Abstract:
A power amplifier apparatus includes: an amplifier configured to amplify an input signal; a sensing circuit connected to the amplifier and configured to sense a bias of the amplifier; and a biasing circuit connected to the sensing circuit and configured to provide a biasing current to the amplifier, wherein the sensing circuit is configured to change the biasing current based on the bias of the amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit under 35 USC 119(a) of Korean Patent Application No. 10-2015-0061419 filed on Apr. 30, 2015 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes. 
     BACKGROUND 
     1. Field 
     The following description relates to a power amplifier and a method for limiting a current in a power amplifier. 
     2. Description of Related Art 
     In general, a power amplifier (PA) amplifying a high signal consumes a large amount of current. Further, in order to output a high signal having a level of a few watts (W), output impedance of the power amplifier needs to be decreased. 
     However, in a case in which the output impedance of the power amplifier is low, the output impedance may be significantly changed even if an operating environment is only changed slightly. As the change of the output impedance is high, a frequency at which an over-current (e.g., current in excess of an intended amount) flows in the power amplifier may be increased. 
     Since the over-current generated by the power amplifier may damage elements included in the power amplifier, a means for breaking the over-current so that the over-current of an allowable value or more does not flow in the power amplifier is desirable. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     According to one general aspect, a power amplifier apparatus includes: an amplifier configured to amplify an input signal; a sensing circuit connected to the amplifier and configured to sense a bias of the amplifier; and a biasing circuit connected to the sensing circuit and configured to provide a biasing current to the amplifier, wherein the sensing circuit is configured to change the biasing current based on the bias of the amplifier. 
     The amplifier may include: a first amplifying circuit configured to amplify the input signal; and a second amplifying circuit configured to amplify the input signal amplified by the first amplifying circuit. The sensing circuit may be connected to the second amplifying circuit. 
     The amplifier may include at least one bipolar junction transistor (BJT) configured to amplify a signal input to a base terminal and output the amplified signal to a collector terminal. The sensing circuit may be configured to sense a bias current of the base terminal of the bipolar junction transistor. 
     The sensing circuit may include: a diode configured to pass a current based on the bias of the amplifier; a first resistor connected between the amplifier and the diode; and a second resistor connected between the diode and the biasing circuit. 
     As a value of a bias current of the amplifier increases, the sensing circuit may decrease a value of the biasing current. 
     The biasing circuit may include at least one source circuit configured to generate a reference current and provide the generated reference current to the sensing circuit and a ground. As a bias current of the amplifier increases, a value obtained by dividing the reference current by a current provided to the sensing circuit may decrease. 
     The source circuit may include a first source circuit configured to generate a first reference current, and a second source circuit configured to generate a second reference current. As a value of a bias current sensed by the sensing circuit increases, a value of a current flowing into the ground from the first source circuit may increase. As the value of the current flowing into the ground from the first source circuit increases, a value of a current flowing into the ground from the second source circuit may increase. 
     The biasing circuit may further include a first semiconductor switch connected between the first source circuit and the ground, the first semiconductor switch being configured to receive the current flowing into the ground from the first source circuit at a first base terminal, and output the current flowing into the ground to a first emitter terminal, and a second semiconductor switch connected between the second source circuit and the ground, the second semiconductor switch being configured to receive the current flowing into the ground from the second source circuit at a second collector terminal, and output the current flowing into the ground to a second emitter terminal. A value of the current flowing in a second base terminal of the second semiconductor switch may be determined based on a value of current flowing in a first collector terminal of the first semiconductor switch. 
     According to another general aspect, a power amplifier includes: a first amplifier configured to amplify an input signal; a second amplifier connected to the first amplifier and configured to amplify the input signal amplified by the first amplifier; a sensing circuit connected to the second amplifier and configured to sense a bias of the second amplifier; and a biasing circuit connected to the sensing circuit and the first amplifier and configured to provide a biasing current to the first amplifier, wherein the sensing circuit is configured to change the biasing current based on the bias of the second amplifier. 
     The second amplifier may include at least one bipolar junction transistor (BJT) configured to amplify a signal input to a base terminal and output the amplified signal to a collector terminal. The sensing circuit may be configured to sense a bias current of the base terminal of the bipolar junction transistor. As a value of a bias current of the second amplifier increases, the biasing circuit may decrease a value of the biasing current provided to the first amplifier. 
     The sensing circuit may include: a diode configured to pass a current based on the bias of the second amplifier; a first resistor connected between the second amplifier and the diode; and a second resistor connected between the diode and the biasing circuit. 
