Abstract:
An error correction amplifier and method of canceling distortion in an amplified signal. The error correction amplifier includes a main amplifier operable to receive a main input signal and generate an amplified signal having a main component and an error component. The error correction amplifier also includes a second amplifier coupled in a feed-forward arrangement to the main amplifier and operable to receive an input signal and to generate an output signal having a main component and an error component. A balancing network is coupled to the main amplifier and to the second amplifier. The balancing network isolates a sample of the output signal of the main amplifier, inverts the sample, and combines the sample with the input signal to the second amplifier. A summing point combines the output signal from the main amplifier and the output signal of the second amplifier in an unequal manner such that the error components of the two output signals substantially cancel one another and the main components of the output signals are added to one another. The method involves dividing an input signal into a first component and a second component; amplifying the first component of the input signal to create an output signal; sampling the output signal to create a sampled signal; combining the sampled signal and the second component of the input signal to create a combined signal; amplifying the combined signal to create a correction signal; and combining the output signal and the correction signal in an unequal combiner to create an amplified signal.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of prior filed co-pending provisional patent application No. 60/199,058 filed on Apr. 22, 2000. This application is a continuation-in-part of co-pending non-provisional application No. 09/557,904 filed Apr. 21, 2000. Non-provisional application No. 09/557,904 claims the benefit of provisional application No. 60/131,484 filed on Apr. 29, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to distortion or error canceling amplifiers. More particularly, the invention relates to a distortion-canceling amplifier that implements unequal power combining of signals. 
     As is known, spectral regrowth refers to the amplification of signals outside a desired frequency range. Ideally, an amplifier would amplify signals without creating noise, particularly noise outside the frequency range of the input signal. In practice, this ideal has not yet been achieved, and spectral regrowth often causes interference between adjacent communication channels. Limiting or reducing spectral regrowth is an important factor to improving spectral efficiency. When spectral regrowth is low, interference is reduced. With reduced interference, channel separation may be narrowed and the number of channels in a given bandwidth may be increased. 
     Feed-forward amplifiers (“FFAs”) use two amplifiers: a main amplifier and a distortion-canceling amplifier. The main amplifier is operated at a relatively high power level and generates an amplified, but distorted or noisy signal. A feed-forward circuit or path is used to estimate the distortion generated by the main amplifier. The estimated distortion is inverted, amplified, and then summed with the output from the main amplifier to remove the distortion in the amplified signal. 
     A balanced error correction (“BEC”) amplifier also uses two amplifiers. In a BEC amplifier, the output power from a main amplifier and an error-canceling amplifier are combined in an equal-power combiner. This results in about a fifty-percent efficiency improvement over feed-forward amplifiers. Nevertheless, the performance of BEC amplifiers is less than ideal. 
     The equal-power combining technique used in a BEC amplifier has at least two efficiency limitations. First, the output coupler or combiner dissipates desired signal power due to the fact that the input signals applied to the combiner can never be exactly equal. The dissipation reduces efficiency. Second, the error-canceling amplifier is necessarily operated at a non-optimal point that requires relatively high levels of the distortion-canceling signal. This higher-than-optimal distortion canceling signal results in further distortion due to a non-linear transfer of the distortion-canceling signal. The error amplifier experiences a reduced canceling capability that can only be compensated by a reduction in the desired output signal power. This, in turn, reduces efficiency. 
     SUMMARY OF THE INVENTION 
     Accordingly, there is a need for an improved amplifier that eliminates the deficiencies in BEC amplifiers. 
     The invention provides an amplifier for communications and other applications in the form of an optimal power combining (“OPC”) amplifier. The OPC amplifier includes a main amplifier operable to receive a main signal and to generate an amplified signal having a main component and an error component. The amplifier also includes an error amplifier coupled in a feed forward arrangement to the main amplifier. The second amplifier receives an input signal and generates an output signal having a main component and an error component. A balancing network is coupled to the main amplifier and to the second amplifier. The balancing network isolates a sample of the error component of the output signal of the main amplifier, inverts the sample and combines the sample with the input signal to the error amplifier. An output combiner combines the output signal from the main amplifier and the output signal of the error amplifier in an unequal fashion such that the error components of the two signals substantially cancel one another. 
     The design of the OPC amplifier reduces losses due to the improvement in efficiency caused by appropriately accounting for unequal signal powers in the input signals of the combiner. Further, the design permits the error-canceling amplifier to be operated in a more linear fashion. 
     As is apparent from the above, it is an advantage of the present invention to provide an amplifier with improved power efficiency. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of an optimal power combining (“OPC”) amplifier embodying the invention. 
     FIG. 2 is a circuit diagram of an OPC amplifier embodying the invention, illustrating the signal terms of signals propagated through the amplifier. 
     FIG. 3 is a circuit diagram of the OPC amplifier shown in FIG. 2, illustrating the spectral shapes of signals propagated through the amplifier. 
     FIG. 4 is a schematic diagram of an OPC amplifier illustrating its signal paths. 
    
