HK1190516A - Power amplifier, power amplifier saturation detection and mobile wireless telecommunication device - Google Patents

Description

Power amplifier, power amplifier saturation detection, mobile radio telecommunication device

The present application is a divisional application of an invention patent application having an application date of 26/10/2009, an application number of 200980152892.4 (international application number of PCT/US 2009/061991), and an invention name of "power amplifier saturation detection".

Technical Field

The disclosure relates to a power amplifier circuit, a power amplifier saturation detection method and a mobile radio device.

Background

Radio Frequency (RF) transmitters of the type used in mobile radiotelephones (also known as cellular telephones) and other portable radio transceivers typically include transmit power control circuitry that adjusts the power of the transmitted RF signals. The power control circuit may adjust the power amplifier to increase or decrease the transmitted RF power. Adjusting the transmitted RF power may be beneficial for some purposes. For example, in many types of cellular telecommunication systems, it is beneficial to transmit higher RF power when the transceiver (also referred to as a handset) is farther from the nearest base station, and lower RF power when the transceiver is closer to the nearest base station. Also, in some types of multi-mode (e.g., dual-mode) transceivers, such as transceivers capable of operating in accordance with both the GSM (global system for mobile communications) standard and the EDGE (enhanced data rates for GSM evolution) standard, the need for transmitted RF power varies depending on whether the transceiver is operating in the GSM mode or the EDGE mode. Similarly, the need for transmitted RF power may differ in multi-band (e.g., dual-band) transceivers, such as transceivers capable of operating in both the GSM "low band" frequency band (e.g., the 880-1910 MHz band used in most places in europe, africa, the middle east, and asia) and the GSM "high band" frequency band (e.g., the 1850-1910MHz band used in the united states). To accommodate the different power amplification requirements of multiple frequency bands, the power amplifier system of the transceiver may correspondingly include multiple power amplifiers.

In some applications, a power amplifier system of a portable radio transceiver includes a negative feedback power control loop to adjust the output power of the power amplifier to a level within an allowable range specified by the mode in which the transceiver is operating. For example, when the transceiver is transmitting in GSM mode, the power control loop strives to maintain the amplifier output power within the allowable range specified by the GSM standard for frequency shift keying modulated (specifically, Gaussian Minimum Shift Keying (GMSK)) signals transmitted according to the GSM standard. Also, when the transceiver is transmitting in EDGE mode, the control loop strives to maintain the amplifier output power within the allowable range specified by the EDGE standard for 8-phase shift keying (8 PSK) modulated signals transmitted in accordance with the EDGE standard. Generally, the feedback loop compares a feedback quantity, such as a detected RF output power level, to a reference control voltage. The difference between the two voltages, also referred to as the difference error, is integrated and applied to the power control port of the power amplifier. For GMSK, the power amplifier power control port is typically a voltage controlled input (V _ PC) that adjusts the power amplifier bias. The RF input level is fixed. For EDGE, the power amplifier power control port is at the RF input level. In EDGE, V _ PC can also be adjusted to optimize efficiency while maintaining linearity. The large loop gain minimizes the error and drives the (drive) output power accuracy to that of the loop feedback circuit and the reference control voltage.

In situations such as insufficient battery power and VSWR (voltage standing wave ratio) load line limit (load linextrame), the power amplifier control loop may not expect the ground voltage to saturate. Such a situation may result in an undesirable decrease in control loop gain, an undesirable increase in differential error, or both. These effects can manifest themselves in a slow control loop response, resulting in a power amplifier output power level drift (drift), or even a complete loss of control loop lock.

Power control loop saturation may also cause handover spectrum degradation and inconsistency with the applicable transmission standard (e.g., GMSK), such as exceeding the power versus time (PvT) measurement specified by the applicable standard. Furthermore, peaks of the amplitude modulated EDGE signal envelope may become clipped (clip), resulting in modulation spectrum degradation.

To avoid power control loop saturation, some power amplifier systems have included circuitry that monitors the loop error voltage and reduces the loop reference voltage until the loop error is cancelled. Alternatively, the power amplifier system may include a saturation detection circuit that detects when the control loop is approaching saturation and activates a "saturation detect" signal. The power control circuit responds to the target output power by reducing it until the saturation detection circuit deactivates the "saturation detect" signal (indicating normal or non-saturated control loop operation).

