Abstract:
A method and apparatus for utilizing the distortion generated within a portion of a balanced amplifier to cancel the distortion generated within the whole balanced amplifier. Samples of the signal and distortion from part of the balanced amplifier are combined with a reference signal such that the two signals destructively combine leaving the distortion from the sampled part of the balanced amplifier. The gain and phase of the distortion is then adjusted so that when it is coupled into the input of the other part of the balanced amplifier the distortion generated by both parts of the balanced amplifier are cancelled.

Description:
REFERENCE TO PRIORITY DOCUMENT  
       [0001]    This application claims priority of co-pending U.S. Provisional Patent Application Serial No. 60/301,927 entitled “Balanced Distortion Reduction Circuit” by Mark Billsberry, filed Jun. 29, 2001. Priority of the filing date of Jun. 29, 2001 is hereby claimed, and the disclosure of the Provisional Patent Application is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates generally to radio frequency amplifiers, and more particularly, to reducing the distortion from a balanced radio frequency power amplifier.  
           [0004]    2. Description of the Related Art  
           [0005]    Radio frequency (RF) amplifiers are used in a wide variety of applications, including communications. Ideally the transfer function of an RF amplifier is linear, with the output of the amplifier being an amplified replica of the input to the amplifier. However, conventional RF amplifiers typically have some degree of non-linearity in their transfer function, particularly at high power levels such that different signal frequencies will be amplified by different amounts. This non-linearity in an RF amplifier produces distortion in the RF amplifier output.  
           [0006]    Distortion in the RF amplifier output can reach levels that are unacceptable. For example, to meet the requirements of many CDMA modulation standards, power amplifier outputs need to exhibit very low distortion. If some type of linearization or distortion reduction technique is not used, the RF amplifier can have poor efficiency or require high cost components.  
           [0007]    Many distortion reduction techniques use a distortion generator to cancel the distortion generated within a main RF power amplifier. These techniques generally assume that the low power distortion generator has a non-linear transfer function that is either similar to, or opposite from, the non-linear transfer function of the main RF amplifier. The output of the distortion generator, which exhibits the non-linear transfer function, is sampled and used to generate a signal that is combined with the output of the main RF power amplifier in an attempt to cancel the distortion present in the RF amplifier output. A drawback to this technique is that, for the signal combining to result in cancellation the non-linear transfer function of the distortion generator must match the non-linear transfer function of the main RF amplifier. This match can be difficult to achieve.  
           [0008]    Another distortion reduction technique, the feedforward technique, overcomes the difficulty in providing a distortion generator that has a non-linear transfer function that matches the main RF amplifier. Rather, the feedforward technique samples the main RF amplifier output, which includes both the desired transmitted signal and amplifier induced distortion. The sampled output of the main RF amplifier is then combined with a sample of the amplifier input signal such that the two sampled signals destructively combine, leaving only the distortion from the main amplifier. The resulting distortion signal is then amplified using an error amplifier and is added at the output of the main RF amplifier with the gain and phase adjusted such that the error amplifier output destructively combines with the main RF amplifier output and cancels the amplifier induced distortion present in the output of the main RF amplifier. A drawback to the feedforward distortion reduction technique is that the power handling capability required of the error amplifier is based on the magnitude of the distortion. The feedforward technique can provide good performance when improving the linearity of low distortion amplifiers, however, when this technique is applied to an amplifier that is operating close to its compression point, the error amplifier power requirement may become excessively large, making this technique impractical and unsatisfactory for many applications.  
           [0009]    From the discussion above, it should be apparent that there is a need for a system that can provide linearization of an amplifier throughout its operating range, for example when it is operating close to its compression point, without requiring excessively large error amplifier power levels.  
         SUMMARY  
         [0010]    A method and apparatus for reducing distortion in the output of a balanced RF power amplifier samples the distortion of the main RF power amplifier output and also reduces the power handling capability required for the error amplifier. A circuit that can provide these features measures the difference between the output from one portion of the balanced RF power amplifier and a sample of the amplifier input. This difference signal, which represents the amplifier distortion, can then be combined at the input to another portion of the balanced RF power amplifier. The amplitude and phase of the difference signal can be adjusted such that the distortion produced from the respective portions of the balanced RF amplifier are cancelled when the respective outputs are combined to produce the balanced RF amplifier output. Because the distortion generated in each portion of the balanced RF power amplifier should be similar, the distortion in one portion of the RF power amplifier is used to minimize the distortion in other portions of the RF power amplifier. In addition, this technique uses one portion of the balanced RF amplifier to amplify the difference signal to the desired level and thereby does not require a powerful error amplifier. Also, because the difference signal is injected into the input signal in front of the power amplifier, this technique does not require additional couplers following the main RF power amplifier output. The elimination of additional couplers after the main RF power amplifier can result in a lower cost design that provides improved efficiency.  