    
     DETAILED DESCRIPTION 
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. In particular, it should be understood that the present invention is not limited to applications in communications, but may be used in a wide variety of applications where amplifiers are needed. 
     FIG. 1 illustrates an OPC amplifier  20  embodying the invention. The OPC amplifier  20  receives an input signal IN at an input node  22 . The input signal IN is split by a coupler  24  causing the input signal IN to be directed to a main path  26  and an error path  28 . A signal S IN  is directed to the main path  26  and is amplified by a main amplifier  30  having a gain G. The main amplifier  30  generates an amplified signal  32  having a main component (GS IN ) and an error or noise component (N). The amplified signal  32  is split by a coupler  34  that causes part of the amplified signal to be diverted down a feed-forward path  36  to a gain and phase control block or assembly  38 . The coupler  34  has a coupling coefficient C of            1   G          [     1   +     1     L   2         ]       .                          
     The gain and phase control assembly  38  includes a gain control amplifier  40  having an adjustable gain. The gain and phase control assembly  38  also has a phase adjuster  42  that provides phase adjustment to compensate for variations in the components used in the OPC amplifier  20 . Preferably, the gain and phase control assembly  38  is adjusted so as to produce an output signal of          -     (       S   IN     +     N   G       )                         (     1   +     1     L   2         )     .                            
     This output signal is delivered to a combiner  44 . 
     The signal from the coupler  24  that is directed to the error path  28  is gain and phase adjusted by a gain and phase control block or assembly  46 . The gain and phase control assembly  46  has a gain control amplifier  48  having an adjustable gain. The gain and phase control assembly  46  also has a phase adjuster  50  that provides phase adjustment to the signal in the error path  28  to compensate for variations inherent in the components used in the OPC amplifier  20 . 
     The output from the gain and phase control assembly  46  is delivered to a delay  52  that provides a time delay approximately equal to the amount of time required for a signal to propagate through the main amplifier  30 . The output of the delay  52  is delivered to the combiner  44 . The combiner  44  adds the signal from the gain and phase control assembly  38  to the output signal of the gain and phase assembly  46 . The signal        (       S   IN     -       N   G          [     1   +     1     L   2         ]         )                          
     exiting the combiner  44  is input to an error or distortion canceling amplifier  58  having a gain G. Alternatively, the signal may be input to a gain adjustment amplifier  59 , prior to being input to the distortion canceling amplifier  58 . The distortion canceling amplifier  58  generates a signal  60  having a main component (GS IN ) and a noise component          (     -     N     L   2         )     .                          
     The signal  60  is input into a summing point or combiner  70 . 
     As noted above, the coupler  34  splits the amplified signal  32  such that part of the amplified signal  32  is diverted down the feed-forward path  36 . Another part of the amplified signal  32  is directed to a delay  75 . The delay  75  provides a time delay equal to the time required for a signal to propagate through the distortion-canceling amplifier  58 . A signal  76  having a main component        (       G   L          S   IN       )                          
     and an error or noise component        N   L                          
     exits the delay  75  and is directed to the combiner  70 , which has a first input  71  and a second input  72 . 
     The signal  60  and the signal  76  are delivered to the combiner  70  and are combined such that their error components substantially cancel each other and their main components are summed together. The resultant output signal  80  is output at an output node  82  and has more power than either the amplified signal  32  or the signal  60 . 
     The advantages of the present invention over standard BEC amplifiers are based, in large part, on adjusting or choosing the gain through the coupler  34  and delay  75  and on the selection of the combiner  70  as an unequal power combiner. Preferably, the main and distortion canceling amplifiers have the same power capabilities. 
     The signal  32  from the main amplifier  30  propagates through the coupler  34  and delay  75 . The signal  32  is reduced in amplitude by a loss through these components. Preferably, the coupler  34  and delay  75  have a gain of        1   L                          
