WO2011159308A1

WO2011159308A1 - Integrated linear power detection in an rf power amplifier

Info

Publication number: WO2011159308A1
Application number: PCT/US2010/039158
Authority: WO
Inventors: Dima P. Prikhodko; Hichem Ben Hedi Abdallah
Original assignee: Skyworks Solutions, Inc.
Priority date: 2010-06-18
Filing date: 2010-06-18
Publication date: 2011-12-22

Abstract

A radio frequency (RF) power amplifier system having a power detection feature includes a balanced power amplifier, an in-phase branch current detector, an out-of- phase branch current detector, and detection circuitry. The balanced power amplifier includes a phase splitter, an in-phase power amplifier branch, an out-of-phase power amplifier branch, and a phase combiner. The in-phase branch current detector provides an indication of current in the in-phase power amplifier branch. The out-of-phase branch current detector provides an indication of current in the out-of-phase power amplifier branch. The detection circuitry combines the indications of current in the in- phase and out-of-phase power amplifier branches to produce an indication of current in the balanced power amplifier.

Description

INTEGRATED LINEAR POWER DETECTION

IN AN RF POWER AMPLIFIER

Background

[0001] Cellular telephones (also referred to as handsets) and other such mobile wireless telecommunications devices continue to be improved so that they can communicate increasing amounts of data. For example, in the emerging markets of 3G/3.9G, linear transceiver systems, such as those that communicate in accordance with standards such as WCDMA, WiMAX, EUTRAN-LTE, and other non-constant envelope modulation methodologies, the requirement for highly efficient radio frequency (RF) power amplifiers that exhibit good linearity and power control under voltage standing wave ratio (VSWR) mismatch continues to present challenges.

[0002] Various types of power amplifiers are known, including single-ended power amplifiers (not shown) and balanced power amplifiers such as that shown in FIG. 1. As illustrated in FIG. 1, a balanced power amplifier 10 comprises a phase splitter 12, an in- phase power amplifier portion 14, an out-of-phase power amplifier portion 16, and a phase combiner 18. As out-of-phase power amplifier portion 16 is typically 90 degrees out of phase with in-phase power amplifier portion 14, in-phase power amplifier portion 14 can also be referred to as the 0° power amplifier portion, and out-of-phase power amplifier portion 16 can also be referred to as the -90° (or +90° in some embodiments) power amplifier portion. A first resistor 20 coupled to ground terminates the out-of- phase input of balanced power amplifier 10, and a second resistor 22 coupled to ground terminates the isolated output port of balanced power amplifier 10. Phase splitter 12 splits the input signal to be amplified (RF_IN) by providing signals at the in-phase (0°) output of phase splitter 12 and the out-of-phase (-90°) output of phase splitter 12 that are equal in amplitude by differ in phase by 90°. In-phase power amplifier portion 14 amplifies the signal it receives from the in-phase output of phase splitter 12. Out-of- phase power amplifier portion 16 amplifies the signal it receives from the out-of-phase output of phase splitter 12. Phase combiner 18 combines these amplified signals. When there is no mismatch between the two signals, the signals are not distorted, and phase combiner 18 produces an output signal (RF_OUT) that represents an equal combination of the two amplified signals. However, when there is mismatch between the two signals, power amplifier portions 14 and 16 experience opposite output phase impedance changes (i.e., one impedance is higher than the other) that partly compensate each other for linearity performance. This compensation helps maintain a constant current in balanced power amplifier 10. In contrast, under mismatch conditions in a single-ended power amplifier (not shown), a constant current would not be maintained.

[0003] Power amplifier power control in mobile wireless telecommunication devices typically involves a feedback loop in which the power amplifier output power is detected. The output power detection scheme is one of the key parameters in maintaining power amplifier linearity and linear power control. Conventionally, a number of output power detection schemes have been used, including: forward power detection using a coupler and a detector; implementing a detector at the power amplifier output, such as a collector voltage detector; and current detection using a current mirror.

[0004] Forward power detection can readily be implemented, but it is less effective in a high peak-to-average ratio (PAR) linear transmitter system than in other transmitter systems (e.g., a transmitter system in which the power amplifier is operated in saturation) because, for high impedance phases of the load, the voltage at the collector terminal of the amplifying transistor must be significantly increased in order to maintain the same linear output power. Increasing the collector voltage in this manner can promote undesirable signal compression in the power amplifier.

