Patent Description:
The present invention relates to envelope tracking, and more particularly, to an envelope tracking supply modulator using a linear amplifier with a segmented output stage and an associated wireless communication system.

A power amplifier (PA) is used to amplify a radio-frequency (RF) signal for radio transmission. The PA is commonly found in a wireless communication device for driving antenna(s) of a transmitter. The power consumption of a PA is critical to a wireless communication device that is battery operated. Traditionally, the PA is biased with a fixed supply voltage. Peak RF output power conditions generally occur when the RF input signal input to the PA is at a maximum level. However, when the PA is backed-off from the peak RF output power conditions, the excess input power must be dissipated by the PA because it is not being transformed into useful RF output power. That is, the traditional fixed PA supply voltage results in significant amount of power loss as heat. Envelope tracking is a technique that requires the supply voltage of the PA to be modulated dynamically with the envelope of the RF input signal. This would make the PA operate closer to the peak level at all times and dramatically improve the efficiency of the PA. That is, the envelope tracking technique modulates the PA supply voltage to track the envelope of the RF input signal to reduce the amount of power dissipated as heat.

In wireless communications, bandwidth is the frequency range occupied by a modulated carrier signal. With the advance of wireless communication technology, a wider bandwidth is used by one modulated carrier signal. Hence, a wide bandwidth linear amplifier is needed by an envelope tracking supply modulator that is used to supply a modulated supply voltage to the PA. However, a typical linear amplifier generally consumes large quiescent current for achieving a wide envelope tracking bandwidth. As a result, a typical wide-bandwidth envelope tracking design is power-hungry.

Thus, there is a need for an innovative design which achieves wide-bandwidth envelope tracking with reduced quiescent current consumption.

<CIT> discloses a transmit apparatus having a supply modulator.

<CIT> discloses an envelope tracking supply modulator comprising a switch mode path and a linear amplifier path.

One of the objectives of the claimed invention is to provide an envelope tracking supply modulator using a linear amplifier with a segmented output stage and an associated wireless communication system.

The present invention is defined by the appended independent claim, to which reference should now be made. Specific embodiments are defined in the dependent claims.

Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms "include" and "comprise" are used in an open-ended fashion, and thus should be interpreted to mean "include, but not limited to. Also, the term "couple" is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

<FIG> is a block diagram illustrating an envelope tracking supply modulator (ETSM) according to an embodiment of the present invention. The ETSM <NUM> is arranged to generate a modulated supply voltage VPA according to an envelope input SENV, and provide the modulated supply voltage VPA to a power amplifier (PA) <NUM>. The PA <NUM> is powered by the modulated supply voltage VPA for amplifying a radio-frequency (RF) signal SRF to generate an output PA_OUT with the desired TX power. In this embodiment, the ETSM <NUM> employs hybrid ETSM architecture, and includes a DC-DC converter <NUM> and a linear amplifier (LA) <NUM>. The DC-DC converter <NUM> is arranged to generate and output a regulated direct current (DC) voltage VDC to an output port N3 of the ETSM <NUM> via an inductor LDC. For example, the DC-DC converter <NUM> may be implemented by a buck converter. The LA <NUM> is arranged to receive an envelope input SENV, and generate an amplifier output VAC to the output port N3 of the ETSM <NUM> via an alternating current (AC) coupling capacitor CAC. The regulated DC voltage VDC and the amplifier output VAC jointly control the modulated supply voltage VPA of the PA <NUM>. Specifically, the regulated voltage VDC decides a DC part of the modulated supply voltage VPA, and the amplifier output VAC decides an AC part of the modulated supply voltage VPA.

<FIG> is a diagram illustrating waveforms of the modulated supply voltage VPA under different power modes of the PA <NUM> according to an embodiment of the present invention. When the PA <NUM> operates under a high power mode PC2 (Power class <NUM>), the DC part VPArms (VPArms=<NUM>. 2V) of the modulated supply voltage VPA is set via the regulated DC voltage VDC, and the AC part of the modulated supply voltage VPA is set via the amplifier output VAC. When the PA <NUM> operates under a lower power mode PC3 (Power class <NUM>), the DC part VPArms (VPArms=<NUM>. 4V) of the modulated supply voltage VPA is set via the regulated DC voltage VDC, and the AC part of the modulated supply voltage VPA is set via the amplifier output VAC.

The LA <NUM> includes a pre-driver stage circuit <NUM> and an output stage circuit <NUM>. The pre-driver stage circuit <NUM> is arranged to receive the envelope input SENV, and generate a pre-driver output SPRE according to the envelope input SENV. The pre-driver stage circuit <NUM> may be implemented using any available pre-driver design. Since the present invention focuses on the output stage design, further description of the pre-driver stage circuit <NUM> is omitted here for brevity.

