Abstract:
A bias control circuit generates a bias control current that is proportional to temperature. The bias control current is drawn from a first node of a bias circuit. The first node of the bias circuit is also configured to receive a relatively large first current that is also proportional to temperature. A bias current is also drawn from the first node, wherein the bias current is equal to the difference between the relatively large first current and the bias control current. The temperature sensitivities of the bias control current and the relatively large first current are matched, such that the bias control current is relatively insensitive to changes in temperature.

Description:
RELATED APPLICATIONS  
       [0001]    The present invention is a continuation-in-part of Provisional U.S. Patent Application Serial No. 60/412,342, which was filed on Sep. 20, 2002. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a bias circuit that operates relatively independent of operating temperature. More specifically, the present invention relates to a bias circuit for biasing a transmitter amplifier stage, wherein the biasing is substantially independent of the operating temperature.  
           [0004]    2. Related Art  
           [0005]    [0005]FIG. 1 is a block diagram of a portion of a conventional transmitter power amplifier circuit  100 . Transmitter power amplifier circuit  100  is typically used in a cellular telephone handset. Transmitter power amplifier circuit  100  is used to amplify a radio frequency signal for transmission from the handset to a nearby receiving station. Transmitter power amplifier circuit  100  includes a bias circuit  101  and a transmitter amplifier stage  102 . Bias circuit  101  is described in more detail in U.S. Pat. No. 6,441,687.  
           [0006]    Bias circuit  101  includes diode element  110 , resistors  111 - 113 , NPN bipolar transistors  121 - 122 , and nodes  104 - 105 , which are connected as illustrated. Transmitter stage  102  includes resistors  130   1 - 130   N  and NPN bipolar transistors  131   1 - 131   N , which are connected as illustrated.  
           [0007]    In general, transistor  121  operates as a reference device. Bias circuit  101  causes a collector current I 3  to flow through transistor  121 . This collector current I 3  is reflected to transistors  131   1 - 131   N , thereby causing corresponding collector currents I C1 , I C2 , . . . I CN  to flow through these transistors  131   1 - 131   N . A DC voltage (not shown) is applied to the collectors of transistors  131   1 - 131   N  Resistor  111  provides most of the current flowing through transistor  121 . However, some of the current through transistor  121  is supplied by resistor  112  and diode  110 . Resistor  112  and diode  110  form a level-shifter, which provides a voltage to the base of transistor  122 . Transistor  122  operates as an emitter-follower to supply base current to transistors  121  and  131   1 - 131   N . The voltage at the emitter of transistor  122  is provided to the base of transistor  121  through resistor  113 , thereby completing a feedback loop that sets the operating point of bias circuit  101 .  
           [0008]    More specifically, bias circuit  101  operates in the following manner. A supply voltage V CC  (e.g., 3.3 Volts) is applied to the collector of transistor  122 , and a reference voltage V REF  is applied to resistors  111  and  112 . The reference voltage V REF  is typically a regulated voltage (e.g., 2.8 V±0.1 V) received from a constant voltage source (not shown), such as a band-gap referenced voltage regulator.  
           [0009]    The voltage (V 112 ) across resistor  112  is defined as follows:  
             V   112   =V   REF −( V   BE1   +V   BE2 )   (1)  
           [0010]    where V BE1  is equal to the base-to-emitter voltage of transistor  121 , and V BE2  is equal to the base-to-emitter voltage of transistor  122 .  
           [0011]    Resistor  112  has a resistance of R 2 . The current (I 2 ) flowing through resistor  112  is therefore defined as follows:  
             I   2   =V   112   /R   2 =( V   REF −( V   BE1   +V   BE2 ))/ R   2    (2)  
           [0012]    Because resistor  111  is connected in parallel with resistor  112  and diode  110 , the voltage across resistor  111  (V 111 ) is equal to the voltage across resistor  112  (V 112 ) plus the voltage across diode  110  (V D1 ). Resistor  111  has a resistance of R 1 . The current (I 1 ) flowing through resistor  111  can therefore be defined as follows:  
             I   1   =V   111   /R   1    (3)  
             I   1 =( V   112   +V   D1 )/ R   1    (4)  
             I   1 =( V   REF −( V   BE1   +V   BE2 )+ V   D1 )/ R   1    (5)  
           [0013]    Assuming that the base current of transistor  122  is negligible, the collector current (I 3 ) flowing through transistor  121  is equal to I 1 +I 2 . Thus, the collector current I 3  can be defined as follows.  