     where L is representative of the loss from the second input  72  of the combiner  70  to the output node  82 . As will be discussed in further detail below, the combiner  70  combines the signals  60  and  76  in a ratio that is equal to the ratio of the desired input powers. So, for example, if the signal  76  is lower than the signal  60  by  1 . 5 SdB, the combiner  70  has 1.5 dB less coupling from the main or second input  72  than the error or first input  71 . 
     The operation and advantages of the invention may be better understood by reference to FIGS. 2 and 3. FIG. 2 illustrates an amplifier  100  having a main amplifier  102  and an error amplifier  104 . The amplifier  100  also includes an input node  106 , a sampling coupler  107 , a first combiner  108 , an output combiner  110 , and an output node  112 . The amplifier  100  is configured in a manner similar to the amplifier  20  shown in FIG.  1 . Accordingly, further discussion of the construction of amplifier  100  will not be provided herein. However, the operation of the amplifier  100  will be discussed. 
     Amplifier distortion of input signals is caused by non-linearities in the amplifier&#39;s characteristics. In many instances, it can safely be assumed that an amplifier produces only a first order term (the amplified or desired signal) and a third order term (a distortion term or component). For example, in the amplifier  100 , the main amplifier  102  has a transfer function that amplifies the input signal V i (t) according to V o (t)=A 1 V i (t)+B 1 V i   3 (t). The desired signal (S 1 ) is A 1 V i (t) and the distortion term (D 1 ) is B 1 V i   3 (t). To perfectly eliminate the distortion component of the amplified signal produced by the main amplifier  102 , the error amplifier  104  must generate a signal that exactly cancels the distortion amplifier. Just like any other amplifier, the error amplifier  104  produces its own third order distortion term. This distortion term plus the distortion term from the main amplifier are added at the output combiner  110 . The input signal that is fed into the error amplifier  104  is adjusted so that the error amplifier provides a cancellation term to the combiner  110  at a certain level. In particular, the input signal to the error amplifier  104  is adjusted such that the output signal of the error amplifier  104  includes a component that is equal in amplitude, but 180 degrees out of phase from the combination of both the main and error amplifier distortion terms. 
     This is best understood by reference to the analysis below. The error amplifier  104  has a transfer function similar to the main amplifier  102 : V oo (t)=A 2 V y (t)+B 2 V y   3 (t). The signal input to the error amplifier  104  includes a sample of the output of the main amplifier and a signal V i (t) based on the signal to the input node  106 . Explained in relation to the transfer function for the error amplifier  104 , V y (t) consists of a distortion term KB 1 V i   3 (t) (which acts as a distortion-canceling signal) and a desired-input signal (C−KA 1 )V i (t). C represents the coupling coefficient of the sampling coupler  107  and K represents the coupling coefficient of the first combiner  108  in the amplifier  100 . When the signal (C−KA 1 )V i (t) is applied to the error amplifier  104 , two terms are produced: 
     1. (C−KA 1 )A 2 V i (t)—this term is the desired signal, S 2 , and has an amplitude that is equal to the amplitude of the desired signal S 1 , or A 1 V i (t). 
     2. B 2 (C−KA 1 ) 3 V i   3 (t)—this is the distortion term D 2 . The distortion term D 2  is cancelled by the distortion-canceling term from the error amplifier  104 . 
     When the signal KB 1 V i   3 (t) is applied to the error amplifier  104 , two other terms are produced: 
     1. KB 1 A 2 V i   3 (t)—this term is used to cancel the distortion. It is set at an amplitude to cancel the third order terms from the error amplifier and the main amplifier  102 . Since the amplitude of KB 1 A 2 V i   3 (t) is proportional to K and A 2 , the signal is a linear signal. 
     2. K 3 B 1 B 2 V i   9 (t)—this term is referred to as an extra error term (“EET”) and is caused by the error cancellation signal distorting in the error amplifier. The EET is proportional to B 2  and K 3 . Thus, the EET increases by three dB for every one dB of cancellation signal increase. 
     Based on the above, it is apparent that increasing the amplitude of the cancellation signal input to the error amplifier  104  increases the EET, preventing the unwanted terms from being reduced to zero. Since unwanted terms cannot be completely eliminated, pure cancellation cannot occur. Nevertheless, improved efficiency results by constructing an amplifier according to the teachings provided herein. 
     The improvement in efficiency is best illustrated by comparing a typical BEC amplifier to an amplifier of the invention. In an amplifier of the invention, with an unequal combiner, there is no loss of the desired signal when the signals input to the output combiner  110  are combined with a ratio equal to the input power ratio and have correct phase. In a BEC amplifier, with an equal combiner, power losses increase as the power difference between the signals input to the output combiner increases. The power lost in an equal combiner is: 
     