It should be noted that, for brevity and simplicity, only the components pertinent to the present invention are shown in <FIG>. In practice, the ETSM <NUM> may include additional component(s), depending upon actual design considerations. For example, the DC-DC converter <NUM> and the LA <NUM> form an envelope tracking modulator (ETM), and the envelope input SENV is provided to the ETM (particularly, LA <NUM>) through an analog filter (not shown) in the ETSM <NUM>.

As mentioned above, a typical wide-bandwidth envelope tracking design is power-hungry due to large quiescent current needed. To address this issue, the present invention proposes an envelope tracking supply modulator using a linear amplifier with a segmented output stage. As shown in <FIG>, the output stage circuit <NUM> includes a plurality of amplifiers 110_1-110_N each coupled between an input port N1 and an output port N2 of the output stage circuit <NUM>, where N is a positive integer not smaller than <NUM> (i.e. N≧<NUM>). For example, the output stage circuit <NUM> may be designed to have only two amplifiers 110_1 and <NUM>10_N (N=<NUM>). For another example, the output stage circuit <NUM> may be designed to have more than two amplifiers 110_1-110_N (N><NUM>). To put it simply, the output stage circuit <NUM> may be regarded as being segmented into multiple amplifiers, where the number of amplifiers may be adjusted, depending upon actual design considerations.

In this embodiment, each of the amplifiers 110_1-110_N may be implemented by a class-AB amplifier. <FIG> is a circuit diagram illustrating a class-AB amplifier used by the output stage circuit <NUM> according to an embodiment of the present invention. The class-AB amplifier design shown in <FIG> may be employed to implement one or more of the amplifiers 110_1-110_N. The class-AB amplifier <NUM> operates under a supply voltage VBB and a ground voltage GND, and includes a plurality of P-type metal oxide semiconductor (PMOS) transistors M10 and M12, a plurality of N-type metal oxide semiconductor (NMOS) transistors M11 and M13, and a plurality of buffers BUF1 and BUF2. In this embodiment, the PMOS transistor M12 and the NMOS transistor M13 are core devices with thin gate oxide, and the PMOS transistor M10 and the NMOS transistor M11 are input/output (I/O) devices with thick gate oxide. The pre-driver output SPRE supplied to the input port N1 of the output stage circuit <NUM> is a differential signal consisting of Vgp and Vgn. Hence, the input port N1 of the output stage circuit <NUM> includes a first node and a second node, where the first node is coupled to a gate electrode of the PMOS transistor M12, and the second node is coupled to a gate electrode of the NMOS transistor M1 <NUM>. A gate voltage of the PMOS transistor M10 is set by one reference voltage Vrefp through the buffer BUF1. A gate voltage of the NMOS transistor M11 is set by another reference voltage Vrefn through the buffer BUF2. A drain electrode of the PMOS transistor M10 and a drain electrode of the NMOS transistor M11 are coupled to the output port N2 of the output stage circuit <NUM>. Hence, when the class-AB amplifier <NUM> is enabled by the output stage circuit <NUM>, the class-AB amplifier <NUM> is involved in setting the amplifier output VAC at the output port N2 of the output stage circuit <NUM> according to the pre-driver output SPRE at the input port N1 of the output stage circuit <NUM>.

It should be noted that the circuit structure shown in <FIG> is for illustrative purposes only, and is not meant to be a limitation of the present invention. For example, another class-AB amplifier design may be employed to implement one or more of the amplifiers 110_1-110_N.

In one exemplary segmented output stage implementation, the amplifiers 110_1-110_N may be identical amplifiers, and therefore have the same output drive capability. In another exemplary implementation, the amplifiers 110_1-110_N may be different amplifiers. For example, the amplifiers 110_1-110_N may have the same circuit design but different transistor sizes, and therefore have different output drive capabilities. For another example, the amplifiers 110_1-110_N may have different circuit designs, and therefore have different output drive capabilities.