             I   3 =( V   REF −( V   BE1   +V   BE2 ))/ R   2 +( V   REF −( V   BE1   +V   BE2 )+ V   D1 )/ R   1    (6)  
           [0014]    The bases of transistors  121  and  131   1 - 131   N  are all biased by the voltage (V BIAS ) on node  105 . Thus, the collector current I 3  of transistor  121  is proportional to the collector currents I C1 -I CN  of transistors  131   1 - 131   N . In this manner, bias circuit  101  selects the collector (DC bias) currents in the transistors  131   1 - 131   N  of transmitter stage  102 .  
           [0015]    The bases of transistors  131   1 - 131   N  are also connected to receive radio frequency (RF) input signals IN 1 -IN N , respectively. Transistors  131   1 - 131   N  provide amplified RF output signals OUT 1 -OUT N  in response to the input signals IN 1 -IN N  and the bias voltage (V BIAS ) on node  105 .  
           [0016]    It is desirable for the collector currents in transistors  131   1 - 131   N  to be constant with respect to varying temperature. Variations in these collector currents undesirably result in variations in the power of the output signals OUT 1 -OUT N . In order for the collector currents I C1 -I CN  of transistors  131   1 - 131   N  to be constant with respect to temperature, the collector current I 3  must be constant with respect to temperature. However, as described below, the collector current I 3  is not constant with respect to temperature. The voltage across a PN semiconductor junction (diode) decreases as the temperature of the junction increases. Thus, as the temperature of bias circuit  101  increases, the (junction) voltages V D1 , V BE1  and V BE2  all decrease. As defined by Equation (6), as the voltages V D1 , V BE1  and V BE2  decrease, the collector current I 3  increases. As a result, the collector currents I C1 -I CN  through transistors  131   1 - 131   N  similarly increase. The increased collector currents through transistors  131   1 - 131   N  undesirably change the operating characteristics of transmitter stage  102 . More specifically, the increased collector currents in transistors  131   1 - 131   N  can undesirably lower the power efficiency of transmitter power amplifier stage  102 . Similarly, decreases in temperature will result in decreased collector currents through transistors  131   1 - 131   N , thereby undesirably reducing the power gain of transmitter power amplifier stage  102 .  
           [0017]    Transmitter power amplifier circuit  100  is typically used in cellular telephone handsets, which are typically required to operate within an extreme range of temperatures (e.g., −30° C. to 85° C.). As the temperature changes, the operating characteristics of bias circuit  101  will change, such that the bias voltage for the transmitter circuit will vary, thereby resulting in considerable variations in the output power gain and power efficiency of the transmitter power amplifier circuit  100 .  
           [0018]    It would therefore be desirable to have a bias circuit for a power amplifier stage that is substantially independent of temperature. It would also be desirable to have a bias circuit for a power amplifier stage that allows for a selectable relationship between bias current and temperature.  
         SUMMARY  
         [0019]    Accordingly, the present invention provides an improved bias circuit having a controlled temperature dependence. In one embodiment, the bias circuit includes a first bias circuit configured to provide a first current to a first node in response to a reference voltage (V REF ). The first current is directly related to the temperature of the bias circuit. Thus, as the temperature increases, the first current increases. In one embodiment, the first current is comprised of the combination of two currents flowing through parallel branches of the bias circuit.  
           [0020]    A bias control circuit is configured to draw a control current from the first node in response to the reference voltage (V REF ). Like the first current, the control current is directly related to the temperature of the bias circuit.  