For example, as shown in fig. 1, the gain or amplification of the power amplifier 10 is controlled by a voltage regulator 12, where the voltage regulator 12 includes an operational amplifier 13, a PFET (p-channel field effect transistor) 14, and associated resistors 16 and 18. The power amplifier 10 may include a plurality of cascaded stages, but for clarity purposes only the transistor 20 of the final stage is shown (other such stages are indicated by the omitted ("…") notation). The voltage regulator 12 is responsive to a power control signal (V _ PC) generated by a power control circuit (not shown for purposes of clarity). Note that the output of operational amplifier 13 is coupled to the collector terminal of transistor 20 via PFET 14. Such an arrangement provides so-called collector voltage amplifier control (COVAC).

The circuitry for generating the "saturation detection" signal includes comparator 22, current source 24 and resistor 26. A supply voltage (V _ BATT) provided by a battery operated power supply (not shown for purposes of clarity) is coupled to a source terminal of PFET14 and one terminal of resistor 26. A power supply dependent reference voltage is applied to one terminal of comparator 22 via resistor 26 and current source 24. The other terminal of comparator 22 receives the drain voltage of PFET 14. If the PFET14 drain voltage exceeds the comparator reference voltage, comparator 22 generates a "saturation detect" signal indicating that the voltage regulator is saturated. The regulator gain bandwidth is not sufficient to accurately follow the V _ PC input signal, resulting in power amplifier PvT time shadowing (mask) and violation of the switching spectrum specifications.

While the technique described above with reference to fig. 1 for detecting when the power amplifier control loop is in or near saturation is beneficial in power amplifier systems having a COVAC transistor arrangement, the technique cannot be used in some other situations. For example, in some power amplifier transistor arrangements, the collector of the final stage transistor is directly connected to the supply voltage (V _ BATT).

Disclosure of Invention

Embodiments of the present invention relate to a power amplifier system in a portable Radio Frequency (RF) transmitter or transceiver, a mobile wireless telecommunication device having such a transceiver, and a method of operating a power amplifier system, wherein the power amplifier system comprises a saturation detector for detecting saturation of the power amplifier.

In an exemplary embodiment, a power amplifier circuit includes a power amplifier, a duty cycle detector, and a comparator section. The power amplifier has at least one output transistor having an output transistor terminal coupled to a supply voltage. The duty cycle detector may provide an indication that the power amplifier is saturated by detecting the duty cycle or the ratio between the amount of time that the waveform produced at the output transistor terminal is negative and the amount of time that the waveform is positive.

In an exemplary embodiment, the duty ratio detector may include a limiter (limiter) part and an averaging filter part. The limiter section is coupled to the output transistor terminal and blocks positive voltage excursions while passing negative voltage excursions. The averaging filter section is coupled to an output of the limiter section. The comparator section generates a saturation detection output signal by comparing the signal output by the averaging filter section with a reference voltage. The saturation detection output signal may be used by the power control circuit to back off or reduce the amplification level of the power amplifier to avoid operating in saturation. The slicer portion, averaging filter portion, and comparator portion, or portions thereof, may be included in any suitable circuit or system, such as discrete circuits formed in an integrated circuit chip, in programmed or configured digital signal processing logic, or in any other suitable circuit or system.

According to an aspect of the invention, there is provided a power amplifier circuit for a radio frequency, RF, transmitter, the power amplifier circuit comprising: a power amplifier having at least one output transistor having an output transistor terminal coupled to a supply voltage; a duty cycle detector having an input coupled to the output transistor terminal and generating an output representing a ratio between an amount of time that an RF signal amplified by a power amplifier is negative and an amount of time that the RF signal is positive; and a comparator section having first and second comparator inputs and generating a saturation detection output signal, one of the first and second comparator inputs being coupled to the output of the duty cycle detector and the other of the first and second comparator inputs being coupled to a reference voltage circuit.

According to another aspect of the invention, there is provided a method for detecting power amplifier saturation in a radio frequency, RF, transmitter, comprising: amplifying the RF signal using an output transistor having an output transistor terminal coupled to a supply voltage; detecting a duty cycle between an amount of time that the amplified RF signal is negative and an amount of time that the amplified RF signal is positive; providing a signal representative of the duty cycle to a first input of a comparator section; providing a reference voltage to a second input of the comparator section; and generating a saturation detection output signal in response to a comparison of a signal representative of the duty cycle with the reference voltage.