The amplifier output VAC generated from the output stage circuit <NUM> is involved in setting the modulated supply voltage VPA of the PA <NUM>. In this embodiment, the output stage circuit <NUM> selects one or more amplifiers from the amplifiers 110_1-110_N for generating the amplifier output VAC. For better understanding of the proposed TX-power based quiescent current reduction technique, the following assumes that the output stage circuit <NUM> may be designed to have only two amplifiers 110_1 and 110_N (N=<NUM>). When the PA <NUM> has a first output power level, the amplifier 110_1 is involved in setting the amplifier output VAC, and the amplifier <NUM>10_N (N=<NUM>) is not involved in setting the amplifier output VAC. When the PA <NUM> has a second output power level different from the first output power level, the amplifiers 110_1 and <NUM>10_N (N=<NUM>) are both involved in setting the amplifier output VAC. For example, the amplifier 110_N (N=<NUM>) is disabled when the PA <NUM> has the first output power level, and is enabled when the PA <NUM> has the second output power level higher than the first output power level. Hence, quiescent current of the LA <NUM> under a first condition that the PA <NUM> has the first output power level is smaller than quiescent current of the LA <NUM> under a second condition that the PA <NUM> has the second output power level higher than the first output power level. In this way, the efficiency of the ETSM <NUM> can be improved for mid-range and low TX power.

<FIG> is a diagram illustrating the relationship between quiescent current LA_IQ of the LA <NUM> (which has only two amplifiers in its output stage) and load current VPA_IL of the PA <NUM> according to an embodiment of the present invention. The quiescent current LA_IQ of the LA <NUM> is dominated by transistors. The load current VPA_IL of the PA <NUM> is positively correlated with the output power level of the PA <NUM>. The TX power of the PA <NUM> is segmented into two non-overlapping output power ranges R<NUM> and R<NUM>, where a maximum output power level within the output power range R<NUM> is lower than a minimum output power level within the output power range R<NUM>.

The characteristic curve CV1 represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> uses only one amplifier 110_1 for generating the amplifier output VAC. The characteristic curve CV2 represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> uses all amplifiers 110_1 and <NUM>10_N (N=<NUM>) for generating the amplifier output VAC. When the proposed TX-power based quiescent current reduction technique is used, the characteristic curve CV3 can be obtained. The characteristic curve CV3 represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> has only one amplifier 110_1 enabled and involved in setting the amplifier output VAC for an arbitrary output power level within the output power range R<NUM>, and has all amplifiers 110_1 and 110_N (N=<NUM>) enabled and involved in setting the amplifier output VAC for an arbitrary output power level within the output power range R<NUM>.

When an output power level of the PA <NUM> is any of the output power levels belonging to the output power range R<NUM> (e.g. PA <NUM> operates in a low power mode), the amplifier 110_1 is enabled and the amplifier 110_N (N=<NUM>) is disabled, such that only one of the amplifiers 110_1 and 110_N (N=<NUM>) is involved in setting the amplifier output VAC that contributes to the modulated supply voltage VPA. When an output power level of the PA <NUM> is any of the output power levels belonging to the output power range R<NUM> (e.g. PA <NUM> operates in a high power mode), the amplifiers 110_1 and 110_N (N=<NUM>) are both enabled and involved in setting the amplifier output VAC that contributes to the modulated supply voltage VPA. The quiescent current of the LA <NUM> is reduced when the TX power level is low. Therefore, the ETSM efficiency can be improved for mid-range and low TX power.

Alternatively, the output stage circuit <NUM> may be designed to have more than two amplifiers 110_1-110_N (N><NUM>). The same concept of using a segmented output stage for LA quiescent current reduction and ETSM efficiency improvement can be applied. <FIG> is a diagram illustrating the relationship between quiescent current LA_IQ of the LA <NUM> (which has more than two amplifiers in its output stage) and load current VPA_IL of the PA <NUM> according to an embodiment of the present invention. The TX power of the PA <NUM> is segmented into N non-overlapping output power ranges R<NUM>, R<NUM>,. , RN-<NUM>, RN, where a maximum output power level within the output power range R<NUM> is lower than a minimum output power level within the output power range R<NUM>, a maximum output power level within the output power range R<NUM> is lower than a minimum output power level within a higher output power range, and so on.

The characteristic curve CV1 represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> uses only one amplifier 110_1 for generating the amplifier output VAC. The characteristic curve CV2' represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> uses all amplifiers 110_1-110_N (N><NUM>) for generating the amplifier output VAC. When the proposed TX-power based quiescent current reduction technique is used, the characteristic curve CV3' can be obtained. The characteristic curve CV3' represents the relationship between load current VPA_IL of the PA <NUM> and quiescent current LA_IQ of the LA <NUM> under a condition that the LA <NUM> has different combinations of amplifiers that are enabled and involved in setting the amplifier output VAC for different output power ranges R<NUM>-RN of the PA <NUM>, respectively. For example, when an output power level of the PA <NUM> is any of the output power levels belonging to the output power range Ri (<NUM> ≦ i≦ N), the amplifiers 110_1-110_i are enabled and involved in setting the amplifier output VAC that contributes to the modulated supply voltage VPA, while the amplifiers 110_(i+<NUM>)-110_N are disabled and not involved in setting the amplifier output VAC.

