Abstract:
A power amplifier circuit disclosed herein comprises an amplifying part transistor including a bipolar transistor to which a first supply voltage is supplied as a driving voltage and which amplifies an input signal inputted to a base of the amplifying part transistor so as to output the input signal; a bias current generating circuit which generates a bias current for biasing the base of the amplifying part transistor and supplies the bias current to the base of the amplifying part transistor when the amplifying part transistor performs an amplification operation; and an additional bias current generating circuit which supplies an additional bias current to the base of the amplifying part transistor in addition to the bias current according to the first supply voltage.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims benefit of priority under 35 U.S.C.§119 to Japanese Patent Application No. 2003-24579, filed on Jan. 31, 2003, the entire contents of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power amplifier circuit which amplifies an inputted input signal and outputs it as an output signal. 
     2. Description of the Related Art 
     FIG. 11 is a diagram showing the configuration of a related power amplifier circuit  5  using bipolar transistors and its peripheral circuit, and FIG. 12 is a diagram showing the static characteristic of an amplifying part transistor Q in FIG.  11 . 
     As shown in FIG. 11, a voltage of 3.6 V, for example, is supplied from a power supply  10  to a DC/DC converter  20 . The DC/DC converter  20  converts the supplied voltage into, for example, 1.5 V and supplies it to a collector of the amplifying part transistor Q in the power amplifier circuit  5 . The amplifying part transistor Q is composed of an NPN bipolar transistor. The voltage which is supplied by the DC/DC converter  20  is variable, and in some cases a high voltage (3.6 V, for example) is supplied, while in other cases a low voltage (1.5 V, for example) is supplied. 
     A high-frequency signal current Isg is supplied to a base of the amplifying part transistor Q from a signal generating circuit  30  via a capacitive element C 1 , and a bias current Ibias from a bias circuit  40  is also supplied thereto. Namely, a base current Ib=Ibias+Isg is supplied to the base. An emitter of the amplifying part transistor Q is grounded. 
     The bias circuit  40  includes NPN bipolar transistors Q 1  to Q 4  and a resistance R 1 . The bipolar transistors Q 1  and Q 2  and the resistance R 1  constitute a switching control circuit  50 , and the bipolar transistors Q 3  and Q 4  constitute a bias current generating circuit  60 . 
     As concerns a control voltage Vcon to be supplied to the switching control circuit  50 , a voltage (3.6 V, for example) is supplied when the power amplifier circuit  5  is on, and no voltage (namely, 0 V) is supplied when it is off. In the bias current generating circuit  60 , the bipolar transistor Q 3  is turned on/off according to the on/off of the control voltage Vcon, and when it is on, the bias current Ibias is supplied from a reference voltage Vref to the base of the amplifying part transistor Q, and when it is off, the bias current Ibias is not supplied. 
     The switching control circuit  50  changes the amount of the bias current Ibias flowing through the bipolar transistor Q 3  by changing the amount of a control current Icon to be supplied to a base of the bipolar transistor Q 3  according to its ambient temperature (ambient temperature of the bipolar transistors Q 1  and Q 2 ). Hence, the switching control circuit  50  prevents thermal runaway of the power amplifier circuit. In addition, the bipolar transistors Q 1  and Q 2  also monitor the ambient temperature of the amplifying part transistor Q. 
     An output node N 0  of this power amplifier circuit is provided on the collector side of amplifying part transistor Q and outputs a voltage output OUT. A load impedance ZL is connected to the output node N 0 . 
     As shown in FIG. 12, the amount of a current Ic flowing through the amplifying part transistor Q is determined by the base current Ib (Ib 1 &lt;Ib 2 &lt;Ib 3 &lt;Ib 4 &lt;Ib 5 &lt;Ib 6 ) which flows into the base of the amplifying part transistor Q. As stated above, the base current Ib is the sum of the bias current Ibias from the bias circuit  40  and the signal current Isg from the signal generating circuit  30 . The bias current Ibias is constant unless temperature changes, and hence the voltage of the voltage output OUT is determined by the high-frequency signal current Isg from the signal generating circuit  30 . 
