Abstract:
A radio frequency (“RF”) power amplifier circuit as described herein is configured to detect and measure an output load mismatch and to adjust the operating characteristics of the RF power amplifier to reduce output signal distortion. The circuit includes a directional RF signal coupler that obtains a coupled reflected RF signal that is indicative of the output load mismatch. The coupled reflected RF signal is processed to generate one or more bias control signals for the RF power amplifier. In operation, a mismatch condition will result in a measurable coupled reflected RF signal and a corresponding reduction in output power from the RF power amplifier. Ultimately, the output power control mechanism strives to maintain the RF power amplifier within a linear operating range.

Description:
TECHNICAL FIELD 
     The present invention relates generally to power amplifier circuits. More particularly, the present invention relates to a radio frequency (“RF”) power amplifier having an output power control feature. 
     BACKGROUND 
     The prior art is replete with RF power amplifiers suitable for use with numerous practical applications. For example, mobile telephones and other wireless communication devices are common applications for RF power amplifiers. In a mobile telephone, the antenna presents a load to the RF power amplifier contained in the transmitter, and the load changes in response to various antenna operating effects and/or environmental conditions. Consequently, the changing load may result in a mismatch for the RF power amplifier. The performance of the RF power amplifier, in particular the linearity of its RF output signal, tends to be strongly dependent upon its loading condition. This practical operating characteristic applies to single-ended amplifier types and to more complex architectures, such as full polar amplifiers. Generally, the variation in loading can result in degradation of the AM-AM response of a full polar amplifier and/or an increase in AM-PM conversion, both of which lead to signal degradation (higher output RF spectrum (“ORFS”) and an increase in error vector magnitude (“EVM”)). 
     Existing power amplifier architectures may employ closed feedback techniques to reduce variations in output power generated by the amplifiers. Such closed loop architectures do not adjust output power based upon dynamically changing load conditions. Moreover, existing closed loop solutions tend to add significant transmitter current for feedback purposes, and such solutions do not directly detect output load mismatches. These limitations can lead to undesirable results in practical applications. 
     Accordingly, it is desirable to have a circuit and a technique for adjusting the output generated by an RF power amplifier, where such adjustment is responsive to changing output load conditions. In addition, it is desirable to have an RF power amplifier circuit that is capable of detecting the presence of an output load mismatch (e.g., high voltage standing wave ratio) and correcting output signal distortions caused by output load mismatches. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a schematic representation of an RF power amplifier circuit configured in accordance with an example embodiment of the invention; 
         FIG. 2  is a schematic representation of a full polar RF power amplifier circuit configured in accordance with an example embodiment of the invention; and 
         FIG. 3  is a flow chart of an output power adjustment process according to an example embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     The invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various integrated circuit components, e.g., memory elements, transistors, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that the present invention may be practiced in conjunction with any number of practical circuits, subsystems, or systems, and that the RF power amplifier deployment described herein is merely one exemplary application for the invention. 
     For the sake of brevity, conventional techniques related to RF power amplifier design, RF signal coupling, RF signal detection, and other functional aspects of the circuits (and the individual operating components of the circuits) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical embodiment. 
     The following description refers to elements or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/feature is directly joined to (or directly communicates with) another element/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/feature, and not necessarily mechanically. Thus, although the circuit schematics shown herein depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an actual embodiment (assuming that the functionality of the circuits are not adversely affected). 
     An RF electronic circuit as described herein employs a technique for improving power amplifier characteristics in response to load mismatch effects, resulting in better amplifier linearity. Under load mismatch conditions, the circuit detects the presence of a reflective load using a directional coupler. The reflected RF signal is detected to generate one adjustment signal (e.g., a voltage), which is proportional to the magnitude of the reflection coefficient. Additionally, the forward incident RF output signal is detected to generate another adjustment signal. In the example embodiment, the two adjustment signals are independently amplified/attenuated and applied in a feedback loop to adjust the bias of the amplifier. Operationally, as the reflected RF signal increases due to high VSWR conditions, the feedback loop adjusts the bias voltage and/or current of one or more amplifier stages. This effectively results in less AM-AM compression, thereby improving output signal quality while reducing variations in incident power as a function of VSWR. Using this technique, the loop parameters can be properly adjusted to reduce gain compression of peak envelope excursions under load mismatch conditions. Consequently, output signal quality is enhanced, as reflected in improved ORFS and EVM measurements. 
