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.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to power amplifiers and, more specifically, to a circuit and method for controlling the bias of a hetero-junction bipolar transistor (HBT) power amplifier.  
         BACKGROUND OF THE INVENTION  
         [0002]    Heterojunction bipolar transistor (HBT) power amplifiers are becoming the standard 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 Vref  130 . This configuration requires that a separate voltage Vref  130  (different from the battery voltage Vbat  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, Rref  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, Vref  230 , would need to be greater than is desirable for mobile communication applications and the solution is therefore not viable.  
           [0005]    What is needed is a method of controlling the bias of an HBT power amplifier transistor used in RF applications that provides stability over temperature and process.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a simplified electrical schematic of a prior art HBT diode based biasing circuit;  
         [0007]    [0007]FIG. 2 is a simplified electrical schematic of another prior art HBT biasing circuit; and  
         [0008]    [0008]FIG. 3 is a simplified electrical schematic of an HBT power amplifier bias controller according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0009]    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 EMODE. Similar reference numerals are used throughout the figures to represent similar features when possible.  
         [0010]    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 PA device to be an HBT transistor, other reference devices and PA devices are contemplated.  
         [0011]    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.  
         [0012]    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 .  
         [0013]    In operation, the present invention uses 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, Vcontrol  350 , adjusts the reference current, Iref  330 , through the reference device  310 . The Vbe of this reference devices 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 Ic  342  reflects the reference current Iref  330  times the ratio of the size difference between the power amplifier  300  and the reference device  310 .  
         [0014]    This configuration of biasing an HBT power amplifier transistor maintains several advantages over traditional methods. The voltage requirements are only 1 Vbe 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 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 Iref 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.  
         [0015]    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.