Abstract:
A power amplifier&#39;s base current is biased by a control circuit that produces a linear relationship across varying temperatures and processes. A voltage to current converter controls a voltage follower configured operational amplifier in response to a reference device to drive the voltage and current of the power amplifier. A slope control circuit is coupled to the reference device to limit a maximum power control slope.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to power amplifiers and, more specifically, to a device and method for controlling the bias of a power amplifier.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radio Frequency (RF) power amplifiers are used as components in many communication devices, including many wireless communication devices, such as base stations and mobile devices such as cell phones. Hetero-junction bipolar transistor (HBT) power amplifiers are a specific type of power amplifier used for cellular applications due to their high power density and reduction in die size. Unfortunately, biasing these transistors with a constant current poses some difficulty. The voltage supply limitation typical to mobile applications combined with a relatively high Vbe of HBT devices make traditional integrated methods unusable.  
           [0003]    [0003]FIG. 1 shows a typical diode based biasing control of an HBT transistor. A power amplifier Qpa HBT  100  is biased by a diode configured transistor  110  where the base and collector are shorted together and receive a current through a resistor  120  and supply voltage V REF    130 . This configuration requires that a separate voltage V REF    130  (different from the battery voltage V BAT    140  supplied to the collector of the power amplifier  110 ) be applied to the diode transistor and the biased base of the power amplifier in order to tightly control the biasing current. This configuration leads to several problems for power amplifier applications in mobile communications. Typically, the power amplifier  100  is N times larger than the diode transistor  110  leading to current stealing. Additionally, R REF    120  needs to be large to provide stability over variations in temperature and process, but needs to be small to provide enough current to properly bias the power amplifier, resulting in a circuit that would require a stable reference which supplies a prohibitively large amount of current and is not a viable circuit for power amplifiers in mobile communications applications.  
           [0004]    Another solution, shown in FIG. 2, solves the problem of current stealing by using a current mirror with an emitter follower to bias the current supplied to the power amplifier&#39;s base. The base of a power amplifier transistor  200  is connected to a base of mirrored transistor  210  and the emitter of a emitter follower transistor  250 . The collector of the mirrored transistor  210  is connected to the base of the emitter follower transistor  250  and is connected to a reference voltage  230  through a reference resistor  220  while the collector of the emitter follower transistor  250  is connected to the battery voltage  240  which is also connected to the collector of the power amplifier transistor  200  through some impedance  270 . However, this type of circuit is not viable because gallium arsenide (GAS) HBT power amplifiers as now used have Vbe&#39;s in the order of 1.4 volts while battery voltage supplies are required to be in the range of 2.7 volts. To control the voltage at the base of the power amplifier, the voltage supply, V REF    230 , would need to be greater than is desirable for mobile communication applications and the solution is therefore not viable.  
           [0005]    In certain applications, RF power amplifiers are placed within feedback control loops to provide for power control. A measurement of the output RF power delivered by the RF power amplifier vs. the input voltage will often indicate a steep slope condition where the RF power amplifier output changes very rapidly with respect to changes in the input voltage. When an RF power amplifier presents the steep slope condition instability in the power control loop and other undesirable overall RF power amplifier breakdown conditions may result. Thus, it would be desirable to provide an RF power amplifier device that addresses the steep slope condition while maintaining high performance operation.  
           [0006]    Accordingly, there is a need for an improved RF power amplifier device and method of operation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a simplified electrical schematic of a prior art HBT diode based biasing circuit;  
         [0008]    [0008]FIG. 2 is a simplified electrical schematic of another prior art HBT biasing circuit;  
         [0009]    [0009]FIG. 3 is a simplified electrical schematic of an HBT power amplifier bias controller according to an embodiment of the invention;  
         [0010]    [0010]FIG. 4 is a block diagram of a power amplifier according to an embodiment of the disclosure;  
         [0011]    [0011]FIG. 5, is a graph illustrating a set of transfer curves; and  
         [0012]    [0012]FIGS. 6 and 7 are simplified schematics of a slope control circuits. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    Referring to FIG. 3, one model of an embodiment of a bias control for a hetero-junction bipolar transistor (HBT) power amplifier is shown. Though the circuit was designed for HBT technology, it is not limited to this technology and could be used in technologies such as enhancement mode metal semiconductor field-effect transistors (MESFETS). Similar reference numerals are used throughout the figures to represent similar features when possible.  
         [0014]    An HBT power amplifier  300  is biased based on the voltage measured on reference HBT transistor  310  by way of a CMOS chip  355 . Although the depiction shows the reference device and power amplifier device to be an HBT transistor, other reference devices and power amplifier devices are contemplated.  
         [0015]    The collector of the HBT power amplifier  300  is tapped for an RF output  385  and is supplied voltage from a battery source  340  and some impedance  374  while the emitter is connected to ground. The base of the HBT power amplifier  300  is connected through some impedance  370  to a first input  352  of the operational amplifier  360 . The connection of the first input  352  is coupled to ground through a capacitor  365 . Additionally, an RF input signal  390  is injected into the base of the HBT power amplifier  300  through some capacitor  380 . Although the RF input and output signals are shown, they are not necessary to the discussion of the operation of the bias control of the power amplifier and are shown only for completeness.  
