# Patent

## Title
Power amplifier controller circuit

## Abstract

A power amplifier controller circuit controls a power amplifier based upon an amplitude correction signal indicating the amplitude difference between the amplitude of the input signal and an attenuated amplitude of the output signal. The power amplifier controller circuit comprises an amplitude control loop and a phase control loop. The amplitude control loop adjusts the supply voltage to the power amplifier based upon the amplitude correction signal. The amplitude correction signal may also be split into two or more signals with different frequency ranges and provided respectively to different types of power supplies with different efficiencies to generate the adjusted supply voltage to the power amplifier. The phase control loop adjusts the phase of the input signal based upon a phase error signal indicating a phase difference between phases of the input signal and the output signal to reduce phase distortion generated by the power amplifier.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation application of, and claims priority under 35 U.S.C. §120 from, U.S. patent application Ser. No. 11/429,119, entitled “Power Amplifier Controller Circuit,” filed on May 4, 2006, which application claims priority under 35 U.S.C. §119(e) from U.S. Provisional Patent Application No. 60/764,947, entitled “RF Power Amplifier with Efficiency Improvement for High Peak to Average Modulation Types,” filed on Feb. 3, 2006, the subject matter of both of which is incorporated by reference herein in its entirety.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a circuit for controlling RF PAs (Radio Frequency Power Amplifiers), and more specifically, to an RF PA controller circuit that controls the supply voltage of a PA using a closed amplitude control loop with an amplitude correction signal.
2. Description of the Related Art
RF transmitters and RF power amplifiers are widely used in portable electronic devices such as cellular phones, laptop computers, and other electronic devices. RF transmitters and RF power amplifiers are used in these devices to amplify and transmit the RF signals remotely. RF PAs are one of the most significant sources of power consumption in these electronic devices, and their efficiency has a significant impact on the battery life on these portable electronic devices. For example, cellular telephone makers make great efforts to increase the efficiency of the RF PA circuits, because the efficiency of the RF PAs is one of the most critical factors determining the battery life of the cellular telephone and its talk time.
FIG. 1 illustrates a conventional RF transmitter circuit, including a transmitter integrated circuit (TXIC) 102 and an external power amplifier (PA) 104. For example, the RF transmitter circuit may be included in a cellular telephone device using one or more cellular telephone standards (modulation techniques) such as UMTS (Universal Mobile Telephony System) or CDMA (Code Division Multiple Access), although the RF transmitter circuit may be included in any other type of RF electronic device. For purposes of illustration only, the RF transmitter circuit will be described herein as a part of a cellular telephone device. The TXIC 102 generates the RF signal 106 to be amplified by the PA 104 and transmitted 110 remotely by an antenna (not shown). For example, the RF signal 106 may be an RF signal modulated by the TXIC 102 according to the UMTS or CDMA standard.
The RF power amplifier 104 in general includes an output transistor (not shown) for its last amplification stage. When an RF modulated signal 106 is amplified by the RF PA 104, the output transistor tends to distort the RF modulated signal 106, resulting in a wider spectral occupancy at the output signal 110 than at the input signal 106. Since the RF spectrum is shared amongst users of the cellular telephone, a wide spectral occupancy is undesirable. Therefore, cellular telephone standards typically regulate the amount of acceptable distortion, thereby requiring that the output transistor fulfill high linearity requirements. In this regard, when the RF input signal 106 is amplitude-modulated, the output transistor of the PA 104 needs to be biased in such a way that it remains linear at the peak power transmitted. This typically results in power being wasted during the off-peak of the amplitude of the RF input signal 106, as the biasing remains fixed for the acceptable distortion at the peak power level.
Certain RF modulation techniques have evolved to require even more spectral efficiency, and thereby forcing the RF PA 104 to sacrifice more efficiency. For instance, while the efficiency at peak power of an output transistor of the PA 104 can be above 60%, when a modulation format such as WCDMA is used, with certain types of coding, the efficiency of the RF PA 104 falls to below 30%. This change in performance is due to the fact that the RF transistor(s) in the RF PA 104 is maintained at an almost fixed bias during the off-peak of the amplitude of the RF input signal 106.
Certain conventional techniques exist to provide efficiency gains in the RF PA 104. One conventional technique is EER (Envelope Elimination and Restoration). The EER technique applies the amplitude signal (not shown in FIG. 1) and the phase signal (not shown in FIG. 1) of the RF input signal 106 separately to 2 ports of the power amplifier 104, i.e., its supply voltage port (Vcc) 108 and its RF input port 107, respectively. However, the EER technique fails to provide significant efficiency gains, because the supply voltage 108 cannot be varied in an energy-efficient way to accommodate the large variations in the amplitude signal of the RF input signal 106 and thus it fails to provide a substantial energy efficiency gain while maintaining the required linear amplification of the RF signal in the RF PA 104. This is mainly due to the difficulty in realizing a fast, accurate, wide range, and energy efficient voltage converter to drive the supply voltage of the RF PA 104.
The conventional EER technique can function better only if a variable power supply with a very large variation range is used to adjust the supply voltage based on the amplitude signal of the RF input signal 106, while not reducing the efficiency of the RF transmitter by power consumed by the power supply itself. However, the variable power supply, which is typically comprised of a linear regulator (not shown in FIG. 1) that varies its output voltage on a fixed current load such as the PA in linear mode, by principle reduces the supply voltage at constant current and by itself consumes the power resulting from its current multiplied by the voltage drop across the linear regulator when there is a large drop in the amplitude signal of the RF input signal 106. This results in no change in the overall battery power being consumed by the RF transmitter, because any efficiency gained in the RF PA 104 is mostly lost in the linear regulator itself. Variations of the EER technique, such as Envelope Following and other various types of polar modulation methods, likewise fails to result in any significant gain in efficiency in the RF transmitter, because the supply voltage is likewise adjusted based on the amplitude signal of the RF input signal 106 which inherently has large variations and thus has the same deficiencies as described above with respect to conventional EER techniques.
Quite often, the conventional methods of controlling a PA fail to address the amplitude-to-phase re-modulation (AM-to-PM) which occurs in a non-frequency linear device such as a PA. Thus, the conventional methods are not suitable for the common types of PAs for use in common mobile telephony or mobile data systems because the required spectral occupancy performance is compromised by the AM to PM distortion.
Finally, PAs are typically used in conjunction with band pass filters that have a high electric coefficient of quality. These filters are typically of the SAW (surface acoustic wave) type. Due to their high coefficient of quality, the filters exhibit a relatively high group delay. The group delay makes it very difficult for a correction loop to work around the arrangement of the SAW filter and the PA while still meeting the high bandwidth requirements needed for the correction of the AM-to-PM.
Thus, there is a need for an RF PA system that is efficient over a wide variety of modulation techniques and results in a significant net decrease in power consumption by the RF PA circuit. There is also a need for a PA controller that can correct the AM to PM effects, while not relying on a PA specially designed for low AM to PM at the expense of efficiency. In addition, there is a need for a PA controller that can exclude the use of SAW filters from the path of the correction loop in the PA circuitry.
SUMMARY OF THE INVENTION
One embodiment of the present invention disclosed is a power amplifier controller circuit for controlling a power amplifier based upon an amplitude correction signal or amplitude error signal. The power amplifier receives and amplifies an input signal to the power amplifier and generates an output signal, and the power amplifier controller circuit controls the power amplifier so that it operates in an efficient manner.
The PA controller circuit comprises an amplitude control loop and a phase control loop. The amplitude control loop determines the amplitude correction signal (also referred to herein as the amplitude error signal), which is indicative of the amplitude difference between the amplitude of the input signal and the attenuated amplitude of the output signal, and adjusts the supply voltage to the power amplifier based upon the amplitude correction signal. The phase control loop determines a phase error signal, which indicates a phase difference between phases of the input signal and the output signal, and adjusts the phase of the input signal based upon the phase error signal to match the phase of the output signal. Thus, the phase control loop corrects for unwanted phase modulation introduced by the AM to PM non-ideality of the power amplifier and thus reduces phase distortion generated by the power amplifier.
In a first embodiment of the present invention, the amplitude control loop comprises an amplitude comparator comparing the amplitude of the input signal with an attenuated amplitude of the output signal to generate an amplitude correction signal, and a power supply coupled to receive the amplitude correction signal and generating the adjusted supply voltage provided to the power amplifier based upon the amplitude correction signal. The power supply can be a switched mode power supply. By using the amplitude correction signal to control the supply voltage to the power amplifier, a high-efficiency yet low-bandwidth power supply such as the switched mode power supply may be used to provide the adjusted supply voltage to the power amplifier.
In a second embodiment of the present invention, the amplitude correction signal is split into two or more signals with different frequency ranges and provided respectively to different types of power supplies with different levels of efficiency to generate the adjusted supply voltage provided to the power amplifier. For example, in the second embodiment, the power supplies include a first power supply with a first efficiency and a second power supply with a second efficiency higher than the first efficiency. The first power supply receives a first portion of the amplitude correction signal in a first frequency range and generates a first adjusted supply output based upon the first portion of the amplitude correction signal, and the second power supply receives a second portion of the amplitude correction signal in a second frequency range lower than the first frequency range and generates a second adjusted supply output based upon the second portion of the amplitude correction signal. The first and second adjusted supply outputs are combined to form the adjusted supply voltage provided to the power amplifier. The first power supply can be a linear regulator, and the second power supply can be a switched mode power supply. By dividing the amplitude correction signal into two or more signals with different frequency ranges, the second embodiment of the present invention has the additional advantage that the switched mode power supply may be implemented with even narrower bandwidth as compared to the first embodiment without significantly sacrificing efficiency. A narrower bandwidth power supply or a variable power supply with a smaller range of voltage variation is easier to implement.
In a third embodiment of the present invention, the amplitude control loop further comprises a gain control module receiving the amplitude correction signal to generate a gain control signal, and a variable gain amplifier adjusting the amplitude of the input signal according to the gain control signal. The third embodiment has the advantage that it is possible to operate the power amplifier at any given depth beyond its compression point, resulting in an extra degree of freedom in designing the PA circuit. This is useful in optimizing the efficiency gain versus spectral occupancy performance. By adding the variable gain amplifier, the amplitude of variation of the control voltage to the PA is further reduced, resulting in further significant efficiency gains.
The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter.