           [0011]    In another aspect, the main RF power amplifier can include multiple amplifying devices that produced a combined output signal. The distortion produced in one of the amplifying devices can be sampled and used to cancel the distortion from all of the multiple amplifying devices to minimize the distortion in the main amplifier output.  
           [0012]    In yet another aspect, a tone, or pilot signal, can be injected into the termination port of a balanced amplifier input splitter. The amplitude of the tone or pilot signal that is present in a balanced RF amplifier output combiner termination port can be detected and used as an input to an adaptation loop to minimize the tone, or pilot signal present in the output combiner termination port. By injecting the tone or pilot signal in the splitter termination port, the tone is isolated from the output of the balanced amplifier.  
           [0013]    Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a block diagram illustrating a balanced distortion reduction circuit.  
         [0015]    [0015]FIG. 2 is a block diagram illustrating a balanced distortion circuit that includes a delay matched first loop.  
         [0016]    [0016]FIG. 3 is a block diagram illustrating a balanced distortion reduction circuit that includes a delayed matched second loop.  
         [0017]    [0017]FIG. 4 is a block diagram that illustrates a balanced reduction circuit applied to an N-way combined amplifier.  
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIG. 1 is a block diagram illustrating a balanced distortion reduction circuit  10  for reducing the distortion from a balanced RF main power amplifier  20  by sampling the output of the main amplifier with reduced power handling requirements for an error amplifier.  
         [0019]    An RF input signal  51  is introduced into the circuit  10  through an RF splitter  25 . The RF splitter  25  directs a portion of the input signal to a input phase shifter and attenuator  26 . The input splitter also directs a portion of the input signal to a distortion phase shifter and attenuator  27 .  
         [0020]    The output  32  of the input phase shifter and attenuator  26  is routed to the main amplifier  20 . The main amplifier  20  includes an input splitter  30  that is configured to receive the output of the input phase shifter and attenuator  26 , divide the received signal to produce two splitter output signals  80 ,  82 , and provide one of the output signals  80 ,  82  into each of two balanced amplifying devices in the main amplifier  20 . For example, in FIG. 1 there are two balanced amplifying devices, a first amplifier  21  and a second amplifier  22  within the main amplifier  20 . In the FIG. 1 embodiment, the input splitter  30  divides the received attenuator/phase shifter signal  32  in half and provides equal-strength signals to the first and second amplifying devices  21 ,  22 . Alternatively, the input splitter  30  can divide the signal  32  into signals of different strengths and can apply the different strength signals to the first and second amplifying devices  21 ,  22 .  
         [0021]    In FIG. 1, the output of the first amplifying device  21  is sampled using a sampling coupler  23 . The output of the sampling coupler  23  is a sampled signal  53 , which includes the distortion introduced by the first amplifying device  21 . The sampled signal  53  is then combined with a sample of the split RF input signal  52  in a distortion combiner  24 . The input signal  52  is received at the combiner  24  from the RF splitter  25 . The input phase shifter and attenuator  26  is controlled such that the gain and phase of the sampled output signal  53  will combine destructively with the sampled input signal  52 , leaving only a difference, or distortion signal  54  output from the distortion combiner  24 . Those of skill in the art will understand how to control the input phase shifter and attenuator  26  so as to achieve the desired effect recited herein. The distortion signal  54  from the combiner  24  is then applied to a distortion phase shifter and attenuator  27 . Depending on the distribution of gain within the main amplifier  20 , a low power error amplifier  28  might be useful to adjust the distortion signal  55  to a desired level. The output of the attenuator and phase shifter  27  and of the optional error amplifier  28  provides an adjusted distortion signal  55 .  