     The biasing circuit may include a first source circuit configured to generate a first reference current and provide the reference current to the sensing circuit and a ground, and a second source circuit configured to generate a second reference current and provide the second reference current to the first amplifier and the ground. As a value of a bias current sensed by the sensing circuit increases, a value of a current flowing into the ground from the first source circuit may increase. As the value of the current flowing into the ground from the first source circuit increases, a value of a current flowing into the ground from the second source circuit may increase. 
     The biasing circuit may further include a first semiconductor switch connected between the first source circuit and the ground, the first semiconductor switch being configured to receive the current flowing into the ground from the first source circuit at a first base terminal and output the current flowing into the ground to a first emitter terminal, and a second semiconductor switch connected between the second source circuit and the ground, the second semiconductor switch being configured to receive the current flowing into the ground from the second source circuit at a second collector terminal and output the current flowing into the ground to a second emitter terminal. A value of a current flowing in a second base terminal of the second semiconductor switch may be determined based on a value of a current flowing in a first collector terminal of the first semiconductor switch. 
     According to another general aspect, a method for limiting current in a power amplifier apparatus includes: providing a biasing current to an amplifier; sensing, at a sensing circuit connected to the amplifier, a bias of the amplifier; and changing the biasing current based on the bias of the amplifier, using the sensing circuit. 
     The sensing circuit may include: a diode configured to pass a current based on the bias of the amplifier; a first resistor connected between the amplifier and the diode; and a second resistor connected between the diode and a biasing circuit providing the biasing current to the amplifier. 
     The method may further include decreasing a value of the biasing current in response to a value of a bias current of the amplifier increasing. 
     The method may further include: providing the biasing current to the amplifier by providing, using a biasing circuit, a reference current to the sensing circuit and a ground; and as a bias current of the amplifier increases, adjusting a current provided to the sensing circuit such that a value obtained by dividing the reference current by the current provided to the sensing circuit decreases. 
     The method may further include, as a value of a bias current sensed by the sensing circuit increases, increasing an amount of current flowing into the ground from the biasing circuit. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a conceptual view illustrating a power amplifier, according to an embodiment. 
         FIG. 2  is a circuit diagram illustrating the power amplifier of  FIG. 1 , according to an embodiment. 
         FIG. 3  is a diagram illustrating an operation of the power amplifier of  FIG. 2 , according to an embodiment. 
         FIG. 4  is a diagram illustrating an operation of limiting a current in the power amplifier of  FIG. 2 , according to an embodiment. 
         FIG. 5  is a conceptual view illustrating a power amplifier, according to an embodiment. 
         FIG. 6  is a circuit diagram illustrating the power amplifier of  FIG. 5 , according to an embodiment. 
         FIG. 7  is a diagram illustrating an operation of the power amplifier of  FIG. 6 , according to an embodiment. 
         FIG. 8  is a diagram illustrating an operation of limiting a current in the power amplifier of  FIG. 6 , according to an embodiment. 
         FIG. 9  is a graph illustrating a current flowing in an amplifier depending on a resistance value of a first resistor or a second resistor included in the power amplifier, according to an embodiment. 
         FIG. 10  is a flow chart illustrating a method for limiting a current in a power amplifier, according to an embodiment. 
         FIG. 11  is a flow chart illustrating the method for limiting a current of  FIG. 10  in greater detail, according to an embodiment. 
     
    
    
     Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience. 
     DETAILED DESCRIPTION 
     The following detailed description is provided to assist the reader in gaining a comprehensive understanding of the methods, apparatuses, and/or systems described herein. However, various changes, modifications, and equivalents of the methods, apparatuses, and/or systems described herein will be apparent to one of ordinary skill in the art. The sequences of operations described herein are merely examples, and are not limited to those set forth herein, but may be changed as will be apparent to one of ordinary skill in the art, with the exception of operations necessarily occurring in a certain order. Also, descriptions of functions and constructions that are well known to one of ordinary skill in the art may be omitted for increased clarity and conciseness. 
     The features described herein may be embodied in different forms, and are not to be construed as being limited to the examples described herein. Rather, the examples described herein have been provided so that this disclosure will be thorough and complete, and will convey the full scope of the disclosure to one of ordinary skill in the art. 
