Abstract:
A device includes: a power amplifier, including a supply voltage terminal, an input port and an output port, and the power amplifier being configured to receive a supply voltage at the supply voltage terminal, an input signal through the input port, to amplify the received input signal, and to output an amplified output signal through the output port; a variable impedance matching circuit having an input terminal connected to the output port of the power amplifier, and having an output terminal for being connected to a load; and a controller including a voltage measuring unit configured to measure the supply voltage, to compare the measured supply voltage with a threshold voltage, and to control the variable impedance matching circuit based on a result of the comparison so as to adjust a load impedance seen by the power amplifier at its output port.

Description:
BACKGROUND 
       [0001]    The present invention relates to a method and apparatus for amplifying a signal. 
         [0002]    As the size of a portable electronic device such as a cellular phone is an important design factor, so also is the capacity or size of a battery installed therein to power the device. Consequently, the power consumption of elements incorporated in the portable electronic device becomes subject to power-efficient design requirements. Such requirements calling for a high power efficiency often become even more stringent in the case of a complicated portable electronic device, such as a smartphone, employing a large number of elements to provide a greater variety of features and functions. In particular, among all the heavy power consuming elements in a portable electronic device, controlling the power efficiency of a power amplifier is important since it operates all the time while the portable electronic device is powered on and its communication capabilities are enabled. 
         [0003]    In designing a power amplifier, the linearity thereof is another important factor in addition to the power efficiency. For instance, on parameter that is often important for a power amplifier is the level of harmonic distortion generated by the power amplifier. Communications systems employing complicated schemes such as HSDPA (High Speed Downlink Packet Access), HSUPA (High Speed Uplink Packet Access), LTE (Long Term Evolution) or the like require high linearity, and often implemented with, for example, a large back-off in order to address the strict linearity requirement. 
         [0004]    However, it has been a major challenge to design a power amplifier to satisfy both high linearity requirements and power efficiency requirements. That is, if a power amplifier is designed to have high linearity, the power efficiency thereof tends to be reduced and vice versa. For instance, a Class A power amplifier has a comparatively good linearity but a relatively poor power efficiency. In contrast, a Class B power amplifier has a comparatively good power efficiency but a rather poor linearity. Thus, engineers have had to settle with a compromise to design a power amplifier having a linearity and a power efficiency capable of achieving certain respective levels. 
       SUMMARY 
       [0005]    It is, therefore, an object of the present invention to provide a method and apparatus for amplifying a signal that addresses both linearity requirements and power efficiency requirements. 
         [0006]    In accordance with one aspect of the invention, there is provided a device comprising: a power amplifier, including a supply voltage terminal, an input port and an output port, and the power amplifier being configured to receive a supply voltage at the supply voltage terminal, an input signal through the input port, to amplify the received input signal, and to output an amplified output signal through the output port; a variable impedance matching circuit having an input terminal connected to the output port of the power amplifier, and having an output terminal for being connected to a load; and a controller including a voltage measuring unit configured to measure the supply voltage, to compare the measured supply voltage with a threshold voltage, and to control the variable impedance matching circuit based on a result of the comparison so as to adjust a load impedance seen by the power amplifier at its output port. 
         [0007]    In accordance with another aspect of the invention, there is provided a method of amplifying a signal. The method comprises: providing the signal to an input port of a power amplifier; providing a supply voltage to the power amplifier; amplifying the signal with the power amplifier and outputting an amplified output signal at an output port of the power amplifier; measuring the supply voltage; comparing the measured supply voltage with a threshold voltage; and adjusting a load impedance seen by the power amplifier at its output port based on a result of the comparison. 
         [0008]    In accordance with yet another aspect of the invention, there is provided a device comprising: a power amplifier, including a supply voltage terminal, an input port and an output port, and the power amplifier being configured to receive a supply voltage at the supply voltage terminal, an input signal through the input port, to amplify the received input signal, and to output an amplified output signal through the output port; and circuitry configured to adjust a load impedance seen by the power amplifier at its output port, wherein when the supply voltage has a first voltage value the circuitry adjusts the load impedance to have a first impedance value, and when the supply voltage has a second voltage value less than the first voltage value, the circuitry adjusts the load impedance to have a second impedance value less than the first impedance value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The above and other objects and features of the present invention will become apparent from the following description of embodiments given in conjunction with the accompanying drawings, in which: 
           [0010]      FIG. 1  illustrates a magnitude of an output voltage supplied to a power amplifier from a battery, varying with time. 
           [0011]      FIG. 2  is a block diagram showing a schematic construction of one embodiment of a signal amplifier. 