[0005] Another type of conventional RF power amplifier control loop detects output power by detecting the voltage at the collector terminal of the amplifying transistor of the power amplifier. Detecting collector voltage can be more preferable than a forward power detection scheme because VSWR matching circuitry that is commonly included can introduce undesirable phase shift. Holding the collector voltage constant in the closed loop can keep the amount of back-off constant for different VSWR mismatch phases. However, this advantage is achieved at the expense of output power. Even though requirements for output power deviation are not as stringent in a linear system as in a non-linear system, unacceptably large power drop can occur under mismatch conditions.

[0006] Still another conventional scheme for detecting RF power amplifier output power uses a current mirror. However, a conventional current mirror scheme cannot readily be used in a linear transmitter system because the current drops for the high impedance phases, causing the input power to increase in the closed loop to maintain the power detection signal equal to the reference signal. This leads to further power amplifier signal compression.

SUMMARY

[0007] Embodiments of the present invention relate to a radio frequency (RF) power amplifier system having a power detection feature. The power amplifier system includes a balanced power amplifier having a phase splitter, an in-phase power amplifier branch, an out-of-phase power amplifier branch, and a phase combiner. The power amplifier system further includes an in-phase branch current detector that provides an indication of current in the in-phase power amplifier branch, an out-of-phase branch current detector that provides an indication of current in the out-of-phase power amplifier branch, and detection circuitry that combines the indications of current in the in-phase and out-of-phase power amplifier branches to produce an indication of current in the balanced power amplifier.

[0008] Other embodiments are also provided. Other systems, methods, features, and advantages of the invention will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE FIGURES

[0009] The invention can be better understood with reference to the following figures.

The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.

[0010] FIG. 1 is a block diagram of a conventional balanced RF power amplifier.

[0011] FIG. 2 is a schematic diagram of a power amplifier system having a power detection feature, in accordance with an exemplary embodiment of the present invention.

[0012] FIG. 3 is a block diagram of an exemplary mobile wireless telecommunications device that includes a power amplifier system in accordance with the present invention.

[0013] FIG. 4 is a schematic diagram of a power amplifier system having a power detection feature, in accordance with another exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0014] As illustrated in FIG. 2 in accordance with an exemplary embodiment of the invention, a radio frequency (RF) balanced power amplifier system having a power detection feature has an in-phase branch 48 and an out-of -phase branch 50. An in-phase input of a phase splitter 52 receives the RF signal to be amplified (RF_IN). A resistor 54 terminates the out-of-phase input of phase splitter 52. The in-phase output of phase splitter 52 is coupled to the input of an in-phase power amplifier portion 56. The out-of- phase output of phase splitter 52 is coupled to the input of an out-of-phase power amplifier portion 58. The in-phase and out-of-phase power amplifier branches 48 and 50 include not only power amplifier portions 56 and 58, respectively, but can also include associated circuit elements, such as inductors 60 and 62 through which a fixed or regulated power supply voltage (VDD) is coupled to power amplifier portions 56 and 58, respectively. The output of in-phase power amplifier portion 56 is coupled to an out-of-phase input of a phase combiner 64. The output of out-of-phase power amplifier portion 58 is coupled to an in-phase input of phase combiner 64. Inductors 60 and 62 also serve as portions of impedance-matching networks (capacitors of which are not shown for purposes of clarity) for matching the output impedances of power amplifier portions 56 and 58 with the input impedances of the in-phase and out-of-phase inputs, respectively, of phase combiner 64. A resistor 66 terminates the isolated output port of phase combiner 64.

[0015] Phase splitter 52 splits the input signal to be amplified (RF_IN) by providing signals at the in-phase (0°) output of phase splitter 52 and the out-of-phase (-90°) output of phase splitter 52 that are equal in amplitude by differ in phase by 90°. In-phase power amplifier portion 56 amplifies the signal it receives from the in-phase output of phase splitter 12. Out-of-phase power amplifier portion 58 amplifies the signal it receives from the out-of-phase output of phase splitter 52. Phase combiner 64 combines these amplified signals and provides the resulting signal (RF_OUT) at an output port.

