Abstract:
A power amplifier having a predistortion linearizer for improving the non-linear property of the power amplifier is disclosed. The powers amplifier includes a predistorter for outputting a distortion compensating signal to the high power amplifier, a delay line for delaying an input signal for a prescribed period of time and outputting a reference signal, a distortion component detection circuit for detecting a distortion component by comparing the output signal of the power amplifier with the reference signal of the delay line, and an adaptive controller for controlling the properties of the predistorter in order to minimize the detected distortion component.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power amplifier, and more particularly, to a power amplifier having a predistortion linearizer for improving a linearity of the power amplifier. 
     2. Background of the Related Art 
     Generally, a power amplifier has a non-linear property in which, as the number of input signals is increased, a gain of the amplifier is reduced and a phase thereof is delayed. Thus, in order to improve the non-linearity of the power amplifier, a wide variety of linearization techniques and algorithms have been developed. These include predistortion, envelope feedback, and feedforward. The predistortion method has been widely employed in mobile communication base stations since it has a simple structure and an excellent efficiency as compared to the feedfoward method. Additionally, unlike the envelope feedback method the predistortion method has no bandwidth limit. 
     FIG. 1 illustrates a related art power amplifier having a predistortion linearizer. As illustrated in FIG. 1, the related art power amplifier having a predistortion linearizer includes a power amplifier  14 , a predistorter  12  for improving the non-linearity of the power amplifier  14 , and a directional coupler  16  for sampling the output signal of the power amplifier  14 . The device also includes a distortion component detection unit  20  for separating a distortion component from the sampled output signals and a power detector  22  determining the amount of the distortion components in the sample. An adaptive controller  24  is additionally provided for controlling the properties of the predistorter  12  according to the amount of the detected distortion components detected in the power detector  22 . 
     The distortion detection unit  20  includes a mixer  20 - 1  for down-converting the output signal and a band pass filter (BPF)  20 - 2  for filtering the output of the mixer  20 - 1  and separating the distortion component from the output signal. 
     The operation of the related art power amplifier having a predistortion linearizer thusly constructed will now be described with reference to the accompanying drawings. 
     When an output signal is inputted through an input port  10 , the predistorter  12  changes the gain and phase of the input signal to counteract the non-linear properties of the power amplifier  14 . That is, the input signal is modified to minimize the non-linear property of the power amplifier  14 . In addition, the input signal whose gain and phase are changed is amplified in the power amplifier  14 , and then is outputted through an output port  18 . 
     If the predistorter  12  operates normally, the output signal of the power amplifier  14  will have no distortion component. However, because it is so difficult to set the property of the predistorter  12  such that it has exactly the inverse of the non-linear property of the power amplifier  14 , a distortion component appears in the output signal. 
     Therefore, in order to minimize the distortion component of the output signal, the adaptive controller  24  periodically checks the generated amount of distortion components detected in the power detector  22 . The adaptive controller  24  then changes the properties of the predistorter  12  in order to minimize the generated amount of distortion components. Specifically, the distortion components are separated from the output signal, as shown in deviated crease line of FIG.  2 . This is done by down-converting (RFout-LO) the output signal using the mixer  20 - 1 , and thereafter passing the same through the BPF  20 - 2 . Therefore, the power detector  22  determines the generated amount of the distortion component upon receipt of the distortion component separated from the BPF  20 - 2  and produces an output signal. 
     As described above, the related art power amplifier has various problems. For example, a distortion component is detected from the down-converted signal using the BPF  20 - 2 . At this time, the output signal of the ideal BPF does not contain an input signal component to be amplified. Because the attenuation property of the filter is restricted, only the input signal component appears in the output of the BPF  20 - 2 . 
     Additionally, in order for the detected distortion component to be used in the adaptive controller  22 , the amplitude of the input component must be smaller than that of the distortion component by more than 15 dB. As a result, considering that the amplitude of the distortion component of a general power amplifier is smaller than that of the input component by approximately 30 dB, the BPF  20 - 2  has to attenuate the input signal component by more than 45 dB. Thus, in the related art, there is a problem that the attenuation property of the filter must be excellent in order to greatly improve the distortion property appearing in the output of the power amplifier. 
     Furthermore, the distortion component generated in the power amplifier  14  is mainly composed of third IMD (Intermodulation distortion). However, as illustrated in FIG. 2, there is a disadvantage that it is very difficult to filter the third IMD because it is directly adjacent to the original input component. 
     Moreover, the related art changes a center frequency of a local oscillation carrier (LO) and performs a down-conversion with respect to the changed center frequency of the local oscillation carrier (LO) in order to maintain the constant center frequency of the BPF  20 - 2 . For this, accurate center frequency information of the input signal is required. However, in the case that the accurate frequency information is not acquired, additional modules for checking the frequency of the input signal are needed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a power amplifier having a predistortion linearizer that substantially obviates problems due to disadvantages in the prior art. 
     It is another object of the present invention to provide a power amplifier having a predistortion linearizer capable of reducing the non-linear property of the power amplifier. 
     It is another object of the present invention to provide a distortion component detection circuit for a power amplifier capable of accurately detecting a distortion component irrespective of the frequency and properties of an input signal. 
