Abstract:
An apparatus and a method for an Envelope Elimination and Restoration (EER) power transmitter are provided. The apparatus includes a signal separator for splitting a transmit signal to an amplitude component and a phase component, an orthogonal modulator for modulating the phase component into a Radio Frequency (RF) signal, a bias modulator for linearly amplifying the amplitude component, for determining a bias voltage according to a magnitude of the amplitude component, and for providing a current generated using the determined bias voltage to a high-efficiency power amplifier and the high-efficiency power amplifier for amplifying the RF signal using the linearly amplified amplitude component as a drain bias voltage and using the generated current as a drain bias current.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Feb. 26, 2008 and assigned Serial No. 10-2008-0017355, the entire disclosure of which is hereby incorporated by reference. 
     JOINT RESEARCH AGREEMENT 
     The presently claimed invention was made by or on behalf of the below listed parties to a joint research agreement. The joint research agreement was in effect on or before the date the claimed invention was made and the claimed invention was made as a result of activities undertaken within the scope of the joint research agreement. The parties to the joint research agreement are Samsung Electronics Co. Ltd. and Postech Foundation. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an apparatus and a method for a power transmitter in a wireless communication system. More particularly, the present invention relates to an apparatus and a method for an Envelope Elimination and Restoration (EER) power transmitter in a wireless communication system. 
     2. Description of the Related Art 
     As wireless communication systems are advancing in accordance with advances in communication technologies, the amount of data transmitted by a wireless communication system and the transmission rate of the data are increasing. Furthermore, with the increased data and transmission rate, a bandwidth of a signal to transmit is widened and a Peak to Average Power Ratio (PAPR) increases. Accordingly, the importance of a high-performance transmitter having high efficiency and high linearity is growing in the wireless communication system. 
     However, since the efficiency and the linearity of a conventional power transmitter are inversely related, it is difficult to meet both the efficiency and the linearity demands of the communication system using the conventional power transmitter. 
     To address this problem, research is being conducted on an Envelope Elimination and Restoration (EER) power transmitter. The EER power transmitter modulates a phase component, from which an amplitude component is removed from the signal to be amplified to a Radio Frequency (RF) signal, and amplifies the modulated signal as shown in  FIG. 1 . This method achieves high-efficiency amplification with a reduction in distortion of the amplifier. 
       FIG. 1  depicts a conventional EER power transmitter. 
     The EER power transmitter of  FIG. 1  includes a signal separator  100 , a bias modulator  110 , an orthogonal modulator  120 , and a high-efficiency power amplifier  130 . 
     The signal separator  100  separates an amplitude component and a phase component of the signal to be amplified. The signal separator  100  outputs the amplitude component to the bias modulator  110  and outputs the phase component to the orthogonal modulator  120 . 
     The bias modulator  110  amplifies the amplitude component fed from the signal separator  100  and outputs the amplified amplitude component to the high-efficiency power amplifier  130 . The bias modulator  110  includes a linear amplitude amplifier (not shown) and a switching regulator (not shown). 
     The orthogonal modulator  120  modulates the phase component fed from the signal separator  110  into an RF signal. 
     The high-efficiency power amplifier  130  amplifies the RF signal output from the orthogonal modulator  120  using the signal output from the bias modulator  110  as the drain bias. By merely amplifying the phase component output from the orthogonal modulator  120 , the high-efficiency power amplifier  130  can reduce the distortion of the amplifier. In addition, using the amplified amplitude component provided from the bias modulator  110  as the drain bias, the high-efficiency power amplifier  130  can restore the amplitude component of the signal to be amplified. 
     As stated above, the EER power amplifier amplifies only the phase component using the amplitude component of the transmit signal as the bias voltage, to thus attain high efficiency and high linearity of the power transmitter. 
     To address both the efficiency and linearity in an EER power amplifier, the bias modulator should be able to accommodate the wide bandwidth and the high PAPR of the amplitude component. In other words, the bias modulator needs to accommodate a rapid change of a slew rate with high efficiency. 
