Abstract:
There is disclosed, for use in an RF amplifier, a biasing circuit for maintaining the quiescent current of an output power transistor at a selected bias level. The biasing circuit comprises a temperature sensor circuit for generating a temperature-sensitive control voltage that varies according to changes in temperature of the output power transistor and a bias voltage generating circuit capable of detecting a variation in the temperature-sensitive control voltage. In response to a detected change in the temperature-sensitive control voltage, the bias circuit adjusts a bias voltage applied to the output power transistor by an amount suitable to offset a change in the selected bias current level caused by a temperature change related to the variation in the temperature-sensitive voltage.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to wireless communications networks and, more specifically, to a system for biasing an RF amplifier in a wireless network base station. 
     BACKGROUND OF THE INVENTION 
     Wireless networks, including cellular telephone networks, have become ubiquitous in society. Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. In order to maximize the number of subscribers that can be serviced in a single cellular system, frequency reuse is increased by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. To maximize usage of the available bandwidth in each cell, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof. 
     Every cellular base station has an RF transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. It is important that the RF power amplifier in a base station transmitter operate in a highly linear manner, especially when amplifying a signal whose envelope changes in time over a wide range, as in CDMA and multi-carrier systems. It also is important that the RF amplifier have good linearity characteristics across a wide range of operating conditions, because wireless systems cannot tolerate large amounts of signal distortion and may not violate adjacent channel power specifications, such as the IS 95 bandwidth requirements, regarding spectral spreading effects. 
     The output stage of an RF amplifier typically contains a high-power transistor, such as a class AB laterally diffused metal-oxide-silicon field-effect transistor (LDMOS FET), a gallium-arsenide (GaAs) FET, or, perhaps, a bipolar junction transistor (BJT). In order to maintain linear operation in the RF amplifier, the bias voltage of the output stage high-power transistor must be adjusted so that the bias current of the high-power transistor remains constant over a range of temperature. 
     For example, in an LDMOS FET, the gate-to-source bias voltage (V gs ) must vary such that the quiescent current (I dq ) remains constant as temperature rises. To maintain constant I dq  over a temperature range, the gate voltage must decrease as temperature increases. The desired slope (mV/C) of the gate voltage varies from one device to the next due to process variation. If I dq  is not constant over temperature, the device linearity or adjacent channel power ratio (ACPR) degrades. If the ACPR degrades, the RF amplifier output power must be reduced to the point at which it again complies with the J-STD-019 spectral mask. This reduction in output power decreases the overall range and capacity of cellular and PCS base stations. 
     One technique for biasing the output power transistor is to use a fixed-bias voltage. The fixed-bias approach is generally implemented with a simple voltage divider or adjustable reference voltage. Unfortunately, this technique is not capable of compensating the bias voltage over temperature, nor is it capable of compensating for lot-to-lot device variations. Furthermore, the fixed-bias technique is subject to thermal runaway. If the bias voltage is not temperature compensated, the bias current becomes very large with increased temperature. Under full RF drive conditions, the increase in bias current may become so large that the device overheats to the point of failure. Regardless of failure, the device mean-time-to-failure (MTTF) degrades with increased current and temperature. 
     Another technique for biasing the output power transistor involves the use of microprocessors and/or electronically programmable resistor arrays. This approach is much more complex and costly and requires input and output data from a master controller card. Furthermore, in order to measure and adjust the quiescent current, the RF input signal to the output power transistor must be temporarily shut off. Obviously, when the RF input signal is removed, the base station no longer transmits and all calls must be dropped. Thus, the base station must go out-of-service just prior to and during adjustment of the bias current. 
     There is therefore a need in the art for improved systems and methods of biasing the output power transistor of an RF amplifier to compensate for temperature variations. In particular, there is a need for temperature-compensated biasing networks for the output power transistor of an RF amplifier that are simple and inexpensive and that do not require that the base station be temporarily put out of service. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an RF amplifier containing an output power transistor having a quiescent current set at a selected bias current level, a biasing circuit for maintaining the quiescent current at the selected bias level. The bias circuit comprises 1) a temperature sensor circuit capable of generating a temperature-sensitive control voltage that varies according to changes in temperature of the output power transistor; and 2) a bias voltage generating circuit capable of detecting a variation in the temperature-sensitive control voltage and, in response thereto, adjusting a bias voltage applied to the output power transistor by an amount suitable to offset a change in the selected bias current level caused by a temperature change related to the variation in the temperature-sensitive voltage. 