         [0022]    The adjusted distortion signal  55  is then applied to a distortion combiner  29  in front of the second amplifying device  22 . The distortion combiner  29  combines the output signal  80  received from the input splitter  30  and the adjusted distortion signal  55  and outputs a combined signal into the second amplifying device  22 . The gain and phase of the distortion phase shifter and attenuator  27  are adjusted to give maximum cancellation of the distortion in the signal at the output  56  of the balanced main amplifier output  20 . Techniques for adjusting the gain and phase of the input phase shifter and attenuator  26  and the distortion phase shifter and attenuator  27  are well known to those skilled in the art, who will be able to select a suitable technique to accommodate various design considerations.  
         [0023]    In another embodiment, an optional tone or pilot signal  57  can be injected into a termination port  59  of the balanced amplifier input splitter  30  to key a reduction in output distortion. The distortion phase shifter and attenuator  27  can be adjusted until the detected tone or pilot signal produced from the balanced amplifier output combiner  31  termination port  58  is minimized. In this arrangement, the injected pilot signal  57  is isolated from the amplifier output. The pilot signal can be a single frequency. In other embodiments, the pilot signal  57  might take other forms, for example, a noise signal, a Code Division Multiple Access (CDMA) signal, or multiple desired frequencies, or a single frequency that changes or hops.  
         [0024]    [0024]FIG. 2 is a block diagram illustrating a balanced distortion circuit  200  that includes a delay matched first loop  202 . In all the drawings, like reference numerals refer to like items, so it should be apparent that the circuit of FIG. 2 has some elements in common with the circuit of FIG. 1. In FIG. 2, however, a delay line  61  in front of the combiner  24  is used to match the delay of the sampled input signal  52  coming from the RF splitter  25  and the sampled power amplifier output signal  53 . The use of the delay line  61  improves the signal cancellation in the first loop  202  over a wider bandwidth.  
         [0025]    [0025]FIG. 3 is a block diagram illustrating a balanced distortion reduction circuit  300  that includes a delayed matched second loop  302  and a delayed matched first loop  304 . In FIG. 3, a delay line  63  is included in front of the second amplifying device  22  of the main amplifier  20 , and thereby enables the cancellation in the second loop  302  to be effective over a wider bandwidth. The addition of the delay line  62  following the first amplifying device  21  of the main amplifier  20  and in front of the output coupler  31  maintains the delay match between the two amplifying devices  21 ,  22  of the main amplifier  20 .  
         [0026]    [0026]FIG. 4 is a block diagram that illustrates a balanced reduction circuit  400  applied to an N-way combined main RF amplifier. In this embodiment, the technique described above can be used in an amplifier that has more than two amplifying devices in the main RF power amplifier. For example, FIG. 4 illustrates the application of this technique to a main RF power amplifier  420  with four power amplifying devices, including a first amplifying device  48 , and second through fourth amplifying devices  42 ,  44 ,  46 , respectively. As before, the input phase shifter and attenuator  26  is controlled such that the gain and phase of the sampled output signal  53  will combine destructively with the sampled input signal  52  in the distortion combiner  24 , leaving only a difference, or distortion signal  54  output from the distortion combiner. In the FIG. 4 embodiment, the output of the error amplifier  28  comprising the adjusted distortion signal is routed to an error signal splitter  32  that divides the error signal output so as to apply the appropriate error signal to a respective distortion combiner  33  located at the input to each of the remaining amplifying devices  42 ,  44 ,  46 .  
         [0027]    The main amplifier input splitter  30  divides the signal  450  from the attenuator and phase shifter  26  into N signals of equal strength to be applied to each of the N power amplifying devices  42 ,  44 ,  46 ,  48 , and the error signal splitter  32  divides the error signal output from the error amplifier  28  into equal strength portions to be applied to the input of each distortion combiner  33 . In other embodiments, the input splitter  30  can divide the input signal  450  into signals of different strengths to be sent to the amplifying devices  42 ,  44 ,  46  whose output is not sampled by the sampling coupler  23 , and the error signal splitter  32  can divide the error signal into appropriate signal strengths to correspond to the respective input signal strengths and to be applied to the remaining amplifying devices  42 ,  44 ,  46 . Thus, the error splitter  32  will apportion the error signal according to the gains of the amplifying devices  42 ,  44 ,  46  and the magnitudes of the signals they will receive from the input splitter  30 . In this way, cancellation of distortion in the signal at the combined RF output  56  of the balanced main amplifier  20 .  
         [0028]    The foregoing description details certain embodiments of a circuit constructed in accordance with the invention. It will be appreciated, however, that no matter how detailed the foregoing appears, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come with the meaning and range of equivalency of the claims are to be embraced within their scope.