     However, the switching regulator, which is the main power source of the bias modulator, cannot supply the current corresponding to the wide change of the slew rate. Accordingly, since the bias modulator provides the current to the high-efficiency power amplifier by increasing the current transfer rate of the linear amplitude amplifier that has lower efficiency than the switching regulator, there is a limitation in improving the total efficiency of the EER power transmitter. 
     Accordingly, there is a need for an improved apparatus and a method that enhances efficiency of a bias modulator in an EER power transmitter. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for enhancing efficiency of a bias modulator in an Envelope Elimination and Restoration (EER) power transmitter. 
     Another aspect of the present invention is to provide an apparatus and a method for increasing a ratio of a current provided from a switching regulator to a high-efficiency power amplifier in a bias modulator of an EER power transmitter. 
     Yet another aspect of the present invention is to provide an apparatus and a method for changing a bias voltage of a switching regulator based on a magnitude detected from an amplitude component so that the switching regulator attains a broad slew rate in an EER power transmitter. 
     According to an aspect of the present invention, an apparatus for an EER power transmitter is provided. The apparatus includes a signal separator for splitting a transmit signal into an amplitude component and a phase component, an orthogonal modulator for modulating the phase component into a Radio Frequency (RF) signal, a bias modulator for linearly amplifying the amplitude component, for determining a bias voltage according to a magnitude of the amplitude component, and for providing a current generated using the determined bias voltage to a high-efficiency power amplifier and the high-efficiency power amplifier for amplifying the RF signal using the linearly amplified amplitude component as a drain bias voltage and using the generated current as a drain bias current. 
     According to another aspect of the present invention, a signal transmission method of an EER power transmitter is provided. The method includes splitting a transmit signal to an amplitude component and a phase component, modulating the phase component into an RF signal, linearly amplifying the amplitude component, generating a current to be used for amplifying the RF signal with a bias voltage determined by a magnitude of the amplitude component and amplifying the RF signal using the linearly amplified amplitude component as a drain bias voltage and using the generated current as a drain bias current. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain exemplary embodiments the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a conventional Envelope Elimination and Restoration (EER) power transmitter; 
         FIG. 2  illustrates an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 3  illustrates a magnitude detector and divider of an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 4  illustrates a bias change of a switching regulator in an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 5  illustrates a method for changing a bias of a switching regulator based on an amplitude magnitude in an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 6  illustrates an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 7  illustrates a performance change of an EER power transmitter according to an exemplary embodiment of the present invention; 
         FIG. 8  illustrates a performance change of an EER power transmitter according to an exemplary embodiment of the present invention; and 
         FIG. 9  illustrates a performance change of an EER power transmitter according to an exemplary embodiment of the present invention; 
     
    
    
     Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     Exemplary embodiments of the present invention provide a technique for enhancing efficiency of a bias modulator in an Envelope Elimination and Restoration (EER) power transmitter. 
     The bias modulator of the EER power transmitter includes a linear amplitude amplifier and a switching regulator. The switching regulator exhibits a high efficiency characteristic and the linear amplitude amplifier exhibits a low efficiency characteristic. 
     Accordingly, the bias modulator can enhance its efficiency by increasing a ratio of the current supplied from the switching regulator to a high-efficiency amplifier and decreasing a ratio of the current supplied from the linear amplitude amplifier to the high-efficiency amplifier. 
     For doing so, the bias modulator needs to constitute the switching regulator so as to support a slew rate desired by the high-efficiency power transmitter. For example, the bias modulator controls the bias voltage of the switching regulator based on a magnitude of an amplitude component as shown in  FIG. 2 . 
       FIG. 2  is a block diagram of an EER power transmitter according to an exemplary embodiment of the present invention. 
     The EER power transmitter of  FIG. 2  includes a signal separator  200 , a bias modulator  210 , an orthogonal modulator  220 , and a high-efficiency power amplifier  230 . 
     The signal separator  200  separates an amplitude component and a phase component of a signal to be amplified. The signal separator  200  sends the amplitude component to the bias modulator  210  and outputs the phase component to the orthogonal modulator  220 . 