     Throughout the specification, it will be understood that when an element, such as a layer, region or wafer (substrate), is referred to as being “on,” “connected to,” or “coupled to” another element, it can be directly “on,” “connected to,” or “coupled to” the other element or other elements intervening therebetween may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element, there may be no elements or layers intervening therebetween. Like numerals refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be apparent that though the terms first, second, third, etc. may be used herein to describe various members, components, regions, layers and/or sections, these members, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one member, component, region, layer or section from another region, layer or section. Thus, a first member, component, region, layer or section discussed below could be termed a second member, component, region, layer or section without departing from the teachings of the embodiments. 
     Spatially relative terms, such as “above,” “upper,” “below,” and “lower” and the like, may be used herein for ease of description to describe one element&#39;s relationship to another element(s) as shown in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “above,” or “upper” other elements would then be oriented “below,” or “lower” the other elements or features. Thus, the term “above” can encompass both the above and below orientations depending on a particular direction of the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly. 
     The terminology used herein is for describing particular embodiments only and is not intended to be limiting of the present inventive concept. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” and/or “comprising” when used in this specification, specify the presence of stated features, integers, steps, operations, members, elements, and/or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, members, elements, and/or groups thereof. 
       FIG. 1  is a conceptual view illustrating a power amplifier  100 , according to an embodiment. Referring to  FIG. 1 , the power amplifier  100  includes an amplifier  110 , a sensing circuit  120 , and a biasing circuit  130 . 
     The amplifier  110  amplifies an input signal. The input signal is input through an input terminal RF in. The signal amplified by the amplifier  110  is output through an output terminal RF out. For example, the output terminal RF out may be connected to an antenna (not illustrated). 
     The sensing circuit  120  is connected to the amplifier  110  to sense a bias of the amplifier  110 . For example, the sensing circuit  120  includes semiconductor elements such as a diode, a transistor, and the like, of which a flowing current changes depending on a bias current or a bias voltage of the amplifier  110 . 
     Further, the sensing circuit  120  changes a value of a current provided to the amplifier  110  by the biasing circuit  130  based on the bias of the amplifier  110 . For example, as a value of the bias current of the amplifier  110  increases, the sensing circuit  120  decreases the value of the current provided to the amplifier  110  by the biasing circuit  130 . 
     The biasing circuit  130  is connected to the sensing circuit  120  and provides the current to the amplifier  110  to bias the amplifier  110 . Here, biasing means that a current or voltage of a specific value is provided so that a bias current flows in a specific block or element, or a bias voltage is applied to the specific block or element. 
     The current or the voltage provided to the amplifier  110  by the biasing circuit  130  changes depending on the bias sensed by the sensing circuit  120 . For example, in a case in which the current provided to the amplifier  110  by the biasing circuit  130  decreases, a current flowing into a ground from the biasing circuit  130  increases. That is, a current or voltage distribution of the biasing circuit  130  changes depending on a state of the sensing circuit  120 . 
     As a result, the power amplifier  100  may break an over-current (e.g., current in excess of an intended amount) of an allowable value or more so as not to flow the over-current in the power amplifier, thereby reducing possibility of damage to elements. 
       FIG. 2  is a circuit diagram illustrating the power amplifier  100  of  FIG. 1 , according to an embodiment. Referring to  FIG. 2 , the amplifier  110  includes a first amplifying circuit  111  and a second amplifying circuit  112 . The first amplifying circuit  111  amplifies the input signal. For example, the first amplifying circuit  111  is a driving amplifier DA amplifying a low signal to have an amplitude within an amplification range of the second amplifying circuit  112 . 
     The second amplifying circuit  112  amplifies the signal amplified by the first amplifying circuit  111 . For example, since the signal amplified by the second amplifying circuit  112  is a high signal having a level of a few watts (W), an over-current may flow in the second amplifying circuit  112 . The power amplifier  100  reduces a frequency at which the over-current flows in the second amplifying circuit  112 . 
     For example, the second amplifying circuit  112  includes at least one bipolar junction transistor (BJT) amplifying a signal input to a base terminal and outputting the amplified signal to a collector terminal. Here, a current flowing in the base terminal of the bipolar junction transistor and a current flowing in the collector terminal of the bipolar junction transistor have a proportional relationship. Thus, the sensing circuit  120  senses a bias current of the base terminal of the bipolar junction transistor, thereby sensing an over-current flowing in the collector terminal of the second amplifying circuit  112 . 
     Referring to  FIG. 2 , the sensing circuit  120  includes a first resistor  121 , a second resistor  122 , and a diode  123 . The first resistor  121  is connected between the second amplifying circuit  112  and the diode  123 . For example, the current flowing in the base terminal of the second amplifying circuit  112  also flows in the second resistor  122 . A voltage corresponding to a product of a resistance value of the first resistor  121  and a current flowing in the first resistor  121  is applied across the first resistor  121 . 