     Incidentally, as shown in FIG. 12, from the characteristics of the bipolar transistor, an amplitude of the voltage output OUT can be widely at high output levels at which the voltage Vc is high, while the amplitude of the voltage output OUT can be only narrowly at low out levels at which the voltage Vc is low. 
     However, as can be seen from FIG.  11  and FIG. 12, even if the voltage Vc is reduced at low output levels, the amount of the current Ic flowing through the amplifying part transistor Q is unchanged. Since power consumption (DC loss) is determined by voltage Vc×current Ic, power consumption is reduced only through a fall in the voltage Vc. 
     When the voltage Vc is at low output levels, however, it does not matter if the amplitude of the voltage output OUT is small, and hence if it is possible to decrease the amount of the current Ic and thereby reduce power consumption, it is more desirable. 
     SUMMARY OF THE INVENTION 
     In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, a power amplifier circuit, comprises: 
     an amplifying part transistor including a bipolar transistor to which a first supply voltage is supplied as a driving voltage and which amplifies an input signal inputted to a base of the amplifying part transistor so as to output the input signal; 
     a bias current generating circuit which generates a bias current for biasing the base of the amplifying part transistor and supplies the bias current to the base of the amplifying part transistor when the amplifying part transistor performs an amplification operation; and 
     an additional bias current generating circuit which supplies an additional bias current to the base of the amplifying part transistor in addition to the bias current according to the first supply voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the configuration of a power amplifier circuit according to a first embodiment and its peripheral circuit; 
     FIG. 2 is a graph showing the static characteristic of an amplifying part transistor in the power amplifier circuit according to the first embodiment; 
     FIG. 3 is a graph showing the static characteristic of the amplifying part transistor in the power amplifier circuit according to the first embodiment and a load line of a voltage output during a high-frequency operation (at high output levels); 
     FIG. 4 is a graph showing the static characteristic of the amplifying part transistor in the power amplifier circuit according to the first embodiment and the load line of the voltage output during the high-frequency operation (at low output levels); 
     FIG. 5A is a diagram showing the configuration of a power amplifier circuit according to a second embodiment and its peripheral circuit; 
     FIG. 5B shows graphs showing the relation between an additional bias current and Vceon(Q 5 ) when the Vbeon(Q 5 ) is constant and the relation between the additional bias current and Vbeon(Q 6 )+Iad×R 2 ; 
     FIG. 6 is a graph showing the static characteristic of an amplifying part transistor in the power amplifier circuit according to the second embodiment; 
     FIG. 7 is a diagram showing a modification of the power amplifier circuit according to the first embodiment; 
     FIG. 8 is a diagram showing a modification of the power amplifier circuit according to the second embodiment; 
     FIG. 9 is a diagram showing another modification of the power amplifier circuit according to the first embodiment; 
     FIG. 10 is a diagram showing another modification of the power amplifier circuit according to the second embodiment; 
     FIG. 11 is a diagram showing the configuration of a related power amplifier circuit and its peripheral circuit; and 
     FIG. 12 is a graph showing the static characteristic of an amplifying part transistor in the related power amplifier circuit. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     First Embodiment 
     In the first embodiment, a reduction in power consumption at low output levels is realized by making the amount of a base current to be supplied to an amplifying part transistor in a power amplifier circuit at low output levels smaller than that at high output levels to thereby reduce a current flowing from a collector to an emitter of the amplifying part transistor. Further details will be given below. 
     FIG. 1 is a diagram showing the configuration of a power amplifier circuit  100  according to this embodiment and its peripheral circuit. As can be seen from FIG. 1, the power amplifier circuit  100  according to this embodiment is configured by adding NPN bipolar transistors Q 5  and Q 6  to the power amplifier circuit  5  in FIG. 11 described above. 