According to another aspect of the present invention there is provided a mobile wireless telecommunications device comprising: a user interface; an antenna; a baseband subsystem coupled to the user interface; and a radio frequency, RF, subsystem coupled to the baseband subsystem and the antenna, the RF subsystem including a transmitter portion and a receiver portion, the transmitter portion including a modulator, an upconverter, and a power amplifier system, the power amplifier system including: a power amplifier having a high-band output transistor and a low-band output transistor; a first duty cycle detector having an input coupled to a high band output transistor terminal; a second duty cycle detector having an input coupled to the low band output transistor terminal; and a comparator section.

Other systems, methods, features and advantages of the invention will be, or will 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 following claims.

Drawings

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

Fig. 1 is a schematic diagram of a portion of a prior art power amplifier system having a saturation detection circuit.

Fig. 2 is a block diagram of a mobile radiotelephone in accordance with an exemplary embodiment of the present invention.

Fig. 3 is a block diagram of the transmitter portion of the mobile radiotelephone shown in fig. 2.

Fig. 4 is a block diagram of the power amplifier system shown in fig. 3.

Fig. 5 is a flow chart illustrating a method of operation of the power amplifier system of fig. 4.

Detailed Description

As shown in fig. 2, a mobile wireless telecommunications device, such as a cellular telephone, in accordance with an exemplary embodiment of the present invention 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 section 38 and a receiver section 40. The user interface section 36 includes a microphone 42, a speaker 44, a display 46, and a keyboard 48, all of which are coupled to the baseband subsystem 34. An output of transmitter section 38 and an input of receiver section 40 are coupled to antenna 32 via a Front End Module (FEM) 50, wherein Front End Module (FEM) 50 allows both transmitted RF signals generated by transmitter section 38 and received RF signals provided to receiver section 40 to pass through simultaneously. However, for the transmitter portion 38, the elements listed above may be of the type conventionally included in such mobile wireless telecommunications devices. As conventional elements, they are well understood by those of ordinary skill in the art to which the present invention pertains, and thus they are not described in further detail in this patent specification ("herein"). However, unlike the conventional transmitter portion of such mobile wireless telecommunication devices, the transmitter portion 38 incorporates power amplifier saturation detection features and methods described in further detail below. It should be noted that while the present invention is described in the context of an exemplary embodiment involving a mobile radiotelephone, the present invention may alternatively be incorporated into other devices including mobile or portable RF transmitters.

As shown in fig. 3, a modulator 52 in the transmitter portion 38 receives a signal input to the transmitter portion 38. The modulator 52 modulates the input signal and supplies the modulated signal to the up-converter 54. An upconverter 54 shifts or upconverts the frequency of the modulated signal from the 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, the power amplifier system 56 may also receive one or more control signals from a system controller, which may be included in the baseband subsystem 34 or other suitable element. Such control signals typically involve adjusting amplifier gain, bias, and other amplifier parameters.

As shown in fig. 4, the power amplifier system 56 is based on a high-band power amplifier 60 and a low-band power amplifier 62. Although the present invention is described with respect to an exemplary embodiment in which the transmitter is of a dual band type capable of transmitting in a selected one of two frequency bands (referred to herein as a high band and a low band), the present invention is applicable to power amplifier systems having as few as a single frequency band and a corresponding single power amplifier. Power amplifiers 60 and 62 may be of conventional type and may include a plurality of cascaded stages, but for purposes of clarity only transistor 64 of the final stage of power amplifier 60 and transistor 66 of the final stage of power amplifier 62 are shown (other such stages and biasing circuitry are represented by the omitted ("…") symbol). The high band power amplifier 60 receives an RF signal 68 to be amplified and the 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; instead, the collector terminals of transistors 64 and 66 are directly coupled to the supply voltage (V _ BATT) via inductors 72 and 74, respectively. (as used herein, the term "coupled" means connected via zero or more intermediate elements.) the supply voltage may be a supply voltage provided by a suitable battery-based supply circuit of the type typically included in mobile wireless telecommunications devices (not shown for purposes of clarity).

The power amplifier system 56 may also include a power amplifier system controller 76 that provides power control signals 78 and 80 to the power amplifiers 60 and 62, respectively. The power amplifier system controller 76 may operate in response to a power control signal 82, the power control signal 82 being received by the power amplifier system controller 76 from a centralized device controller (not shown) in the baseband subsystem 34 (fig. 2) or other suitable portion of the mobile wireless telecommunication device. The power amplifier system controller 76 is also operable in response to feedback signals 84 and 86 representing the power of the transmitted high band and low band RF signals, respectively. Since such feedback control loops are conventional in mobile wireless telecommunication devices, they are well understood by those of ordinary skill in the art and therefore will not be described in further detail herein.