     To achieve at least the above objects in whole or in parts, there is provided a power amplifier having a predistortion linearizer, which includes a predistorter for outputting a distortion compensating signal to the power amplifier; a delay line for delaying an input signal for a predetermined time and generating a reference signal; a distortion component detection circuit for detecting a distortion component by comparing the output signal of the power amplifier with the reference signal of the delay line; and an adaptive controller for controlling the properties of the predistorter in order to minimize the detected distortion component. The above distortion component detection circuit adjusts the output signal so that it has the same amplitude as the reference signal and the phase difference between the output signal and the reference signal is 180 degrees, and thereafter separates distortion components from the output signal of the power amplifier by vector operation of the output signal and the reference signal. 
     To further achieve the above objects in whole or in parts, there is provided a distortion component detection circuit having a vector modulator for controlling the gain and phase of an output signal; a branch line coupler for receiving an output signal and reference signal of the vector modulator and adding vector components of both signals; a 2-way combiner for combining outputs of the branch line coupler and outputting a distortion component; and a control signal generator for producing the difference in gain and phase by computing the output signal and reference signal of the vector modulator, the output signal of the branch line coupler, and the power of the distortion component and outputting the control voltage proportional to the difference in the outputted gain and phase to the vector modulator. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
     FIG. 1 is a diagram illustrating a power amplifier having a predistortion linearizer in the related art; 
     FIG. 2 is a diagram illustrating a distortion signal contained in an output signal of the power amplifier of FIG. 1; 
     FIG. 3 is a diagram illustrating a power amplifier having a predistortion linearizer in accordance with a preferred embodiment of the present invention; 
     FIG. 4 is a diagram illustrating a distortion component detection unit of FIG. 3; 
     FIG. 5A is a diagram illustrating the reference signal generated by the power amplifier having a predistortion linearizer of FIG. 3; 
     FIG. 5B is a diagram illustrating the distortion component of the output signal of the power amplifier having a predistortion linearizer of FIG. 3; 
     FIGS.  6 A˜ 6 D illustrate signals on each port of the distortion component detection unit of FIG. 3 represented as vector components. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 3, a power amplifier having a predistortion linearizer in accordance with a preferred embodiment of the present invention is illustrated. It includes a predistorter  12 , a power amplifier  14 , and a directional coupler  16 . It further includes a power detector  22  and an adaptive controller  24 . These elements are similar to those of the related art and have thus been identified with the same reference numerals. Next, the device includes a delay line  26 , and a distortion component detection circuit  28 . 
     The delay line  26  outputs a reference signal R by delaying an input signal for a prescribed period of time, and the distortion component detection circuit  28  outputs a distortion component by adjusting an output signal M of the power amplifier  14 . The distortion component is such that it has the same amplitude as the reference signal R and the phase difference between the reference signal and the output signal is 180 degrees. Since the operation of the other portions is similar to the related art, the detailed description thereof is omitted. 
     The distortion component detection circuit  28  includes a vector modulator  28 - 1 , and a distortion component detection unit  28 - 2  coupled to receive the output signal M of the power amplifier  14  and reference signal R, and detect a distortion component. The distortion component detection unit  28 - 2  outputs gain and phase control voltages V I  and V Q  to the vector modulator  28 - 1 . 
     Referring to FIG. 4, the distortion component detection unit  28 - 2  includes a branch line coupler  100  to receive the output signal M and the reference signal R. It further includes a 2-way combiner  101  coupled to receive the output signals X, Y of the branch line coupler. Additionally, it includes a control signal generator  102  which receives an output signal V of the two way combiner  101 , an output signal Y of the branch line coupler  100 , the output signal M, and the reference signal R. 
     The operation of the thusly constructed power amplifier having a predistortion linearizer in accordance with the present invention will now be described with reference to the accompanying drawings. 
     As illustrated in FIG. 5A, the distortion component detection circuit  28  receives the reference signal R outputted from the delay line  26 , as well as the output signal M of the power amplifier. The distortion component detection circuit  28  separates a distortion component B from the output signal M of the power amplifier  14 , as illustrated in FIG. 5B, by adjusting the output signal M so that it has the same amplitude as the reference signal R and a phase difference between the reference signal R and the output signal M of 180 degrees. 
     As illustrated in FIG. 4, the branch line coupler  100  of the distortion component detection unit  28 - 2  adjusts the path of the two inputted signals M and R. At this time, M a is a signal in which a part of the output of the power amplifier  14  is sampled to have the same amplitude as the reference signal R, and R is the input signal that is delayed in the delay line  26  so that it has a phase difference of 180 degrees from the signal M. 
     Therefore, when there is no difference in amplitude and phase between the two signals M and R, components A of the two signals M and R are represented as vectors as illustrated in FIG.  6 A. The outputs of the branch line coupler with respect to the vectors M and R are M X , M Y , R X , and R Y . Signal X is a resultant vector of M Y  and R X , and signal Y is a resultant vector of M X  and R Y . However, because the output V of the 2-way combiner  101  is a resultant vector of signals X and Y, the output V of the 2-way combiner  101  becomes 0 if there is no difference in amplitude and phase generated between M and R. Thus only the distortion component B generated in the power amplifier  14  appears. 
     A difference in amplitude and phase is generated between M and R due to the non-linearity of the power amplifier  14 . In such an instance, the components A of signals M and R are represented as vectors, as shown in FIG.  6 B. The output V of the 2-way combiner  101  is thus represented as a resultant vector V of X′ and Y′, as shown in FIG.  6 C. The control signal generator  102  accordingly detects the signals M and R, and the power Y of the branch line coupler  100  and the power V of the 2-way combiner  101 . It then produces the difference in amplitude and phase. 
     V, a resultant vector of X′ and Y′ in FIG. 6C can be represented as the following Expression 1. 
     