           [0021]    A bias current, which is equal to the difference between the first current and the control current, is drawn from the first node. Because both the first current and the control current are directly related to temperature, the bias current can be made relatively insensitive to variations in temperature. That is, because an increase in temperature results in an increase in both the first current (into the first node) and the control current (out of the first node), the bias current remains relatively unchanged if the variations of the first current and the control current are matched. In other embodiments, the bias current can be controlled to increase or decrease with respect to an increasing temperature.  
           [0022]    The bias circuit is further configured to provide a bias voltage (V BIAS ) on a second node in response to the reference voltage (V REF ). An amplifier stage can be coupled to the second node, such that a current proportional to the bias current flows in the amplifier stage in response to the bias voltage. The amplifier stage can be, for example, a portion of a cellular telephone wireless transmitter handset.  
           [0023]    In accordance with one embodiment, the bias control circuit includes a first transistor having a collector coupled to the first node, and an emitter coupled to ground. A resistive element has a first terminal coupled to receive the reference voltage, and a second terminal coupled to a base of the first transistor. A second bipolar transistor has a collector coupled to the second terminal of the resistive element, an emitter coupled to ground, and a base coupled to the base of the first transistor. A diode element can be coupled between the second terminal of the resistive element and the collector of the second transistor to adjust the temperature sensitivity of the control current.  
           [0024]    The present invention also includes a method that includes the steps of (1) generating a bias control current that increases as temperature increases, wherein the bias control current flows out of a first node of a bias circuit, (2) generating a first current that increases as temperature increases, wherein the first current is greater than the bias control current, and flows into the first node of the bias circuit, (3) providing a bias current from the first node of the bias circuit to a transistor of the bias circuit, wherein the bias current is equal to the difference between the first current and the bias control current.  
           [0025]    The present invention will be more fully understood in view of the following description and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 is a circuit diagram of a portion of a conventional transmitter circuit, including a bias circuit and a transmitter amplifier stage.  
         [0027]    [0027]FIG. 2 is a circuit diagram of a portion of a transmitter circuit in accordance with one embodiment of the present invention.  
         [0028]    [0028]FIG. 3 is a graph that illustrates the relationship between quiescent collector current and temperature for the transmitter circuits of FIGS. 1 and 2.  
         [0029]    [0029]FIG. 4 is a block diagram of the output stage of a power amplifier in a cellular telephone handset in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]    Embodiments are described using NPN-type bipolar junction transistors (BJTs), which are illustrative of various transistor types that may be used in other embodiments of the invention. Some embodiments are formed on an integrated circuit chip having a gallium arsenide (GaAs) substrate, and use heterojunction BJTs (HBTs) for at least some transistors. Other embodiments are implemented using other types of semiconductor material technology. In the drawings, like-numbered or labeled elements represent the same or substantially similar elements. It should be understood that electrical components described as being coupled together are electrically coupled. It should also be understood that the scope of the enclosed invention includes embodiments in which additional electrical components may be coupled between elements described herein, and that such components are omitted from the description so as to more clearly illustrate the invention. The chassis ground symbols shown in the drawings are illustrative of various electrical grounds that may be used as a reference electrical potential.  
         [0031]    [0031]FIG. 2 is a circuit diagram of a portion of a transmitter power amplifier circuit  200  in accordance with one embodiment of the present invention. Transmitter power amplifier circuit  200  includes bias circuit  201  and amplifier stage  202 . Amplifier stage  202  can be, for example, a final amplifying stage in a wireless cellular telephone handset transmitter power amplifier integrated circuit. In another instance, amplifier stage  202  can be a driving stage in such an integrated circuit.  
         [0032]    Because portions of transmitter power amplifier circuit  200  (FIG. 2) are similar to portions of transmitter circuit  100  (FIG. 1), similar elements in FIGS. 1 and 2 are labeled with similar reference numbers. Thus, amplifier circuit  202  includes bias resistors  130   1 ,  130   2 , . . .  130   N , and associated NPN bipolar transistors  131   1 ,  131   2 , . . .  131   N  (where N is an integer). Although the present example shows three or more pairs of bias resistors/bipolar transistors, it is understood that other numbers of resistor/transistor pairs can be used in amplifier circuit  102  in other embodiments.  
