POWER AMPLIFIER SATURATION DETECTION

Abstract

In a portable radio transceiver, a power amplifier system includes a saturation detector that detects power amplifier saturation in response to duty cycle of the amplifier transistor collector voltage waveform. The saturation detection output signal can be used by a power control circuit to back off or reduce the amplification level of the power amplifier to avoid power amplifier control loop saturation.

Claims

1. (canceled)

2. A saturation detection system for detecting saturation of a power amplifier, the saturation detection system comprising: a first circuit in electrical communication with a first power amplifier, the first circuit configured to block a positive cycle portion of an amplified radio frequency signal output by the first power amplifier and pass a negative cycle portion of the amplified radio frequency signal output by the first power amplifier to a first averaging filter; and a second circuit including a comparator configured to output a saturation detection signal based at least in part on a comparison of an output of the first averaging filter to a reference signal, the reference signal being based at least in part on a signal from a second power amplifier.

3. The saturation detection system of claim 2 wherein an amplification level of the first power amplifier is adjusted based at least in part on the saturation detection signal.

4. The saturation detection system of claim 2 wherein the second circuit further includes a power amplifier controller configured to adjust a power level of the first power amplifier based at least in part on the saturation detection signal.

5. The saturation detection system of claim 2 wherein the second power amplifier is configured to process a different frequency band than the first power amplifier.

6. The saturation detection system of claim 2 wherein the first circuit includes a diode that clips the positive cycle portion of the amplified radio frequency signal output by the first power amplifier and passes the negative cycle portion of the amplified radio frequency signal output by the first power amplifier to the first averaging filter.

7. The saturation detection system of claim 2 wherein the first averaging filter is a low-pass filter.

8. The saturation detection system of claim 2, wherein the second circuit further includes a first switch and a second switch, the first switch configured to electrically connect the output of the first averaging filter to a first input of the comparator in response to a band-select signal being received, and the second switch configured to electrically connect an output of a second averaging filter corresponding to the second power amplifier to a second input of the comparator in response to the band-select signal being received.

9. The saturation detection system of claim 8 wherein, in response to a second band-select signal being received, the first switch is configured to electrically disconnect the output of the first averaging filter from the first input of the comparator and the second switch is configured to electrically disconnect the output of the second averaging filter from the second input of the comparator.

10. A transmitter comprising: a modulator; an upconverter; and a power amplifier system configured to receive a radio frequency signal after it passes through the modulator and the upconverter, the power amplifier system including a first power amplifier configured to amplify the radio frequency signal to produce an amplified radio frequency signal, a first circuit in electrical communication with the first power amplifier, and a second circuit, the first circuit configured to block a positive cycle portion of the amplified radio frequency signal and pass a negative cycle portion of the amplified radio frequency signal to a first averaging filter, and the second circuit including a comparator configured to output a saturation detection signal based at least in part on a comparison of an output of the first averaging filter to a reference signal, the reference signal being based at least in part on a signal from a second power amplifier.

11. The transmitter of claim 10 wherein an amplification level of the first power amplifier is adjusted based at least in part on the saturation detection signal.

12. The transmitter of claim 10 wherein the power amplifier system further includes a power amplifier controller configured to adjust a power control signal based at least in part on the saturation detection signal and to provide the power control signal to the first power amplifier to cause the first power amplifier to adjust a power level of the radio frequency signal based at least in part on the power control signal.

13. The transmitter of claim 10 wherein the second power amplifier is configured to process a different frequency band than the first power amplifier.

14. The transmitter of claim 10 wherein the second circuit further includes a first switch and a second switch, the first switch configured to electrically connect the output of the first averaging filter to a first input of the comparator in response to a band-select signal being received, and the second switch configured to electrically connect an output of a second averaging filter corresponding to the second power amplifier to a second input of the comparator in response to the band-select signal being received.

15. The transmitter of claim 14 wherein, in response to a second band -select signal being received, the first switch is configured to electrically disconnect the output of the first averaging filter from the first input of the comparator and the second switch is configured to electrically disconnect the output of the second averaging filter from the second input of the comparator.

16. A wireless device, comprising: a user interface; an antenna; a baseband subsystem in communication with the user interface; and a radio frequency subsystem coupled to the baseband subsystem and the antenna, the radio frequency subsystem including a transmitter, the transmitter including a modulator, an upconverter, and a power amplifier system, the power amplifier system configured to receive a radio frequency signal after it passes through the modulator and the upconverter, the power amplifier system including a first power amplifier, a first circuit in electrical communication with the first power amplifier, and a second circuit, the first power amplifier configured to amplify the radio frequency signal to produce an amplified radio frequency signal, the first circuit configured to block a positive cycle portion of the amplified radio frequency signal and pass a negative cycle portion of the amplified radio frequency signal to a first averaging filter, and the second circuit including a comparator configured to output a first saturation detection signal based at least in part on a comparison of an output of the first averaging filter to a reference signal, the reference signal being based at least in part on a signal from a second power amplifier.