BRIEF DESCRIPTION OF THE DRAWINGS
The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings.
FIG. 1 illustrates a conventional RF transmitter circuit.
FIG. 2 illustrates an RF transmitter circuit including the PA controller in accordance with the present invention.
FIG. 3A illustrates an RF power amplifier circuit, in accordance with a first embodiment of the present invention.
FIG. 3B illustrates a method of controlling the amplitude control loop of a RF PA circuit, in accordance with the first embodiment of the present invention.
FIG. 4A illustrates an RF power amplifier circuit, in accordance with a second embodiment of the present invention.
FIG. 4B illustrates a method of controlling the amplitude control loop of a RF PA circuit, in accordance with the second embodiment of the present invention.
FIG. 5A illustrates an RF power amplifier circuit, in accordance with a third embodiment of the present invention.
FIG. 5B illustrates a method of controlling the amplitude control loop of a RF PA circuit, in accordance with the third embodiment of the present invention.
FIG. 6 illustrates a method of controlling the phase control loop of a RF power amplifier circuit in accordance with the present invention.
FIG. 7 illustrates simulation results of the changes in the waveform of the supply voltage 208 to the PA corresponding to the conventional polar control method, the first embodiment of FIG. 3A, and the third embodiment of FIG. 5A, for a typical commercial WCDMA PA with 3.4 V nominal supply voltage and WCDMA modulation using 3.84 Mchips per second.
FIG. 8 illustrates the simulation results of an example of a time domain waveform present at the node 509 of FIG. 5A for a typical commercial WCDMA PA with 3.4 V nominal supply voltage and WCDMA modulation using 3.84 Mchips per second.
FIG. 9 illustrates the simulation results of an example of a time domain waveform present at nodes 401 and 403 of FIG. 5A for a typical commercial WCDMA PA with 3.4 V nominal supply voltage and WCDMA modulation using 3.84 Mchips per second.