     According to one embodiment of the present invention, the bias voltage generating circuit comprises amplification means for scaling a voltage change in the temperature-sensitive control voltage to match a required voltage change in the bias voltage. 
     According to another embodiment of the present invention, the bias voltage generating circuit further comprises an adjustable voltage divider circuit for further scaling the voltage change in the temperature-sensitive control voltage to match the required voltage change in the bias voltage. 
     According to still another embodiment of the present invention, the bias voltage generating circuit comprises an operational amplifier having a first input coupled to an output of the temperature sensor circuit. 
     According to yet another embodiment of the present invention, the operational amplifier has a second input coupled to an output of a precision voltage reference circuit. 
     According to a further embodiment of the present invention, an output of the operational amplifier is proportional to a difference between the precision voltage reference circuit output and the temperature sensor circuit output. 
     According to another embodiment of the present invention, the precision voltage reference circuit output provides a DC offset voltage in the operational amplifier output. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 illustrates an exemplary wireless network according to one embodiment of the present invention; 
     FIG. 2 illustrates in greater detail an exemplary base station according to one embodiment of the present invention; 
     FIG. 3 illustrates in greater detail a temperature compensated bias network for use in an exemplary Class A/B RF amplifier in the RF transceiver in FIG. 2 in accordance with one embodiment of the present invention; 
     FIG. 4A illustrates a curve which represents the output response over temperature of an exemplary temperature sensor in accordance with one embodiment of the present invention; and 
     FIG. 4B illustrates a curve which represents the bias voltage response over temperature on the gate of an exemplary LDMOS FET according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 through 4, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged wireless network and any suitably arranged RF transmitter, including RF transmitters used to transmit television signals and commercial radio signals. 
     FIG. 1 illustrates exemplary wireless network  100  according to one embodiment of the present invention. The wireless telephone network  100  comprises a plurality of cell sites  121 - 123 , each containing one of the base stations, BS  101 , BS  102 , or BS  103 . Base stations  101 - 103  are operable to communicate with a plurality of mobile stations (MS)  111 - 114 . Mobile stations  111 - 114  may be any suitable cellular devices, including conventional cellular telephones, PCS handset devices, portable computers, metering devices, and the like. 
     Dotted lines show the approximate boundaries of the cell sites  121 - 123  in which base stations  101 - 103  are located. The cell sites are shown approximately circular for the purposes of illustration and explanation only. It should be clearly understood that the cell sites may have other irregular shapes, depending on the cell configuration selected and natural and man-made obstructions. 
     In one embodiment of the present invention, BS  101 , BS  102 , and BS  103  may comprise a base station controller (BSC) and a base transceiver station (BTS). Base station controllers and base transceiver stations are well known to those skilled in the art. A base station controller is a device that manages wireless communications resources, including the base transceiver station, for specified cells within a wireless communications network. A base transceiver station comprises the RF transceivers, antennas, and other electrical equipment located in each cell site. This equipment may include air conditioning units, heating units, electrical supplies, telephone line interfaces, and RF transmitters and RF receivers. For the purpose of simplicity and clarity in explaining the operation of the present invention, the base transceiver station in each of cells  121 ,  122 , and  123  and the base station controller associated with each base transceiver station are collectively represented by BS  101 , BS  102  and BS  103 , respectively. 
     BS  101 , BS  102  and BS  103  transfer voice and data signals between each other and the public telephone system (not shown) via communications line  131  and mobile switching center (MSC)  140 . Mobile switching center  140  is well known to those skilled in the art. Mobile switching center  140  is a switching device that provides services and coordination between the subscribers in a wireless network and external networks, such as the public telephone system. Communications line  131  may be any suitable connection means, including a T1 line, a T3 line, a fiber optic link, a network backbone connection, and the like. In some embodiments of the present invention, communications line  131  may be several different data links, where each data link couples one of BS  101 , BS  102 , or BS  103  to MSC  140 . 