       FIG. 1  is a schematic representation of an RF power amplifier circuit  100  configured in accordance with an example embodiment of the invention. Circuit  100  generally includes an RF power amplifier  102  and an output power control architecture  104  coupled to RF power amplifier  102 . In this example, which is suitable for use with a wireless communication device, RF power amplifier  102  receives an input signal  106  and generates an RF output signal  108  having desired output characteristics. In practice, the frequency, amplitude, phase, and other characteristics of RF output signal  108  are dictated by the particular application. RF power amplifier  102  drives an RF antenna  110  for transmission of RF output signal  108 . As mentioned above, RF antenna  110  represents an output load for RF power amplifier  102 , and in operation the output impedance presented by RF antenna  110  may vary in response to changing operating, environmental, and other conditions. In other words, RF antenna  110  represents a potentially variable output load for RF power amplifier  102 . 
     RF power amplifier circuit  100  preferably includes an RF coupler  112 , which is suitably configured to obtain a coupled reflected signal  114  and a coupled incident signal  116  for output power control architecture  104 . In practice, coupled reflected signal  114  is based upon a reflected component of RF output signal  108 , while coupled incident signal  116  is based upon a forward incident component of RF output signal  108 . No RF energy will reflect under ideal conditions where antenna  110  presents a matched output load for RF power amplifier  102 . Under such ideal conditions, therefore, coupled reflected signal  114  will not be detected. 
     RF coupler  112  can be realized as a directional coupler having an incident port and a reflected port. In a practical implementation, RF coupler  112  can be integrated into an output harmonic filter for RF power amplifier  102 , thus minimizing insertion loss and conserving physical space. RF coupler  112  may incorporate an RF transmission line that provides a suitable amount of coupling relative to RF output signal  108 . For example, RF coupler  112  may be realized as a −20 dB coupler using any suitable construction. 
     Briefly, output power control architecture  104  is configured to adjust operating characteristics of RF power amplifier  102  in response to coupled reflected signal  114  (and, in this example embodiment, in response to coupled incident signal  116 ). Although not depicted in  FIG. 1 , output power control architecture  104  may include or communicate with suitable control or processing logic that influences its operation, sets initial parameter settings, or the like. Output power control architecture  104  is suitably configured to generate at least one control signal  118  for RF power amplifier  102 , where the control signal(s)  118  have characteristics influenced by coupled reflected signal  114  (and, in this example embodiment, influenced by coupled incident signal  116 ). As depicted in  FIG. 1 , output power control architecture  104  can generate any number (N) of control signals  118 , where the actual number depends upon the particular application. Furthermore, a given control signal  118  may be a bias voltage, a bias current, a supply voltage, a supply current, or a digital control signal that influences bias or supply voltages or currents, and a given control signal  118  may be applied to any number of amplifier stages associated with RF power amplifier  102 . 
     Output power control architecture  104  is suitably configured to adjust output power of RF power amplifier  102  in response to coupled reflected signal  114 . Output power control architecture  104  is designed to dynamically adjust output power of RF power amplifier  102  to reduce the amplitude of the reflected component of RF output signal  108 . For example, in response to the presence of a mismatch condition that causes RF signal reflection, circuit  100  can reduce the output power level to increase the linearity of RF power amplifier  102 . Circuit  100  may utilize different methodologies to carry out the output power adjustment. For example, output power control architecture  104  may be suitably configured to: adjust biasing of RF power amplifier  102  in response to coupled reflected signal  114 ; adjust gain of RF power amplifier  102  in response to coupled reflected signal  114 ; adjust linearity of RF power amplifier  102  in response to coupled reflected signal  114 ; or the like. 
     In one practical embodiment of the invention, power amplifier  102  is realized as a full polar amplifier. In this regard,  FIG. 2  is a schematic representation of a full polar RF power amplifier circuit  200  configured in accordance with an example embodiment of the invention. Circuit  200  is a highly simplified diagram illustrating one preferred architecture for implementing a full polar power amplifier with a closed loop output power control feature. Circuit  200  is suitable for use in transmitters of cell phones supporting, for example, EDGE/GSM standards. The following description summarizes the general operation of the full polar amplifier. 
     The input RF signal to be amplified can be written in terms of amplitude envelope (m(t)) and phase (Φ(t)) constituents: V in (t)=m in (t)cos(ω o t+Φ in )(t)). 
     As illustrated in  FIG. 2 , the phase modulated RF carrier (without amplitude modulation) is applied at the input of the RF power amplifier. The amplitude constituent is applied through a summing junction to modulate the biases of the amplifier stages, thereby reconstituting the envelope on the phase modulated signal. The signal at the output of the amplifier therefore takes the form: V out (t)=m out (t)cos(ω o t+Φ out (t)+Φ offset ), where Φ offset  represents a phase offset caused by time delay through the RF power amplifier. 
     For the signal to be linearly amplified, the envelope and phase properties are maintained as follows:
 