17. The wireless device of claim 16 wherein the power amplifier system further includes a power amplifier controller configured to adjust a power control signal based at least in part on the first saturation detection signal and to provide the power control signal to the first power amplifier to cause the first power amplifier to adjust a power level of the radio frequency signal based at least in part on the power control signal.

18. The wireless device of claim 16 wherein the second power amplifier is configured to process a different frequency band than the first power amplifier.

19. The wireless device of claim 16 wherein the first saturation detection signal indicates that the first power amplifier is operating in a saturation state.

20. The wireless device of claim 19 wherein the comparator is further configured to output a second saturation detection signal indicating that the second power amplifier is operating in the saturation state, and the second circuit further includes a switching circuit configured to cause the comparator to output one of the first saturation detection signal or the second saturation detection signal based at least in part on a band-selection signal.

21. The wireless device of claim 16 wherein the second circuit further includes a first switch and a second switch, the first switch configured to electrically connect the output of the first averaging filter to a first input of the comparator in response to a band-select signal being received, and the second switch configured to electrically connect an output of a second averaging filter corresponding to the second power amplifier to a second input of the comparator in response to the band-select signal being received.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0014] 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.

[0015] FIG. 1 is a schematic diagram of a portion of a prior power amplifier system having saturation detection circuitry.

[0016] FIG. 2 is a block diagram of a mobile wireless telephone, in accordance with an exemplary embodiment of the present invention.

[0017] FIG. 3 is a block diagram of the transmitter portion of the mobile wireless telephone shown in FIG. 2.

[0018] FIG. 4 is a block diagram of the power amplifier system shown in FIG. 3.

[0019] FIG. 5 is a flow diagram illustrating a method of operation of the power amplifier system of FIG. 4.

DETAILED DESCRIPTION

[0020] As illustrated in FIG. 2, in accordance with an exemplary embodiment of the invention, a mobile wireless telecommunication device, such as a cellular telephone, includes a radio frequency (RF) subsystem 30, an antenna 32, a baseband subsystem 34, and a user interface section 36. The RF subsystem 30 includes a transmitter portion 38 and a receiver portion 40. User interface section 36 includes a microphone 42, a speaker 44, a display 46, and a keyboard 48, all coupled to baseband subsystem 34. The output of transmitter portion 38 and the input of receiver portion 40 are coupled to antenna 32 via a front-end module (FEM) 50 that allows simultaneous passage of both the transmitted RF signal produced by transmitter portion 38 and the received RF signal that is provided to receiver portion 40. But for transmitter portion 38, the above-listed elements can be of the types conventionally included in such mobile wireless telecommunication devices. As conventional elements, they are well understood by persons of ordinary skill in the art to which the present invention relates and, accordingly, not described in further detail in this patent specification (herein). However, unlike conventional transmitter portions of such mobile wireless telecommunication devices, transmitter portion 38 embodies power amplifier saturation detection features and methods, described in further detail below. It should be noted that while the invention is described in the context of an exemplary embodiment relating to a mobile wireless telephone, the invention alternatively can be embodied in other devices that include mobile or portable RF transmitters.

[0021] As illustrated in FIG. 3, a modulator 52 in transmitter portion 38 receives the signal that is input to transmitter portion 38. Modulator 52 modulates the input signal and provides the modulated signal to an upconverter 54. Upconverter 54 shifts or upconverts the frequency of the modulated signal from a baseband frequency to a transmit frequency and provides the upconverted signal to a power amplifier system 56. Although not shown in FIG. 2 or 3 for purposes of clarity, power amplifier system 56 can also receive one or more control signals from a system controller, which can be included in baseband subsystem 34 or other suitable element. Such control signals typically relate to adjusting amplifier gain, bias, and other amplifier parameters.