[0016] An in-phase branch current detector 68, an out-of-phase branch current detector

70, and detection circuitry 72 together detect the output power of the balanced RF power amplifier, i.e., the combined power output of in-phase power amplifier portion 56 and out-of-phase power amplifier portion 58. The output power of the balanced RF power amplifier is related to the sum of the current in in-phase power amplifier portion 56 and out-of-phase power amplifier portion 58. In the embodiment illustrated in FIG. 2, in- phase branch current detector 68 provides an indication of the current in in-phase power amplifier portion 56, out-of-phase branch current detector 70 provides an indication of the current in out-of-phase power amplifier portion 58, and detection circuitry 72 combines these indications by generating a signal representing their sum. In the embodiment illustrated in FIG. 2, the indications of the currents in in-phase power amplifier portion 56 and out-of-phase power amplifier portion 58 are themselves currents (signals). However, as described below with regard to an alternative embodiment, currents in the in-phase and out-of-phase power amplifier portions can alternatively be indicated by voltage signals.

[0017] In-phase branch current detector 68 includes replica circuitry comprising a transistor 78 that corresponds to transistor 74 in in-phase power amplifier portion 56, a bias resistor 80 that corresponds to a bias resistor 82 in in-phase power amplifier portion 56, and a coupling capacitor 84 that corresponds to a coupling capacitor 86 in in-phase power amplifier portion 56. The circuitry in in-phase branch current detector 68 thus substantially replicates the amplifying circuitry of in-phase power amplifier portion 56 but for a scaling factor N, where N is a number greater than one. The collector current in transistor 78 of in-phase branch current detector 68 can be one-Nth the collector current of transistor 74 of in-phase power amplifier portion 56. The values of components of the replica circuitry of in-phase branch current detector 68 can be similarly proportional to those of the amplifying circuitry of in-phase power amplifier portion 56. For example, the value or capacitance of coupling capacitor 86 of in-phase power amplifier portion 56 can be N times the value or capacitance of coupling capacitor 84 of in-phase branch current detector 68. Similarly, the value or resistance of bias resistor 82 of in-phase power amplifier portion 56 can be one-Nth the value or resistance of bias resistor 80 of in-phase branch current detector 68. Also, the area of transistor 78 of in-phase branch current detector 68 can be one-Nth the area of transistor 74 (i.e., the area on the chip or die (not shown) on which transistors 74 and 78 are formed).

[0018] Accordingly, the in-phase output of phase splitter 52 is coupled via coupling capacitor 86 to the base of transistor 74 of in-phase power amplifier portion 56 and coupled via coupling capacitor 84 to the base of transistor 78 of in-phase branch current detector 68. A bias current V_BIAS biases the base of transistor 74 through bias resistor 82 and also biases the base of transistor 78 through bias resistor 80. A filter capacitor 88 is coupled between the emitter and collector of transistor 78 to filter RF components out of the signal at the collector of transistor 78. The collector of transistor 78 is coupled to the input of a current mirror 89 comprising two FETs 90 and 92. The output of current mirror 89 provides an in-phase current mirror signal Ic^in, which is equal to Ic_in/N, where Ic_in is the collector current of transistor 74. Although in the exemplary embodiment current mirror 89 comprises FETs 90 and 92, in other embodiments such a current mirror can comprise bipolar transistors or any other suitable circuitry.

Out-of-phase branch current detector 70 includes replica circuitry comprising a transistor 94 that corresponds to transistor 76 in out-of-phase power amplifier portion 58, a bias resistor 96 that corresponds to a bias resistor 98 in out-of-phase power amplifier portion 58, and a coupling capacitor 100 that corresponds to a coupling capacitor 102 in out-of-phase power amplifier portion 58. The circuitry in out-of-phase branch current detector 70 thus substantially replicates the amplifying circuitry of out-of- phase power amplifier portion 58 but for the above-referenced scaling factor N. The collector current in transistor 94 of out-of-phase branch current detector 70 can be one- Nth the collector current of transistor 76 of out-of-phase power amplifier portion 58. The values of components of the replica circuitry of out-of-phase branch current detector 70 can be similarly proportional to those of the amplifying circuitry of out-of-phase power amplifier portion 58. For example, the value or capacitance of coupling capacitor 102 of out-of-phase power amplifier portion 58 can be N times the value or capacitance of coupling capacitor 100 of out-of-phase branch current detector 70. Similarly, the value or resistance of bias resistor 98 of out-of-phase power amplifier portion 58 can be one-Nth the value or resistance of bias resistor 96 of out-of-phase branch current detector 70. Also, the area of transistor 94 of out-of-phase branch current detector 70 can be one-Nth the area of transistor 76 (i.e., the area on the chip or die (not shown) on which transistors 94 and 98 are formed).

