Abstract:
A control circuit for controlling a bias circuit coupled to an amplifier is disclosed. An exemplary bias control circuit comprises means for receiving a control voltage, and means for actively adjusting an equivalent resistance of the bias control circuit responsive to the control voltage, wherein the equivalent resistance is established between the first node and a reference voltage. In one embodiment, when the control voltage is increased, the equivalent resistance is gradually decreased and a current drawn by the bias control circuit is gradually increased, resulting in a quiescent current of the amplifier transistor being gradually increased.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention is generally in the field of semiconductors. More specifically, the invention is in the field of semiconductor circuits and amplifiers.  
         [0003]     2. Related Art  
         [0004]     Amplifiers based on bipolar technology are widely used in a variety of applications, including wireless communication, such as radio frequency (“RF”) communication, for example. Bias circuits perform an important function by supplying a base bias current to bipolar transistors for controlling the operation modes of the bipolar transistors in amplifiers.  
         [0005]     Digital mode control circuits have been used to reduce current and power consumption for low power mode operation in high-power amplifiers. Digital mode controls circuits, however, have a single and abrupt transition point from low power mode to high power mode, which substantially limits current consumption savings, particularly during very low power mode operation.  
         [0006]     In an effort to improve current consumption savings, CMOS circuitry in an additional CMOS die have been employed in high-power amplifiers. With this arrangement, CMOS circuitry can provide improved analog control voltage into the base bias of the bipolar transistor of the amplifier, resulting in a substantially continuous quiescent current transition from a very low power level. In this way, current consumption can be greatly reduced even at low power modes. The addition of a separate CMOS die to the amplifier, however, results in increased device size and increased costs, both of which are undesirable.  
         [0007]     Accordingly, there is a strong need in the art for a quiescent current control circuit for high-power amplifiers.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is directed to a quiescent current control circuit for high-power amplifiers. In one exemplary embodiment, the control circuit controls a bias circuit coupled to an amplifier, such as a high-power CDMA amplifier. The bias circuit includes a first bias transistor, a second bias transistor, and a third bias transistor, wherein a base of the amplifier transistor is coupled to an emitter of the second bias transistor, a base of the second bias transistor is coupled to a base of the first bias transistor and to a collector of the third bias transistor, and a base of the third bias transistor is coupled to an emitter of the first bias transistor and to the bias control circuit at a first node.  
         [0009]     In one embodiment, the bias control circuit comprises means for receiving a control voltage, and means for actively adjusting an equivalent resistance of the bias control circuit responsive to the control voltage, wherein the equivalent resistance is established between the first node and a reference voltage, such as ground. For example, in one embodiment, when the control voltage is increased, the equivalent resistance is gradually decreased and a current drawn by the bias control circuit is gradually increased, resulting in a quiescent current of the amplifier transistor being gradually increased. As such, continuous quiescent current control of the amplifier transistor is achieved, resulting in significant current and power consumption savings.  
         [0010]     According to one embodiment, the bias control circuit, the bias circuit and the amplifier transistor are based on bipolar technology. As such, the bias control circuit, the bias circuit and the amplifier transistor can be integrated into a single die, resulting in significant reduction in device size and device cost.  
         [0011]     Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a circuit diagram of an exemplary bias circuit for a high-power amplifier according to one embodiment of the present invention.  
         [0013]      FIG. 2  shows a circuit diagram of an exemplary control circuit according to one embodiment of the present invention.  
         [0014]      FIG. 3  shows a circuit diagram of an exemplary control circuit according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The present invention is directed to a quiescent current control circuit for high-power amplifiers. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.  
         [0016]     The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the invention which use the principles of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings.  
         [0017]     Referring to  FIG. 1 , there is shown a circuit diagram of exemplary bias circuit  102  including control circuit  106  according to one embodiment of the present invention. In  FIG. 1 , bias circuit  102  is coupled to and supplies base bias current  108  (“Ib  108 ”) to amplifier transistor  110  of amplifier  104 . Amplifier  104  may, for example, be a high-power amplifier, such as a high-power CDMA handset amplifier, and amplifier transistor  110  may for example, be a large heterojunction bipolar transistor (“HBT”). As discussed below, control circuit  106  is based on bipolar technology and be integrated into the same die as bias circuit  102  and amplifier  104 . Also discussed below, control circuit  106  achieves dynamic and continuous control of quiescent current  112  (“Icq  112 ”) of amplifier transistor  110 , resulting in significantly reduced current and power consumption.  
         [0018]     As shown in  FIG. 1 , bias circuit  102  comprises bias transistors  114 ,  116  and  118 , and resistors  120  and  122 . Bias transistors  114 ,  116  and  118  comprise bipolar transistors, wherein a base of bias transistor  114  is connected at node  128  to a base of bias transistor  116  and to a collector of bias transistor  118 . Bias transistor  114  further has an emitter connected at node  126  to control circuit  106  and to a base of bias transistor  118 . An emitter of bias transistor  118  is connected to a reference voltage, such as ground  132 . Bias transistor  116  further has an emitter connected at node  130  to a base of amplifier transistor  110 . An emitter of amplifier transistor  110  is connected to a reference voltage, such as ground  132 .  