DETAILED DESCRIPTION OF EMBODIMENTS
The Figures (FIG.) and the following description relate to preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the claimed invention.
Reference will now be made to several embodiments of the present invention(s), examples of which are illustrated in the accompanying figures. Wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. The figures depict embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein.
FIG. 2 illustrates an RF transmitter circuit including the PA controller 202 in accordance with the present invention. The PA controller 202 is placed between the transmitter IC 102 and the PA 104 to receive the RF signal 204 from the TXIC 102 and provide the RF signal 206 to the PA 104, while controlling the PA 104 by way of an adjusted supply voltage 208. The PA controller 202 is also placed between the power supply line (Vcc) 210 and the PA 104. The PA 104 amplifies the RF signal 206 to output the amplified RF output signal 110, which is also provided as a feedback signal back to the PA controller 202. As will be explained below with reference to FIGS. 3A, 3B, 4A, 4B, 5A, and 5B, the adjusted supply voltage 208 is generated by the PA controller 202 based on an amplitude correction signal (not shown in FIG. 2) indicative of the difference between the attenuated amplitude of the feedback RF output signal 110 and the amplitude of the RF input signal 204. Note that the term “amplitude correction signal” is used herein synonymously with the term “amplitude error signal.” The PA controller 202 adjusts the supply voltage (Vcc) 210 based upon the amplitude correction signal to generate the adjusted supply voltage 208 provided to the PA 104, to optimize the efficiency of the PA 104. An advantage of the PA controller 202 is that existing signal connections to the PA 104 and the TXIC 102 need not change when the PA controller 202 is inserted between the TXIC 102, the PA 104, and the supply voltage (Vcc) 210.
The PA controller circuit 202 may also adjust the phase and amplitude of the signal 204 to allow for power control and PA ramping, in accordance with information received through the configuration signals 209. Since the PA controller circuit 202 is aware of the voltage at the output and the current in the power amplifier 104, it can also adjust for load variations at an antenna (not shown herein) that may be used with the PA. If a directional coupler (not shown) is used to feed the attenuated amplitude of the signal 204, the PA controller 202 can adjust the forward power while controlling the PA operation point as it is aware of the voltage and current at node 208.
FIG. 3A illustrates an RF PA circuit, according to a first embodiment of the present invention. The RF PA circuit includes the PA 104, and the PA controller 202 including a closed amplitude control loop and a closed phase control loop.
The phase control loop includes two limiters 312, 314, a phase comparator 316, a loop filter (PLF (Phase Loop Filter)) 318, and a phase shifter 320. To achieve stability over all conditions, the phase comparator 316 is of an adequate type with a capture range greater than 2*PI. To achieve this, a combination of adjustable delay elements and frequency dividers may be used. Also a phase sub-ranging system can be used since the dynamic phase variations that the phase correction loop processes are limited in amplitude. A sub-ranging phase control block (not shown) could be one of the constituents of the phase comparator 316 used with this system. Advantages of using sub-ranging in the phase comparator 316 are stability and good noise.
The amplitude control loop includes an adjusted variable attenuator (RFFA (RF Feedback Attenuator)) 306, two matched amplitude detectors 302, 304, a comparator 308, and a switched mode power supply (SMPS) 310. Note that the limiter 312 and the detector 302, and the limiter 314 and the detector 304, can be combined into a single limiter/power detector blocks without altering the functionality of the system.
Referring to FIG. 3A, the phase control loop monitors the RF input signal 204 from the transmitter IC 102 (not shown in FIG. 3A) and compares the phase of the RF input signal 204 with the phase of the output signal 110 of the PA 104 attenuated 326 by the adjusted variable attenuator (RFFA) 306, resulting in a control signal 319 that varies the phase of the RF signal 206 coming out of the phase shifter 320. More specifically, the limiter 312 receives the RF input signal 204 from the TXIC 102 and outputs to the comparator 316 an amplitude limited signal 324 mathematically representative of the phase of its input signal. The limiter 314 also receives the output signal 110 of the PA 104 as attenuated 326 by the adjusted variable attenuator (RFFA) 306, and outputs its phase signal 325 to the comparator 316. The comparator 316 compares the phases of the output signals 324, 325 of the two limiters 314, 316, and generates a phase error signal 317. Note that the term “phase error signal” is used herein synonymously with the term “phase correction signal.” The phase error signal 317 is filtered by the loop filter (PLF) 318 to generate the phase control signal 319. The loop filter 318 completes the phase loop and provides the necessary gain, bandwidth limitation, and loop stability required for the phase loop to function properly. The particular loop filter used here can be of any type, and can include multiple integration and derivation stages so as to satisfy the best loop performance. The types of the loop filter may include classical types I, II, and the like. A particularity of this phase loop design is that the group delay through the PA 104 must be taken into account for stability reasons. This is achieved by choosing the proper pole-zero placement in the loop filter and may include delay compensation. The phase control signal 319 is input to the phase shifter 320 to control the shifting of the phase of the input RF signal 206 so that the phase of the output signal 110 dynamically matches the phase of the transmitter signal 204.
The function of the phase control loop is to counteract the AM (Amplitude Modulation) to PM (Phase Modulation) characteristics of the PA 104, which is part of the normal distortion characteristics of transistor-based amplifiers, allowing for the phase of the RF signal to be the same at the output 110 of the PA 104 as it is at the input 204 of the phase shifter 320 and thus reducing phase distortion generated by the PA 104. This phase control loop contributes to linearizing the PA 104 as the AM to PM phase shift of the PA 104 tends to become higher at higher power levels. By limiting the effects of AM to PM of the PA 104, the phase control loop allows the PA 104 to function at higher power levels with less distortion for the output signal 110, thus allowing the use of the PA 104 in more favorable efficiency conditions. In addition, the phase control loop also helps in correcting any additional AM to PM characteristics that the amplitude control loop (described below) may cause. While FIG. 3A shows the phase shifter circuit 320 controlling the input to the PA 104, it is also possible to place the phase shifter 320 at the output of the PA 104 with the same benefits.
Note that the phase control loop is of the error correction only type. In other words, the phase control loop does not modify the phase of the input signal 204 to the PA 104 unless the PA 104 or the amplitude control loop introduces a phase error. Since the noise contributions of the feedback loops affect the overall signal quality of the RF transmitter, an error correction only loop such as the phase control loop shown in FIG. 3A by definition introduces only a small correction, hence has a low noise contribution.
The amplitude control loop is also of the error correction only type, and thus is referred to herein as the amplitude correction loop. Thus, amplitude control loop and amplitude correction loop are used synonymously herein. Referring to FIG. 3A, the amplitude of the RF input signal 204 is monitored through the amplitude detector 302 and compared by the comparator 308 with the amplitude at the output 110 of the PA 104 as attenuated 326 by the adjusted variable attenuator (RFFA) 306, seen through a matched amplitude detector 304. The attenuator 306 is adjusted such that the output 110 of the PA 104 is at a desired level. This can be achieved though programming 321 the variable attenuator (RFFA) 306 by either a digital input to the PA controller 202 or by analog control of the variable attenuator (RFFA) 306. The comparator 308 generates an error signal 309 indicating the difference between the amplitude of the input RF signal 204 and the attenuated amplitude 326 of the output RF signal 110, referred to herein as the “amplitude correction signal” 309. The amplitude correction signal 309 is fed into power supply 310, which is a switch mode power supply (SMPS). The SMPS 310 generates an adjusted supply voltage 208 provided to one or more supply voltage pins of the PA 104 based upon the amplitude correction signal 309. The adjusted supply voltage 208 in essence operates as a bias control signal that controls the operating point of the PA 104.