[0020] Accordingly, the out-of-phase output of phase splitter 52 is coupled via coupling capacitor 102 to the base of transistor 76 of out-of-phase power amplifier portion 58 and coupled via coupling capacitor 100 to the base of transistor 94 of out-of-phase branch current detector 70. A bias current V_BIAS biases the base of transistor 76 through bias resistor 98 and also biases the base of transistor 94 through bias resistor 96. A filter capacitor 104 is coupled between the emitter and collector of transistor 94 to filter RF components out of the signal at the collector of transistor 94. The collector of transistor 94 is coupled to the input of a current mirror 105 comprising two FETs 106 and 108. The output of current mirror 105 provides an out-of-phase current mirror signal

which is equal to Ic_out/N, where Ic_out is the collector current of transistor 74. (In FIG. 2, the signals Ic^in and Ic^out are labeled "Icm_in" and "Icm_out" for readability.) Although in the exemplary embodiment current mirror 105 comprises FETs 106 and 108, in other embodiments such a current mirror can comprise bipolar transistors or any other suitable circuitry.

[0021] In the embodiment shown in FIG. 2, detection circuitry 72 can comprise a resistor 110, the transistor pair 90 and 92 defining current mirror 89, and the transistor pair 106 and 108 defining current mirror 105. The currents Ic^in and Ic^out provided by the two current mirrors 89 and 105 are summed at a circuit node 112, yielding a detection signal V_DET across resistor 110 at circuit node 112. The detection signal V_DET has a value or voltage that represents or indicates the output power in the balanced power amplifier as a whole. The detection signal V_DET in FIG. 2 can be provided to elements of a power amplifier control loop in a manner similar to that in which similar power amplifier output power detection signals are conventionally used in a power amplifier control loop to generate a feedback or error signal to control the power amplifier. Although control of the balanced power amplifier is not shown in FIG. 2 for purposes of clarity, this aspect is well understood by persons skilled in the art to which the invention relates. An example of power amplifier power control in a mobile wireless telecommunication device is described in U.S. Patent No. 6,670,849. Although power amplifier control is in itself well understood, FIG. 3 illustrates the inclusion of the above-described balanced power amplifier system, with its power detection feature, in an exemplary mobile wireless telecommunication device 114 (transceiver), such as a cellular telephone handset or similar device.

[0022] As illustrated in FIG. 3, mobile wireless telecommunication device 114 includes an RF transmitter 116, an RF receiver 118, a baseband subsystem 120, a digital-to- analog converter (DAC) 122, and an analog-to-digital converter (ADC) 124 that can communicate with one another via a digital bus. Transmitter 116 can include a modulator, an upconverter, and any other functional elements that modulate and upconvert a baseband signal. The receiver 118 includes filter circuitry, downconverter circuitry and demodulation circuitry that enable the recovery of the information signal from the received RF signal. Mobile wireless telecommunication device 114 also includes a power amplifier system 126 and a power amplifier controller 128. The output of transmitter 116 is provided to power amplifier system 126. Receiver 118 and power amplifier system 126 are connected to a front end module 130. Front end module 130 can be a duplexer, a diplexer, or any element that separates the transmit signal from the receive signal. Front end module 130 is connected to an antenna 132. In transmit mode, the output of power amplifier system 126 (RF_OUT) is provided to front end module 130. In receive mode, front end module 130 provides a receive signal to receiver 118.

[0023] In transmit mode, baseband subsystem 120 can select a power level at which power amplifier 126 is to transmit by providing an indication of the power level to DAC 122, which in turn provides a corresponding power control voltage (V_DAC) to power amplifier controller 128. Power amplifier controller responds to the control voltage by providing a power control signal 133 (V_PC) to power amplifier system 126 that causes power amplifier system 126 to amplify an input signal (RF_IN) at the selected amplification level or gain. In accordance with the above-described feedback control principles, power amplifier system 126 provides the output power detection signal (V_DET) 134 to power amplifier controller 128. In response to the output power detection signal, power amplifier controller 128 can adjust control signal 132 to maintain the output power at the selected level.