     In the exemplary wireless network  100 , MS  111  is located in cell site  121  and is in communication with BS  101 , MS  113  is located in cell site  122  and is in communication with BS  102 , and MS  114  is located in cell site  123  and is in communication with BS  103 . The MS  112  is also located in cell site  121 , close to the edge of cell site  123 . The direction arrow proximate MS  112  indicates the movement of MS  112  towards cell site  123 . At some point, as MS  112  moves into cell site  123  and out of cell site  121 , a “handoff” will occur. 
     As is well know, the “handoff” procedure transfers control of a call from a first cell to a second cell. For example, if MS  112  is in communication with BS  101  and senses that the signal from BS  101  is becoming unacceptably weak, MS  112  may then switch to a BS that has a stronger signal, such as the signal transmitted by BS  103 . MS  112  and BS  103  establish a new communication link and a signal is sent to BS  101  and the public telephone network to transfer the on-going voice, data, or control signals through BS  103 . The call is thereby seamlessly transferred from BS  101  to BS  103 . An “idle” handoff is a handoff between cells of a mobile device that is communicating in the control or paging channel, rather than transmitting voice and/or data signals in the regular traffic channels. 
     FIG. 2 illustrates in greater detail exemplary base station  101  in accordance with one embodiment of the present invention. Base station  101  comprises base station controller (BSC)  210  and base transceiver station (BTS)  220 . Base station controllers and base transceiver stations were described previously in connection with FIG.  1 . BSC  210  manages the resources in cell site  121 , including BTS  220 . BTS  120  comprises BTS controller  225 , channel controller  235 , which contains representative channel element  240 , transceiver interface (IF)  245 , RF transceiver unit  250 , antenna array  255 , and channel monitor  260 . 
     BTS controller  225  comprises processing circuitry and memory capable of executing an operating program that controls the overall operation of BTS  220  and communicates with BSC  210 . Under normal conditions, BTS controller  225  directs the operation of channel controller  235 , which contains a number of channel elements, including channel element  240 , that perform bi-directional communications in the forward channel and the reverse channel. A “forward” channel refers to outbound signals from the base station to the mobile station and a “reverse” channel refers to inbound signals from the mobile station to the base station. Transceiver IF  245  transfers the bi-directional channel signals between channel controller  240  and RF transceiver unit  250 . 
     Antenna array  255  transmits forward channel signals received from RF transceiver unit  250  to mobile stations in the coverage area of BS  101 . Antenna array  255  also sends to transceiver  250  reverse channel signals received from mobile stations in the coverage area of BS  101 . In a preferred embodiment of the present invention, antenna array  255  is multi-sector antenna, such as a three sector antenna in which each antenna sector is responsible for transmitting and receiving in a 120° arc of coverage area. Additionally, transceiver  250  may contain an antenna selection unit to select among different antennas in antenna array  255  during both transmit and receive operations. 
     FIG. 3 illustrates in greater detail a temperature compensated bias network, generally designated  300 , for use in an exemplary RF amplifier in RF transceiver  250  in accordance with one embodiment of the present invention. Bias network  300  maintains a constant desired quiescent current, I dq , and device linearity in class AB laterally diffused metal-oxide-silicon field-effect transistor (LDMOS FET)  301 . Although the discussion that follows is directed toward the biasing of a class AB LDMOS FET, it will be understood by those skilled in the art that the teachings of this disclosure may easily be adapted to bias a GaAs FET device or a BJT device. However, for the sake of simplicity, the following discussion will be limited to a class AB LDMOS FET device. 
