,603and605are on. Both capacitors606and607are charged by the input DC voltage Vbatt. In addition, the controller regulates the boost converter to operate at a lower duty cycle D1(e.g., 0%).

When the input RF signals RFin, level is at a medium or high level, the envelope power supply400operates at a medium or high power mode, where the RF PA402is biased by a medium or high voltage (e.g., 2Vbattor 3Vbatt). When the input RF signal RFintransitions to the high level, the controller couples the capacitive network608to the input DC voltage Vbatt, turns off the bypass switch403, and regulates the boost DC converter601to 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 PA402is ensured to amplify the input RF signal RFinat different levels. As such, the ET PA600is ensured to track the signal envelope of the input RF signal RFin. When being coupled to the input DC voltage Vbatt, the capacitive network608may be configured to provide different levels of boosting voltages (e.g., Vbatt, or 2Vbatt). The DC bias voltage Vcccan be increased instantaneously to various levels (e.g., 2Vbattor 3Vbatt) to meet different amount of power needed by the RF PA402to maintain linear operation.

As one example, when the RF input signal RFintransitions into the medium level, the controller turns off the switches603and605and subsequently turns off the bypass switch403. Switch604was off and remains off. The controller subsequently turns on the switch601to couple the capacitor606to be in series with the input DC voltage Vbatt. As such, the DC bias voltage Vcc is instantaneously boosted by the voltage across the capacitor606.

As another example, when the RF input signal RFin, transitions into a high level, to further boost the voltage, the controller turns off the switch602and subsequently turns on the switch604. As such, the capacitor607is coupled in series with the capacitor606, both of which are coupled in series with the input DC voltage Vbatt. In both cases, the capacitor606or the capacitors606and607are discharged by supplying a current to the RF PA402. The controller may regulate the boost DC converter401by increasing its duty cycle to stabilize the DC bias voltage Vcc.

When the RF input signal RFin, 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 Vbattfrom 3Vbatt, the controller turns off the switch604and subsequently turns on the switches403,602,603, and605.

The ET PA600can be adapted to include an envelope tracking power that can vary the output voltage in finer steps, such as the example illustrated inFIG. 7.FIG. 7is a block diagram of an example high-speed envelope tracking radio frequency power amplifier system700, according to yet another embodiment. The illustrated ET PA700comprises a high-speed envelope tracking power supply730and an RF PA402. The envelope tracking power supply730is configured to provide a bias voltage Vcc to the RF PA402and comprises a boost DC converter401, a bypass switch403, a capacitive network708, and a controller (not shown). The capacitive network708includes switch and capacitor ladders701and702. Similar to the envelope power supply600illustrated inFIG. 6, the envelope power supply700may boost the DC bias voltage to different levels (e.g., 2Vbattand 3Vbatt). In addition, the envelope tracking power supply730can provide finer steps of voltage boosting. When being coupled to the input DC voltage Vbatt, the capacitive network708may be configured to provide different levels of boosting voltages (e.g., 1/3*Vbatt, 2/3*Vbatt, Vbatt, 4/3*Vbatt, 5/3*Vbatt, or 2Vbatt). The DC bias voltage Vcccan be increased instantaneously to various levels (e.g., 4/3*Vbatt, 5/3*Vbatt, 2Vbatt, 7/3*Vbatt, 8/3*Vbatt, or 3Vbatt) to meet different amount of power needed by the RF PA402to 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.