 V   out ( t )/ V   in ( t )= K , where  K  is a constant
 
Φ out ( t )=Φ in ( t )+Φ offset 
 
     For this architecture, the phase requirement (represented by the immediately preceding expression) is achieved by careful design of the RF power amplifier such that it inherently exhibits low AM-to-PM conversion. Preserving the envelope characteristics is achieved via the power control techniques described in more detail herein. 
     Circuit  200  addresses the issue of detecting and correcting the deleterious effects on amplifier performance resulting from antenna mismatch loading of the RF amplifier. Essentially, the antenna presents a load (of reflection coefficient Γ L  at angle θ) to the amplifier, and that load varies (both Γ L  and angle θ) due to various antenna and/or environmental effects. Antenna load effects are quantified in terms of VSWR presented at the output of the amplifier: 
     
       
         
           
             VSWR 
             = 
             
               
                 1 
                 + 
                 
                    
                   
                     Γ 
                     L 
                   
                    
                 
               
               
                 1 
                 - 
                 
                    
                   
                     Γ 
                     L 
                   
                    
                 
               
             
           
         
       
     
     Typical specifications require the amplifier to achieve a specified level of linearity in the presence of VSWR, typically up to 4:1 (for any angle θ from zero to 2π). In general, amplifier linearity suffers with increasing VSWR, and will be worse for a certain angle of θ. For EDGE modulation, both ORFS and EVM are typically cited linearity requirements, and both suffer with increasing VSWR loading. 
     In response to VSWR loading, an envelope feedback loop functions primarily to maintain constant incident power at the amplifier output. That is, the envelope of the output voltage (m out (t)) is sampled, scaled, and then compared to the input envelope signal (m in (t)). Hence, the load line presented to the amplifier (assuming infinite coupler directivity) varies from shallow to steep. Significant compressions of peak amplitude excursions occur (for certain load lines due to limited headroom of the amplifier) which then leads to an increase in ORFS and EVM. The situation is made worse when the coupler exhibits limited directivity. In a small foot print module environment, the directivity of such a coupler is on the order of 10 dB to 15 dB. Hence, the feedback signal generated by the coupler includes the vector sum of the incident wave plus a portion of the reflected signal (due to coupler directivity). When the signal phases are aligned, the feedback error voltage results in a drop of output power. Alternatively, when they are 180 degrees apart, the feedback error voltage drives the RF power amplifier further into compression leading to even higher levels of ORFS and EVM. 
     Referring again to  FIG. 2 , circuit  200  generally includes an RF power amplifier  202 , an RF antenna  203 , an RF coupler  204 , a first RF attenuator/gain element  206 , a second RF attenuator/gain element  208 , a first amplitude detector  210 , a second amplitude detector  212 , a summer  214 , an integrator  216 , and a bias signal generator  218 . In a practical embodiment, summer  214  and integrator  216  may be realized as a single element or component. The components depicted in  FIG. 2  (other than RF power amplifier  202 , RF antenna  203 , and RF coupler  204 ) collectively may be considered to be an output power control architecture as described above in connection with circuit  100 . 
     In this example embodiment, RF power amplifier  202  is a full polar amplifier that operates in the manner described above. As depicted in  FIG. 2 , the amplitude constituent of the RF input signal serves as one input to summer  214 , and the phase constituent of the RF input signal serves as an input to RF power amplifier  202 . RF power amplifier  202  is suitably configured to generate an RF output signal  220 , which is utilized to drive RF antenna  203  in this example. RF coupler  204 , which may be configured to operate as described above in connection with RF coupler  112 , obtains a coupled reflected signal  222  associated with RF output signal  220  (along with a coupled incident signal  224  in this example). Coupled reflected signal  222  may serve as an input to RF attenuator/gain element  206 , and coupled incident signal  224  may serve as an input to RF attenuator/gain element  208 . 