[0022] As illustrated in FIG. 4, power amplifier system 56 is based upon a high-band power amplifier 60 and a low-band power amplifier 62. Although the invention is described with regard to an exemplary embodiment in which the transmitter is of the dual-band type, capable of transmitting in a selected one of two frequency bands (referred to herein as high band and low band), the invention is applicable to power amplifier systems having as few as a single band and corresponding single power amplifier. Power amplifiers 60 and 62 can be of a conventional type and can include a number of cascaded stages, but only the transistor 64 of the final stage of power amplifier 60 and the transistor 66 of the final stage of power amplifier 62 are shown for purposes of clarity (other such stages and biasing circuitry being indicated by the ellipsis (. . . ) symbol). High-band power amplifier 60 receives an RF signal 68 to be amplified, and low-band power amplifier receives an RF signal 70 to be amplified. Note that power amplifiers 60 and 62 are not of the COVAC type; rather, the collector terminals of transistors 64 and 66 are coupled directly to the power supply voltage (V_BATT) via inductances 72 and 74, respectively. (As used herein, the term coupled means connected via zero or more intermediate elements.) The power supply voltage can be that which is provided by a suitable battery-based power supply circuit (not shown for purposes of clarity) of the type commonly included in mobile wireless telecommunication devices.

[0023] Power amplifier system 56 can further include a power amplifier system controller 76 that provides power control signals 78 and 80 to power amplifiers 60 and 62, respectively. Power amplifier system controller 76 can operate in response to power control signals 82 that it receives from a centralized device controller (not shown) in baseband subsystem 34 (FIG. 2) or other suitable portion of the mobile wireless telecommunication device. Power amplifier system controller 76 can further operate in response to feedback signals 84 and 86 that are respectively representative of the transmitted high-band and low-band RF signal power. As such a feedback control loop is conventional in mobile wireless telecommunication devices, it is well understood by persons of ordinary skill in the art and, accordingly, not described in further detail herein.

[0024] When a transistor 64 or 66 is not operating in its saturation region, its collector voltage waveform is sinusoidal. It has been found in accordance with the present invention that as a transistor 64 or 66 enters the bipolar device saturation region of its operation, the negative cycle portion of its collector voltage waveform becomes increasingly deformed from a sinusoidal shape. That is, entry into the saturation region affects the negative cycle portion more than the positive cycle portion. As transistor operation moves deeper and deeper into the saturation region, the positive cycle portion remains substantially sinusoidal, but the negative cycle portion becomes increasingly square and increases in duty cycle. Accordingly, a value that represents the ratio between the amount of time the collector voltage waveform is negative and the amount of time the collector voltage waveform is positive, i.e., the duty cycle, can provide an indication of saturation depth. Similarly, it can be noted that a value that represents the approximate average or mean voltage of the negative cycle portion can also provide an indication of saturation depth. In the exemplary embodiment of the invention, the value is determined as described below. It should be noted that although the term average or averaging is used herein for convenience, the term is not limited to the mathematical average (or mean) or a mathematical process, and encompasses within its scope of meaning all quantities that approximate or correspond to such an average or mean, as illustrated by the operation of the exemplary averaging circuitry described below.

[0025] A first limiter circuit 87 that is coupled to the output of high-band power amplifier 60 includes a first diode 88. A first averaging filter 90 that is coupled to the output of first limiter circuit 87 includes a capacitor 92 and two resistors 94 and 96. Biasing resistors 98 and 100 and the voltage provided by a voltage regulator 102 bias diode 88 and define the quiescent operating point of diode 88 to be substantially at the knee voltage of diode 88. In this manner, diode 88 turns on or conducts in response to even small positive voltage excursions or cycle portions of the RF signal at the output of power amplifier 60. When conducting, diode 88 clips the positive voltage excursion or cycle portion of the signal at a value of about one diode drop (0.7 V). Diode 88 is turned off or does not conduct in response to negative voltage excursions or cycle portions of the signal. Thus, first limiter circuit 87 passes the negative cycle portion and blocks or clips the positive cycle portion. A filter capacitor 104 inhibits the RF signal from interfering with the operation of other circuitry.

[0026] First averaging filter 90 receives the RF signal negative cycle portion that first limiter circuit 87 passed and low-pass filters or averages it. The output of first averaging filter 90 thus represents an average of the negative cycle portion of the RF signal that is output by high-band power amplifier 60. Stated another way, the output of first averaging filter 90 is representative of the duty cycle, i.e., the ratio between the amount of time that the RF signal that is output by high -band power amplifier 60 is negative and the amount of time that the RF signal that is output by high-band power amplifier 60 is positive. The combination of first limiter circuit 87 and first averaging filter 90 defines a first duty cycle detector.