       
           V   2   =X′   2   +Y′   2 −2 X′Y ′ cos (π−δΦ)  Equation 1 
       
     
     δΦ is a phase difference generated in the power amplifier  14 . 
     Additionally, in order to express the difference in amplitude generated in the power amplifier  14 , the above equation 1 can be represented as the following Equation 2. 
     
       
           V   2   =X′   2 +( X′+δA ) 2 −2 X ′( X+δA )cos(π−δΦ)  Equation 2 
       
     
     In FIG. 4, the output signal M of the power amplifier  14  can be represented as the sum of the input signal R and distorted the component V, so the difference in amplitude can be obtained by the following Equation 3. 
     
       
         Amplitude difference= P   M −( P   R +2 P   V )  Equation 3 
       
     
     At this time, P M , P R , and 2P V  are powers of M, R, and V, which are represented as follows. 
     
       
           P   M =( A   2   +B   2 ) 
       
     
     
       
           P   R =( A+δA ) 2   
       
     
     
       
           P   V =½ {A   2 +( A+δA ) 2 −2 A ( A+δA )cos(π−δΦ)+ B   2 } 
       
     
     Thus, Equation 3 results in the following Equation 4 by the above term. 
     
       
         Amplitude difference=−2( A+δA )[ A (1−cos δΦ)+δ A ]≈−2 AδA  if  A&gt;&gt;δΦ≈ 0  Equation 4 
       
     
     Phase difference is obtained by the definition that the sum of the two input powers P M  and P R  of the branch line coupler  100  is two times the output power of the branch line coupler  100 . 
     
       
         Phase difference= P   M   +P   R −2 P   Y   Equation 5 
       
     
     As illustrated in FIG. 6D, P Y  is obtained from Y′. Y′ and P Y  are expressed as follows. 
     
       
           Y′   2   =R   Y   2   +M′   X   2 −2 R   Y   M′   X  cos(π/2−δΦ) 
       
     
     
       
           P   Y =½ {A   2 +( A+δA ) 2 −2 A ( A+δA )cos(π/2−δΦ)+ B   2 } 
       
     
     Thus, Equation 5 results in the following Equation 6 by the above terms. 
     
       
         Amplitude difference=−2 A ( A+δA ) sin δΦ≈−2 A   2 δΦ if δ A&lt;&lt;A , δΦ, &lt;&lt;1  Equation 6 
       
     
     Thus, the control signal generator  102  generates control voltages V I  and V Q  proportional to the difference in amplitude and phase, and outputs the same to the vector modulator  28 - 1 . The vector modulator  28 - 1  adjusts the gain and phase of the output signal M so that it has the same amplitude as the reference signal and a phase difference between the output signal and the reference signal of 180 degrees. 
     If the output signal M has the same amplitude as the reference signal R and the phase difference therebetween is 180 degrees, only the distortion components B are separated in the distortion component detection unit  28 - 2 . The power detector  22  thus detects only the generation amount of distortion components based on the separated distortion components. Therefore, the adaptive controller  24  controls the predistorter  12  so that the generation amount of distortion components detected in the power detector  22  is minimized, thereby improving the non-linear property of the power amplifier  14 . 
     As described herein, the power amplifier having a predistortion linearizer in accordance with the present invention detects a distortion component by adjusting an output signal of the power amplifier so that the output signal has the same amplitude as an input signal and the phase difference therebetween is 180 degrees. It therefore compensates the non-linear property of the power amplifier according to the detected distortion component. Consequently, the present invention has an advantage that a distortion component can be accurately detected regardless of the frequency of the signal inputted into the power amplifier, and the properties of the filter, and the non-linear property of the power amplifier can be greatly improved by the accurate detection of the distortion component. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.