[0024] An alternative embodiment of the system described above with regard to FIG. 2 is shown in FIG. 4. Descriptions of elements of this system that are identical to those described above with regard to FIG. 2, which are indicated by the "prime" symbol in the reference numerals, are not repeated here. The difference between the embodiment shown in FIG. 2 and the embodiment shown in FIG. 4 is that in the embodiment shown in FIG. 2 each current mirror couples only DC and low frequency signals, i.e., the RF signal is filtered out, while in the embodiment shown in FIG. 4 resistors 136 and 144 convert the respective collector currents of transistors 78' and 94' into voltages, which are then detected by peak detectors (diodes) 140 and 148, respectively.

[0025] More specifically, in the illustrated embodiment the collector of transistor 78' of in-phase branch current detector 68' is coupled to a battery voltage (V_BATT) or similar voltage via a resistor 136. A coupling capacitor 138 couples the output of the collector of transistor 78' to the input of diode 140. A bias resistor 142 is coupled between ground potential and the input of diode 140. Similarly, the collector of transistor 94' of out-of-phase branch current detector 70' is coupled to the battery voltage (V_BATT) via a resistor 144. A coupling capacitor 146 couples the output of the collector of transistor 70' to the input of diode 148. A bias resistor 150 is coupled between ground potential and the input of diode 148.

[0026] Detection circuitry 152 in this embodiment includes the above-described resistors 136 and 144, diodes 140 and 148, and a capacitor 156 and resistor 158. Capacitor 156 and resistor 158 are in parallel with each other and coupled between circuit node 154 and ground. Capacitor 156 and resistor 158 serve to sum the voltages provided detected by peak detectors (diodes) 140 and 148. The sum of these voltages yields a detection signal V_DET at circuit node 154 having a voltage that indicates the output power in the balanced power amplifier as a whole.

While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention. For example, the invention is not limited to a specific type of radio transmitter o power amplifier. Embodiments of the invention are applicable to different types of radio transmitters and power amplifiers and are applicable to any transmitter that transmits a non- constant envelope signal.

Claims

CLAIMS What is claimed is:

1. A radio frequency (RF) power amplifier system with power detection, comprising:

a balanced power amplifier comprising an in-phase power amplifier portion and an out-of-phase power amplifier portion; and

a power detector, the power detector providing an indication of current in the balanced power amplifier in response to a combination of current in the in-phase power amplifier portion and current in the out-of-phase power amplifier portion.

2. The RF power amplifier system claimed in claim 1, wherein the balanced power amplifier comprises a phase splitter, an in-phase power amplifier branch, an out-of- phase power amplifier branch, and a phase combiner, and the power detector comprises: an in-phase branch current detector having an in-phase replica circuit replicating amplifying circuitry of the in-phase power amplifier branch, the in-phase branch current detector providing an indication of current in the in-phase replica circuit;

an out-of-phase branch current detector having an out-of-phase replica circuit replicating amplifying circuitry of the out-of-phase power amplifier branch, the out-of- phase branch current detector having an output providing an indication of current in the out-of-phase replica circuit; and

detection circuitry coupled to the outputs of the in-phase branch current detector and the out-of-phase branch current detector, the detection circuitry providing an indication of current in the balanced power amplifier in response to a combination of the indication of current in the in-phase replica circuit and the indication of current in the out-of-phase replica circuit.

3. The RF power amplifier system with power detection claimed in claim 2, wherein:

the indication of current in the in-phase replica circuit comprises a first current signal;

the indication of current in the out-of-phase replica circuit comprises a second current signal; and

the detection circuitry comprises a current-summing node, the current-summing node producing a voltage indicating current in the balanced power amplifier in response to a sum of the first current signal and the second current signal.

4. The RF power amplifier system with power detection claimed in claim 2, wherein:

the in-phase power amplifier portion comprises a transistor having a base coupled to the in-phase output of the phase splitter and a collector providing the output of the in-phase power amplifier branch;

the out-of-phase power amplifier portion comprises a transistor having a base coupled to the out-of-phase output of the phase splitter and a collector providing the output of the out-of-phase power amplifier branch current.

the in-phase replica circuit comprises a transistor having a base coupled to the in-phase output of the phase splitter and a collector coupled to the output of the in-phase branch current detector; and

the out-of-phase replica circuit comprises a transistor having a base coupled to the in-phase output of the phase splitter and a collector coupled to the output of the out- of-phase branch current detector.