When 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 when transistor 64 or 66 enters the saturation region of the bipolar device in which it operates, the negative periodic portion of its collector voltage waveform becomes progressively distorted from a sinusoidal shape. That is, entering 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 periodic portion remains substantially sinusoidal, while the negative periodic portion gradually becomes square and increases the duty cycle. Thus, a value (i.e., duty cycle) representing the ratio between the amount of time that the collector voltage waveform is negative and the amount of time that the collector voltage waveform is positive may provide an indication of the saturation depth. Similarly, it may be noted that a value representing the approximate average or mean voltage of the negative periodic portion may also provide an indication of the saturation depth. In an exemplary embodiment of the present invention, the value is determined as follows. It should be noted that although the term "averaging" or "averaging" is used herein for convenience, the term is not limited to mathematical averaging or mathematical processes and includes within its meaning all quantities that approximate or correspond to such an average, as illustrated by the operation of the exemplary averaging circuit described below.

The first limiter circuit 87, which is coupled to the output of the high band power amplifier 60, comprises a first diode 88. A first averaging filter 90 coupled to the output of the first limiter circuit 87 comprises a capacitor 92 and two resistors 94 and 96. Bias resistors 98 and 100 and the voltage provided by voltage regulator 102 bias diode 88 and define the quiescent operating point of diode 88 as being substantially at the knee voltage (knee voltage) of diode 88. In this manner, diode 88 turns on or conducts in response to a very small positive voltage excursion or periodic portion of the RF signal at the output of power amplifier 60. When conducting, diode 88 will shift or clip the positive voltage of the signal for a portion of the cycle to a value of about one diode drop (0.7V). Diode 88 is turned off or non-conducting in response to a negative voltage excursion or a portion of the cycle of the signal. Thus, the first limiter circuit 87 passes the negative periodic portion and blocks or limits the positive periodic portion. The filter capacitor 104 inhibits the RF signal from interfering with the operation of other circuitry.

The first averaging filter 90 receives the negative periodic portion of the RF signal passed by the first limiter circuit 87 and low-pass filters or averages the negative periodic portion of the RF signal. The output of the first averaging filter 90 thus represents the average of the negative periodic portion of the RF signal output by the high-band power amplifier 60. In other words, the output of the first averaging filter 90 represents a duty cycle, i.e., the ratio between the amount of time that the RF signal output by the high-band power amplifier 60 is negative and the amount of time that the RF signal output by the high-band power amplifier 60 is positive. The combination of the first limiter circuit 87 and the first averaging filter 90 defines a first duty cycle detector.

The second limiter circuit 105, which is coupled to the output of the low band power amplifier 62, includes a second diode 106. The second averaging filter 108 coupled to the output of the second limiter circuit 105 comprises a capacitor 110 and two resistors 112 and 114. Bias resistors 116 and 118 and the voltage provided by voltage regulator 102 bias diode 106 and define the quiescent operating point of diode 106 as substantially the knee voltage of diode 106. When conducting, diode 106 clips the positive voltage excursion or portion of the cycle of the signal in the same manner as described above with respect to diode 88. Diode 106 is turned off or non-conducting in response to the negative portion of the cycle. Thus, the second limiter circuit 105 passes the negative periodic portion and blocks or limits the positive periodic portion. The filter capacitor 119 inhibits the RF signal from interfering with the operation of other circuitry.

The second averaging filter 108 receives the negative periodic part of the RF signal passed by the second limiter circuit 105 and low-pass filters or averages the negative periodic part of the RF signal. The output of the second averaging filter 108 thus represents the average of the negative periodic portion of the RF signal output by the low-band power amplifier 62. In other words, the output of the second averaging filter 108 represents a duty cycle, i.e., the ratio between the amount of time that the RF signal output by the low band power amplifier 62 is negative and the amount of time that the RF signal output by the low band power amplifier 62 is positive. The combination of the second limiter circuit 105 and the second averaging filter 108 defines a second duty cycle detector.