     The second resistor  122  is connected between the diode  123  and the biasing circuit  130 . A voltage corresponding to a product of a resistance value of the second resistor  122  and a current flowing in the second resistor  122  is applied across the second resistor  122 . 
     A current based on the bias of the amplifier  110  flows in the diode  123 . For example, in a case in which a voltage across the diode  123  is lower than a threshold voltage, the current flowing in the diode  123  is low. For example, in a case in which the voltage across the diode  123  is higher than the threshold voltage, the current flowing in the diode  123  increases as a voltage across a plurality of terminals increases. That is, the current flowing in the diode  123  is determined based on a direct current (DC) voltage determined by currents flowing in the first resistor  121 , the second resistor  122 , and the sensing circuit  120 . 
     The diode  123  may be implemented as a transistor of which a base terminal and a collector terminal are connected to each other, and may also be implemented as a field effect transistor of which a gate terminal and a drain terminal are connected to each other. Thus, the diode  123  is not limited to a diode element formed by a single p-n junction. 
     A value of the current flowing in the sensing circuit  120  is adjusted depending on the resistance value of the first resistor  121  or the second resistor  122 . A detailed description the adjustment of the value of the current flowing in the sensing circuit  120  will be provided below with reference to  FIG. 9 . 
     Referring to  FIG. 2 , the biasing circuit  130  includes a first semiconductor switch  131 , a second semiconductor switch  132 , a first source circuit  136 , and a second source circuit  137 . The first semiconductor switch  131  is connected between the first source circuit  136  and a ground, and receives a current flowing into the ground from the first source circuit  136  at a base terminal and outputs the received current to an emitter terminal. Here, the source circuit is a current source circuit or a voltage source circuit. 
     The second semiconductor switch  132  is connected between the second source circuit  137  and the ground, and receives a current flowing into the ground from the second source circuit  137  at a collector terminal and outputs the received current to an emitter terminal. 
     A value of the current flowing in the base terminal of the second semiconductor switch  132  is determined based on a value of the current flowing in the collector terminal of the first semiconductor switch  131 . Specifically, the current flowing in the collector terminal of the first semiconductor switch  131  and the current flowing in the base terminal of the second semiconductor switch  132  have a proportional relationship. 
     The first source circuit  136  generates a first reference current and provides the first reference current to the sensing circuit  120  and the ground. As a resistance value of the sensing circuit  120  increases, a value of a current flowing into the ground from the first source circuit  136  increases. 
     The second source circuit  137  generates a second reference current and provides the second reference current to the sensing circuit  120  and the ground. As the value of the current flowing into the ground from the first source circuit  136  increases, the current flowing into the ground from the second source circuit  137  increases. 
     Specifically, the value of the current flowing into the ground from the first source circuit  136  and a base current of the first semiconductor switch  131  are proportional. The base current of the first semiconductor switch  131  is proportional to a collector current of the first semiconductor switch  131 . The collector current of the first semiconductor switch  131  and a base current of the second semiconductor switch  132  are proportional. The base current of the second semiconductor switch  132  and a collector current of the second semiconductor switch  132  are proportional. Thus, as the current flowing into the ground from the first source circuit  136  increases, the current flowing into the ground from the second source circuit  137  increases. 
     Increasing the current flowing into the ground from the source circuit means that a value obtained by dividing a value of the reference current by a value of the current provided to the sensing circuit  120  decreases. As a result, a current flowing into the amplifier  110  is reduced, and an occurrence frequency of the over-current is reduced. 
       FIG. 3  is a diagram illustrating an operation of the power amplifier  100  of  FIG. 2 , according to an embodiment. Referring to  FIG. 3 , a flow of a current in an overall circuit diagram of the power amplifier  100  in a circumstance in which the over-current does not flow in the amplifier  110  is illustrated. Here, a region in which a current of a relatively high value flows is indicated by an arrow. Hereinafter, a description will be provided on the basis of the arrow. 
