Abstract:
A radio frequency (RF) transmitter is provided. The RF transmitter includes first and second drivers that are configured to receive first and second sets of complementary RF signals. Restoration circuits are coupled to the first and second drivers, and a bridge circuit is coupled to the first and second restoration circuits. By having the restoration circuits and the bridge circuit, a common mode impedance and a differential impedance can be provided, where the common mode impedance is lower than the differential impedance.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to Linear Amplification with Nonlinear Components (LINC) transmitter and, more particularly, to a LINC transmitter with compensation circuits to improve efficiency. 
       BACKGROUND 
       [0002]    Turning to  FIGS. 1 and 2 , an example of a conventional LINC transmitter  100  can be seen. In operation, a input signal S(t) (which has a varying envelope) is provided to the signal generator  102 , which can be represented as: 
         [0000]        S ( t )= A ( t ) e   iθ(t) ,  (1)
 
         [0000]    where A(t) is the signal envelope and θ(t) is the signal phase. The signal generator  102  is then able to generate signals S 1 (t) and S 2 (t) from signal S(t), which can be represented as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           S 
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                         = 
                           
                          
                         
                           
                             1 
                             2 
                           
                            
                           
                             ( 
                             
                               
                                 
                                   S 
                                   1 
                                 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   S 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             c 
                             2 
                           
                            
                           
                             ( 
                             
                               
                                  
                                 
                                    
                                    
                                   
                                     ( 
                                     
                                       
                                         θ 
                                          
                                         
                                           ( 
                                           t 
                                           ) 
                                         
                                       
                                       + 
                                       
                                         ϕ 
                                          
                                         
                                           ( 
                                           t 
                                           ) 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                               + 
                               
                                  
                                 
                                    
                                    
                                   
                                     ( 
                                     
                                       
                                         θ 
                                          
                                         
                                           ( 
                                           t 
                                           ) 
                                         
                                       
                                       - 
                                       
                                         ϕ 
                                          
                                         
                                           ( 
                                           t 
                                           ) 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           c 
                            
                           
                               
                           
                            
                           
                             
                                
                               
                                 θ 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                             ( 
                             
                               
                                 
                                    
                                   
                                     ϕ 
                                      
                                     
                                       ( 
                                       t 
                                       ) 
                                     
                                   
                                 
                                 + 
                                 
                                    
                                   
                                     - 
                                     
                                       ϕ 
                                        
                                       
                                         ( 
                                         t 
                                         ) 
                                       
                                     
                                   
                                 
                               
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         
                           = 
                             
                            
                           
                             c 
                              
                             
                                 
                             
                              
                             
                                
                               
                                 θ 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                             
                              
                             
                               cos 
                               ( 
                               
                                 ϕ 
                                  
                                 
                                   ( 
                                   t 
                                   ) 
                                 
                               
                               ) 
                             
                           
                         
                         , 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where c is radius shown in  FIG. 2  and φ(t) is the out-phasing angle. When combining equations (1) and (2) and solving for the out-phasing angle φ(t), it becomes: 
         [0000]    
       
         
           
             
               
                 
                   
                     ϕ 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       arccos 
                        
                       
                         ( 
                         
                           
                             A 
                              
                             
                               ( 
                               t 
                               ) 
                             
                           
                           c 
                         
                         ) 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Since the arccosine function is limited to a domain between −1 and 1, then: 
         [0000]        c ≧max( A ( t )),  (4)
 