5. The RF power amplifier system with power detection claimed in claim 4, wherein the detection circuitry further comprises an in-phase current mirror having an input coupled to the collector of the transistor of the in-phase replica circuit, an out-of- phase current mirror having an input coupled to the collector of the transistor of the out- of-phase replica circuit, and a resistance coupled at a current-summing node to an output of the in-phase current mirror and an output of the out-of-phase current mirror.

6. The RF power amplifier system with power detection claimed in claim 4, wherein:

the in-phase replica circuit further comprises a capacitor coupled between the collector and an emitter of the transistor of the in-phase replica circuit, the capacitor filtering RF components from the signal coupled to the output of the in-phase branch current detector; and

the out-of-phase replica circuit further comprises a capacitor coupled between the collector and an emitter of the transistor of the out-of-phase replica circuit, the capacitor filtering RF components from the signal coupled to the output of the out-of- phase branch current detector.

7. The RF power amplifier system with power detection claimed in claim 4, wherein:

a collector current in the transistor of the in-phase replica circuit is one-Nth the collector current of the transistor of the in-phase power amplifier portion, where N is an integer;

the in-phase replica circuit further comprises a replica circuit coupling capacitor coupled between the in-phase output of the phase splitter and the base of the transistor of the in-phase replica circuit and a replica circuit bias resistor coupled between the base of the transistor of the in-phase replica circuit and a fixed bias voltage, the replica circuit coupling capacitor having a capacitance C, the replica circuit bias resistor having a resistance R; the in-phase amplifier portion further comprises an amplifier coupling capacitor coupled between the in-phase output of the phase splitter and the base of the transistor of the in-phase amplifier portion and an amplifier bias resistor coupled between the base of the transistor of the in-phase replica circuit and the fixed bias voltage, the amplifier coupling capacitor having a capacitance CxN and the replica circuit bias resistor having a resistance R N.

8. The RF power amplifier system with power detection claimed in claim 4, wherein:

a collector current in the transistor of the out-of-phase replica circuit is one-Nth the collector current of the transistor of the out-of-phase power amplifier portion, where N is an integer;

the out-of-phase replica circuit further comprises a replica circuit coupling capacitor coupled between the out-of-phase output of the phase splitter and the base of the transistor of the out-of-phase replica circuit and a replica circuit bias resistor coupled between the base of the transistor of the out-of-phase replica circuit and a fixed bias voltage, the replica circuit coupling capacitor having a capacitance C, the replica circuit bias resistor having a resistance R;

the out-of-phase amplifier portion further comprises an amplifier coupling capacitor coupled between the out-of-phase output of the phase splitter and the base of the transistor of the out-of-phase amplifier portion and an amplifier bias resistor coupled between the base of the transistor of the out-of-phase replica circuit and the fixed bias voltage, the amplifier coupling capacitor having a capacitance CxN and the replica circuit bias resistor having a resistance R/N.

9. The RF power amplifier system with power detection claimed in claim 2, wherein:

the indication of current in the in-phase replica circuit comprises a first voltage signal;

the indication of current in the out-of-phase replica circuit comprises a second voltage signal; and

the detection circuitry comprises an in-phase diode-based current-to-voltage converter, an out-of-phase diode-based current-to-voltage converter, and a voltage summer, the voltage summer providing an indication of current in the balanced power amplifier defined by a sum of the first voltage signal and the second voltage signal.

10. The RF power amplifier system with power detection claimed in claim 9, wherein:

the in-phase replica circuit comprises a transistor having a base coupled to the first phase output of the phase splitter and a collector coupled to the output of the in- phase branch current detector;

11. The RF power amplifier system with power detection claimed in claim 10, wherein the detection circuitry comprises a first resistance coupled to the collector of the transistor of the in-phase replica circuit, a second resistance coupled to the collector of the transistor of the out-of-phase replica circuit, a first detection diode coupled to the collector of the transistor of the in-phase replica circuit, and a second detection diode coupled to the collector of the transistor of the out-of-phase replica circuit.