     When the over-current does not flow in the second amplifying circuit  112 , a value of a bias current of the second amplifying circuit  112  is relatively small, and a bias voltage of the base terminal of the second amplifying circuit  112  is relatively high. In addition, a voltage dropped by the first resistor  121  and the second resistor  122  is low. In addition, since a voltage across the diode  123  is high, a current flowing in the diode  123  is high. That is, since the current flowing in the sensing circuit  120  is high, the majority of the current generated by the first source circuit  136  flows into the sensing circuit  120 . In addition, the value of the current flowing into the first semiconductor switch  131  from the first source circuit  136  is low. In addition, the value of the current flowing into the second semiconductor switch  132  is low. In addition, the majority of the current generated by the second source circuit  137  flows into the sensing circuit  120 , not the second semiconductor switch  132 . Thus, the majority of the current generated by the first source circuit  136  and the second source circuit  137  flows into the second amplifying circuit  112 . 
     The bias current of the first amplifying circuit  111  may be independent of the bias current of the second amplifying circuit  112 . For example, the bias current supplied to the first amplifying circuit  111  may have a constant value. 
       FIG. 4  is a diagram illustrating an operation of limiting a current in the power amplifier  100  of  FIG. 2 , according to an embodiment. Referring to  FIG. 4 , a flow of a current in an overall circuit diagram of the power amplifier is illustrated in a circumstance in which the over-current flows in the amplifier  110 . Here, a region in which a current of a relatively high value flows is indicated by an arrow. Hereinafter, a description will be provided on the basis of the arrow. 
     When the over-current flows in the second amplifying circuit  112 , a value of a bias current of the second amplifying circuit  112  is relatively high, and a bias voltage of the base terminal of the second amplifying circuit  112  is relatively low. In addition, a voltage dropped by the first resistor  121  and the second resistor  122  is high. In addition, since a voltage across the diode  123  is low, a current flowing in the diode  123  is low. That is, since the current flowing in the sensing circuit  120  is low, the majority of the current generated by the first source circuit  136  flows into the first semiconductor switch  131 , not the sensing circuit  120 . In addition, the value of the current flowing into the second semiconductor switch  132  is high. In addition, the majority of the current generated by the second source circuit  137  flows into the second semiconductor switch  132 , not the sensing circuit  120 . 
     Thus, the majority of the current generated by the first source circuit  136  and the second source circuit  137  flows into the ground, rather than the second amplifier  112 . As a result, since a value of the current supplied to the second amplifying circuit  112  decreases, the second amplifying circuit  112  reaches a current limit. 
       FIG. 5  is a conceptual view illustrating a power amplifier, according to another embodiment. Referring to  FIG. 5 , the power amplifier  200  includes a first amplifier  211 , a second amplifier  212 , a sensing circuit  220 , and a biasing circuit  230 . 
     The first amplifier  211  amplifies an input signal. The input signal is input through an input terminal RF in. For example, the first amplifier  211  is a driving amplifier DA amplifying a low signal to have an amplitude within an amplification range of the second amplifier  212 . 
     In a case in which a value of a bias current of the first amplifier  211  decreases, a gain of the first amplifier  211  or energy of an output signal deceases. As a result, energy of a signal input to the second amplifier  212  decreases, and a value of a current flowing in an output terminal of the second amplifier  212  also decreases. 
     The second amplifier  212  is connected to the first amplifier  211  and amplifies the signal amplified by the first amplifier  211 . The signal amplified by the second amplifier  211  is output through an output terminal RF out. For example, since the signal amplified by the second amplifier  212  is a high signal having a level of a few watts (W), an over-current may flow in the second amplifier  212 . The power amplifier  200  reduces a frequency at which the over-current flows in the second amplifier  212 . 
     The second amplifier  212  includes at least one bipolar junction transistor (BJT) amplifying a signal input to a base terminal and outputting the amplified signal to a collector terminal. As a result, the sensing circuit  220  senses a bias current of the base terminal of the bipolar junction transistor. 
     The sensing circuit  220  is connected to the second amplifier  212  to sense a bias of the second amplifier  212 . For example, the sensing circuit  220  includes semiconductor elements such as a variable resistor, a diode, a transistor, and the like, of which a resistance value changes depending on a current or a voltage. Specifically, the sensing circuit  220  includes semiconductor elements such as a diode, a transistor, and the like, of which a flowing current changes depending on a bias current or a bias voltage of the second amplifier  212 . 
     The biasing circuit  230  is connected to the sensing circuit  220  and the first amplifier  211 , and may provide the current to the first amplifier  211  to bias the first amplifier  211 . Here, the biasing means that a current or voltage of a specific value is provided so that a bias current flows in a specific block or element or a bias voltage is applied to the specific block or element. 