         [0000]    which means that the signals S 1 (t) and S 2 (t) have a generally constant envelope. As a result, high-efficiency, nonlinear power amplifiers (PAs) can be used as PAs  104 - 1  and  104 - 2  to generate signals O 1 (t) and O 2 (t), which can then be combined with combiner  106  to produce signal O(t) that has a variable envelope. 
         [0003]    One issue with LINC transmitter  100  is that there is an efficiency loss (due in part to combiner  106 ), so, as an alternative, an Asymmetric Mutlilevel Outphasing (AMO) transmitter  200  can be employed, as shown in  FIG. 3 . In operation, the AMO modulator  202  (which generally includes predistortion that is adjusted by the predistortion trainer  212 ) generates amplitude signals AMP- 1  and AMP- 2  and phase signals φ- 1  and φ- 2  from input amplitude signal AMP and input phase signal φ. The phase signals φ- 1  and φ- 2  are provided to the digital-to-radio-frequency phase converter (DRFPC)  204  that produces generally constant envelope signals for PAs  208 - 1  and  208 - 2  (similar to signals generator  102 ), and the amplitude signals AMP- 1  and AMP- 2  are used to control the power level applied to PAs  208 - 1  and  208 - 2  from supplies  206 - 1  and  206 - 2  to achieve higher efficiency. As shown in  FIG. 4 , the power is switched in regions where the probability distribution function (PDF) is the largest. This allows the AMO transmitter  200  to have greater overall efficiency than the conventional LINC transmitter  100  and a multilevel LINC (ML-LINC) transmitter but less overall efficiency than an power added efficiency (PAE). Because the efficiency of the AMO transmitter  200  is still relatively low, however, there is a need for an RF transmitter with improved efficiency. 
         [0004]    Some examples of conventional circuits are: Chung et al. “Asymmetric Multilevel Outphasing Architecture for Multi-standard Transmitters,” 2009  IEEE Radio Frequency Integrate Circuits Symposium , pp. 237-240; Godoy et al., “A Highly Efficient 1.95-GHz,  18 -W Asymmetric Multilevel Outphasing Transmitter for Wideband Applications,”  Microwave Symposium Digest  ( MTT ), 2011  IEEE MTT - S International , Jun. 5-10, 2011, pp. 1-4; U.S. Pat. No. 6,366,177; and U.S. Pat. No. 7,260,157. 
       SUMMARY 
       [0005]    An embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises a first driver that is configured to receive a first set of complementary radio frequency (RF) signals; a second driver that is configured to receive a second set of complementary RF signals; a first restoration circuit that is coupled to the first driver; a second restoration circuit that is coupled to the second driver; a bridge circuit that is coupled to the first and second restoration circuits; and an output circuit that is coupled to the first and second restoration circuits, wherein the first restoration circuit, the second restoration circuit, and the bridge circuit provide a common mode impedance and a differential impedance, wherein the common mode impedance is lower than the differential impedance. 
         [0006]    In accordance with an embodiment of the present invention, the first and second restoration circuits further comprise first and second inductor-capacitor (LC) circuits. 
         [0007]    In accordance with an embodiment of the present invention, the apparatus further comprises: a first cancellation circuit that is coupled to the first driver; and a second cancellation circuit that is coupled to the second driver, wherein the first and second cancellation circuits increase peak efficiency. 
         [0008]    In accordance with an embodiment of the present invention, the bridge circuit further comprises an inductor that is coupled between the first and second restoration circuits. 
         [0009]    In accordance with an embodiment of the present invention, there is a free-fly interval between consecutive pulses from the first set of RF pulses and between consecutive pulses from the second set of RF pulses, and wherein at least one of the first and second cancellation circuits are configured to provide harmonic restoration during each free-fly interval. 
         [0010]    In accordance with an embodiment of the present invention, the first and second cancellation circuits further comprise third and fourth LC circuits. 
         [0011]    In accordance with an embodiment of the present invention, the output circuit further comprises a combiner. 
         [0012]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a signal generator that is configured to receive an input signal and that is configured to generate a plurality of sets of complementary RF signals; a plurality of drivers that are coupled to the signal generator, wherein each driver is configured to receive at least one of the sets of complementary RF signals; a plurality of restoration circuits, wherein each restoration circuit is coupled to at least one of the drivers; a bridge circuit that is coupled to each of the restoration circuits; and an output circuit that is coupled to each restoration network, wherein the plurality of restoration circuits and the bridge circuit provide a common mode impedance and a differential impedance, wherein the common mode impedance is lower than the differential impedance. 
         [0013]    In accordance with an embodiment of the present invention, each restoration circuit further comprises an LC circuit. 