     More specifically, the bipolar transistor Q 5  and the bipolar transistor Q 6  are connected in series to constitute an additional bias current generating circuit  100  in this embodiment. 
     A voltage Vc 1  is supplied to a collector of the bipolar transistor Q 6  from a DC/DC converter  20 . In this embodiment, this voltage Vc 1  is equivalent to a voltage Vc 2  which is supplied to a collector of an amplifying part transistor Q. However, a node which supplies the voltage Vc 1  and a node which supplies the voltage Vc 2  are short-circuited in a high frequency manner so as not to be affected by each other in the DC/DC converter  20 . 
     Moreover, to the collector of the bipolar transistor Q 6 , its own base is connected. In other words, the bipolar transistor Q 6  functions as a diode. An emitter of this bipolar transistor Q 6  is connected to a collector of the bipolar transistor Q 5 . 
     A base of the bipolar transistor Q 5  is connected to a collector of a bipolar transistor Q 1 . An emitter of the bipolar transistor Q 5  is connected to a base of the amplifying part transistor Q. Accordingly, an additional bias current Iad which flows into the base of the amplifying part transistor Q through the bipolar transistor Q 5  is controlled by the amount of a control current Icon which flows into the base of the bipolar transistor Q 5  from the collector of the bipolar transistor Q 1 . 
     Similarly to FIG. 11 described above, a switching control circuit  50  includes a resistance R 1  and bipolar transistors Q 1  and Q 2 . A control voltage Vcon is supplied to one end of the resistance R 1 , and the other end of the resistance R 1  is connected to the collector of the bipolar transistor Q 1 . To the collector of the bipolar transistor Q 1 , its own base is also connected. In other words, the bipolar transistor Q 1  functions as a diode. An emitter of the bipolar transistor Q 1  is connected to a collector of the bipolar transistor Q 2 . 
     To the collector of the bipolar transistor Q 2 , its own base is also connected. Namely, the bipolar transistor Q 2  functions as a diode. An emitter of the bipolar transistor Q 2  is connected to a ground. As can be seen from the above, the switching control circuit  50  has a configuration in which two diodes (Q 1 , Q 2 ) are connected in series to one end of the resistance R 1 . 
     Similarly to FIG. 11 described above, a bias current generating circuit  60  includes bipolar transistors Q 3  and Q 4 . A reference voltage Vref is supplied to a collector of the bipolar transistor Q 3 . A base of the bipolar transistor Q 3  is connected to the collector of the bipolar transistor Q 1 , and an emitter of the bipolar transistor Q 3  is connected to a collector of the bipolar transistor Q 4  and the base of the amplifying part transistor Q. Accordingly, a bias current Ibias which flows into the base of the amplifying part transistor Q through the bipolar transistor Q 3  is controlled by the amount of the control current Icon which flows into the base of the bipolar transistor Q 3  from the collector of the bipolar transistor Q 1 . 
     To the collector of the bipolar transistor Q 4 , its own base is also connected. In other words, the bipolar transistor Q 4  functions as a diode. An emitter of this bipolar transistor Q 4  is connected to a ground. 
     The bias current which flows into the base of the amplifying part transistor Q from a point between the bipolar transistor Q 3  and the bipolar transistor Q 4  is Ibias, the additional bias current which flows into the base of the amplifying part transistor Q from the bipolar transistor Q 5  is Iad, and a signal current which flows into the base of the amplifying part transistor Q from a signal generating circuit  30  is Isg, whereby the whole base current Ib flowing into the base of the amplifying part transistor Q is expressed as follows. 
       Ib=Ibias+Iad+Isg   
     It is noted that the high-frequency signal current Isg is inputted to the base of the amplifying part transistor Q but the signal current Isg is not inputted to the additional bias current generating circuit  110  or the bias current generating circuit  60 . Moreover, the signal generating circuit  30  and the power amplifier circuit  100  are isolated from each other in a direct current manner but they are short-circuited in a high frequency manner. 
     The configuration of the power amplifier circuit  100  has been described above, and next the operation of this power amplifier circuit  100  will be explained. 