     For example, as a value of the bias current of the second amplifier  212  increases, the biasing circuit  230  decreases the value of the current provided to the first amplifier  211 . That is, a current or voltage distribution of the biasing circuit  230  changes depending on a state of the sensing circuit  220 . 
     As a result, the power amplifier  200  breaks an over-current of an allowable value or more so as not to flow the over-current in the power amplifier, thereby reducing the possibility of damage to elements. 
       FIG. 6  is a circuit diagram illustrating the power amplifier of  FIG. 5 , according to an embodiment. Referring to  FIG. 6 , the sensing circuit  220  includes a first resistor  221 , a second resistor  222 , and a diode  223 . 
     The first resistor  221  is connected between the second amplifier  212  and the diode  223 . For example, the current flowing in the base terminal of the second amplifier  212  also flows in the second resistor  222 . A voltage corresponding to a product of a resistance value of the first resistor  221  and a current flowing in the first resistor  221  is applied across the first resistor  221 . 
     The second resistor  222  is connected between the diode  223  and the biasing circuit  230 . A voltage corresponding to a product of a resistance value of the second resistor  222  and a current flowing in the second resistor  222  is applied across the second resistor  222 . 
     A current based on the bias of the second amplifier  212  flows in the diode  223 . For example, in a case in which a voltage across the diode  223  is lower than a threshold voltage, the current flowing in the diode  223  is low. For example, in a case in which the voltage across the diode  223  is higher than the threshold voltage, the current flowing in the diode  223  increases as a voltage across a plurality of terminals increases. That is, the current flowing in the diode  223  changes based on a direct current (DC) voltage determined by currents flowing in the first resistor  221 , the second resistor  222 , and the sensing circuit  220 . 
     Meanwhile, the diode  223  may be implemented as a transistor of which a base terminal and a collector terminal are connected to each other, and may also be implemented as a field effect transistor of which a gate terminal and a drain terminal are connected to each other. Thus, the diode  223  is not limited to a diode element formed by a single p-n junction. 
     A value of the current flowing in the sensing circuit  220  is adjusted depending on the resistance value of the first resistor  221  or the second resistor  222 . A detailed description of the adjustment of the value of the current flowing in the sensing circuit  220  will be provided below with reference to  FIG. 9 . 
     Referring to  FIG. 6 , the biasing circuit  230  includes a first semiconductor switch  231 , a second semiconductor switch  232 , a first source circuit  236 , and a second source circuit  237 . The first semiconductor switch  231  is connected between the first source circuit  236  and a ground, and receives a current flowing into the ground from the first source circuit  236  at a base terminal and outputs the received current to an emitter terminal. 
     The second semiconductor switch  232  is connected between the second source circuit  237  and the ground, and receives a current flowing into the ground from the second source circuit  237  at a collector terminal and outputs the received current to an emitter terminal. 
     Here, a value of the current flowing in the base terminal of the second semiconductor switch  232  is determined based on a value of the current flowing in the collector terminal of the first semiconductor switch  231 . Specifically, the current flowing in the collector terminal of the first semiconductor switch  231  and the current flowing in the base terminal of the second semiconductor switch  232  have a proportional relationship. 
     The first source circuit  236  generates a first reference current and provides the first reference current to the sensing circuit  220  and the ground. Here, as a value of a bias current sensed by the sensing circuit  220  is high, a value of a current flowing into the ground from the first source circuit  236  decreases. 
     The second source circuit  237  generates a second reference current and provides the second reference current to the sensing circuit  220  and the ground. As the value of the current flowing into the ground from the first source circuit  236  increases, the current flowing into the ground from the second source circuit  237  increases. 
     Specifically, the value of the current flowing into the ground from the first source circuit  236  and a base current of the first semiconductor switch  231  are proportional to each other. The base current of the first semiconductor switch  231  are proportional to a collector current of the first semiconductor switch  231 . The collector current of the first semiconductor switch  231  and a base current of the second semiconductor switch  232  are proportional to each other. The base current of the second semiconductor switch  232  and a collector current of the second semiconductor switch  232  are proportional to each other. 
     Thus, as the current flowing into the ground from the first source circuit  236  increases, the current flowing into the ground from the second source circuit  237  increases. Increasing the current flowing into the ground from the source circuit means that a value obtained by dividing the reference current by the current provided to the sensing circuit  220  decreases. As a result, a current flowing into the second amplifier  212  is reduced, and an occurrence frequency of the over-current is reduced. 
       FIG. 7  is a diagram illustrating an operation of the power amplifier of  FIG. 6 , according to an embodiment. 