           [0012]      FIG. 3  shows on a current-voltage plane an exemplary characteristic curve of a general power amplifier and a change of an exemplary load line. 
           [0013]      FIG. 4A  provides a first embodiment of a variable impedance matching circuit. 
           [0014]      FIG. 4B  exhibits on a Smith chart a change in the magnitude of a load impedance presented to a power amplifier by the variable impedance matching circuit of  FIG. 4A  depending on control of a switch. 
           [0015]      FIG. 5A  presents a second embodiment of a variable impedance matching circuit. 
           [0016]      FIG. 5B  displays on a Smith chart a change in the magnitude of a load impedance presented to a power amplifier by the variable impedance matching circuit of  FIG. 5A  depending on control of a switch. 
           [0017]      FIG. 6A  represents a third embodiment of the variable impedance matching circuit. 
           [0018]      FIG. 6B  depicts on a Smith chart a change in the magnitude of a load impedance presented to a power amplifier by the variable impedance matching circuit of  FIG. 6A  depending on control of a varactor diode. 
           [0019]      FIG. 7  is a block diagram showing a schematic construction of another embodiment of a signal amplifier. 
           [0020]      FIG. 8  demonstrates an exemplary relationship between the temperature of a power amplifier and a threshold voltage for controlling a variable impedance matching circuit for the power amplifier. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparati and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparati are clearly within the scope of the present teachings. 
         [0022]    Unless otherwise noted, when a first device is said to be connected to a second device, this encompasses cases where one or more intermediate devices may be employed to connect the two devices to each other. However, when a first device is said to be directly connected to a second device, this encompasses only cases where the two devices are connected to each other without any intermediate or intervening devices. Similarly, when a signal is said to be coupled to a device, this encompasses cases where one or more intermediate devices may be employed to couple the signal to the device. However, when a signal is said to be directly coupled to a device, this encompasses only cases where the signal is directly coupled to the device without any intermediate or intervening devices 
         [0023]    Hereinafter, certain preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
         [0024]    The inventors have recognized the following. When a battery supplies an output voltage to a device (e.g., a mobile phone) including power amplifier, the output voltage does not remain constant: as the electric energy charged in the battery is consumed, the output voltage decreases. On the other hand, since the power amplifier is designed to meet a minimum linearity requirement so as to ensure the power amplifier to operate normally even when the output voltage becomes low, there tends to exist an excess linearity over the minimum linearity requirement when the output voltage is comparatively high. Such excess linearity may entail an undesirable or unnecessary sacrifice in the power efficiency. 
         [0025]      FIG. 1  illustrates how a magnitude of a battery supply voltage, V SUPPLY , varies with time between recharges. When the battery is fully charged, the maximum magnitude of the supply voltage V SUPPLY  is denoted by V MAX . As the battery supplies the supply voltage V SUPPLY  to a device including a power amplifier, over time the magnitude of the supply voltage V SUPPLY  decreases. The minimum magnitude of the supply voltage V SUPPLY  at which the power amplifier is still required to meet its minimum linearity requirement so as to ensure the power amplifier operates normally is denoted by V MIN . Since the power amplifier is not required or expected to operate normally in a situation where the magnitude of the supply voltage V SUPPLY  falls below V MIN , the power amplifier may be designed to stop its operation in such a situation or to notify a user of such a situation. 
         [0026]    Since the minimum magnitude of the supply voltage V SUPPLY  which meets the minimum linearity requirement ensuring the power amplifier operates normally is V MIN  as set forth above, an excess linearity of the power amplifier over the minimum linearity requirement is obtained during a range where the magnitude of the supply voltage V SUPPLY  is greater than V MIN . Further, the larger that the supply voltage V SUPPLY  is, the larger the amount of excess linearity that exists for the power amplifier. Recognizing that there is a trade-off between linearity and power efficiency, it can be expected that when the excess linearity becomes large due to the supply voltage V SUPPLY  having a large magnitude, the power efficiency of the power amplifier would be reduced. Embodiments of a device including a power amplifier will now be described which improve the power efficiency of the power amplifier by way of reducing the excess linearity during times when the supply voltage V SUPPLY  is significantly greater than V MIN . 