     The control voltage Vcon to be supplied to the switching control circuit  50  is a predetermined voltage (for example, 3.6 V) when the power amplifier circuit  100  is operated, while it is 0 V when the power amplifier circuit  100  is not operated, that is, it is off. Since the bipolar transistors Q 3  and Q 5  are turned off when the control voltage Vcon is 0 V, the amplifying part transistor Q is also turned off, and thereby the power amplifier circuit  100  does not operate. Consequently, power consumption of the power amplifier circuit  100  on standby can be reduced. The standby means a state that a predetermined voltage (for example, 3.6 V) is applied to the control voltage Vcon and the signal current Isg is not supplied from the signal generating circuit  30 . 
     On the other hand, when the control voltage Vcon is the predetermined voltage (for example, 3.6 V), the bipolar transistors Q 3  and Q 5  are turned on, and the bias current Ibias and the additional bias current Iad are supplied to the base of the amplifying part transistor Q. 
     The amount of the current flowing through the bipolar transistors Q 1  and Q 2  constituting the diodes of the switching control circuit  50  changes depending on the ambient temperature of the bipolar transistors Q 1  and Q 2 . In addition, the bipolar transistors Q 1  and Q 2  also monitor the ambient temperature of the amplifying part transistor Q. Namely, the flowing current increases if the ambient temperature rises, while the flowing current reduces when the ambient temperature falls. Hence, the voltage of a node N 1  changes according to the temperature. In other words, if the temperature rises, the current flowing through the resistance R 1  increases, whereby voltage drop in the resistance R 1  increases, resulting in a fall in the voltage of the node N 1 . Consequently, the control current Icon which flows into the bases of the bipolar transistors Q 3  and Q 5  reduces. Hence, if the ambient temperature rises, the bias current Ibias and the additional bias current Iad reduce, and thereby the base current Ib which flows into the base of the amplifying part transistor Q also reduces. 
     On the other hand, if the temperature falls, the current flowing through the resistance R 1  reduces, whereby the voltage drop in the resistance R 1  reduces. Thereby the voltage of the node N 1  rises, and the control current Icon which flows into the bases of the bipolar transistors Q 3  and Q 5  increases. Hence, if the ambient temperature falls, the bias current Ibias and the additional bias current Iad increase, and the base current Ib which flows into the base of the amplifying part transistor Q also increases. In other words, the switching control circuit  50  has a temperature compensation function. However, this temperature compensation function is not always necessary, and it is also possible to omit it. In this case, the bipolar transistors Q 1  and Q 2  become unnecessary. 
     The bias current generating circuit  60  supplies the bias current Ibias to the base of the amplifying part transistor Q according to the control current Icon which flows into the base of the bipolar transistor Q 3 . 
     The additional bias current generating circuit  110  supplies the additional bias current Iad to the base of the amplifying part transistor Q according to the control current Icon which flows into the base of the bipolar transistor Q 5 . In this embodiment, in particular, the additional bias current generating circuit  110  is configured in such a manner to supply the additional bias current Iad when the voltage Vc 1  supplied from the DC/DC converter  20  is high (3.6 V, for example) and not to supply the additional bias current Iad when the voltage Vc 1  is low (1.5 V, for example). To realize this, the additional bias current generating circuit  110  according to this embodiment satisfies the following conditions. 
     If in each bipolar transistor, the base-emitter on-voltage which is an on-voltage between the base and the emitter is taken as Vbeon, and the collector-emitter on-voltage which is an on-voltage between the collector and the emitter is taken an Vceon, it is recommended to set the voltage Vc 1  as follows. 
     At High Output Levels (When the Voltage Vc 1  is High) 
     The base(B)-emitter(E) on-voltage of the amplifying part transistor Q is taken as Vbeon(Q), the collector(C)-emitter(E) on-voltage of the bipolar transistor Q 5  is taken as Vceon(Q 5 ), and the base(B)-emitter(E) on-voltage of the bipolar transistor Q 6  is taken as Vbeon(Q 6 ). In this case, the additional bias current Iad can be supplied by satisfying the following condition. 
     