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

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

In low-band operation, as the saturation depth of the low-band power amplifier 62 increases, the voltage at the output of the second averaging filter 108 (which may be referred to as the Vsat lo signal) decreases. When the reduced Vsat _ lo signal crosses the low-band saturation detection threshold, the comparator 120 generates a high or binary "1" output signal, indicating that the low-band power amplifier 62 is operating in saturation (or at least substantially in saturation). This saturation detection output signal may be provided to the power amplifier system controller 76, which the power amplifier system controller 76 responds by adjusting the power control signal 80 to indicate a decrease in the target amplifier power level. Alternatively, in other embodiments, the saturation detection output signal may be provided to another element, such as a centralized device controller (not shown) in the baseband subsystem 34 (fig. 2), which may then respond by adjusting the power control signal 82 received by the power amplifier system controller 76. In such embodiments, the power amplifier system controller 76 then responds to the adjusted control signal 82 by adjusting the power control signal 80 to indicate a decrease in the target amplifier power level.

The Vsat lo signal increases when the low-band power amplifier 62 responds to a change in the power control signal 80 by decreasing the power level of its output RF signal. When the increased Vsat _ lo signal crosses the low-band saturation detection threshold, the comparator 120 toggles (toggle) to produce a low or binary "0" output signal, indicating that the low-band power amplifier 62 is no longer operating in saturation.

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

In high-band operation, as the saturation depth of the high-band power amplifier 60 increases, the voltage at the output of the first averaging filter 90 (which may be referred to as the Vsat — hi signal) decreases. When the reduced Vsat _ hi signal crosses the high-band saturation detection threshold, comparator 120 generates a high or binary "1" output signal, indicating that high-band power amplifier 60 is operating in saturation (or at least substantially in saturation). This saturation detection output signal may be provided to a power amplifier system controller 76, which power amplifier system controller 76 responds by adjusting a power control signal 78 to indicate a decrease in the target amplifier power level. As described above with respect to low band operation, the saturation detection output signal may alternatively be provided to a centralized device controller or other element, which may then respond by adjusting the power control signal 82 received by the power amplifier system controller 76. In such embodiments, the power amplifier system controller 76 then responds to the adjusted control signal 82 by adjusting the power control signal 78 to indicate a decrease in the target amplifier power level.

The Vsat hi signal increases when the high-band power amplifier 60 responds to a change in the power control signal 78 by decreasing the power level of its output RF signal. When the increased Vsat _ hi signal crosses the high-band saturation detection threshold, comparator 120 triggers to generate a low or binary "0" output signal, indicating that high-band power amplifier 60 is no longer operating in saturation.

Note that similar variations in reference voltage, diode and resistor values between the high band and low band circuits are cancelled out by the common mode rejection characteristics of comparator 120.

The above-described elements may be distributed across two or more integrated circuit chips 136 and 138 to take advantage of the benefits of different chip processing techniques. For example, chip 136 may be formed using indium gallium phosphide (InGaP) Heterojunction Bipolar Transistor (HBT) technology, and chip 138 may be formed using silicon BiCMOS technology, which may advantageously integrate bipolar and CMOS devices.

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

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, while in the illustrated or exemplary embodiments described above, the limiter section, averaging filter section, and comparator section are shown as being included in discrete circuits for illustrative purposes, those skilled in the art will appreciate that some or all of such sections and their elements may alternatively be included in suitably programmed or configured digital signal processing logic. Accordingly, the invention is not limited except as by the appended claims.

Claims (24)

1. A power amplifier circuit for a radio frequency, RF, transmitter, the power amplifier circuit comprising:

a power amplifier having at least one output transistor having an output transistor terminal coupled to a supply voltage;

a duty cycle detector having an input coupled to the output transistor terminal and generating an output representing a ratio between an amount of time that an RF signal amplified by a power amplifier is negative and an amount of time that the RF signal is positive; and

a comparator section having first and second comparator inputs and producing a saturation detection output signal, one of said first and second comparator inputs being coupled to the output of the duty cycle detector and the other of said first and second comparator inputs being coupled to a reference voltage circuit.

2. The power amplifier circuit of claim 1, wherein the duty cycle detector comprises:

a limiter portion coupled to the output transistor terminal, the limiter portion blocking positive voltage excursions and passing negative voltage excursions; and

an averaging filter section coupled to an output of the slicer section.