     RF attenuator/gain element  206 , which has an input coupled to RF coupler  204 , is suitably configured to adjust the magnitude of coupled reflected signal  222  to a level appropriate for the current operating conditions. In practice, the amount of attenuation or gain provided by RF attenuator/gain element  206  may be influenced by the amount of attenuation or gain provided by RF attenuator/gain element  208 . In addition, the amount of attenuation or gain provided by RF attenuator/gain element  206  is influenced by the amount of desired correction for RF power amplifier  202 . RF attenuator/gain element  206  adjusts the level of coupled reflected signal  222  to increase the dynamic range of amplitude detector  210 . In a practical embodiment, circuit  200  may utilize a suitable control scheme to initialize RF attenuator/gain elements  206 / 208  in accordance with the desired level for RF output signal  220 . Thereafter, the initial settings can be altered as the desired level for RF output signal  220  changes to suit the dynamic needs of the particular application. 
     In a practical embodiment of circuit  200 , RF attenuator/gain element  206  is realized as an adjustable or programmable component having a suitable adjustment range for setting the average power level of coupled reflected signal  222 . In one example embodiment, RF attenuator/gain element  206  provides 28 dB of programmable attenuation/gain. In operation, RF attenuator/gain element  206  is controlled to attenuate or amplify coupled reflected signal  222  as needed to set the loop parameters of circuit  200 . Thus, RF attenuator/gain element  206  produces an attenuated/amplified RF signal  226 . RF attenuator/gain element  208  is similarly configured to provide a desired amount of attenuation or gain for coupled incident signal  224 , resulting in an attenuated/amplified RF signal  228 . 
     Amplitude detector  210  has an input coupled to the output of RF attenuator/gain element  206 . Amplitude detector  210  is suitably configured to quantify the amplitude of attenuated/amplified RF signal  226 . Moreover, amplitude detector  210  is preferably configured to generate an adjustment signal  230  that is indicative of an output load mismatch for RF power amplifier  202 . In the example embodiment, amplitude detector  210  is a linear amplitude detector that is capable of detecting a number of discrete amplitude levels corresponding to attenuated/amplified RF signal  226 , where adjustment signal  230  is indicative of the particular amplitude level. Alternatively, amplitude detector  210  may be realized as a logarithmic detector. In practice, adjustment signal  230  may be a voltage signal, where the particular voltage level represents the detected amplitude of attenuated/amplified RF signal  226 . Amplitude detector  212  is similarly configured to generate a second adjustment signal  232  indicative of the detected amplitude of attenuated/amplified RF signal  228 . 
     Summer  214  adjusts the amplitude constituent of the RF input signal in response to adjustment signals  230 / 232 . In this example, the output of summer  214  corresponds to the amplitude constituent of the RF input signal minus adjustment signals  230 / 232 . The output of summer  214  may serve as an input to integrator  216 , which is suitably configured to perform averaging or filtering of its input to obtain an average power level indication. In practice, integrator  216  can be realized as a gain stage that is also configured to provide a suitable amount of loop gain for circuit  200 . 