         [0027]      FIG. 2  schematically shows a construction of one embodiment of a signal amplifier  10 . The signal amplifier  10  includes a power amplifier  100 , a battery  200 , a variable impedance matching circuit  300  and a controller  400 . Power amplifier  100  includes an input port  102 , an output port  104 , and a supply voltage terminal  106 . Supply voltage (or a drive voltage) V SUPPLY  is supplied to supply voltage terminal  106  from battery  200 . Power amplifier  100  receives an input signal at input port  102  and amplifies the input signal to output an output signal at output port  104 . The amplified signal is transmitted via variable impedance matching circuit  300  to a load, such as for example an antenna (not shown). Controller  400  has a voltage measuring unit  410  for measuring the supply voltage V SUPPLY  and controls variable impedance matching circuit  300  based on the measured supply voltage V SUPPLY . 
         [0028]    For example, signal amplifier  10  may be included in a portable electronic device, such as a cellular phone, and battery  200  may be a rechargeable battery of such a device for supplying the supply voltage V SUPPLY  to power amplifier  100 . An exemplary operation of variable impedance matching circuit  300  controlled by controller  400  will be explained below in detail. 
         [0029]    First, an appropriate value between V MAX  and V MIN , is selected to be a threshold voltage, V CRITICAL . The voltage measuring unit  410  measures the supply voltage V SUPPLY  and controller  400  controls variable impedance matching circuit  300  based on the measured supply voltage V SUPPLY  to adjust the magnitude of a load impedance Z O  which is seen by power amplifier  100  at its output port  104 . In a particular embodiment, when the supply voltage V SUPPLY  lies between V MAX  and the threshold voltage V CRITICAL  controller  400  adjusts the magnitude of Z O  to be larger than the magnitude of the load impedance Z O  when the supply voltage V SUPPLY  lies between the threshold voltage V CRITICAL  and V MIN . How the linearity and power efficiency of power amplifier  100  vary with the adjustment of the load impedance Z O  will be explained below in detail. 
         [0030]      FIG. 3  illustrates on a current-voltage plane an exemplary characteristic curve C of a general power amplifier and exemplary load lines L 1  and L 2 . If a load impedance of the power amplifier becomes larger, the load line changes from L 1  to L 2 . Further, when the load line changes as above, the operating point also changes from O 1  to O 2  and both the current swing and the voltage swing become smaller. As a result of that, the output power of the power amplifier transferred to a load is also reduced, and the linearity of the power amplifier is decreased. On the other hand, since the dissipation power of the power amplifier is also reduced as the output power is reduced, the power efficiency of the power amplifier is improved. To sum up, by increasing the load impedance as seen by power amplifier  100  at its output port, power efficiency can be increased at the expense of reduced linearity. 
         [0031]    Based on such a principle, when the supply voltage V SUPPLY  lies between V MAX  and the threshold voltage V CRITICAL , controller  400  controls variable impedance matching circuit  300  to adjust the load impedance Z O  to have a comparatively large magnitude so that the linearity of power amplifier  100  is reduced, while the power efficiency thereof is increased. In a situation where the supply voltage V SUPPLY  is greater than the threshold voltage V CRITICAL , an excess linearity exists and, therefore, there is no problem in meeting a minimum linearity requirement for normal operation of power amplifier  100  even if the linearity is somewhat reduced as described above. In contrast, when the supply voltage V SUPPLY  lies between the threshold voltage V CRITICAL  and V MIN , controller  400  controls variable impedance matching circuit  300  to adjust the load impedance Z O  to have a comparatively small magnitude so that the linearity is increased and, thus, there occurs no problem in meeting the minimum linearity requirement for normal operation of power amplifier  100  even when the supply voltage V SUPPLY  is less than the threshold voltage V CRITICAL . 
         [0032]    As presented in  FIG. 1  in general, it can be seen that the supply voltage V SUPPLY  of a battery decreases slowly for a certain period of time and then, for a remaining period of time the supply voltage V SUPPLY  decreases quickly. Further, in general the period of time where the supply voltage V SUPPLY  decreases slowly is much longer than the period of time of where the supply voltage V SUPPLY  decreases more rapidly. Therefore, if the threshold voltage V CRITICAL  is set in the vicinity of a point where the rate of which the supply voltage V SUPPLY  is decreasing changes abruptly from a lower rate to a higher rate, the power efficiency of the power amplifier  100  can be improved during most of the discharging time of battery  200  in which the supply voltage V SUPPLY  is larger than the threshold voltage V CRITICAL . 
         [0033]    Presented below are example embodiments of variable impedance matching circuit  300  which can be controlled by controller  400  to adjust the magnitude of the load impedance Z O . 