For a given output power, adjusting the supply voltage 208 of the PA 104 has the effect of varying its gain, as well as changing its efficiency. For a given output power, lowering the supply voltage 208 to the PA 104 provides better efficiency for the PA 104. The adjusted supply voltage 208 of the PA 104 is adjusted to ensure that the PA 104 stays in its most efficient amplification zone. Because adjusting the supply voltage 208 of the PA 104 does make a change to the gain of the PA 104, the output amplitude of the PA 104 changes with the supply voltage 208 from the SMPS 310, and the amplitude control loop can be closed. The principles of such operation can be explained as follows.
When the input to the PA 104 increases, the output of the PA 104 also increases. As the PA 104 stays in its linear region of operation, which corresponds to small input signals, its output will increase linearly with its input. Thus, both inputs to the comparator 308 will rise by the same amount, resulting in no error correction and no change to the supply voltage 208. This is the case when the output power is relatively small and well below the saturation point.
As the input power continues to rise at the input of PA 104, there will be a point beyond which the output of the PA 104 will no longer be directly proportional with the input to the PA 104. The amplitude control loop will detect this error between the output and input of the PA 104, and raise the supply voltage to the PA 104 such that the initially-desired output power is delivered, resulting in linear operation of the system, even with a non-linear PA 104.
In a practical application, the PA 104 will be fully or partially saturated from its Vcc, for example, the highest 10 dB of its output power range, and as the RF modulation of the RF signal 204 forces the amplitude to vary, the amplitude control loop will only be actively controlling the supply voltage 208 to the PA 104 when the highest powers are required. For lower input power, the amplitude control loop will leave the supply voltage 208 at a fixed level because it detects no gain error, resulting in a fixed gain for the PA 104. The depth beyond compression can be adjusted by setting the level of the input signal 204 and the level of the attenuator 306, as well as the default supply voltage Vcc (not shown in FIG. 3A) to the PA 104. This behavior is illustrated in FIG. 7 where simulation results compare the behavior of the conventional polar architecture (with no feedback) where the supply voltage to the PA swings between 0.1 V and 2.9 V and reaches a minimum value around 0.1 V as shown with curve 701, while the supply voltage 208 to the PA 104 in the first embodiment of FIG. 3A using the amplitude correction signal 309 does not drop below 0.5 V as shown with curve labeled 702. The amplitude swing in the dual gain control method is clearly further reduced as indicated by curve 703, as will be explained in detail below with respect to the third embodiment of the present invention with reference to FIGS. 5A and 5B.
Varying the supply voltage to the PA 104 also results in a phase change. Thus, the phase control loop described above operates in conjunction with the amplitude control loop to maintain the accuracy of RF modulation at the output signal of the PA 104. Note that the phase control loop is also an error correction loop only, and therefore minimally contributes to noise.
Furthermore, the amplitude correction loop has the advantage that an SMPS 310, which does not consume any significant power by itself and thus actually increases the efficiency of the overall RF power amplifier circuit, can be used to generate the adjusted supply voltage 208 to the PA 104. This is possible because the adjusted supply voltage 208 to the PA 104 is generated by the SMPS 310 based upon the amplitude correction signal 309 which by nature has a much narrower range of variation or fluctuation rather than the actual amplitude of the RF input signal 204 which by nature has a much wider range of variation or fluctuation. An SMPS 310 is easier to implement to follow the amplitude correction signal 309 with a narrow range of variation, but would be more difficult to implement if it had to follow the unmodified amplitude of the RF input signal 204. This is related to the fact that the amplitude signal itself has its fastest variations when the amplitude itself is low. The amplitude correction loop does not need to make any changes to its output when the PA is operating in linear mode. For example, the amplitude correction signal 309 may be only active for the highest 10 dB of the actual output power variation. In contrast, the amplitude signal itself may vary by 40 dB, and varies much faster between −10 dBc to −40 dBc than it does between 0 dBc to −10 dBc. Thus the bandwidth requirements on the SMPS 310, which are coupled with the rate of change of the voltage, are reduced when an amplitude correction signal 309 rather than the amplitude signal itself is used to control the supply of the PA 104. The SMPS 310 does not consume any significant power by itself, and thus does not significantly contribute to usage of the battery power, and actually increases the efficiency of the RF power amplifier circuit. In contrast, a conventional polar modulation technique typically utilizes the amplitude signal itself to adjust the supply voltage to the PA 104, which prevents the use of an SMPS 310 for wideband RF signals because of the higher bandwidth requirements. Therefore, conventional RF power amplifier control systems typically use linear regulators (rather than an SMPS) to adjust the supply voltage to the PA 104. Such a linear regulator by itself consumes power resulting from its current multiplied by the voltage drop across the linear regulator. When there is a large drop in the amplitude signal, this can result in significant power being lost and results in none or little reduction in the overall battery power being consumed by the RF transmitter. This is because any efficiency gained in the RF PA is mostly lost in the linear regulator itself FIG. 3B illustrates a method of controlling the amplitude control loop of a RF PA 104 in an RF PA circuit, according to the first embodiment of the present invention. Referring to both FIGS. 3A and 3B, as the process begins 352, the comparator 308 compares 354 the amplitude 323 of the RF input signal 204 with the attenuated amplitude 322 of the RF output signal 110 from the PA 104 to generate an amplitude correction signal 309. The SMPS 310 generates 358 an adjusted supply voltage 208 provided to the PA 104 based upon the amplitude correction signal 309, and the process ends. 360.
FIG. 4A illustrates an RF PA circuit, according to a second embodiment of the present invention. The RF PA circuit illustrated in FIG. 4A is substantially the same as the RF transmitter circuit illustrated in FIG. 3A, except that (i) the amplitude correction signal 309 is split into two signals, a high frequency amplitude correction signal 401 that is fed into a high frequency path including a linear regulator 402 and a low frequency amplitude correction signal 403 that is fed into a low frequency path including an SMPS 404 and that (ii) the outputs of the linear regulator 402 and the SMPS 404 are combined in the adder block 406 to generate the adjusted supply voltage 208 to the PA 104. For example, a simple current adding node, a small, high frequency transformer or other types of active electronic solutions can be used as the adder block 406.
The high frequency amplitude correction signal 401 is input to the linear regulator 402, which generates the high frequency part 405 of the adjusted supply voltage 208. The low frequency amplitude correction signal 403 is input to the SMPS 404, which generates the low frequency part 407 of the adjusted supply voltage 208. The adder block 406 combines the high frequency part 405 and the low frequency part 407 to generate the adjusted supply voltage 208 to the PA 104 in order to keep the PA 104 in an efficient operation range.
The amplitude correction signal 309 is split into the high frequency amplitude correction signal 401 and the low frequency amplitude correction signal 403 using the high pass filter 410 and the low pass filter 411, respectively. The high frequency amplitude correction signal 401 comprised of components of the amplitude correction signal 309 higher than a predetermined frequency and the low frequency amplitude correction signal 403 is comprised of components of the amplitude correction signal 309 lower than the predetermined frequency. The predetermined frequency used to split the amplitude correction signal 309 can be set at any frequency, but is preferably set at an optimum point where the efficiency of the overall RF transmitter circuit becomes sufficiently improved. For example, the predetermined frequency can be as low as 1/20th of the spectrally occupied bandwidth for the RF signal. In other embodiments, the predetermined frequency may not be fixed but may be adjusted dynamically to achieve optimum performance of the RF transmitter circuit.
Power consumed by the linear regulator 401 from a power source such as a battery (not shown) for a given control voltage 208 on the PA 104 can be approximated as follows:




  
    P
    bat
  
  ≈
  
    
      
        I
        pa
      
      ×
      
        V
        pa
      
    
    +
    
      Effl
      ×
      
        (
        
          Vcc
          -
          
            V
            pa
          
        
        )
      
      ×
      
        I
        pa
      
    
  
  ≈
  
    Effl
    ×
    Vcc
    ×
    
      I
      pa
    
  




with Effl=1.05, which is sufficiently close to 1 to allow for this approximation, where Pbat is the power from the battery, Ipa is the input current to the PA 104, Vpa is the input supply voltage to the PA 104, and Vcc is the supply voltage of the battery. In addition, power consumed by the SMPS 404 from a power source such as a battery (not shown) for a given control voltage 208 on the PA 104 can be approximated as follows:

Pbat=Effs*Ipa*Vpa 

with Effs=1.1,
and the efficiency of the switch (not shown) in the SMPS generally exceeding 90%.
If the average input voltage Vpa to the PA 104 is significantly lower than supply voltage Vcc of the battery, the SMPS 404 achieves much lower power consumption. While the linear regulator 402 is generally less efficient than the SMPS 404, the linear regulator 402 processing the high frequency part 401 of the amplitude correction signal 309 does not make the overall RF PA circuit inefficient in any significant way, because most of the energy of the amplitude correction signal 309 is contained in the low frequency part 403 rather than the high frequency part 401. This is explained below with reference to FIGS. 8 and 9.
Using both a high efficiency path comprised of the SMPS 404 carrying the low frequency portion 403 of the amplitude correction signal 309 and a low efficiency path comprised of the linear regulator 402 carrying the high frequency portion 401 of the amplitude correction signal 309 has the advantage that it is possible to use an SMPS 404 with a limited frequency response. In other words, the SMPS 404 need not accommodate for very high frequencies but just accommodates for a limited range of lower frequencies of the amplitude correction signal 309, making the SMPS 404 much easier and more cost-effective to implement. Combining the SMPS 404 with the linear regulator 402 enables high bandwidths of operation accommodating for full frequency ranges of the amplitude correction signal 309 without sacrificing the overall efficiency of the RF PA circuit in any significant way, since most of the energy of the amplitude correction signal 309 that is contained in the low frequency part 403 of the amplitude correction signal 309 is processed by the more efficient SMPS 404 rather than the less efficient linear regulator 402.
For example, Table 1 below illustrates the percentage of energy contained in the various frequency ranges in a hypothetical simple 4QAM (Quadrature Amplitude Modulation) signal used in WCDMA cellular telephones and the overall efficiency that can be expected to be achieved by the RF transmitter according to the embodiment of FIG. 4A with the assumptions of the particular operating conditions as illustrated in Table 1. The combined amplitude and phase spectrum is 4 MHz wide.