     The bias modulator  210  amplifies the amplitude component output from the signal separator  200  and outputs the amplified amplitude component to the drain bias voltage of the high-efficiency power amplifier  230 . For example, the bias modulator  210  includes a linear amplitude amplifier  211 , a magnitude detector and divider  213 , a switch controller  215 , a bias power  217 , and a switching regulator  219 . 
     The linear amplitude amplifier  211  linearly amplifies the amplitude component fed from the signal separator  200 . 
     The magnitude detector and divider  213  amplifies the magnitude of a signal voltage-dropped by a resistor serially connected to the output stage of the linear amplitude amplifier  211  and divides the magnitude of the amplitude component fed from the signal separator  200  based on a plurality of voltage levels. Herein, as the current supplied by the linear amplitude amplifier  211  increases, the voltage reduction by the resistor increases. In contrast, when the linear amplitude amplifier  211  supplies less current or the switching regulator  219  supplies unnecessary current, the voltage reduction decreases. 
     Next, the magnitude detector and divider  213  generates a control signal to control the bias power  217  based on the divided voltage level. An exemplary magnitude detector and divider  213  will be shown below with reference to  FIG. 3 . 
     The switch controller  215  connects one or more of a plurality of power devices of the bias power  217 , based on the magnitude of the amplitude component, to the switching regulator  219  according to the control signal output from the magnitude detector and divider  213 . Since the switch controller  215  controls the switch connected to the voltage devices, it can reduce a switching loss by lowering the switching speed. 
     The bias power  217  includes the plurality of power devices having different bias voltages. Of course, the bias voltages may differ from each other by a consistent amount or may differ from each other by a varying amount. 
     The switching regulator  219  generates the current to be supplied to the high-efficiency power amplifier  230  using the bias voltage fed from the voltage device connected under the control of the switch controller  215 . As the bias voltage is fed according to the magnitude of the amplitude component, the switching regulator  219  can regulate the change of the slew rate of the output current. Thus, the switching regulator  219  can address the issue of unnecessary current caused by a ripple effect when a smaller inductor is designed to increase the slew rate. 
     The orthogonal modulator  220  modulates the phase component output from the signal separator  200  into an RF signal. 
     The high-efficiency power amplifier  230  amplifies the RF signal output from the orthogonal modulator  220  using the signal fed from the bias modulator  210  as the drain bias. In more detail, the high-efficiency power amplifier  230  amplifies the RF signal using the amplitude component linearly amplified by the linear amplitude amplifier  211  of the bias modulator  210  as the drain bias voltage and using the current generated by the switching regulator  219  as the drain bias current. Herein, the high-efficiency power amplifier  230  represents all kinds of the switching and saturation high-efficiency power amplifier including class D, class E, class F, class J, class E/F series, class J/E, inverting class D, and inverting class F. 
     As stated above, the switching regulator  219  can support the slew rate of the current as desired by the high-efficiency power amplifier  230  by supplying the drain bias current of the high-efficiency power amplifier  230  using the bias voltage based on the magnitude of the amplitude component. 
     The magnitude detector and divider  213 , which examines the magnitude of the amplitude component, may be constructed as shown in  FIG. 3 . 
       FIG. 3  is a block diagram of a magnitude detector and divider of an EER power transmitter according to an exemplary embodiment of the present invention. 
     The magnitude detector and divider  213  of  FIG. 3  includes an amplifier  300 , a comparator  310 , and a control signal generator  320 . 
     The amplifier  300  amplifies the signal voltage-reduced by the resistor serially connected to the output stage of the linear amplitude amplifier  211  and outputs the amplified signal to the comparator  310 . 
     The comparator  310  divides the magnitude of the amplitude component into a plurality of different voltage levels using a plurality of hysteresis comparators  311  through  315 , each hysteresis comparator having a hysteresis value that does not overlap with another. Herein, the comparator  310  includes a number of hysteresis comparators that is greater than the number of voltage devices of the bias power  217  by one. 