         [0014]    In accordance with an embodiment of the present invention, the apparatus further comprises a plurality of cancellation circuits, wherein each cancellation circuit is coupled to at least one of the drivers, and wherein the plurality of cancellation circuits increases peak efficiency. 
         [0015]    In accordance with an embodiment of the present invention, the bridge circuit further comprises an inductor that is coupled between the plurality of restoration circuits. 
         [0016]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a signal generator that is configured to receive an input signal and that is configured to generate first, second, third, and fourth RF signals, wherein the input signal has a variable envelope, and wherein the first and second RF signals are complementary, and wherein the third and fourth RF signals are complementary, and wherein there is a free-fly interval between consecutive pulses of the first, second, third, and fourth RF signals; a first driver having: a first transistor having a first passive electrode, a second passive electrode, and a control electrode, wherein the control electrode of the first transistor is coupled to the signal generator so as to receive the first RF signal; and a second transistor having a first passive electrode, a second passive electrode, and a control electrode, wherein the control electrode of the second transistor is coupled to the signal generator so as to receive the second RF signal, and wherein the first passive electrode of the second transistor is coupled to the second passive electrode of the first transistor at a first node; a second driver having: a third transistor having a first passive electrode, a second passive electrode, and a control electrode, wherein the control electrode of the third transistor is coupled to the signal generator so as to receive the third RF signal; and a fourth transistor having a first passive electrode, a second passive electrode, and a control electrode, wherein the control electrode of the fourth transistor is coupled to the signal generator so as to receive the fourth RF signal, and wherein the first passive electrode of the fourth transistor is coupled to the second passive electrode of the third transistor at a second node; a first restoration circuit that is coupled to the first node; a second restoration circuit that is coupled to the second node; an output circuit that is coupled to the first and second restoration circuits at third and fourth nodes, respectively; and a bridge circuit that is coupled to the third and fourth nodes, wherein the first restoration circuit, the second restoration circuit, and the bridge circuit provide a common mode impedance and a differential impedance, wherein the common mode impedance is lower than the differential impedance. 
         [0017]    In accordance with an embodiment of the present invention, the first and third transistors further comprise first and second PMOS transistors, respectively, and wherein the second and fourth transistors further comprise first and second NMOS transistors, respectively. 
         [0018]    In accordance with an embodiment of the present invention, the bridge circuit further comprises an inductor that is coupled to the third and fourth nodes. 
         [0019]    In accordance with an embodiment of the present invention, the inductor further comprises a first inductor, and wherein the first restoration circuit further comprises: a second inductor that is coupled between the first and third nodes; and a first capacitor that is coupled between the first and third nodes; and wherein the second restoration circuit further comprises: a third inductor that is coupled between the second and fourth nodes; and a second capacitor that is coupled between the second and fourth nodes. 
         [0020]    In accordance with an embodiment of the present invention, the apparatus further comprises: a first cancellation circuit that is coupled to the first node; and a second cancellation circuit that is coupled to the second node. 
         [0021]    In accordance with an embodiment of the present invention, the first cancellation circuit further comprises a third capacitor and a fourth inductor coupled in series with one another, and wherein the second cancellation circuit further comprises a fourth capacitor and a fifth inductor coupled in series with one another. 
         [0022]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0024]      FIG. 1  is a diagram of an example of a conventional LINC transmitter; 
           [0025]      FIG. 2  is a vector diagram for the LINC transmitter of  FIG. 1 ; 
           [0026]      FIG. 3  is a diagram of an example of a conventional AMO transmitter; 
           [0027]      FIG. 4  is a diagram illustrating the efficiency of the LINC transmitter of  FIG. 1  and the AMO transmitter of  FIG. 3 ; 
           [0028]      FIG. 5  is a diagram of an example of a LINC transmitter in accordance with the present invention; 
           [0029]      FIG. 6  is a more detailed example of the LINC transmitter of  FIG. 5 ; and 
           [0030]      FIG. 7  is a diagram depicting the efficiency of the LINC transmitter of  FIG. 6  compared to the LINC transmitter of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