         [0034]      FIG. 4A  shows a first embodiment of a variable impedance matching circuit. An input port and an output port of the variable impedance matching circuit of  FIG. 4A , which may be externally connected to an output port of power amplifier  100  and a load, respectively, are connected to each other by a microstrip line M (for example, a 50-ohm microstrip line). One end of a first shunt capacitor C 1  is connected to an appropriate point of the microstrip line M and the other end thereof is connected to ground via a switch  310 . One end of a second capacitor C 2  is connected to the output port of the variable impedance matching circuit of  FIG. 4A  and the other end thereof is connected to ground. 
         [0035]      FIG. 4B  shows a change of the magnitude of the load impedance Z O  on a Smith chart depending on the control of switch  310 . When the measured supply voltage V SUPPLY  lies between V MAX  and the threshold voltage V CRITICAL , controller  400  controls switch  310  to be open so that the load impedance Z O  is placed at the point ‘A’ along one curve on the Smith chart. By contrast, when the measured supply voltage V SUPPLY  lies between the threshold voltage V CRITICAL  and V MIN , controller  400  controls switch  310  to be closed so that the load impedance Z O  is placed at the point ‘B’ along the other dotted curve on the Smith chart. Since the magnitude of the load impedance Z O  at the point ‘A’ is greater than the magnitude of the load impedance Z O  at the point ‘B’, it can be understood that excess linearity of power amplifier  100  is sacrificed at higher supply voltages V SUPPLY  to achieve increased power efficiency. 
         [0036]      FIG. 5A  shows a second embodiment of a variable impedance matching circuit. An input port and an output port of the variable impedance matching circuit of  FIG. 5A , which are externally connected to an output port of power amplifier  100  and a load, respectively, are connected to each other by a microstrip line M (for example, a 50-ohm microstrip line). One end of a first shunt capacitor C 3  is connected to an appropriate point of the microstrip line M and the other end thereof is connected to ground. One end of a second capacitor C 4  is connected to the output port of the variable impedance matching circuit and the other end thereof is connected to ground via a switch  320 . 
         [0037]      FIG. 5B  shows a change of the magnitude of the load impedance Z O  on a Smith chart depending on the control of switch  320 . When the supply voltage V SUPPLY  lies between V MAX  and the threshold voltage V CRITICAL , controller  400  controls switch  320  to be open so that the load impedance Z O  is placed at the point ‘A’ along one curve on the Smith chart. By contrast, when the supply voltage V SUPPLY  lies between the threshold voltage V CRITICAL  and V MIN , the controller  400  controls switch  320  to be closed so that the load impedance Z O  is placed at the point ‘B’ along the other dotted curve on the Smith chart. Since the magnitude of the load impedance Z O  at the point ‘A’ is greater than the magnitude of the load impedance Z O  at the point ‘B’, it can be understood that excess linearity of power amplifier  100  is sacrificed at higher supply voltages V SUPPLY  to achieve increased power efficiency. 
         [0038]      FIG. 6A  shows a third embodiment of a variable impedance matching circuit. An input port and an output port of the variable impedance matching circuit of  FIG. 6A , which are externally connected to an output port of power amplifier  100  and a load, respectively, are connected each other by a microstrip line M (for example, a 50-ohm microstrip line). One end of a varactor diode  330  is connected to an appropriate point of the microstrip line M and the other end thereof is connected to ground. One end of a capacitor C 5  is connected to the output port of variable impedance matching circuit  300  and the other end thereof is connected to ground. 
         [0039]      FIG. 6B  shows a change of the magnitude of the magnitude of the load impedance Z O  on a Smith chart depending on the control of a capacitance of varactor diode  330 . The controller  400  controls the varactor diode  330  so that the capacitance of varactor diode  330  in when the supply voltage V SUPPLY  lies between V MAX  and the threshold voltage V CRITICAL  is less than the capacitance of varactor diode  330  when the supply voltage V SUPPLY  lies between the threshold voltage V CRITICAL  and V MIN . As a result, the load impedance Z O  is placed at the point ‘A’ along one curve on the Smith chart when the supply voltage V SUPPLY  is greater than the threshold voltage V CRITICAL , and the load impedance Z O  is placed at the point ‘B’ along the other dotted curve on the Smith chart when the supply voltage V SUPPLY  is less than the threshold voltage V CRITICAL . Since the magnitude of the load impedance Z O  at the point ‘A’ is greater than he magnitude of the load impedance Z O  at the point ‘B’, it is can be understood that excess linearity of power amplifier  100  is sacrificed at higher supply voltages V SUPPLY  to achieve increased power efficiency. 
         [0040]    The embodiments of  FIGS. 4A ,  5 A and  6 A are just some examples of variable impedance matching circuit  300 , and variable impedance matching circuit  300  can be implemented by other circuitry capable of adjusting the magnitude of the load impedance Z O . 