     In an exemplary implementation, each of the hysteresis comparators  311  through  315  operate when the input signal is greater than or equal to the hysteresis corresponding to the respective hysteresis comparator. For example, if the comparator  310  includes three hysteresis comparators and the respective hysteresis of the hysteresis comparators is 1˜2V, 3˜4V, and 5˜6V in order, when the input signal momentarily swings from 0V to 4.5V, the first hysteresis comparator and the second hysteresis comparator output ‘1’ and the third hysteresis comparator outputs ‘0’. In this situation, the interval of the output ‘1’ of the second hysteresis comparator is smaller than the interval of the output ‘1’ of the first hysteresis comparator. 
     The control signal generator  320  includes control signal generation parts  321  through  325 . There are as many control signal generation parts as there are voltage devices of the bias power  217 . Herein, each of the control signal generation parts  321  through  325  are respectively connected to one voltage device and control the connection between the voltage device and the switching regulator  219 . 
     The control signal generation parts  321  through  325  control to connect the corresponding voltage device to the switching regulator  219 . To determine if a connection is to be made between the corresponding voltage device and the switching regulator  219 , each of the control signal generation parts  321  through  325  performs a logical operation on the output signals of two consecutive hysteresis comparators of the comparator  310 . In an exemplary implementation, the logical operation is an Exclusive OR (XOR) operation. For example, the first control signal generation part  321  may control the connection of a first voltage device having the lowest bias voltage in the bias power  217  through an XOR operation of the output signals from the first hysteresis comparator  311  and the second hysteresis comparator  313 . Similarly, the second control signal generation part  323  controls the connection of the second voltage device having the second lowest bias voltage in the bias power  217  through an XOR operation of the output signals from the second hysteresis comparator  313  and the third hysteresis comparator. 
     As such, the bias modulator of the EER power transmitter regulates the bias voltage of the switching regulator based on the magnitude of the amplitude component to address the broad slew rate. For example, the bias modulator controls the bias voltage of the switching regulator as illustrated in  FIG. 4 . 
       FIG. 4  illustrates a bias variation of a switching regulator in an EER power transmitter according to an exemplary embodiment of the present invention. 
       FIG. 4A  illustrates the magnitude of the amplitude component and  FIG. 4B  illustrates the bias voltage fed to the switching regulator based on the magnitude of the amplitude component. 
     In  FIG. 4A , the magnitude detector and divider  213  divides the magnitude of the amplitude component, separated by the signal separator  200 , into the plurality of the voltage levels. 
     Next, the magnitude detector and divider  213  controls to connect the voltage device of the bias power  217  corresponding to the magnitude of the amplitude component to the switching regulator  219 . 
     Hence, the switching regulator  219  receives the bias voltage according to the magnitude of the amplitude component as shown in  FIG. 4B  and generates the current to be supplied to the high-efficiency power amplifier  230 . 
     Now, descriptions explaining exemplary methods for controlling the bias voltage of the switching regulator based on the magnitude of the amplitude component for the broad slew rate in the bias modulator of the EER power transmitter are provided. 
       FIG. 5  is a flowchart outlining a method for changing a bias of a switching regulator based on an amplitude magnitude in an EER power transmitter according to an exemplary embodiment of the present invention. 
     In step  501 , the EER power transmitter splits the signal to be amplified into an amplitude component and a phase component. 
     In step  503 , the EER power transmitter linearly amplifies the amplitude component separated in step  501 . 
     In step  505 , the EER power transmitter divides the magnitude of the linearly amplified amplitude component into the plurality of voltage levels. For example, the EER power transmitter divides the magnitude of the amplitude component by the magnitude of the bias voltage that can be provided by the bias power  217  of  FIG. 2 . 
     In step  507 , the EER power transmitter determines the bias voltage to be supplied to the switching regulator according to the magnitude of the divided amplitude component. For example, the EER power transmitter selects the power device which supplies the bias voltage according to the divided magnitude in the bias power  217 , and connects the selected power device to the switching regulator. 
     In step  509 , the EER power transmitter generates the current according to the bias voltage using the switching regulator. 