         [0032]    Turning to  FIG. 5 , an example of a LINC transmitter  300  in accordance with the present invention can be seen. LINC transmitter  300  has a similar operation to LINC transmitter  100  in that it employs a signal generator  102  to produce signals S 1 (t) and S 2 (t) (which have a generally constant envelope) from signal S(t) (which has a variable envelope). These signal S 1 (t) and S 2 (t), for LINC transmitter  300 , are then applied to drivers  302 - 1  and  302 - 2  so as to generate signal O(t) from output network  310  (which can include, for example, an inductor-capacitor network having capacitors CM 1  and CM 2  and inductor LM in  FIG. 6 ). This output signal O(t) can then be applied to an radio frequency load (i.e., resistor RL of  FIG. 6 ), such as an antenna. To improve the performance of the LINC transmitter  300  compared to other conventional transmitters, cancellation circuits  308 - 1  and  308 - 2 , restoration circuits  304 - 1  and  304 - 2 , and bridge circuit  306  are employed. 
         [0033]    In  FIG. 6 , a more detailed example of the LINC transmitter  300  can be seen, which can assist in illustrating how the cancellation circuits  308 - 1  and  308 - 2  can improve performance. As shown, the drivers  302 - 1  and  302 - 2  are comprised of transistors Q 1 - 1 , Q 2 - 1 , Q 1 - 2 , and Q 2 - 2  (where transistors Q 1 - 1  and Q 1 - 2  are shown to be PMOS transistors and transistors Q 2 - 1  and where Q 2 - 2  are shown to be NMOS transistors) having parasitic capacitances CP 1 - 1 , CP 2 - 1 , CP 1 - 2 , and CP 2 - 2 , respectively. These transistors Q 1 - 1 , Q 2 - 1 , Q 1 - 2 , and Q 2 - 2  are, respectively, driven by RF input signal pairs RFINU- 1 /RFIND- 1  and RFINU- 2 /RFIND- 2  (which generally correspond to signals S 1 (t) and S 2 (t)). This signal pairs RFINU- 1 /RFIND- 1  and RFINU- 2 /RFIND- 2  are generally complementary pulse width modulated (PWM) input signals that are able to activate transistors Q 1 - 1 /Q 2 - 1  and Q 1 - 2 /Q 2 - 2 , but for transmitter  300 , these signals are not “adjacent” to one another, meaning that these signals are truly complementary from a timing perspective. Between consecutive pulses for the signal pairs RFINU- 1 /RFIND- 1  and RFINU- 2 /RFIND- 2 , there is a free-fly or dead time interval, meaning that there is an interval between consecutive activations of transistors Q 1 - 1  and Q 2 - 1  in driver  302 - 1  and between consecutive activations of transistors Q 1 - 2  and Q 2 - 2  in driver  302 - 2 . As a result of using this free-fly interval, cancellation circuits  308 - 1  and  308 - 2  (which are generally comprised of inductor-capacitor circuits CC- 1 /LC- 1  and CC- 2 /LC- 2 ) can then control the harmonic content at the output of drivers  302 - 1  and  302 - 2  by providing a cancellation current as described in co-pending U.S. patent application Ser. No. 13/106,611, which is entitled “CLASS D POWER AMPLIFIER” and which is incorporated by reference herein. Essentially, the cancellation circuit  308 - 1  and  308 - 2  can provide harmonic restoration with its provision of the cancellation current that allows for an increase in peak efficiency. 
         [0034]    With the restoration circuits  304 - 1  and  304 - 2  and bridge circuit  306 , these circuits can vary the impedance of the transmitter  300  so as to increase the back-off efficiency. As shown, the restoration circuits  304 - 1  and  304 - 2  are generally comprise capacitors CHR 1  and CHR 2  and inductors LHR 1  and LHR 2 , and the restoration circuits  304 - 1  and  304 - 2  are typically tuned (i.e., capacitors CHR 1  and CHR 2  and inductors LHR 1  and LHR 2  are properly dimensioned) to isolate the third harmonic (although tuning to other harmonics may also be possible) to generally function as a harmonic rejection filter. The bridge circuit  306 , as shown, is generally comprised of an inductor LBC that is tuned or dimensioned to “tune-out” the effects of the parasitic capacitors CP 1 - 1 , CP 2 - 1 , CP 1 - 2 , and CP 2 - 2  at the first harmonic (although other harmonics may be chosen). 
         [0035]    Looking back to the vector diagram of  FIG. 2  for transmitter  100  (which would be similar to a vector diagram for transmitter  300 ), the vectors representing each of the signals S 1 (t) and S 2 (t) has both an out-of-phase component and an in-phase component that combine to form the vector representing signal S(t). To achieve higher total efficiency, it is desirable to have a higher impedance for the out-of-phase components (which can be referred to as the differential impedance) because the higher impedance lowers the current drawn. It is also desirable to have a lower impedance for the in-phase components (which can be referred to as the common mode impedance) because there is a decrease in the switching losses for transmitter  300 . By having the restoration circuits  304 - 1  and  304 - 2  tuned to the third harmonic (for example) and having the bridge circuit  306  tuned to the first harmonic (for example), there can be a high differential impedance and a low common mode impedance, allowing for lower power consumption and increase efficiency. 
         [0036]    With the combined use of cancellation circuits  308 - 1  and  308 - 2 , restoration circuits  304 - 1  and  304 - 2 , and bridge circuit  306  in transmitter  300 , an efficiency improvement can be seen as compared to transmitter  100  in  FIG. 7 . As shown, there is nearly a 50% increase in efficiency at low power and about a 10% increase at high power. This improvement can also be achieved with passive components (i.e., resistors, capacitors, and inductors), avoiding the costs and penalties associated with other active systems (like AMO transmitter  200 ). Additionally, a bulky combiner (as used with transmitter  100 ) can also be eliminated. 
         [0037]    Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.