         [0033]    Bias circuit  201  includes diode  110 , resistors  111 - 113 , NPN bipolar transistors  121 - 122  and nodes  104 - 105 , which were described above in connection with FIG. 1. In addition, bias circuit  201  includes a bias control circuit  250 , which includes resistor  114 , diode  115  and NPN bipolar transistors  124  and  125 . In one embodiment, diodes  110  and  115  are diode-connected transistors. The first terminal of resistor  114  is connected to reference voltage supply terminal (V REF ) and a second terminal of resistor  114  is coupled to the anode of diode  115 . The cathode of diode  115  is coupled to the collector and base of transistor  124  and the base of transistor  125 . The emitters of transistors  124  and  125  are coupled to the ground supply terminal. The collector of transistor  125  is coupled to node  104 .  
         [0034]    Bias circuit  201  operates in the following manner. Currents I 1  and I 2  are established in the manner defined above in Equations (5) and (2), respectively. Conceptually, these currents I 1  and I 2  form a first current that flows into node  104 . Combined, these currents I 1  and I 2  exhibit a temperature dependence that is defined above in Equation (6).  
         [0035]    Because the bases of transistors  124  and  125  are commonly coupled to the cathode of diode  115 , and the emitters of transistors  124  and  125  are commonly coupled to the ground supply terminal, the base-to-emitter voltage V BE4  of transistor  124  is equal to the base-to-emitter voltage V BE5  of transistor  125 . The collector current I 4  of transistor  124  is therefore directly proportional to the collector current I 5  of transistor  125 . More specifically, if the emitter area of transistor  124  is equal to A4, and the emitter area of transistor  125  is equal to A5, then the collector currents I 4  and I 5  are related in the following manner.  
           I   5   =I   4 ( A 5/ A 4)   (7)  
         [0036]    If the voltage drop across diode  115  is designated V D2 , then the voltage drop across resistor  114  (V 114 ) can be defined as follows.  
           V   114   =V   REF −( V   D2   +V   BE4 )   (8)  
         [0037]    If resistor  114  has a resistance of R 4 , then collector current I 4  can be defined as follows.  
           I   4   =V   114   /R   4 =( V   REF −( V   D2   +V   BE4 ))/ R   4    (9)  
         [0038]    Thus, the collector current I 5  through transistor  125  can be defined as follows.  
           I   5 =( A 5/ A 4)( V   REF −( V   D2+   V   BE4 ))/ R   4    (10)  
         [0039]    Using Kirchoff&#39;s current law, the sum of the currents flowing into node  104  (I 1  and I 2 ) is equal to the sum of the currents flowing out of node  104  (I 3  and I 5 ). Thus, the relationship between currents I 1 , I 2 , I 3  and I 5  can be written as follows.  
           I   3   =I   1   +I   2   −I   5    (11)  
         [0040]    Substituting Equations (2), (5) and (10) into Equation (11) provides the following.  
           I   3 =( V   REF −( V   BE1   +V   BE2 ))/ R   2 +( V   REF −( V   BE1   +V   BE2 )+ V   D1 )/ R   1 −( A 5/ A 4)( V   REF −( V   D2   +V   BE4 ))/ R   4    (12)  
         [0041]    As the temperature of bias circuit  201  increases, the (junction) voltages V D1 , V D2 , V BE1 , V BE2  and V BE4  all decrease. As a result, each of the currents I 1 , I 2  and I 5  increases. However, because the increase in current I 5  is effectively subtracted from the increases in currents I 1  and I 2  (Equations 11-12), the current I 3  remains relatively constant as the temperature increases.  
         [0042]    Conversely, as the temperature of bias circuit  201  decreases, the voltages V D1 , V D2 , V BE1 , V BE2  and V BE4  all increase. As a result, each of the currents I 1 , I 2  and I 5  decreases. However, because the decrease in current I 5  is effectively subtracted from the decreases in currents I 1  and I 2  (Equations 11-12), the current I 3  remains relatively constant as the temperature decreases.  