         [0019]     According to one embodiment, resistor  120  is approximately 1 to 2 kiloOhms (kΩ) and is connected across reference voltage (“Vref”)  124  and node  128 , and resistor  122  is approximately 0.5 to 1 kΩ and is connected across node  130  and a reference voltage, such as ground  132 . According to another embodiment, resistor  122  may be omitted, wherein the emitter of bias transistor  116  is connected only to the base of amplifier transistor  110 . Nodes  134 ,  136  and  138  may be connected to a bias voltage or may be directly connected to a supply voltage (“VCC”), as is known in the art.  
         [0020]     Control circuit  106  is connected across node  126  and a reference voltage, such as ground  132 . As shown in  FIG. 1 , control circuit  106  comprises bias control transistor  140  and resistors  142 ,  144 ,  146  and  148 . Resistor  142  is connected across node  126  and node  158  and, according to one embodiment, is approximately 2 kΩ. Resistor  144  is connected across node  158  and a reference voltage, such as ground  132  and, according to one embodiment, is approximately 100 kΩ. Resistor  146  is connected across an emitter of bias control transistor  140  and a reference voltage, such as ground  132  and, according to one embodiment, is approximately 100 Ω. Bias control transistor  140  comprises a bipolar transistor and has a collector connected to node  158  and a base connected to node  160 . Resistor  148  is connected across node  160  and a control voltage (“Vcont”)  156  and, according to one embodiment, is approximately 10 kΩ.  
         [0021]     In operation, control circuit  106  receives Vcont  156  and provides a “reference” resistance corresponding to an equivalent resistance (“Req”) across node  126  and ground  132 . Req determines the status of bias circuit  102 , which in turn determines the status of Icq  112  of amplifier  104 . In control circuit  106 , bias control transistor  140  operates as an active resistor controlled by Vcont  156 , such that as Vcont  156  is increased from a low level to a high level, Req is gradually decreased. Vcont  156 , for example, may have a low level of approximately 0 to 1.1 volts (“V”) and a high level of approximately 2 to 3 V. Resistor  142  establishes the primary resistance of Req for high mode operation and operates to restrict Icq  112  at high Vcont  156 , and resistor  144  establishes the primary resistance of Req for low mode operation and operates for baseline Icq  112  at very low Vcont  156 .  
         [0022]     With this arrangement, as Vcont  156  is increased from a low level, bias control transistor  140  is gradually turned on, resulting in a gradual increase of collector current (“Ic”)  162  of bias control transistor  140 . As Ic  162  is gradually increased, Req of control circuit  106  is dynamically reduced such that control circuit  106  draws increased current  164 , resulting in a decrease in base current (“Ib”)  166  and Ic  168  of bias transistor  118 . Decreased Ic  168  results in increased  1   b    170  and Ic  172  of bias transistor  116 , further resulting in increased Vb of amplifier transistor  110  at node  130 , and further in increased Ib  108  and Icq  112  of amplifier transistor  110 .  
         [0023]     Due to the particular arrangement of control circuit  106  and bias circuit  102 , significantly improved analog control over Vb of amplifier transistor  110  by control circuit  106  is achieved, such that continuous Icq  112  transition from a very low power level can be provided, which results in significant current savings. Since control circuit  106  is based on bipolar technology, control circuit  106  may be integrated in to the same die as bias circuit  102  and amplifier  104 , resulting in substantial cost savings and significantly reduced device size.  
         [0024]     As shown in  FIG. 1 , control circuit  106  may further include temperature compensation circuit  150  comprising resistor  152  and diode  154 . Resistor  152  is connected across node  160  and an anode of diode  154  and, according to one embodiment, is approximately 2 to 5 kΩ. Diode  154  may, for example, be an HBT diode, and further has a cathode connected to a reference voltage, such as ground  132 . In the absence of temperature compensation circuit  150 , at high temperatures, the requisite forward bias voltage (corresponding to the base-to-emitter voltage (“Vbe”)) of bias control transistor  140  drops, resulting in an increase in Ic  162  of bias control transistor  140  and a corresponding decrease in the Req of control circuit  106 . However, with resistor  152  and diode  154  coupled to the base of bias control transistor  140  at node  160 , diode  154  offsets any increase in Ic  162  by drawing a corresponding increased current  174  from node  160  to ground  132 , since at high temperatures, the requisite forward bias voltage for diode  154  decreases for the same reason that the requisite forward bias voltage of bias control transistor  140  drops. As a result, greater control and accuracy of Req of control circuit  106  are achieved even at high temperatures, which, as discussed above, provides significantly improved analog control over Vb of amplifier transistor  110  and improved continuous control of Icq  112  of amplifier transistor  110 , resulting in significantly reduced current and power consumption.  
         [0025]     Referring now to  FIG. 2 , exemplary control circuit  206  according another embodiment of the present invention is shown. Control circuit  206  may be used to control bias circuit  102  of  FIG. 1  and to provide continuous control of quiescent current  112  of amplifier transistor  110  as described above, wherein control circuit  206  replaces control circuit  106  of  FIG. 1 , and wherein node  226 , Vcont  256  and ground  232  respectively corresponds to node  126 , Vcont  156  and ground  132  of  FIG. 1 .  