     Referring to  FIG. 7 , a flow of a current in an overall circuit diagram of the power amplifier is illustrated in a circumstance in which the over-current does not flow in the second amplifier  212 . 
     For example, when the over-current does not flow in the second amplifier  212 , a voltage of a base terminal of the second amplifier  212  is 1.25V, and a threshold voltage of the diode  223  is 1.3V. As a result, the diode  223  is in an off-state. A region in which a current of a relatively high value flows is indicated by an arrow. Hereinafter, a description will be provided on the basis of the arrow. 
     When the over-current does not flow in the second amplifier  212 , a value of a bias current of the second amplifier  212  is relatively small, and a bias voltage of the base terminal of the second amplifier  212  is relatively high. In addition, a voltage dropped by the first resistor  221  and the second resistor  222  is low. In addition, since a voltage across the diode  223  is high, a current flowing in the diode  223  is high. That is, since the current flowing in the sensing circuit  220  is high, the majority of the current generated by the first source circuit  236  flows into the sensing circuit  220 . In addition, the value of the current flowing into the first semiconductor switch  231  from the first source circuit  236  is low. In addition, the value of the current flowing into the second semiconductor switch  232  is low. In addition, the majority of the current generated by the second source circuit  237  flows into the first amplifier  211 , not the second semiconductor switch  232 . 
     Thus, the majority of the current generated by the first source circuit  236  flows into the second amplifier  212  and the majority of the current generated by the second source circuit  237  flows into the first amplifier  211 . 
     An overall bias current of the second amplifier  212  is supplemented through an additional source circuit  238  as well as the first source circuit  236 . For example, energy of an output signal of the second amplifier  212  is higher than the energy output of an output signal of the first amplifier  211 . As a result, a value of the bias current of the second amplifier  212  is higher than a value of the bias current of the first amplifier  211 . Thus, the second amplifier  212  is also supplied with the bias current through the additional source circuit  238 . Here, the additional source circuit  238  may supply a current having a constant value, or a variable current according to an embodiment described above with reference to  FIGS. 1 through 4 . 
       FIG. 8  is a diagram illustrating an operation of limiting a current in the power amplifier  200  of  FIG. 6 , according to an embodiment. Referring to  FIG. 8 , a flow of a current in an overall circuit diagram of the power amplifier is illustrated in a circumstance in which the over-current flows in the second amplifier  212 . 
     For example, when the over-current of 1.5 A or more flows in the second amplifier  212 , a voltage of a base terminal of the second amplifier  212  decreases to 1V or less. As a result, the diode  223  is in an off-state. A region in which a current of a relatively high value flows is indicated by an arrow. Hereinafter, a description will be provided on the basis of the arrow. 
     When the over-current flows in the second amplifier  212 , a value of a bias current of the second amplifier  212  is relatively high, and a bias voltage of the base terminal of the second amplifier  212  is relatively low. In addition, a voltage dropped by the first resistor  221  and the second resistor  222  is high. In addition, since a voltage across the diode  223  is low, a current flowing in the diode  223  may be low. That is, since the current flowing in the sensing circuit  220  is low, the majority of the current generated by the first source circuit  236  flows into the first semiconductor switch  231 , not the sensing circuit  220 . In addition, the value of the current flowing into the second semiconductor switch  232  is high. Further, the majority of the current generated by the second source circuit  237  flows into the second semiconductor switch  232 , not the first amplifier  211 . 
     Thus, the majority of the current generated by the first source circuit  236  flows into the ground, rather than the second amplifier  212 . Here, in a case in which a value of a bias current of the first amplifier  211  decreases, a gain of the first amplifier  211  or energy of an output signal decreases. As a result, energy of a signal input to the second amplifier  212  decreases, and a value of a current flowing in an output terminal of the second amplifier  212  also decreases. As a result, the second amplifier  212  becomes a current limiter. 
       FIG. 9  is a graph illustrating a current flowing in an amplifier depending on a resistance value of a first resistor or a second resistor included in the power amplifier, according to an embodiment described herein. 
     Referring to  FIG. 9 , the horizontal axis denotes power of an input signal, and the vertical axis denotes a current flowing in the amplifier. At a load of the amplifier, a voltage standing wave ratio (VSWR) is assumed to be 10:1. Four curves illustrated in the graph illustrate a case in which the resistance value of the first resistor or the second resistor is very small (Oohm, 1 kohm, 1.5 kohm, and 4 kohm, respectively). 