         [0043]    Stated another way, the collector current I 5  of transistor  125  increases as temperature increases. By controlling the magnitude of current I 5 , the magnitude of current I 3  through transistor  121  is controlled. Skilled persons will understand that diode  115  provides a relatively high temperature dependence slope for current I 5 . Skilled persons will also understand that diode  115  is illustrative of various embodiments in which one or more, or in one instance zero, diodes are used, depending on the magnitude of the reference voltage V REF .  
         [0044]    To make current I 3  temperature independent, the temperature dependence of current T 5  should be equal to the combined temperature dependences of currents I 1  and I 2 . Further, to make current I 3  increase with temperature, the temperature dependence of current I 5  should be less than the combined temperature dependences of currents I 1  and I 2 . Similarly, to make current I 3  decrease with temperature, the temperature dependence of current I 5  should be larger than the combined temperature dependences of currents I 1  and I 2 .  
         [0045]    The relationship between collector current I 3  and temperature can be selected by the circuit designer by appropriately selecting the resistances R 1 -R 4 , the emitter areas of transistors  121 - 125  and the junction areas of diodes  110  and  115 . Table 1 below lists these parameters for one embodiment of the present invention.  
                           TABLE 1                                       Transistor/Diode   Emitter Area (μm 2 )                       121   405           122   2,430           131 1 -131 N  (N = 16)   405           124   135           125   405           110   405           115   135                       Resistors   Resistance (Ohms)                       111 (R 1 )   300           112 (R 2 )   120           113   40           130 1 -130 N  (N = 16)   125           114 (R 4 )   225                      
 
         [0046]    [0046]FIG. 3 is a graph  300  showing the relationship between the measured quiescent collector current I 3  (milliamperes (mA)) versus temperature (°C.). Line  301 , shown plotted by diamonds, represents the current/temperature relationship of bias circuit  101  of FIG. 1. As illustrated, the collector current I 3  of bias circuit  101  varies from about 63 mA at −30° C. to about 102 mA at 85° C. This variation represents an increase of about 62 percent over the expected temperature range of a cellular telephone. Line  302 , shown plotted by squares, represents the current/temperature relationship of the bias circuit  201  of the present invention (FIG. 2). As illustrated, the collector current I 3  of bias circuit  201  varies from about 62 mA at −30° C. to about 68 mA at 85° C. This variation represents an increase of less than about 10 percent over the expected temperature range of a cellular telephone.  
         [0047]    [0047]FIG. 4 is a block diagram of the output stage of a power amplifier in a cellular telephone handset  400  in accordance with one embodiment of the present invention. As shown in FIG. 4, amplifier stage  202  receives a temperature-stabilized base bias voltage (V BIAS1 ) from bias circuit  201  as described above. Reference voltage supply  402  supplies reference voltage V REF  to bias circuit  201 , and power supply  401  supplies voltage V CC  to bias circuit  201  and amplifier stage  202 . Amplifier stage  202  outputs an amplified signal from the parallel-connected collector terminals of transistors  131   1 - 131   N  (illustrated as output terminal  404 ). This amplified signal is transmitted to antenna  405  via conventional impedance matching circuit  403 . Bias circuit  201  allows transmitter  400  to broadcast a signal having relatively constant power over an extreme range of anticipated operating temperatures.  
         [0048]    Although the invention has been described in connection with several embodiments, it is understood that this invention is not limited to the embodiments disclosed, but is capable of various modifications, which would be apparent to one of ordinary skill in the art. For example, the circuit topology that includes resistor  114 , diode  115 , and transistors  124 - 125  may be used with other bias circuits that are temperature dependent. That is, this topology may be used to provide temperature dependent control for current exiting from a particular node. Thus, current for a particular device coupled at the node is controlled as a function of temperature. In addition, capacitors can be added to power amplifier circuit  200  to achieve the goals of RF bypass, RF decoupling and/or loop bandwidth adjustment. The connections and sizes of such capacitors would be obvious to those of ordinary skill in the art. Thus, the invention is limited only by the following claims.