TABLE 1





4QAM Signal
Below
Above



PA current = 100 mA
100 KHz
100 KHz (up to


Adjusted supply voltage
(Through
40 MHz )


208 to PA = 60% of Vbat
SMPS
(Through Linear
All


on average
404)
Regulator 402)
Frequencies







Percentage of energy in
83%
17%
100%


adjusted supply voltage


208 to PA 104 in


designated bandwidth


Efficiency of
90%
57%
 71%


conversion at 60% of Vbat


Current from battery
66.66 mA
17.85 mA
84.51 mA








Power supply system
71%


efficiency using high and


low bandwidth paths









Despite the extremely narrow bandwidth (100 KHz) of the SMPS 404 shown in the example of Table 1, 71% efficiency in the RF power amplifier supply system according to the embodiment of FIG. 4A can be expected under the above hypothetical conditions by using a 90% efficient SMPS 404 combined with a 57% efficient linear regulator 402. This is a very significant improvement over conventional PA controller systems that would typically use only a linear regulator under the same operating conditions and thus would be only 57% efficient. By using an SMPS 404 with an increased bandwidth, it is possible to improve the efficiency of the RF power amplifier even further.
FIG. 4B illustrates a method of controlling the amplitude control loop of a RF PA in an RF PA circuit, in accordance with the second embodiment of the present invention. FIG. 4B is explained in conjunction with FIG. 4A. Referring to both FIGS. 4A and 4B, as the process begins 452, the comparator 308 compares 454 the amplitude 323 of the RF input signal 204 with the attenuated amplitude 322 of the RF output signal 110 from the PA 104 to generate an amplitude correction signal 309. The low frequency part 403 of the amplitude correction signal 309 is applied 456 to the high efficiency SMPS 404 while the high frequency part 401 of the amplitude correction signal 309 is applied 456 to the low efficiency linear regulator 402. The supply voltage 208 to the PA 104 is adjusted 460 based upon the combination of the outputs 407, 405 of the high efficiency SMPS 404 and the low efficiency linear regulator 402, and the process ends 462.
FIG. 5A illustrates an RF PA circuit, according to a third embodiment of the present invention. The RF transmitter circuit illustrated in FIG. 5A is substantially the same as the RF transmitter circuit illustrated in FIG. 4A, except that the gain control block 506 and the variable gain amplifier 502 are added to provide an additional means to control the efficiency of the PA 104 and the overall RF transmitter circuit. Although the third embodiment of FIG. 5A is illustrated herein as an improvement to the second embodiment of FIG. 4A, note that the same concepts of the third embodiment of FIG. 5A can also be used to improve the first embodiment of FIG. 3A.
More specifically, the gain control block 506 receives the amplitude correction signal 309 and adjusts the gain of the variable gain amplifier 502 based upon the amplitude correction signal 309, as well as passing the low frequency and high frequency parts 403, 401 of the amplitude correction signal 309 to the SMPS 404 and the linear regulator 402, respectively, to generate the adjusted supply voltage 208 as explained above with reference to FIG. 4A. By monitoring the amplitude of the amplitude correction signal 309 input to the gain control block 506, a control signal 504 is created to further compensate the gain of the variable gain amplifier 502 before the PA 104. This arrangement allows the use of even lower bandwidth for the PA controller system as compared to that of the second embodiment described in FIG. 4A above. Also the programmability of the output power can now be entirely left to the PA controller 202, while in the embodiment of FIG. 4A changing the output power required a change in gain in the transmitter IC 102.
With the addition of the variable gain amplifier 502 and the gain control block 506, it is possible to use the PA 104 at any given depth beyond its compression point. The term “depth beyond compression” is used herein to refer to the difference between the averaged input compression level of the PA 104 and the actual averaged input power at the PA 104. For instance, when the peak output power is required, the input to the PA 104 can be overdriven by 10 dB beyond the 1 dB compression point of the PA 104. It is also possible to adjust the supply voltage of the PA 104 at the instant when the peak power is required, such that the 1 dB compression point is set higher and it is only necessary to overdrive the PA 104 input by 3 dB to obtain the same output peak power. A dynamic adjustment of both the input level and the supply voltage allows this loop system to reduce significantly further the amplitude of the control voltage 208.
In the embodiment of FIG. 5A, the independent programming of gain and compression point by the closed amplitude control loop also makes it possible to reduce the amount of high frequency energy that the power supply system (linear regulator) has to deliver to the PA 104. This can be done by having the variable gain amplifier 502 correct for some of the gain error at a higher speed than the Vcc control loop (closed on node 208) can do, thus reducing the amount of correction that is to be done by the low efficiency, high frequency branch (linear regulator 401). Thus, the bandwidth of the signals at nodes 208 and 504 can be made to be significantly different. Since only a small fraction of the energy resides at high frequencies, there is only a small penalty in efficiency for reducing the bandwidth of the control at node 208 relative to the bandwidth at node 504. The ratio of the two active bandwidths is part of the design trade-off for the whole system. The gain control block 506 adjusts the compression point while the gain loop remains closed through the variable gain amplifier 502. This allows the RF controller system to search an optimum depth beyond compression (as measured by the absolute value of the amplitude correction signal 309 or alternatively by the averaged value of the gain control 504) and efficiency with less effect on the resulting signal quality. The search for the optimum depth beyond compression can be made by a slow control loop which monitors the absolute value of the amplitude correction signal 309, as well as its derivative. Another alternative is to monitor the averaged value of the gain control signal 504. In order to control the relative action of both amplitude controls 504 and 208, and in particular control the maximum voltage at node 208, a control system for the compression level of the variable gain amplifier 502 can be implemented. Because in the embodiment of FIG. 5A both the supply voltage 208 to the PA 104 and the input 508 to the PA 104 can be adjusted, this embodiment inherently offers greater flexibility in design by exploiting two sources of signal information for control. This allows to further reduce the amplitude of the variation of the voltage control signal 208, as shown on FIG. 7, where the voltage with the smallest variation is the signal labeled 703, corresponding to this third embodiment of FIG. 5A.