     It can be confirmed from the graph that a maximum current flowing in the amplifier is varied depending on the resistance value of the first resistor or the second resistor. For example, in a case in which a design of the power amplifier having a maximum current of 2.5 A or less flowing in an output terminal is required, the power amplifier may be designed so that the resistance value of the first resistor or the second resistor (or a summation of the resistance value of the first resistor and the resistance value of the second resistor) is 1.5 kohm. 
     That is, the maximum current of the power amplifier may be precisely adjusted depending on the resistance value of the first resistor or the second resistor. As a result, the power amplifier according to an embodiment disclosed herein precisely controls the maximum current and is designed to be optimized for a maximum current standard or a communications standard of internal elements. 
     Hereinafter, a method for limiting a current in a power amplifier, according to an embodiment, will be described. Since the method for limiting the current is performed by the power amplifier  100  described above with reference to  FIG. 1  or the power amplifier  200  described above with reference to  FIG. 5 , a description of features that are the same as or correspond to those described above will be omitted. 
       FIG. 10  is a flow chart illustrating a method for limiting a current in a power amplifier  100  or  200 , according to an embodiment. Referring to  FIG. 10 , the method for limiting the current includes an operation of providing a current (S 10 ), an operation of sensing the current (S 20 ), and an operation of distributing the current (S 30 ). 
     For example, the method for limiting the current may be autonomously performed by an internal control circuit of the power amplifier, or may be performed by an external control circuit. 
     In the operation of providing a current (S 10 ), the power amplifier is provided with a current for operation of the power amplifier. 
     In the operation of sensing the current (S 20 ), when the power amplifier is operated, the power amplifier senses a current flowing in the power amplifier. 
     In the operation of distributing the current (S 30 ), in a case in which a value of the current flowing in the power amplifier is higher than a preset value, the power amplifier distributes the current so that a portion of the current provided to the power amplifier is dropped, using a sensing circuit sensing a bias of the power amplifier. 
     The preset value may be a maximum current standard of the power amplifier or an internal element of the power amplifier. For example, the preset value may be adjusted by adjusting the resistance value of the sensing circuit as described above with reference to  FIG. 9 . Further, the sink (dropping of the portion of the current provided to the power amplifier) means that the current leaks to other blocks, external elements, a ground, or the like, rather than the power amplifier. 
     According to the method for limiting the current, since the power amplifier reduces an occurrence frequency of an over-current, the possibility of damage to elements of the power amplifier is reduced. 
       FIG. 11  is a flow chart illustrating the method for limiting the current of  FIG. 10  in greater detail, according to an embodiment. 
     Referring to  FIG. 11 , a control circuit controlling the power amplifier provides the current to a first amplifier and a second amplifier to operate the power amplifier (operation S 11 ) and senses a current flowing in the second amplifier (operation S 21 ). 
     In a case in which the current flowing in the second amplifier is higher than a preset current (operation S 22 ), a current flowing in the sensing circuit is decreased (operation S 31 ), a first semiconductor switch is in an on-state (operation S 32 ), a second semiconductor switch is in the on-state (operation S 33 ), and a portion of a current flowing in the first amplifier flows into the ground (operation S 34 ). 
     In a case in which the current flowing in the second amplifier is smaller than the preset current (operation S 22 ), the current flowing in the sensing circuit is increased (operation S 35 ), the first semiconductor switch is in an off-state (operation S 36 ), and the second semiconductor switch is in the off-state (operation S 37 ). 
     As set forth above, according to the embodiments disclosed herein, the power amplifier breaks the over-current of the allowable value or more so as not to flow the over-current in the power amplifier, thereby reducing possibility of damage to elements in the power amplifier. 
     Further, the power amplifier precisely controls the maximum current and is designed to be optimized for the maximum current standard or the communications standard of the internal elements of the power amplifier. 
     While this disclosure includes specific examples, it will be apparent to one of ordinary skill in the art that various changes in form and details may be made in these examples without departing from the spirit and scope of the claims and their equivalents. The examples described herein are to be considered in a descriptive sense only, and not for purposes of limitation. Descriptions of features or aspects in each example are to be considered as being applicable to similar features or aspects in other examples. Suitable results may be achieved if the described techniques are performed in a different order, and/or if components in a described system, architecture, device, or circuit are combined in a different manner, and/or replaced or supplemented by other components or their equivalents. Therefore, the scope of the disclosure is defined not by the detailed description, but by the claims and their equivalents, and all variations within the scope of the claims and their equivalents are to be construed as being included in the disclosure.