3. The power amplifier circuit of claim 2, wherein:

the power amplifier includes a high-band output transistor and a low-band output transistor that are selectively activatable in response to a band selection signal, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to the supply voltage;

the limiter portion includes a first limiter coupled to the high-band output transistor terminal that blocks positive voltage excursions and passes negative voltage excursions and a second limiter coupled to the low-band output transistor terminal that blocks positive voltage excursions and passes negative voltage excursions;

the averaging filter section includes a first averaging filter coupled to an output of the first limiter and a second averaging filter coupled to an output of the second limiter; and

the comparator section also includes a switching section responsive to the band select signal, the switching section coupling the first comparator input to the output of the first averaging filter and the second comparator input to a reference voltage circuit when the band select signal indicates high band operation, the switching section also coupling the first comparator input to the output of the second averaging filter and the second comparator input to a reference voltage circuit when the band select signal indicates low band operation.

4. The power amplifier circuit of claim 3, wherein:

the first limiter comprises a first diode and a first bias circuit biasing the first diode to turn on during positive voltage excursions at the high-band output transistor terminal and to turn off during negative voltage excursions at the high-band output transistor terminal; and

the second limiter includes a second diode and a second bias circuit that biases the second diode to turn on during positive voltage excursions at the low-band output transistor terminal and to turn off during negative voltage excursions at the low-band output transistor terminal.

5. The power amplifier circuit of claim 3, wherein:

the first averaging filter comprises at least one resistor and at least one capacitor; and

the second averaging filter comprises at least one resistor and at least one capacitor.

6. A power amplifier circuit as claimed in claim 3, wherein the switching section comprises:

a first switch responsive to the band select signal, the first switch having a pole terminal coupled to the first comparator input, a first throw terminal coupled to the output of the first averaging filter and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter and a portion of the reference voltage circuit;

a second switch responsive to the band select signal, the second switch having a pole terminal output coupled to the second comparator input, a first throw terminal coupled to the output of the first averaging filter and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter and a portion of the reference voltage circuit.

7. The power amplifier circuit of claim 6, wherein the reference voltage circuit comprises:

a first current source responsive to the band select signal and coupled to the output of the first averaging filter, a first throw terminal of the first switch, and a first throw terminal of the second switch; and

a second current source responsive to the band select signal and coupled to the output of the second averaging filter, a second throw terminal of the first switch, and a second throw terminal of the second switch.

8. A method for detecting power amplifier saturation in a radio frequency, RF, transmitter, comprising:

amplifying the RF signal using an output transistor having an output transistor terminal coupled to a supply voltage;

detecting a duty cycle between an amount of time that the amplified RF signal is negative and an amount of time that the amplified RF signal is positive;

providing a signal representative of the duty cycle to a first input of a comparator section;

providing a reference voltage to a second input of the comparator section; and

a saturation detection output signal is generated in response to a comparison of a signal representative of the duty cycle with the reference voltage.

9. The method of claim 8, wherein detecting the duty cycle of the amplified RF signal comprises:

limiting a positive voltage excursion of the amplified RF signal while passing a negative voltage excursion of the amplified RF signal to produce a limited signal; and

the limited signal is averaged to produce an averaged signal.

10. The method of claim 9, wherein:

amplifying the RF signal includes: amplifying a high-band RF signal using a high-band output transistor of the power amplifier and amplifying a low-band RF signal using a low-band output transistor, the high-band output transistor having a high-band output transistor terminal coupled to a supply voltage, the low-band output transistor having a low-band output transistor terminal coupled to the supply voltage;

limiting the positive voltage excursion comprises: limiting a positive voltage excursion of the amplified high band RF signal while passing a negative voltage excursion of the amplified high band RF signal to produce a first limited signal, and limiting a positive voltage excursion of the amplified low band RF signal while passing a negative voltage excursion of the amplified low band RF signal to produce a second limited signal;

averaging the limited signal includes: averaging the first limited signal to generate a first average signal and averaging the second limited signal to generate a second average signal; and

providing a signal representative of the duty cycle comprises: the first average signal is coupled to a first input of a comparator section and a reference voltage is coupled to a second input of the comparator section when the band select signal indicates high band operation, and the second average signal is coupled to the second input of the comparator section and a reference voltage is coupled to the first input of the comparator section when the band select signal indicates low band operation.

11. The method of claim 10, wherein:

limiting a positive voltage excursion of the amplified high band RF signal while passing a negative voltage excursion of the amplified high band RF signal to produce a first limited signal comprises: the first diode is turned on during positive voltage excursions and turned off during negative voltage excursions; and

limiting the positive voltage excursion of the amplified low band RF signal to produce a second limited signal while passing the negative voltage excursion of the amplified low band RF signal comprises: the second diode is turned on during positive voltage excursions and turned off during negative voltage excursions.