       
           Lc =( P   err   ×P   main ) ½ −( P   err   +P   main )/2 
       
     
     where Lc is the power lost in the combiner, P err  is the power provided to the combiner from the error amplifier, and P main  is the power provided to the coupler from the main amplifier. For example, if P err =P main  then Lc=0. If P err  or P main  is zero then the loss is ½ of the remaining signal. In the equal-combiner case, the desired signal output power is: 
     
       
           P   err   +P   main   −Lc= 3/2 ×( P   err   +P   main )−( P   err   ×P   main ) ½   
       
     
     In an amplifier of the present invention, the desired signal output power is: 
     
       
         
           P 
           err 
           +P 
           main 
         
       
     
     The ratio of equal combiner power to unequal combiner power is: 
     
       
         3/2−( P   err   ×P   main ) ½ /( P   err   +P   main ) 
       
     
     For example, if there is 1.5 dB loss from the main amplifier  102  to the output combiner  110 , the unequal combiner is 0.7% more efficient than a standard BEC amplifier. 
     Even better results may be achieved by operating the amplifier  100  with a relatively high amplitude cancellation signal. As was discussed above, the error amplifier  104  can only cancel a limited amount of distortion; the limit is a result of ninth order terms or EET that the error amplifier creates. The performance improvements that are possible by driving the error amplifier with a relatively high amplitude cancellation signal are evident from the discussion below. 
     The main amplifier  102  and the error amplifier  104  have equal desired signal levels and produce the same distortion power. That is, ∥S 1 ∥=∥S 2 ∥ and ∥D 1 ∥=∥D 2 ∥. However, the main distortion signal, D 1 , is reduced in amplitude to D 1 /L, where L is representative of the loss from main amplifier output to the combiner input. In the amplifier  100 , the distortion ∥D 1 ∥ is reduced to ∥D 1 ∥/L 2  since the output combiner  110  provides a reduced coupling from the input of the combiner  110  to the output node  112 . As noted, the reduced coupling in the combiner  110  is equal to the loss incurred from main amplifier to the combiner input. For example, if there is a 1.5 dB loss from the main amplifier  102  to the output combiner  110 , then there is a 3 dB total loss to the distortion signal ∥D∥ to the output node  112  of the amplifier  100 . 
     The total distortion signal at the combiner  110  (prior to cancellation) is the sum of each distortion signal: ∥D 1 ∥(1+1/L 2 ). In the 1.5 dB output-loss example, the total pre-cancellation distortion power is 1.5 times the distortion power at the output of the amplifier  102 . In contrast, in an equal-power combiner BEC amplifier, the pre-cancellation power is set to ∥D 1 ∥(1+1/L). 
     The distortion canceling capability of the error amplifier  104  approaches a limit imposed by the EET. The limit occurs when the EET power is at the distortion limit. The EET power is set by the distortion-canceling power applied to the input of the error amplifier  104 . So, reducing the required distortion cancellation allows higher system output power. The system output power may be increased to the point where the requisite distortion-canceling signal returns to the previous level. Since ∥D 1 ∥ increases by a ratio of 3:1 relative to the desired signal, the net increase in desired power is ½ of the reduction in the required distortion cancellation. For the amplifier  100 , the requisite pre-cancellation distortion power is ∥D 1 ∥/(1+1/L 2 ). As noted, in the equal-combiner case the pre-cancellation distortion power is ∥D 1 ∥(1+1/L). The ratio is of the two is: 
     
       
         (1+1 /L )/(1+1 /L   2 ) 
       
     
     Thus, in the 1.5 dB output loss example, the relative pre-distortion level is 1.14 or 0.6 dB. 
     Since the amplifier  100  exhibits an increased distortion canceling capability, additional power is available from the main and error amplifiers, as compared to a typical BEC amplifier. In the amplifier  100 , the pre-cancellation distortion level increases 3:1 over the desired signal. As such, the distortion approaches the desired signal power at a rate of 2:1. Therefore, the unequal power combiner amplifier  100  has an increased desired signal power of: 
     
       
         ½×(1+1 /L )/(1+1 /L   2 ) 
       
     
     In the 1.5 dB output loss example, the output power may be increased by 0.3 dB over the equal-combining method. 
     The increase in efficiency in the amplifier  100  is the product of the improvement from the two reduced losses in an unequal combiner and the ability to drive the error amplifier with a higher amplitude cancellation signal. In the example discussed herein, the total improvement is 7.8%. 
     FIG. 3 illustrates the spectral shapes of the signals propagated through the amplifier  100 . As can be seen, the signal V y  (t) includes a desired signal and a distortion cancellation term that falls outside of the desired spectrum. Once amplified by the error amplifier  104 , the spectral regrowth expands due to the creation of the EET. At the output node  112 , the output signal still exhibits spectral regrowth in the form of an EET of K 3 B 1 B 2 V 1   9 (t), but the amplitude of the EET is diminished. 
     FIG. 4 illustrates a more generalized form of an OPC amplifier  150 . The amplifier  150  includes an input node  152 , a balancing network  154  that provides gain and phase adjustments to various signals propagated through the amplifier  150 . The amplifier  150  also includes a main amplifier  156 , a sampling coupler  158 , an error amplifier  160 , and an output combiner  162 . As can be appreciated, the gain and phase adjustment components shown specifically in FIGS. 1 and 2 could be implemented, given the teachings of the circuit shown in FIGS. 1 and 2, in different ways. For example, the gain and phase components could be separated and placed in different locations in the circuit. A way of showing this variation possibility is in the balancing network  154 . 
     As can be seen from the above, the present invention provides an amplifier that has higher efficiency than many known amplifiers. Various features and advantages of the invention are set forth in the following claims.