     In this example, bias signal generator  218  is coupled to RF power amplifier  202  and to integrator  216 . In this regard, bias signal generator  218  is also coupled to amplitude detectors  210 / 212 . Briefly, bias signal generator  218  is configured to generate, in response to adjustment signal  230  (and in response to adjustment signal  232  in this example), at least one bias control signal  234  for RF power amplifier  202 . Thus, bias control signal(s)  234  are generated in response to coupled reflected signal  222  and, in this example, in response to coupled incident signal  224 . As described above, bias control signal(s)  234  influence the output power of RF power amplifier  202 , and bias control signal(s)  234  may be realized as a bias voltage, a bias current, a supply voltage, a supply current, a digital control signal that influences bias or supply voltages or currents, or the like. Furthermore, multiple bias control signals  234  may be utilized to independently control separate stages of a practical RF power amplifier  202 . Circuit  200  may utilize different methodologies to carry out the output power adjustment. For example, bias signal generator  218  may be suitably configured to: adjust biasing of RF power amplifier  202  in response to coupled reflected signal  222 ; adjust gain of RF power amplifier  202  in response to coupled reflected signal  222 ; adjust linearity of RF power amplifier  202  in response to coupled reflected signal  222 ; or the like. 
     In operation, if RF antenna  203  presents an ideal 50 Ohm termination for RF power amplifier  202 , then coupled reflected signal  222  has no measurable amplitude and, therefore, the loop is unaffected. Under high VSWR conditions, however, the amplitude of coupled reflected signal  222  is proportional to the VSWR and to the output power level. Circuit  200  detects coupled reflected signal  222  and generates a feedback voltage in the form of adjustment signal  230 . The higher feedback voltage in the loop has the effect of reducing the drive on the bias controls of RF power amplifier  202 . Consequently, RF power amplifier  202  is driven less deeply into compression, resulting in an improvement in linearity. 
       FIG. 3  is a flow chart of an output power adjustment process  300  according to an example embodiment of the invention. Process  300  may be performed by an RF electronic circuit as described above. The various tasks performed in connection with process  300  may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description of process  300  may refer to elements mentioned above in connection with  FIG. 1  and  FIG. 2 . In practical embodiments, portions of process  300  may be performed by different elements of the described system, e.g., the power amplifier, the RF coupler, or the output power control architecture. It should be appreciated that process  300  may include any number of additional or alternative tasks, the tasks shown in  FIG. 3  need not be performed in the illustrated order, and process  300  may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. 
     Process  300  may begin by generating a suitable RF output power signal (task  302 ) with a power amplifier. Generally, process  300  detects the presence of an output load mismatch using techniques described above. For example, process  300  may obtain a coupled reflected signal that is based upon a reflected component of the RF output signal (task  304 ). In this example, process  300  also obtains a coupled incident signal that is based upon the forward transmit component of the RF output signal (task  306 ). Process  300  continues by adjusting the magnitude of the coupled reflected signal and/or the magnitude of the coupled incident signal (task  308 ), and by detecting the magnitude of the coupled signals (task  310 ). As described above, the circuit may utilize one or more programmable attenuation/gain elements to perform task  308 . 
     Process  300  may also generate an adjustment signal that is indicative of an output load mismatch (task  312 ). In practice, this adjustment signal may be generated in response to the adjusted coupled reflected signal resulting from task  308 . In addition, process  300  may generate another adjustment signal that is indicative of the incident output power of the RF output signal (task  314 ). In practice, this adjustment signal may be may be generated in response to the adjusted coupled incident signal resulting from task  308 . These adjustment signals may be utilized to generate at least one bias control signal for the power amplifier (task  316 ), where the bias control signals have characteristics that are influenced by the coupled reflected signal and/or by the coupled incident signal. Eventually, process  300  adjusts operating characteristics of the power amplifier in response to the coupled reflected signal and/or in response to the coupled incident signal (task  318 ). In practice, task  318  may adjust the output power level of the power amplifier using the techniques described in more detail above. 