         [0041]    Furthermore, in an alternative embodiment of signal amplifier  10  where variable impedance matching circuit  300  employs a varactor diode (e.g.,  FIG. 6A ), controller  400  may control the voltage that is applied to the varactor diode (and therefore the capacitance of the varactor diode) to be a function of the measured supply voltage V SUPPLY , rather than comparing the supply voltage V SUPPLY  to a threshold voltage. For example, in one embodiment controller  400  may include a look-up table which maps various values of the measured supply voltage V SUPPLY  to corresponding values of a control voltage to be applied to the varactor diode in impedance matching circuit  300  such that a desired load impedance is presented to output port  104  of power amplifier  100  for all values of the supply voltage V SUPPLY . In another example, a processor may employ an equation (e.g., a polynomial fit) to calculate a value of a control voltage to be applied to the varactor diode in response to a value of the measured supply voltage V SUPPLY  to control the variable impedance matching circuit  300  to present a desired load impedance Z O  to output port  104  of power amplifier  100 . 
         [0042]      FIG. 7  depicts a schematic configuration of another embodiment of a signal amplifier  20 . Elements in  FIG. 7  having the same reference numerals or characters as those in  FIG. 1  may be the same as each other, and specific descriptions of these elements will be omitted in the description of  FIG. 7 . 
         [0043]    Signal amplifier  20  measures the temperature of power amplifier  100  and adjusts the threshold voltage V CRITICAL  based on the measured temperature. Since the power efficiency of a practical power amplifier is not 100%, the power amplifier is inevitably heated and, consequently, the linearity thereof is reduced due to the heat. The reduced linearity at higher temperatures might cause what would be an excess linearity at lower temperatures to be reduced or even to disappear even when the supply voltage V SUPPLY  is larger than the threshold voltage V CRITICAL . Therefore, to compensate for the fact that the excess linearity is reduced at higher temperatures, the threshold voltage V CRITICAL  may be increased when the temperature of the power amplifier  100  becomes higher. Consequently, in embodiments of signal amplifier  20  the linearity and the power efficiency of power amplifier  100  are exchanged only when the supply voltage V SUPPLY  is larger than the increased threshold voltage V CRITICAL  and thus the excess linearity is great enough. 
         [0044]    When compared with signal amplifier  10 , the controller  700  of signal amplifier  20  further includes a temperature measuring unit  420 . Temperature measuring unit  420  measures the temperature of the power amplifier  100 . Controller  700  adjusts the threshold voltage V CRITICAL  based on the measured temperature and adjusts the load impedance Z O  such that the magnitude of the load impedance Z O  when the supply voltage V SUPPLY  lies between V MAX  and the adjusted threshold voltage V CRITICAL  is greater than the magnitude of the load impedance Z O  when the supply voltage V SUPPLY  lies between the adjusted threshold voltage V CRITICAL  and V MIN . In adjusting the threshold voltage V CRITICAL , it is beneficial that the threshold voltage V CRITICAL  is proportional to the temperature of the power amplifier  100 , even though it is not necessary to be directly proportional. Furthermore, the threshold voltage V CRITICAL  may be predefined as a function of the temperature of power amplifier  100 . Furthermore, as shown in  FIG. 8 , it is also possible to preset T CRITICAL  in advance and to maintain the threshold voltage V CRITICAL  constantly when the temperature of the power amplifier is smaller than T CRITICAL  and to adjust the threshold voltage V CRITICAL  to have a proportional magnitude to the temperature only where the temperature is greater than T CRITICAL . 
         [0045]    The temperature measuring unit  420  is illustrated as being provided separately from the voltage measuring unit  410  in signal amplifier  20 . However, in other embodiments it may be implemented as a single element with the voltage measuring unit  410 . 
         [0046]    As explained above, when an output voltage of a battery, which is supplied to a power amplifier, is comparatively high, a power efficiency of the power amplifier can be improved by taking advantage of, and sacrificing, an excess linearity of the power amplifier above a specified requirement. 
         [0047]    Furthermore, it is possible to control the power efficiency and linearity of the power amplifier by adjusting a load impedance seen from an output port of the power amplifier with simple elements, for example, such as a capacitor and a switch. 
         [0048]    Furthermore, by measuring the temperature of the power amplifier and adjusting the load impedance based on the measured temperature, it is possible to improve the power efficiency without failing to meet a minimum linearity requirement for normally operating the power amplifier even in a situation where the temperature becomes somewhat higher. 
         [0049]    While the invention has been shown and described with respect to the preferred embodiments only, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.