[0027] A second limiter circuit 105 that is coupled to the output of low -band power amplifier 62 includes a second diode 106. A second averaging filter 108 that is coupled to the output of second limiter circuit 105 includes a capacitor 110 and two resistors 112 and 114. Biasing resistors 116 and 118 and the voltage provided by voltage regulator 102 bias diode 106 and define the quiescent operating point of diode 106 to be substantially at the knee voltage of diode 106. When conducting, diode 106 clips the positive voltage excursion or cycle portion of the signal, in the same manner as described above with regard to diode 88. Diode 106 is turned off or does not conduct in response to negative cycle portions. Thus, second limiter circuit 105 passes the negative cycle portion and blocks or clips the positive cycle portion. A filter capacitor 119 inhibits the RF signal from interfering with the operation of other circuitry.

[0028] Second averaging filter 108 receives the RF signal negative cycle portion that second limiter circuit 105 passed and low-pass filters or averages it. The output of second averaging filter 108 thus represents an average of the negative cycle portion of the RF signal that is output by low-band power amplifier 62. Stated another way, the output of second averaging filter 108 is representative of the duty cycle, i.e., the ratio between the amount of time that the RF signal that is output by low-band power amplifier 62 is negative and the amount of time that the RF signal that is output by low-band power amplifier 62 is positive. The combination of second limiter circuit 105 and second averaging filter 108 defines a second duty cycle detector.

[0029] A comparator circuit includes a comparator 120 and a switching circuit that comprises two single-pole double-throw switch devices 122 and 124. The pole terminal of the first switch device 122 is connected to a first input (e.g., the inverting input) of comparator 120. The pole terminal of the second switch device 124 is connected to a second input (e.g., the non-inverting input) of comparator 120. The first throw terminal of first switch device 122 is coupled to the output of first averaging filter 90 via a resistor 126 and is also connected to a first current source 128. The second throw terminal of first switch device 122 is coupled to the output of second averaging filter 108 via a resistor 130 and is also connected to a second current source 132. The first throw terminal of second switch device 124 is similarly coupled to the output of first averaging filter 90 via resistor 126 and is also connected to first current source 128. The second throw terminal of second switch device 124 is similarly coupled to the output of second averaging filter 108 via resistor 130 and is also connected to second current source 132. Switch devices 122 and 124 and current sources 128 and 132 are responsive to a band-select signal 134. The state of band-select signal 134 indicates either low-band operation or high-band operation. Although not shown for purposes of clarity, other circuitry, which may be included, for example, in baseband subsystem 34 (FIG. 2), generates band-select signal 134 in response to operating conditions in the manner that is conventional and well understood in dual-band mobile wireless telecommunication devices. It can also be noted that, at any given time while the device is transmitting, one of high-band power amplifier 60 and low-band power amplifier 62 is active and the other is inactive in accordance with band-select signal 134. That is, high-band power amplifier 60 and low-band power amplifier 62 are selectably activatable in response to band-select signal 134.

[0030] When band-select signal 134 indicates low-band operation, first switch device 122 connects the output of second averaging filter 108 (via resistor 130) to the first input (e.g., the inverting input) of comparator 120, and second switch device 124 connects the output of first averaging filter 90 (via resistor 126) to the second input (e.g., the non-inverting input) of comparator 120. (Band-select signal 134 and the corresponding switch positions are shown in FIG. 4 in the low -band state.) In addition, when band-select signal 134 indicates low-band operation, current source 128 is active, and current source 132 is inactive. However, as high -band power amplifier 60 is inactive during low-band operation, the voltage at the output of first averaging filter 90 is constant. This voltage is level-shifted by the effect of resistor 126 and current source 128. The level-shifted voltage serves as the comparator reference voltage and defines the low-band saturation detection threshold. (Including resistor 126 in the exemplary embodiment provides a convenient means for selecting or setting the low-band saturation detection threshold.)

[0031] In low-band operation, as the saturation depth of low-band power amplifier 62 increases, the voltage at the output of second averaging filter 108 (which can be referred to as the Vsat_lo signal) decreases. As the decreasing Vsat_lo signal crosses the low-band saturation detection threshold, comparator 120 produces a high or binary 1 output signal, thereby indicating that low-band power amplifier 62 is operating in (or at least substantially in) saturation. This saturation detection output signal can be provided to power amplifier system controller 76, which responds by adjusting power control signal 80 to indicate a reduction in the target amplifier power level. Alternatively, in other embodiments the saturation detection output signal can be provided to another element, such as a centralized device controller (not shown) in baseband subsystem 34 (FIG. 2), which can in turn respond by adjusting power control signals 82 that power amplifier system controller 76 receives. In such an embodiment, power amplifier system controller 76 in turn responds to the adjusted control signals 82 by adjusting power control signal 80 to indicate a reduction in the target amplifier power level.