     After splitting the signal to amplify to the amplitude component and the phase component in step  501 , the EER power transmitter modulates the phase component separated in step  501 , to the RF signal in step  511 . 
     In step  513 , the EER power transmitter amplifies the RF signal using the linearly amplified amplitude component as the drain bias. More particularly, the high-efficiency power amplifier of the EER power transmitter amplifies the RF signal using the linearly amplified amplitude component as the drain bias voltage and using the current generated with the bias voltage determined by the magnitude of the amplitude component as the drain bias current. 
     Next, the EER power transmitter finishes this process. 
     As above, when the bias voltage of the switching regulator is controlled based on the magnitude of the amplitude component, a delay may be generated in the feedback loop from the magnitude detector and divider  213  to the switching regulator  219 . To address the delay, the EER power transmitter may be constituted as illustrated in  FIG. 6 . 
       FIG. 6  is a block diagram of an EER power transmitter according to an exemplary embodiment of the present invention. 
     The EER power transmitter of  FIG. 6  includes a signal separator  600 , a bias modulator  610 , an orthogonal modulator  620 , and a high-efficiency power amplifier  630 . 
     The signal separator  600  separates the amplitude component and the phase component of a signal to be amplified. The signal separator  600  sends the amplitude component to the bias modulator  610  and outputs the phase component to the orthogonal modulator  620 . 
     The bias modulator  610  amplifies the amplitude component output from the signal separator  600  and outputs the amplified amplitude component to the drain bias voltage of the high-efficiency power amplifier  630 . For example, the bias modulator  610  includes a linear amplitude amplifier  611 , a magnitude detector and divider  612 , a switch controller  613 , a bias power  614 , a switching regulator  615 , a feedback part  616 , a delay controller  617 , and a delay checker  618 . 
     The linear amplitude amplifier  611  amplifies the amplitude component fed from the signal separator  600 . 
     The magnitude detector and divider  612  amplifies the signal voltage-dropped by a resistor serially connected to the output stage of the linear amplitude amplifier  611  and divides the magnitude of the amplitude component fed from the signal separator  600  based on a plurality of voltage levels. 
     Next, the magnitude detector and divider  612  generates a control signal to control the bias power  614  based on the divided voltage level. 
     The switch controller  613  connects one of power devices of the bias power  614 , based on the magnitude of the amplitude component, to the switching regulator  615  according to the control signal provided from the magnitude detector and divider  612 . Since the switch controller  613  controls the switch connected to the voltage devices, it can reduce the switching loss by lowering the switching speed. 
     The bias power  614  includes a plurality of power devices having different bias voltages. 
     The switching regulator  615  generates the current to be supplied to the high-efficiency power amplifier  630  using the bias voltage fed from the voltage device connected under the control of the switch controller  613 . As the bias voltage is supplied according to the magnitude of the amplitude component, the switching regulator  615  can control the change of the slew rate of the output current. Thus, the switching regulator  615  can address an unnecessary current caused by a ripple effect when a smaller inductor is designed to increase the slew rate. 
     The feedback part  616  detects the output of the bias modulator  610  to check for a delay and provides the detected output to the delay checker  618 . 
     The delay checker  618  determines the delay time by comparing the signal fed from the magnitude detector and divider  612  with the signal fed from the feedback part  616 . 
     The delay controller  617  compensates for the delay confirmed by the delay checker  618 . 
     The orthogonal modulator  620  modulates the phase component output from the signal separator  600  to an RF signal. 
     The high-efficiency power amplifier  630  amplifies the RF signal output from the orthogonal modulator  620  using the signal fed from the bias modulator  610  as the drain bias. In more detail, the high-efficiency power amplifier  630  amplifies the RF signal using the amplitude component linearly amplified by the linear amplitude amplifier  611  of the bias modulator  610  as the drain bias voltage and using the current generated by the switching regulator  615  as the drain bias current. 
     When the bias modulator of the EER power transmitter controls the input bias voltage of the switching regulator based on the magnitude of the amplitude component, the performance change is now illustrated. 