In addition, the third embodiment of FIG. 5A is also well suited to process directly a polar representation of the RF signal. In this case, an amplitude signal from the TXIC 102 would couple to the amplitude detector 302 and a phase only signal from the TXIC 102 would be coupled to the variable gain amplifier 502 and the limiter 312.
FIG. 5B illustrates a method of controlling the amplitude control loop of a RF PA in an RF transmitter circuit, in accordance with the third embodiment of the present invention. The method illustrated in FIG. 5B is substantially the same as the method illustrated in FIG. 4B, except that step 512 is added. In step 512, the input signal 508 to the PA 104 is adjusted, by use of a variable gain amplifier 502, based upon the amplitude correction signal 309. Therefore, the method of FIG. 5B is provided with an additional means for controlling the efficiency of the PA 104 and the overall RF PA circuit.
FIG. 6 illustrates a method of controlling the phase control loop of a RF PA in an RF PA circuit in accordance with the present invention. The phase control method of FIG. 6 can be used with any one of the methods of controlling the amplitude correction loops described in FIGS. 3B, 4B, and 5B, as shown in FIGS. 3A, 4A, and 5A. The method of FIG. 6 will be explained in conjunction with FIGS. 3A, 4A, and 5A.
As the process begins 602, the comparator 316 compares 604 the phase of the RF input signal 204 with the phase of the attenuated RF output signal 326 from the PA 104 to generate the phase error signal 317. The phase error signal 316 is filtered 606 by the loop filter (PLF) 318 to generate the phase control signal 319. The phase of the input RF signal 204 is shifted 608 based upon the phase control signal 319 so that the phase of the input signal 204 dynamically matches the phase of the output RF signal 110, and the process ends 610.
FIG. 7 illustrates simulation results of the changes in the waveform of the supply voltage 208 to the PA corresponding to the conventional polar control method, the first embodiment of FIG. 3A, and the third embodiment of FIG. 5A, for a typical commercial WCDMA PA with 3.4 V nominal supply voltage and WCDMA modulation using 3.84 Mchips per second. As explained previously, the adjusted supply voltage 208 generated by a conventional polar system as indicated by curve 701 varies the most with wide fluctuations, the adjusted supply voltage 208 generated by the first embodiment of FIG. 3A as indicated by curve 702 varies less than the curve 701, and the adjusted supply voltage 703 generated by the third embodiment of FIG. 5A varies the least with only a little fluctuation.
FIG. 8 illustrates the simulation results of an example of a time domain waveform present at node 509 (which voltage would be the same as the voltage at node 309) of FIG. 5A, and FIG. 9 illustrates the simulation results of an example of a time domain waveform present at nodes 401 and 403 of FIG. 5A, both for a typical commercial WCDMA PA with 3.4 V nominal supply voltage and WCDMA modulation using 3.84 Mchips per second. The loop voltage versus time on FIG. 8 shows that the loops maintain a voltage much lower than 2.5 V most of the time, except for some short instants. This is due to the signal's amplitude characteristics which require high peaks but a much lower average. In FIG. 9, the voltages 401 and 403 are shown. They correspond to the voltage 309 (or 509) after filtering by a 100 kHz high pass filter 410 and a 100 kHz low pass filter 411, respectively. It can be seen that the low pass filtered signal 403 is almost a DC signal of value 1.9 V, while the high pass filtered signal 401 is a band limited waveform having a low DC value and an rms value of only 0.2V. If the 1.9V is generated with an efficiency of 90% by an easy-to-realize low output bandwidth SMPS 404, and the 0.2V is generated with an efficiency of 60% using a linear amplifier 402, the signal 309 can be generated with a combined efficiency of (1.9+0.2)/(1.9/0.9+0.2/0.6)=87.5%. This is much better than generating the signal 309 using a linear regulator with an average efficiency of (1.9/3.4)/1.05=53%. While it should be understood that the calculations presented herein are engineering approximations, the potential benefit in battery life is clearly apparent through this example.
Upon reading this disclosure, those of skill in the art will appreciate still additional alternative structural and functional designs for the RF power amplifier controller through the disclosed principles of the present invention. For example, although the embodiment in FIG. 4B splits the amplitude correction signal 309 into two frequency ranges, it is possible to split the amplitude correction signal 309 into more than two different frequency ranges for separate processing by adjustable power supply components. The power amplifier controller circuit can be used with any type of power amplifier for many different types of electronic devices, although the embodiments are described herein with respect to a RF PA controller used in cellular telephone applications. Examples of these applications include video signals and Manchester coded data transmissions.
For another example, digital techniques can be used to process some of the signals of the PA system described herein. Whether a signal is represented in an analog form or a digital form will not change the functionality or principles of operation of amplitude and phase control loops of the PA system according to various embodiments of the present invention. For instance, based on the observation of the amplitude error signal 309, one could calculate a typical transfer function for the PA 104 and construct the signals that drive the PA at nodes 206, 208, which is still a form of closed loop control.
Thus, while particular embodiments and applications of the present invention have been illustrated and described, it is to be understood that the invention is not limited to the precise construction and components disclosed herein and that various modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the method and apparatus of the present invention disclosed herein without departing from the spirit and scope of the invention as defined in the appended claims.

## Claims

1. A power amplifier controller circuit for controlling a power amplifier, the power amplifier coupled to receive and amplify an input signal to generate an output signal, the power amplifier controller circuit comprising:
an amplitude control loop determining an amplitude correction signal indicative of an amplitude difference between an amplitude of the input signal to the power amplifier and an attenuated amplitude of the output signal of the power amplifier and adjusting a supply voltage or bias to the power amplifier based upon the amplitude correction signal to control an operating point of the power amplifier, wherein adjusting the supply voltage or bias to the power amplifier changes a gain of the power amplifier resulting in a gain error; and
a variable gain amplifier adjusting the amplitude of the input signal to the power amplifier based upon a gain control signal derived from the amplitude correction signal, wherein the variable gain amplifier corrects for at least some of the gain error of the power amplifier at a higher speed than the amplitude correction loop.