     Bias network  300  comprises differential operational amplifier (OA)  305 , which receives a first signal on a non-inverting input from voltage reference circuit  310  and a second buffered control signal on an inverting input from temperature sensor  315 . The output of temperature sensor  315  is buffered by non-inverting, unity gain OA  320 . The output of unity gain OA  320  is scaled by a gain factor determined by resistor  330  (referred to below as “R1”) and resistor  325  (referred to below as “R2”). The resultant output of differential OA  305  is subsequently scaled by a voltage divider comprised of resistor  335  and variable resistor (potentiometer)  340 . The RF input signal (RF IN) is supplied to the gate of LDMOS FET  301  by RF coupling capacitor  345 . 
     FIG. 4A illustrates curve  400 , which represents the voltage output of temperature sensor  315  across a range of temperatures in accordance with one embodiment of the present invention. As shown, temperature sensor  315  provides an output voltage (V TS ) that increases linearly with temperature (Temp) as depicted by the slope of curve  400 . As previously described, OA  320  provides non-inverting, unity gain for the output voltage of temperature sensor  315 . OA  320 , in conjunction with R2, adjusts the high output impedance of temperature sensor  315  to an appropriate level for input to differential OA  305 . This prevents the output impedance of temperature sensor  315  from negatively impacting the performance of differential OA  305 . Since OA  320  provides unity gain, the output of OA  320  to R2 is similar to curve  400 . Thus, the output of OA  320  varies linearly with the temperature sensor  315  output (control signal). 
     Voltage reference  310  provides a precise non-varying output voltage (V REF ) for input to the positive terminal of differential OA  305 . The positive input terminal of differential OA  305  provides an output gain to the V REF  signal equal to G P , where G P =1+R1/R2. The negative input terminal of differential OA  305  provides an output gain to the V TS  signal equal to G n,  where G n =−R1/R2. Therefore, differential OA  305  provides an output voltage (V 0 ) that is represented by: 
     
       
           V   0 =(1+ R   1   /R   2 ) V   REF −( R   1   /R   2 ) V   TS   =V   REF +( R   1   /R   2 )( V   REF   −V   TS )  
       
     
     The above equations in conjunction with FIG. 4A show that V 0  decreases linearly with the increases in temperature. Thus, V 0  has the required characteristic for providing stable I dq  over changes in temperature. 
     As shown by FIG. 3, V 0  is applied to a voltage divider comprised of resistor  335  and multi-turn potentiometer  340 . When properly adjusted, potentiometer  340  provides the desired quiescent current, I dq , and nominal operating voltage. In the case of the LDMOS FET, potentiometer  340  is adjusted, at room temperature (25° C.) for example, while monitoring the FET&#39;s quiescent current. Once I dq  is obtained, potentiometer  340  is no longer changed. 
     FIG. 4B illustrates curve  410 , which represents the gate-source bias voltage (V GS ) response over temperature on the gate of LDMOS FET  301  in accordance with one embodiment of the present invention. As shown by correlation of FIGS. 4A and 4B, when potentiometer  340  is adjusted at temperature T(1), for example 25° C., to produce I dq,  the resultant bias voltage V GS =V(2). As previously described for bias network  300 , V(2) is a function of V TS  at T(1), which is shown as equal to V(1) in FIG.  4 A. Once adjusted, temperature sensitive bias network  300  provides constant I dq  for various values of V TS  and V GS  across the indicated temperature range. 
     Besides providing means for stable output of I dq  across various temperature ranges, bias network  300  also provides the means for compensation of manufacturing, lot-to-lot FET (device) variations. Further, bias network  300  prevents FET thermal runaway by reducing the gate voltage as the temperature increases. Fixed-bias designs are not capable of such dynamic control. 
     One of the primary advantages of bias network  300  is its ability to provide a wide range of nominal quiescent currents and output voltages across temperature with a single adjustment. Bias network  300  also provides the ability to obtain more output power from a given device without complex, expensive bias circuitry. Without bias network  300 , the power amplifier must be over-sized to ensure adequate performance over temperature. An over-sized power amplifier results in lower efficiency and higher cost, lower mean-time-to failure (MTTF), and larger and more costly heat sinking. Thus, bias network  300  allows the amplifier to operate at nominal output power over a wide temperature range. 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.