       FIGS. 7A and 7B  are graphs showing the performance change of an EER power transmitter according to an exemplary embodiment of the present invention. Herein, to examine the performance change, it is assumed that the EER power transmitter uses a mobile WiMAX amplitude signal of the Institute of Electrical and Electronics Engineers (IEEE) 802.16e standard with a 10 MHz band and that the Peak to Average Power Ratio (PAPR) is 8 dB with a Crest Factor Reduction (CFR) adopted. 
       FIG. 7A  shows the output current of the bias modulator when a constant bias voltage is applied to the switching regulator, and  FIG. 7B  shows the output current of the bias modulator when the bias voltage of the switching regulator is controlled based on the magnitude of the amplitude component according to an exemplary embodiment of the present invention. 
     When the bias modulator supplies the two-step bias voltage of zero (0) and Vds to the switching regulator according to the magnitude of the amplitude signal, the slew rate of the output current  700  of the switching regulator does not significantly change as shown in  FIG. 7A . Hence, the bias modulator utilizes the output current  710  of the linear amplitude amplifier to meet the slew rate of the current  720  desired by the high-efficiency power amplifier. In so doing, because the linear amplitude amplifier having low efficiency consumes a large amount of power, the entire efficiency of the bias modulator deteriorates. 
     When the bias modulator supplies the divided bias voltages to the switching regulator according to the magnitude of the amplitude signal, the slew rate of the output current  700  of the switching regulator significantly changes as shown in  FIG. 7B . Thus, the bias modulator utilizes the output current  700  of the switching regulator to meet the slew rate of the current  720  desired by the high-efficiency power amplifier. If the output current  700  of the switching regulator is insufficient, the bias modulator supplies the current  720  desired by the high-efficiency power amplifier by additionally using the output current  710  of the linear amplitude amplifier. 
     As mentioned above, when the bias modulator supplies the divided bias voltages to the switching regulator and increases the slew rate change of the current, the current consumption can be reduced as shown in  FIG. 8 . 
       FIG. 8  is a graph showing the performance change of an EER power transmitter according to an exemplary embodiment of the present invention. 
       FIG. 8A  shows the output current of the linear amplitude amplifier, and  FIG. 8B  shows the unnecessary current of the switching regulator lost to the linear amplitude amplifier. 
     When the bias modulator supplies the multiple divided bias voltages to the switching regulator based on the magnitude of the amplitude signal, the slew rate of the output current  700  of the switching regulator changes significantly as shown in  FIG. 7B . Thus, the bias modulator can reduce the output current of the linear amplitude amplifier as shown in  FIG. 8A . 
     Since less unnecessary current is produced by significantly changing the slew rate of the output current of the switching regulator, the current lost to the linear amplitude amplifier can be decreased as shown in  FIG. 8B . 
     As described above, the bias modulator employs the linear amplitude amplifier and the switching regulator as the power source. The ratio of the currents of the linear amplitude amplifier and the switching regulator affects the entire efficiency of the bias modulator. For example, provided that the efficiency of the linear amplitude amplifier is 45% and the efficiency of the switching regulator is 90%, the total efficiency of the bias modulator is shown in  FIG. 9 . 
       FIG. 9  is a graph showing the performance change of an EER power transmitter according to an exemplary embodiment of the present invention. 
     When the output current of the bias modulator is normalized to one (1) in  FIG. 9 , the sum of the rates of the currents of the linear amplitude amplifier and the switching regulator of the bias modulator is one (1) as indicated by the first graph  900 . 
     When the ratio of the linear amplitude amplifier and the switching regulator is regulated, the efficiency of the bias modulator is yielded as indicated by the second graph  910 . That is, the greater ratio of the switching regulator, the better efficiency of the bias modulator. 
     As set forth above, the EER power transmitter raises the ratio of the current supplied to the high-efficiency power amplifier by changing the bias voltage of the switching regulator based on the magnitude detected from the amplitude component. Therefore, the bias modulator can support the broad slew rate and the entire efficiency of the EER power transmitter can be enhanced. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.