To put it another way, in response to an output power level of the PA <NUM>, the output stage circuit <NUM> selects one or more amplifiers from the amplifiers 110_1-110_N for generating the amplifier output VAC. For example, the number of amplifiers selected from amplifiers 110_1-110_N and involved in setting the amplifier output VAC under a first condition that the PA <NUM> has a first output power level is smaller than the number of amplifiers selected from amplifiers 110_1-110_N and involved in setting the amplifier output VAC under a second condition that the PA <NUM> has a second output power level that is different from (e.g. higher than) the first output power level.

In addition to the amplifiers 110_1-110_N, the output stage circuit <NUM> may include an adjustable compensation circuit <NUM> that is arranged to change its compensation setting when the selection of amplifiers involved in setting the amplifier output VAC changes due to the output power level change of the PA <NUM>. <FIG> is a circuit diagram illustrating an adjustable compensation circuit according to an embodiment of the present invention. By way of example, but not limitation, the adjustable compensation circuit <NUM> shown in <FIG> may be implemented by the adjustable compensation circuit <NUM> shown in <FIG>. The adjustable compensation circuit <NUM> is coupled between the input port N1 and the output port N2 of the output stage circuit <NUM>, and includes a plurality of resistors R1, R2 and a plurality of variable capacitors C1, C2. The pre-driver output SPRE supplied to the input port N1 of the output stage circuit <NUM> is a differential signal consisting of Vgp and Vgn. Hence, the input port N1 of the output stage circuit <NUM> includes a first node and a second node, where the first node is coupled to one end of the resistor R1, and the second node is coupled to one end of the resistor R2. The capacitance values of the variable capacitors C1 and C2 can be dynamically adjusted to change the compensation setting of the adjustable compensation circuit <NUM>. For example, when the PA <NUM> has the first output power level, the adjustable compensation circuit <NUM> is configured to have a first compensation setting optimized for the output stage circuit <NUM>. When the PA <NUM> has the second output power, the adjustable compensation circuit <NUM> is configured to have a second compensation setting optimized for the output stage circuit <NUM>, where the second compensation setting is different from the first compensation setting due to capacitance adjustment made via variable capacitors C1 and C2.

It should be noted that the adjustable compensation circuit <NUM> may be optional. For example, the output stage circuit <NUM> may be modified to have the adjustable compensation circuit <NUM> replaced with a compensation circuit with a fixed compensation setting. Any envelope tracking supply modulator using a linear amplifier with a segmented output stage falls within the scope of the present invention.

<FIG> is a block diagram illustrating a wireless communication system according to an embodiment of the present invention. For example, the wireless communication system <NUM> may be a <NUM>-NR system or a <NUM>-LTE system, and an envelope tracking supply modulator of the wireless communication system <NUM> may have a digitally controlled linear amplifier with a segmented output stage for RF transmitter efficiency optimization. As shown in <FIG>, the wireless communication system <NUM> includes a transmit (TX) circuit <NUM>, an envelope tracking circuit <NUM>, and a modulator/demodulator circuit (labeled as "MODEM") <NUM>. The modulator/demodulator circuit <NUM> may be a part of a digital baseband circuit. The TX circuit <NUM> is arranged to receive a TX baseband signal TX_BBfrom the modulator/demodulator circuit <NUM>, generate a radio-frequency (RF) signal SRF according to the TX baseband signal TX_BB, and output the RF signal SRF to an antenna <NUM> via a power amplifier (PA) <NUM>. For example, the TX baseband signal TX_BB is a digital signal, the RF signal SRF is an analog signal, and the TX circuit <NUM> includes a TX digital front end circuit (labeled as "TXDFE") <NUM>, a digital-to-analog converter (DAC) <NUM> (in-phase (I) path), a digital-to-analog converter (DAC) <NUM> (quadrature (Q) path), an RF circuit (labeled as "TXRF") <NUM>, and the PA <NUM>. The TX digital front end circuit <NUM> may include a digital pre-distortion block, an upsampling block (I-path), an upsampling block (Q-path), etc. The RF circuit <NUM> may include an analog filter (I-path), an analog filter (Q-path), an upconverter, etc..

The envelope tracking circuit <NUM> is arranged to derive an envelope input SENV from the baseband signal TX_BB, and generate a modulated supply voltage VPA according to the envelope input SENV. For example, the TX baseband signal TX_BB is a digital signal, the envelope input SENV is an analog signal, and the envelope tracking circuit <NUM> includes an envelope tracking digital baseband circuit (labeled as "ETDBB") <NUM>, a DAC <NUM>, and an envelope tracking supply modulator (ETSM) <NUM>, where the ETSM <NUM> includes an analog filter <NUM> and an envelope tracking modulator (ETM) <NUM>. The envelope tracking digital baseband circuit <NUM> may include an envelope detection block, a power scaling block, a lookup table, an upsampling block, etc..