[0032] As low-band power amplifier 62 responds to the change in power control signal 80 by reducing the power level of its output RF signal, the Vsat_lo signal increases. As the increasing Vsat_lo signal crosses the low-band saturation detection threshold, comparator 120 toggles to produce a low or binary 0 output signal, thereby indicating that low-band power amplifier 62 is no longer operating in saturation.

[0033] When band-select signal 134 indicates high-band operation, first switch device 122 connects the output of first averaging filter 90 (via resistor 126) to the first input (e.g., the inverting input) of comparator 120, and second switch device 124 connects the output of second averaging filter 108 (via resistor 130) to the second input (e.g., the non-inverting input) of comparator 120. In addition, when band-select signal 134 indicates high-band operation, current source 132 is active, and current source 128 is inactive. However, as low-band power amplifier 62 is inactive during high-band operation, the voltage at the output of second averaging filter 108 is constant. This voltage is level-shifted by the effect of resistor 130 and current source 132. The level-shifted voltage serves as the comparator reference voltage and defines the high-band saturation detection threshold. (Including resistor 130 in the exemplary embodiment provides a convenient means for selecting or setting the high-band saturation detection threshold.)

[0034] In high-band operation, as the saturation depth of high-band power amplifier 60 increases, the voltage at the output of first averaging filter 90 (which can be referred to as the Vsat_hi signal) decreases. As the decreasing Vsat_hi signal crosses the high-band saturation detection threshold, comparator 120 produces a high or binary 1 output signal, thereby indicating that high-band power amplifier 60 is operating in (or at least substantially in) saturation. This saturation detection output signal can be provided to power amplifier system controller 76, which responds by adjusting power control signal 78 to indicate a reduction in the target amplifier power level. As described above with regard to low-band operation, the saturation detection output signal alternatively can be provided to a centralized device controller or other element, which can in turn respond by adjusting power control signals 82 that power amplifier system controller 76 receives. In such an embodiment, power amplifier system controller 76 in turn responds to the adjusted control signals 82 by adjusting power control signal 78 to indicate a reduction in the target amplifier power level.

[0035] As high-band power amplifier 60 responds to the change in power control signal 78 by reducing the power level of its output RF signal, the Vsat_hi signal increases. As the increasing Vsat_hi signal crosses the high-band saturation detection threshold, comparator 120 toggles to produce a low or binary 0 output signal, thereby indicating that high-band power amplifier 60 is no longer operating in saturation.

[0036] Note that similar variations in reference voltage, diode and resistor values between the high-band and low-band circuitry are canceled by the common -mode rejection properties of comparator 120.

[0037] The above-described elements can be distributed over two or more integrated circuit chips 136 and 138 to take advantage of benefits of different chip process technologies. For example, chip 136 can be formed using Indium-Gallium -Phosphide (InGaP) Heterojunction Bipolar Transistor (HBT) technologies, and chip 138 can be formed using silicon BiCMOS technologies that can advantageously integrate bipolar and CMOS devices.

[0038] The operation of the above-described power amplifier system 56 is presented in flow diagram form in FIG. 5. As indicated by block 140, depending on whether the transmitter is operating in high-band or low-band mode, i.e., depending on which of high-band power amplifier 60 or low-band power amplifier 62 is active, either the high-band RF signal or low-band RF signal is amplified. As indicated by block 142, the amplified signal is limited by blocking positive voltage excursions (i.e., the positive cycle portion of the amplified voltage waveform) while passing negative voltage excursions (i.e., the negative cycle portion of the amplified voltage waveform). As indicated by block 144, the limited signal, representing only the negative cycle portion of the waveform, is averaged. This average or mean value provides an indication of saturation depth. As indicated by block 146, this value is compared with a threshold value (e.g., a reference voltage). A reference voltage can be obtained from the output of the inactive one of power amplifiers 60 and 62, which can function as part of a reference voltage circuit in combination with a current source. As indicated by block 148, if the average value falls below the threshold value, a saturation detection signal can be produced. The saturation detection signal can be used by power control circuitry to back off or reduce the target power level for the active one of power amplifiers 60 and 62.

[0039] 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 this invention. For example, although in the illustrated or exemplary embodiment described above the limiter section, averaging filter section, and comparator section are shown for purposes of illustration as embodied in discrete circuitry, persons skilled in the art will appreciate that some or all of such sections and elements thereof alternatively can be embodied in suitably programmed or configured digital signal processing logic. Accordingly, the invention is not to be restricted except in light of the following claims.