         [0026]     As shown in  FIG. 2 , control circuit  206  comprises bias control transistor  240 , resistors  242 ,  248 ,  276 ,  278  and  280 , and diode  282 . Resistor  242  is connected across node  226  and node  258  and, according to one embodiment, is approximately 2 kΩ. Resistor  276  is connected across node  258  and node  284  and, according to one embodiment, is approximately 100 kΩ. Resistor  278  is connected across node  284  and an anode of diode  282  and, according to one embodiment, is approximately 10 to 20 Ω. Diode  282  may, for example, be a Schottky diode having a turn on forward bias voltage of approximately 0.5 V, and further has a cathode connected to a reference voltage, such as ground  232 . Resistor  280  is connected across node  284  and a reference voltage, such as ground  232  and, according to one embodiment, is approximately 100 Ω. Bias control transistor  240  comprises a bipolar transistor and has a collector connected to node  258  and an emitter connected to node  284 . Resistor  248  is connected across a base of bias control transistor  240  and Vcont  156  and, according to one embodiment, is approximately 10 kΩ.  
         [0027]     In operation, control circuit  206  operates in substantially the same manner as described above in conjunction with control circuit  106  of  FIG. 1 . Thus, as Vcont  256  is increased from a low level, bias control transistor  240  is gradually turned on, resulting in a gradual increase of collector current (“Ic”)  262  of bias control transistor  240 . As Ic  262  is gradually increased, Req of control circuit  206  is dynamically reduced such that control circuit  206  draws increased current  264 , and as discussed above in conjunction with  FIG. 1 , further results in increased Vb of amplifier transistor  110  at node  130 , and in increased Ib  108  and Icq  112  of amplifier transistor  110 . Due to the particular arrangement of control circuit  206 , significantly improved analog control over Vb of amplifier transistor  110  by control circuit  106  is achieved, such that continuous Icq  112  transition from a very low power level can be provided, which results in significant current savings.  
         [0028]     Control circuit  206  of  FIG. 2  further includes resistor  278  and diode  282  connected across node  284  and ground  232 . In this particular arrangement, resistor  278  and diode  282  operate to reduce the requirement of having very high Vcont  252  for high mode operation. According to another embodiment, temperature compensation circuit  150  of  FIG. 1  could be connected between resistor  248  and the base of bias control transistor  240  of  FIG. 2  to provide temperature compensation and improved continuous control of quiescent current  112  of amplifier transistor  110  as described above.  
         [0029]     Referring now to  FIG. 3 , exemplary control circuit  306  according another embodiment of the present invention is shown. Control circuit  306  may be used to control bias circuit  102  of  FIG. 1  and to provide continuous control of quiescent current  112  of amplifier transistor  110  as described above, wherein control circuit  306  replaces control circuit  106  of  FIG. 1 .  
         [0030]     In  FIG. 3 , Vcont  356 , node  326 , ground  332 , bias control transistor  340  and resistors  342 ,  344 ,  336  and  348  respectively correspond to Vcont  156 , node  126 , ground  132 , bias control transistor  140  and resistors  142 ,  144 ,  136  and  148  in  FIG. 1 . Also shown in  FIG. 3 , temperature compensation circuit  350  is connected at node  360  to the base of bias control transistor  340 . Temperature compensation circuit  350  comprises resistor  352  and diodes  353  and  355 . Resistor  352  is connected across node  360  and an anode of diode  353 . A cathode of diode  353  is connected to an anode of diode  355 , and a cathode of diode  355  is connected to a reference voltage, such as ground  332 . Diode  353  and  355  may, for example, be Schottky diodes, each diode  353  and  355  having a turn on forward bias voltage of approximately 0.5 V. In this way, diodes  353  and  355  have a functionally equivalent turn on forward bias voltage (i.e., measured across the anode of diode  353  and the cathode of diode  355 ) of approximately 1 to 1.2 V. Thus, operation of control circuit  306  operates in substantially the same manner described above in conjunction with control circuit  106  of  FIG. 1 .  
         [0031]     In sum, a quiescent current control circuit for high-power amplifiers is achieved according to various embodiments of the present invention, whereby significant analog continuous control over the quiescent current of an amplifier is achieved, resulting in significantly reduced current and power consumption, particularly for low mode operation. Furthermore, improved temperature compensation is achieved by the control circuit of the present invention, resulting in improved control over the quiescent current of an amplifier. Moreover, the control circuit of the present invention is based on bipolar technology, allowing the control circuit to be integrated into the same die as the bias circuit and the amplifier, resulting in significant cost savings and reduced device size.  
         [0032]     From the above description of exemplary embodiments of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would recognize that changes could be made in form and detail without departing from the spirit and the scope of the invention. For example, the particular resistive values for bias circuit  102  and control circuits  106 ,  206  and  306  discussed above can be modified without departing from the scope of the present invention. The described exemplary embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular exemplary embodiments described herein, but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention.  
         [0033]     Thus, a quiescent current control circuit for high-power amplifiers has been described.