2. The power amplifier controller circuit of claim 1, wherein the amplitude control loop comprises:
an amplitude comparator comparing the amplitude of the input signal with the attenuated amplitude of the output signal to generate the amplitude correction signal; and
a power supply coupled to receive the amplitude correction signal and generating the adjusted supply voltage provided to the power amplifier based upon the amplitude correction signal.

3. The power amplifier controller circuit of claim 2, wherein the amplitude control loop further comprises:
a first amplitude detector coupled to the amplitude comparator and detecting the amplitude of the input signal; and
a second amplitude detector coupled to the amplitude comparator and detecting the attenuated amplitude of the output signal.

4. The power amplifier controller circuit of claim 3, wherein the amplitude control loop further comprises:
a variable attenuator coupled to the second amplitude detector and the power amplifier, the variable attenuator attenuating the output signal of the power amplifier and providing the attenuated output signal to the second detector.

5. The power amplifier controller circuit of claim 2, wherein the power supply is a switched mode power supply.

6. The power amplifier controller circuit of claim 2, wherein the power supply comprises:
a first power supply with a first efficiency, the first power supply coupled to the amplitude comparator and receiving a first portion of the amplitude correction signal in a first frequency range and generating a first adjusted supply output based upon the first portion of the amplitude correction signal; and
a second power supply with a second efficiency higher than the first efficiency, the second power supply coupled to the amplitude comparator and receiving a second portion of the amplitude correction signal in a second frequency range lower than the first frequency range and generating a second adjusted supply output based upon the second portion of the amplitude correction signal, the adjusted supply voltage including a combination of the first adjusted supply output and the second adjusted supply output.

7. The power amplifier controller circuit of claim 6, wherein:
the first power supply is a linear regulator; and
the second power supply is a switched mode power supply.

8. The power amplifier controller circuit of claim 6, wherein the amplitude control loop further comprises:
an adder circuit coupled to the first power supply and the second power supply, the adder circuit combining the first adjusted supply output and the second adjusted supply output to generate the adjusted supply voltage provided to the power amplifier.

9. The power amplifier controller circuit of claim 2, wherein the amplitude control loop further comprises:
a gain control module coupled to the amplitude comparator and receiving the amplitude correction signal to generate the gain control signal.

10. The power amplifier controller circuit of claim 9, wherein the gain control module is coupled between the amplitude comparator and the power supply.

11. The power amplifier controller circuit of claim 1, further comprising:
a phase control loop determining a phase error signal indicative of a phase difference between phases of the input signal to the power amplifier and the output signal of the power amplifier and adjusting the phase of the input signal to the power amplifier to reduce phase distortion generated by the power amplifier, wherein the phase control loop comprises:
a phase comparator comparing the phase of the input signal to the power amplifier with the phase of the output signal of the power amplifier to generate the phase error signal; and
a phase shifter coupled to the phase comparator and an input of the power amplifier, the phase shifter shifting the phase of the input signal to the power amplifier based upon the phase error signal.

12. The power amplifier controller circuit of claim 11 further comprising:
a phase control loop determining a phase error signal indicative of a phase difference between phases of the input signal to the power amplifier and the output signal of the power amplifier and adjusting the phase of the input signal to the power amplifier to reduce phase distortion generated by the power amplifier, wherein the phase control loop further comprises:
a filter coupled between the phase comparator and the phase shifter, the filter filtering the phase error signal and providing the filtered phase error signal to the phase shifter.

13. The power amplifier controller circuit of claim 1, wherein the input signal is a radio frequency (RF) signal.

14. A power amplifier controller circuit for controlling a power amplifier, the power amplifier coupled to receive and amplify an input signal to generate an output signal, the power amplifier controller circuit comprising:
an amplitude control loop determining an amplitude correction signal indicative of an amplitude difference between an amplitude of the input signal to the power amplifier and an attenuated amplitude of the output signal of the power amplifier, the amplitude control loop comprising a digital control element receiving the amplitude correction signal and applying a transfer function to generate an adjusted input signal to the power amplifier based upon the amplitude correction signal and a signal to drive a supply voltage or bias to the power amplifier based upon the amplitude correction signal.

15. The power amplifier controller circuit of claim 14, wherein the amplitude control loop comprises:
an amplitude comparator comparing the amplitude of the input signal with the attenuated amplitude of the output signal to generate the amplitude correction signal.

16. The power amplifier controller circuit of claim 15, wherein the amplitude control loop further comprises:
a first amplitude detector coupled to the amplitude comparator and detecting the amplitude of the input signal; and
a second amplitude detector coupled to the amplitude comparator and detecting the attenuated amplitude of the output signal.

17. The power amplifier controller circuit of claim 16, wherein the amplitude control loop further comprises:
a variable attenuator coupled to the second amplitude detector and the power amplifier, the variable attenuator attenuating the output signal of the power amplifier and providing the attenuated output signal to the second detector.

18. The power amplifier controller circuit of claim 14, further comprising:
a phase control loop determining a phase error signal indicative of a phase difference between phases of the input signal to the power amplifier and the output signal of the power amplifier and adjusting the phase of the input signal to the power amplifier to reduce phase distortion generated by the power amplifier.

19. The power amplifier controller circuit of claim 18, wherein the phase control loop comprises:
a phase comparator comparing the phase of the input signal to the power amplifier with the phase of the output signal of the power amplifier to generate the phase error signal; and
a phase shifter coupled to the phase comparator and an input of the power amplifier, the phase shifter shifting the phase of the input signal to the power amplifier based upon the phase error signal.

20. The power amplifier controller circuit of claim 19, wherein the phase control loop further comprises:
a filter coupled between the phase comparator and the phase shifter, the filter filtering the phase error signal and providing the filtered phase error signal to the phase shifter.