     In summary, systems, devices, and methods configured in accordance with example embodiments of the invention relate to: 
     An RF electronic device comprising a power amplifier configured to generate an RF output signal, an RF coupler configured to obtain a coupled reflected signal based upon a reflected component of the RF output signal, and an output power control architecture coupled to the power amplifier and to the RF coupler, the output power control architecture being configured to adjust operating characteristics of the power amplifier in response to the coupled reflected signal. The output power control architecture may be configured to adjust output power of the power amplifier in response to the coupled reflected signal. The output power control architecture may be configured to adjust output power of the power amplifier to reduce amplitude of the reflected component. The output power control architecture may be configured to adjust biasing of the power amplifier in response to the coupled reflected signal. The output power control architecture may be configured to adjust gain of the power amplifier in response to the coupled reflected signal. The output power control architecture may be configured to adjust linearity of the power amplifier in response to the coupled reflected signal. The RF coupler may be configured to obtain a coupled incident signal based upon the RF output signal, and the output power control architecture may be configured to adjust operating characteristics of the power amplifier in response to the coupled incident signal. The power amplifier may be a closed loop polar feedback amplifier. The output power control architecture may comprise an adjustable RF attenuator/gain element configured to adjust magnitude of the coupled reflected signal. The output power control architecture may be configured to generate at least one control signal for the power amplifier, the at least one control signal having characteristics influenced by the coupled reflected signal. 
     A method of adjusting output power of an RF power amplifier configured to generate an RF output signal, the method comprising detecting the presence of an output load mismatch, obtaining a coupled reflected signal based upon a reflected component of the RF output signal, and adjusting operating characteristics of the RF power amplifier in response to the coupled reflected signal. Adjusting operating characteristics of the RF power amplifier may comprise adjusting biasing of the RF power amplifier. Adjusting operating characteristics of the RF power amplifier may comprise adjusting gain of the RF power amplifier. The method may further comprise obtaining a coupled incident signal based upon the RF output signal, wherein adjusting operating characteristics of the RF power amplifier is responsive to the coupled incident signal. The method may further comprise adjusting the magnitude of the coupled reflected signal. The method may further comprise generating at least one control signal for the RF power amplifier, the at least one control signal having characteristics influenced by the coupled reflected signal. 
     An RF electronic device comprising a power amplifier configured to generate an RF output signal, an RF coupler configured to obtain a coupled reflected signal based upon a reflected component of the RF output signal, an amplitude detector coupled to the RF coupler, the amplitude detector being configured to generate an adjustment signal indicative of an output load mismatch, and a bias signal generator coupled to the amplitude detector, the bias signal generator being configured to generate, in response to the adjustment signal, at least one bias control signal for the power amplifier. The RF electronic device may further comprise an RF antenna coupled to the power amplifier, the RF antenna representing a potentially variable output load for the power amplifier. The at least one bias control signal influences output power of the power amplifier. The RF coupler may be configured to obtain a coupled incident signal based upon the RF output signal, and the bias signal generator may be configured to generate the at least one bias control signal in response to the coupled incident signal. 
     While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. For example, circuit techniques in the digital domain can be utilized to produce equivalent RF electronic circuits. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.