       
         Vc 1 &gt;Vbeon( Q )+Vceon( Q   5 )+Vbeon( Q   6 ) 
       
     
     Consequently, the additional bias current Iad can be supplied to the base of the amplifying part transistor Q, and thus the bias point of the amplifying part transistor Q can be set high. 
     At Low Output Levels (When the Voltage Vc 1  is Low) 
     At low output levels, contrary to the aforementioned high output levels, it becomes possible not to supply the additional bias current Iad by satisfying the following condition. 
     
       
         Vc 1 &lt;Vbeon( Q )+Vceon( Q   5 )+Vbeon( Q   6 ) 
       
     
     Consequently, it becomes possible not to supply the additional bias current Iad to the base of the amplifying part transistor Q, and thus the bias point of the amplifying part transistor Q can be set low. 
     FIG. 2 is a graph showing the static characteristic of the amplifying part transistor Q according to this embodiment. In FIG. 2, voltage Vc 1 =voltage Vc 2 =voltage Vc is a precondition as stated above. As shown in FIG. 2, according to this embodiment, the bias point of the amplifying part transistor Q at low output levels is lower than the bias point of the amplifying part transistor Q at high output levels. Therefore, the amount of the current Ic which flows through the amplifying part transistor Q from its collector to its emitter at low output levels becomes smaller, whereby power consumption which is Ic×Vc can be reduced. 
     FIG. 3 is a graph showing the high-frequency static characteristic of the amplifying part transistor Q at high output levels, and FIG. 4 is a graph showing the high-frequency static characteristic of the amplifying part transistor Q at low output levels. 
     As shown in FIG. 3, at high output levels, the bias point is high, a current output ic with large amplitude is obtained, where the ic is a current flowing the collector side of the amplifying part transistor Q. Therefore, a voltage output OUT with large amplitude is obtained. Namely, the voltage output OUT forms large amplitude according to the amplitude of the signal current Isg from the signal generating circuit  30 . Contrary to this, at low output levels, the bias point is low as shown in FIG. 4, the current output ic with small amplitude is obtained so that the voltage output OUT with small amplitude is obtained. Namely, the amplitude of the voltage output OUT at low output levels becomes smaller than the amplitude of the voltage output OUT at high output levels. In the power amplifier circuit  100  according to this embodiment, the amplitude of the voltage output OUT at low output levels is small as described above, and therefore, even if the bias point is lowered, no problem occurs in terms of its operation. 
     Second Embodiment 
     In the second embodiment, by inserting a resistance on the collector side of the bipolar transistor Q 6 , the current Ic shows a linear slope with respect to the voltage Vc 1  in the region of Vc 1 &gt;Vbeon(Q)+Vceon(Q 5 )+Vbeon(Q 6 ) in the aforementioned first embodiment. Further details will be given below. 
     FIG. 5A is a diagram showing the configuration of a power amplifier circuit  200  according to this embodiment and its peripheral circuit, and FIG. 5A corresponds to FIG. 1 described above. As shown in FIG. 5A, in the power amplifier circuit  200  according to this embodiment, a resistance R 2  is additionally inserted between the collector of the bipolar transistor Q 6  and the DC/DC converter  20 . Therefore, the voltage Vc 1  is supplied from the DC/DC converter  20  to one end of the resistance R 2  and thereafter supplied from the other end of the resistance R 2  to the collector of the bipolar transistor Q 6 . The configuration other than this is the same as that in the aforementioned first embodiment. 
     The voltage Vc 1  in this embodiment can be expressed as follows. 
     
       
         Vc 1 =Vbeon( Q )+Vceon( Q   5 )+Vbeon( Q   6 )+Iad×R 2   
       
     
     Moreover, it is defined that if the base(B)-emitter(E) on-voltage of the bipolar transistor Q 5  is taken as Vbeon(Q 5 ), this Vbeon(Q 5 ) is determined by the switching control circuit  50  and it is constant. A graph showing the relation between the additional bias current Iad and Vceon(Q 5 ) in this case and a graph showing the relation between the additional bias current Iad and Vbeon(Q 6 )+Iad×R 2  are shown in FIG.  5 B. 
     Referring to this FIG. 5B, the static characteristic of the bipolar transistor Q 5  is considered divided into three regions: a region A (The additional bias current Iad is almost 0); a region B (saturation region); and a region C (linear region). 
     In the region A, the additional bias current Iad is almost 0, whereby the bias current of the amplifying part transistor Q is determined by the bias current generating circuit  60 . Hence, the bias point of the amplifying part transistor Q is low. 
     In the region B, Vceon(Q 5 ) is determined according to the amount of the flowing additional bias current Iad, and thereby Vbeon(Q 6 )+Iad×R 2  is determined. The current flowing through the bipolar transistor Q 5  is herein the additional bias current Iad, and the current flowing through the bipolar transistor Q 6  and the resistance R 2  is also the additional bias current Iad, whereby both are equal to each other. Therefore, from two graphs in FIG. 5B, Vceon(Q 5 ) and Vbeon(Q 6 )+Iad×R 2  can be found. As a result, Vc 1 =Vbeon(Q)+Vceon(Q 5 )+Vbeon(Q 6 )+Iad×R 2  can be calculated. 
     In the region C, since Vbeon(Q 5 ) is constant, the current flowing through the bipolar transistor Q 5  from its collector to its emitter is constant, and in other words, the additional bias current Iad is constant. Since the additional bias current Iad is constant, Vbeon(Q 6 )+Iad×R 2  is also constant. Vceon(Q 5 ) at this time is expressed as follows. 
     