12. The method of claim 10, wherein:

averaging the first limited signal includes: low pass filtering the first limited signal; and

averaging the second limited signal includes: the second limited signal is low pass filtered.

13. A mobile wireless telecommunications device comprising:

a user interface;

an antenna;

a baseband subsystem coupled to the user interface; and

a Radio Frequency (RF) subsystem coupled to the baseband subsystem and the antenna, the RF subsystem including a transmitter portion and a receiver portion, the transmitter portion including a modulator, an upconverter, and a power amplifier system, the power amplifier system including: a power amplifier having a high-band output transistor and a low-band output transistor; a first duty cycle detector having an input coupled to a high band output transistor terminal; a second duty cycle detector having an input coupled to the low band output transistor terminal; and a comparator section.

14. The mobile wireless telecommunication device of claim 13, wherein the high-band output transistor of the power amplifier includes a high-band output transistor terminal coupled to a supply voltage, and the low-band output transistor includes a low-band output transistor terminal coupled to the supply voltage.

15. The mobile wireless telecommunication device of claim 14, wherein the first duty cycle detector generates an output representing a ratio between an amount of time that the RF signal amplified by the high-band output transistor is negative and an amount of time that the RF signal is positive.

16. The mobile wireless telecommunication device of claim 15, wherein the first duty cycle detector comprises: a first limiter portion coupled to the high-band output transistor terminal, the first limiter portion blocking positive voltage excursions and limiting negative voltage excursions from passing; and a first averaging filter section coupled to an output of the first limiter section.

17. The mobile wireless telecommunication device of claim 16, wherein the second duty cycle detector generates an output representing a ratio between an amount of time that the RF signal amplified by the high-band output transistor is negative and an amount of time that the RF signal is positive.

18. The mobile wireless telecommunication device of claim 17, wherein the second duty cycle detector comprises: a second limiter portion coupled to the low band output transistor terminal, the second limiter portion blocking positive voltage excursions and passing negative voltage excursions; and a second averaging filter section coupled to an output of the second limiter section.

19. The mobile wireless telecommunications device of claim 18, wherein the comparator section has first and second comparator inputs and produces a saturation detection output signal.

20. The mobile wireless telecommunication device of claim 19, wherein the comparator section further comprises a switching section responsive to a band select signal, the switching section coupling the first comparator input to the output of the first averaging filter and the second comparator input to a reference voltage circuit when the band select signal indicates high band operation, the switching section further coupling the first comparator input to the output of the second averaging filter and the second comparator input to a reference voltage circuit when the band select signal indicates low band operation.

21. The mobile wireless telecommunications device of claim 18, wherein:

the first limiter section includes a first diode and a first bias circuit that biases the first diode to turn on during positive voltage excursions at the high-band output transistor terminal and to turn off during negative voltage excursions at the high-band output transistor terminal; and

the second limiter portion includes a second diode and a second bias circuit that biases the second diode to turn on during positive voltage excursions at the low-band output transistor terminal and to turn off during negative voltage excursions at the low-band output transistor terminal.

22. The mobile wireless telecommunications device of claim 18, wherein:

the first averaging filter section includes at least one resistor and at least one capacitor; and

the second averaging filter section comprises at least one resistor and at least one capacitor.

23. The mobile wireless telecommunication device of claim 20, wherein the handover portion comprises:

a first switch responsive to the band select signal, the first switch having a pole terminal coupled to the first comparator input, a first throw terminal coupled to the output of the first averaging filter section and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter section and a portion of the reference voltage circuit;

a second switch responsive to the band select signal, the second switch having a pole terminal output coupled to the second comparator input, a first throw terminal coupled to the output of the first averaging filter portion and a portion of the reference voltage circuit, and a second throw terminal coupled to the output of the second averaging filter portion and a portion of the reference voltage circuit.

24. The mobile wireless telecommunication device of claim 23, wherein the reference voltage circuit comprises:

a first current source responsive to the band select signal and coupled to the output of the first averaging filter, a first throw terminal of the first switch, and a first throw terminal of the second switch; and

a second current source responsive to the band select signal and coupled to the output of the second averaging filter, a second throw terminal of the first switch, and a second throw terminal of the second switch.