The PA <NUM> shown in <FIG> may be the PA <NUM> shown in <FIG>. The ETSM <NUM> (particularly, ETM <NUM>) may be implemented by using the hybrid ETSM architecture shown in <FIG>. Hence, the ETM <NUM> may include the DC-DC converter <NUM> and the linear amplifier <NUM> shown in <FIG>. In this embodiment, the ETSM <NUM> can be automatically adjusted according to digital detection of the TX power level for system efficiency optimization. As shown in <FIG>, the modulator/demodulator circuit <NUM> includes a TX power detection circuit <NUM> arranged to perform digital detection of the TX power level. The TX power detection circuit <NUM> may receive an output PA_OUT of the PA <NUM> via a coupler <NUM> and a receive (RX) path, and may detect the output power level of the PA <NUM> (i.e. TX power of the wireless communication system <NUM>), for example, by processing the output PA_OUT in a digital domain. The modulator/demodulator circuit <NUM> generates a control signal S_CTRL according to the detected output power level of the PA <NUM>, and outputs the control signal S_CTRL to the ETSM <NUM> (particularly, output stage circuit <NUM> of LA <NUM>). In response to the control signal S_CTRL, the output stage circuit <NUM> selects one or more amplifiers from the amplifiers 110_1-110_N for generating the amplifier output VAC, and/or configures the compensation setting of the adjustable compensation circuit <NUM>.

In the embodiment shown in <FIG>, the ETSM <NUM> has the LA <NUM> coupled to the output port N3 through the AC coupling capacitor CAC. However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. <FIG> is a block diagram illustrating another envelope tracking supply modulator according to an embodiment of the present invention. The major difference between ETSMs <NUM> and <NUM> is that the ETSM <NUM> has the LA <NUM> coupled to the output port N3 without via any AC coupling capacitor. In other words, the amplifier output VAC is direct current (DC) coupled to the output port N3. The same objective of using the regulated DC voltage Voc and the amplifier output VAC to jointly control the modulated supply voltage VPA of the PA <NUM> is achieved. Since a person skilled in the art can readily understand details of the ETSM <NUM> after reading above paragraphs directed to the ETSM <NUM>, further description is omitted here for brevity. Furthermore, regarding the wireless communication system <NUM> shown in <FIG>, the ETSM <NUM> (particularly, ETM <NUM>) may be implemented by using the hybrid ETSM architecture shown in <FIG>. Hence, the ETM <NUM> may include the DC-DC converter <NUM> and the linear amplifier <NUM> shown in <FIG>.

Claim 1:
An envelope tracking supply modulator (<NUM>) comprising:
a linear amplifier (<NUM>), comprising:
a pre-driver stage circuit (<NUM>), arranged to receive an envelope input, and
generate a pre-driver output according to the envelope input; and
an output stage circuit (<NUM>), arranged to receive the pre-driver output, and
generate an amplifier output of the linear amplifier (<NUM>) according to the pre-driver output, wherein the amplifier output is involved in setting a modulated supply voltage of a power amplifier, and the output stage circuit (<NUM>) comprises:
a plurality of amplifiers (110_1-110_N) each coupled between an input port (N1) and an output port (N2) of the output stage circuit (<NUM>), comprising a first amplifier and a second amplifier, wherein the envelope tracking supply modulator is configured to enable the first amplifier and disable the second amplifier if the power amplifier has a first output power level; and configured to enable both the first and second amplifier if the power amplifier has a second output power level different from the first output power level;
wherein an input port of the output stage circuit (<NUM>) is configured to receive the pre-driver output, an output port of the output stage circuit (<NUM>) is configured to generate the amplifier output, and the output stage circuit (<NUM>) further comprises:
an adjustable compensation circuit, coupled between the output port and the input port of the output stage circuit (<NUM>), wherein the adjustable compensation circuit comprises resistors (R1, R2) and variable capacitors (C1, C2); wherein the envelope tracking supply modulator is configured such that if power amplifier has the first output power level, the variable capacitors (C1, C2) have first capacitance values, and the adjustable compensation circuit is configured to have a first compensation setting; and if the power amplifier has the second output power level, the variable capacitors (C1, C2) have second capacitance values,
and the adjustable compensation circuit has a second compensation setting different from the first compensation setting due to capacitance adjustment made via the variable capacitors (C1, C2).