       
         Vceon( Q   5 )=Vc 1 −Vbeon( Q )−Vbeon( Q   6 )− Iad×R   2   
       
     
     FIG. 6 is a graph showing the static characteristic of the amplifying part transistor Q according to this embodiment. Similarly to the aforementioned first embodiment, also in FIG. 6, voltage Vc 1 =voltage Vc 2 =voltage Vc is a precondition. As shown in FIG. 6, also in this embodiment, the bias point of the amplifying part transistor Q at low output levels is lower than the bias point of the amplifying part transistor Q at high output levels. Therefore, the amount of the current Ic which flows through the amplifying part transistor Q from its collector to its emitter at low output levels becomes smaller, whereby power consumption which is Ic×Vc can be reduced. 
     Moreover, owing to the existence of the resistance R 2 , in the region of Vc 1 &gt;Vbeon(Q)+Vceon(Q 5 )+Vbeon(Q 6 ), the current Ic which flows from the collector to the emitter of the amplifying part transistor Q comes to show linearity. Hence, the voltage Vc at which the power amplifier circuit  200  operates can be set between high output levels at which the voltage Vc (=Vc 1 =Vc 2 ) is high and low output levels at which the voltage Vc is low. Namely, a plurality of, for example, three, four, five, voltages Vc can be set as bias points, and thus the plurality of voltages Vc can be set as the operation voltages of the power amplifier circuit  200 . 
     It should be noted that the present invention is not limited to the aforementioned embodiments, and various changes may be made therein. For example, the configuration of the aforementioned switching control circuit  50  with the temperature compensation function is not limited to that in the aforementioned embodiments. For example, as shown in FIG. 7, in place of the switching control circuit  50  in the first embodiment, a switching control circuit  54  including a bipolar transistor Q 11  which is connected to the bipolar transistor Q 4  to form a current mirror is also suitable. In the switching control circuit  54  configured as shown in FIG. 7, the temperature compensation function is realized by the bipolar transistor Q 11 . 
     The above also applies to the aforementioned second embodiment. If the configuration of the switching control circuit  54  is applied to the power amplifier circuit  200  according to the second embodiment, such a circuit configuration as shown in FIG. 8 is obtained. 
     Moreover, the number of the bipolar transistors Q 6  in the aforementioned embodiments is arbitrarily selected depending on the set value of the voltage Vc 1  which switches between high output and low output. Namely, the diode-connected bipolar transistor Q 6  may be omitted or plural ones may be provided. In other words, the number N of the bipolar transistors Q 6  may be 0, one, two, three, ***. When the bipolar transistor Q 6  is omitted, the power amplifier circuit  100  according to the first embodiment has such a configuration as shown in FIG. 9, and the power amplifier circuit  200  according to the second embodiment has such a configuration as shown in FIG.  10 . 
     In other words, the number N of the bipolar transistors Q 6  has only to be 0 or a positive integer. In this case, the operating condition at high output levels (when the voltage Vc 1  is high) and the operating condition at low output levels (when the voltage Vc 1  is low) can be expressed as follows, for example, in the first embodiment. 
     At High Output Levels (When the Voltage Vc 1  is High) 
     
       
         Vc 1 &gt;Vbeon( Q )+Vceon( Q   5 )+Vbeon( Q   6 )× N   
       
     
     At Low Output Levels (When the Voltage Vc 1  is Low) 
     
       
         Vc 1 &lt;Vbeon( Q )+Vceon( Q   5 )+Vbeon( Q   6 )× N