         [0016]    The output  353  of the operational amplifier is fed back and connected to the first input  352  of the operational amplifier in order to cause the operational amplifier to function as a voltage follower where the voltage appearing on a second input  351  of the operational amplifier  360  appears some minimal time later on the output  353  of the operational amplifier  360 . The second input  351  of the operational amplifier  360  is connected to an output of a voltage-to-current converter  368  as well as to the base through some impedance  372  and to the collector of the reference device  310 , in this case another HBT transistor. A control  366  of the voltage-to-current converter is connected to a voltage control signal  350  and the battery supply  340  is used to supply voltage to the voltage-to-current converter  368  through another input  367 .  
         [0017]    In operation, the present disclosure can use an external CMOS chip and bias control  355 , consisting of an operational amplifier  360  and a voltage-to-current converter  368  to bias the HBT power amplifier  300 . An analog voltage, V CONTROL    350 , adjusts the reference current, I REF    330 , through the reference device  310 . The VBE of this reference device is measured by the operational amplifier  360  and applied to the base of the HBT power amplifier  300 . The HBT power amplifier&#39;s collector current I C    342  reflects the reference current I REF    330  times the ratio of the size difference between the power amplifier  300  and the reference device  310 .  
         [0018]    This configuration of biasing a power amplifier transistor maintains several advantages over traditional methods. The voltage requirements are only 1 V BE  plus the overhead of the current source that typically is only a few hundred millivolts. Also, current through the reference device  310  is significantly less temperature dependent due to the high output impedance of the current source compared to a resistor. Additionally, the reference device  310  can be sourced from the normal battery source operating the power amplifier rather than having to create an independent stable reference. Other advantages are that I REF  is not a function of the battery voltage or of process leading to more stabilized control and linearity of the bias control. Additionally, the control voltage Vcontrol can operate the bias as low as Vcontrol=0 volts.  
         [0019]    [0019]FIG. 4 illustrates, in block diagram form a specific embodiment of the present disclosure that illustrates a biased power amplifier module  400 , such as that illustrated in FIG. 3, and a slope control circuit  405 .  
         [0020]    To assure appropriate resolution at their outputs and adequate stability under all conditions, power amplifiers are often specified to have a maximum power control slope. This maximum power control slope is the slope of the transfer function of output power as a function of control voltage. However, the use of power amplifiers with control voltages described herein results in a transfer curve having very steep transfer functions at specific certain control voltages. To decrease the power control slope, a slope smoothing circuit is used in the circuit of FIG. 4 to remove bias current from the biased power amplifier  400 . The amount of current that is removed is based on the voltage on the control electrode, e.g., the base-collector node of Q REF    310 . The amount of bias current that is removed is roughly proportional to the control voltage until Q REF    310  is turned completely on. After Q REF    310  is turned on, the amount removed is fairly constant. This removal of bias current in this manner results in the power amplifier turning on more slowly, resulting in a smoother power control slope, i.e., a smaller maximum power control slope. This can be better understood with reference to FIGS. 3-7.  
         [0021]    In operation, a control voltage is applied to the biased power amplifier module  400 , at an input labeled BIAS CTL. The biased power amplifier  400  receives an RF INPUT signal, at an input labeled RFIN, that is amplified to produce the signal RF OUTPUT at the output labeled RFOUT. The slope control circuit  405  receives a sink current I from an output of the biased power amplifier module  400  labeled Slope CTL C. The current I affects the output of the power amplifier and bias circuit  400  such that the transfer function from the control voltage to the RF OUTPUT will be smoother, as compared with the power amplifier and bias circuit without the slope control circuit. For example, Curve  410  of FIG. 5 represents the V CONTROL  to power output transfer function of a power amplifier device without the slope smoothing circuitry, while the curve  415  represents the transfer function of a power amplifier device with the slope smoothing circuitry. The transform function observed with the slope smoothing circuitry is a much smoother curve and a slope of approximately one-tenth the magnitude.  
         [0022]    [0022]FIG. 6 illustrates a specific embodiment of a slope control circuit  405  coupled to the power amplifier of FIG. 3. The slope control circuit of FIG. 6 comprises resistive element  431  coupled in series with a voltage reference source  435 , labeled V SLOPE,CTL . By selecting the value of V SLOPE,CTL  to be less than the threshold voltage, e.g. the reference voltage, of the reference device Q REF    310  a portion of the current supplied by the bias circuit  355  to the conductive element coupled to the collector of Q REF  is provided to the resistive element  431 . This results in less current being provided to the reference device Q REF . In one embodiment the value of V SLOPE,CTL  can be zero (0) volts. In other words, only a resistor  431  is needed in one embodiment.  
         [0023]    [0023]FIG. 7 illustrates another specific embodiment of the slope control circuit of FIG. 6, where the voltage supply device  435  has been implemented using a transistor  445  and an amplifier  446  as the voltage supply  435 . Specifically, the transistor  445  has a first current electrode coupled to the resistive element  431 , a second current electrode tied to a reference, such as ground, and a control electrode coupled to the first current electrode output of amplifier  446 . The amplifier  446  is a differential amplifier having a positive input coupled to the first electrode of the transistor  445 , and a negative electrode coupled to the voltage reference source V SLOPE,CTL .  
         [0024]    While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, the slope smoothing techniques can be used with various power amplifiers and power transistors.