12. The RF power amplifier system with power detection claimed in claim 11, wherein the first detection diode is coupled to the collector of the transistor of the in- phase replica circuit via a first coupling capacitor, and the second detection diode is coupled to the collector of the transistor of the out-of-phase replica circuit via a second coupling capacitor.

13. The RF power amplifier system with power detection claimed in claim 10, wherein:

the in-phase replica circuit further comprises a replica circuit coupling capacitor coupled between the in-phase output of the phase splitter and the base of the transistor of the in-phase replica circuit and a replica circuit bias resistor coupled between the base of the transistor of the in-phase replica circuit and a fixed bias voltage, the replica circuit coupling capacitor having a capacitance C, the replica circuit bias resistor having a resistance R;

the in-phase amplifier portion further comprises an amplifier coupling capacitor coupled between the in-phase output of the phase splitter and the base of the transistor of the in-phase amplifier portion and an amplifier bias resistor coupled between the base of the transistor of the in-phase replica circuit and the fixed bias voltage, the amplifier coupling capacitor having a capacitance CxN and the replica circuit bias resistor having a resistance R/N.

14. The RF power amplifier system with power detection claimed in claim 10, wherein:

15. A mobile wireless telecommunication device, comprising:

an antenna;

a radio frequency (RF) receiver system coupled to the antenna;

an RF transmitter system; a baseband subsystem coupled to the RF transmitter system and the RF receiver system;

a power amplifier system having an input coupled to an output of the RF transmitter system and an output coupled to the antenna, the power amplifier system comprising a balanced power amplifier and a power detector, the balanced power amplifier having an in-phase power amplifier portion and an out-of-phase power amplifier portion, the power detector providing an indication of current in the balanced power amplifier to the baseband subsystem in response to a combination of current in the in-phase power amplifier portion and current in the out-of-phase power amplifier portion.

16. The mobile wireless telecommunication device claimed in claim 15, wherein: the balanced power amplifier comprises a phase splitter, an in-phase power amplifier branch, an out-of-phase power amplifier branch, and a phase combiner; and the power detector comprises:

an in-phase branch current detector having an in-phase replica circuit replicating amplifying circuitry of the in-phase power amplifier branch, the in- phase branch current detector providing an indication of current in the in-phase replica circuit;

an out-of-phase branch current detector having an out-of-phase replica circuit replicating amplifying circuitry of the out-of-phase power amplifier branch, the out-of-phase branch current detector having an output providing an indication of current in the out-of-phase replica circuit; and

17. The mobile wireless telecommunication device claimed in claim 16, wherein: the indication of current in the in-phase replica circuit comprises a first current signal;

18. A method for detecting power in a radio frequency (RF) power amplifier system, the RF power amplifier system comprising having a balanced power amplifier having a phase splitter, an in-phase power amplifier branch, an out-of-phase power amplifier branch, and a phase combiner, the method comprising:

providing an indication of current in the in-phase power amplifier branch;

providing an indication of current in the out-of-phase power amplifier branch, and

combining the indication of current in the in-phase power amplifier branch and the indication of current in the out-of-phase power amplifier branch to produce an indication of current in the balanced power amplifier.

19. The method claimed in claim 18, wherein the balanced power amplifier comprises a phase splitter, an in-phase power amplifier branch, an out-of-phase power amplifier branch, and a phase combiner, and wherein: providing an indication of current in the in-phase power amplifier branch comprises providing an indication of current in an in-phase replica circuit of an in-phase branch current detector, the in-phase replica circuit replicating amplifying circuitry of the in-phase power amplifier branch;

providing an indication of current in the out-of-phase power amplifier branch comprises providing an indication of current in an out-of-phase replica circuit of an out- of-phase branch current detector, the out-of-phase replica circuit replicating amplifying circuitry of the out-of-phase power amplifier branch; and

combining the indication of current in the in-phase power amplifier branch and the indication of current in the out-of-phase power amplifier branch comprises providing an indication of current in the balanced power amplifier in response to a combination of the indication of current in the in-phase replica circuit and the indication of current in the out-of-phase replica circuit.

20. The method claimed in claim 19, wherein the indication of current in the in- phase replica circuit comprises a first current signal, and the indication of current in the out-of-phase replica circuit comprises a second current signal, and wherein:

providing an indication of current in the balanced power amplifier comprises producing a voltage indicating current in the balanced power amplifier in response to a sum of the first current signal and the second current signal.