Abstract:
A power amplifier incorporates base ballasted hetero-junction bipolar transistors (HBTs) in parallel. A transistor pair adjusts the voltage applied to the base ballast if it senses that the voltage appearing between one of the HBT transistors and its base ballast is drooping to a level not strong enough to keep the HBTs active.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to power amplifiers and, more specifically, to a circuit and method for adjusting the voltage at the commonly connected ballasted ends of a plurality of base ballasted HBTs forming a power amplifier when the voltage appearing at one of the bases of the plurality of HBTs drops below a threshold.  
         BACKGROUND OF THE INVENTION  
         [0002]    Hetero-junction bipolar transistor (HBT) power amplifiers are becoming the standard for cellular applications due to their high power density and reduction in die size. HBT&#39;s can draw substantial base current during high power operation. Since multi-finger power devices suffer from thermally related current collapse they cannot be operated without ballast resistors on either the emitter or base. Emitter ballasting is not practical for power amplifiers because of the high emitter currents and small resistor values, so base ballasting must be used. Because an individual HBT is a very small device, it is required to be paralleled with multiple HBTs to achieve high power operation required for most power amplifier applications.  
           [0003]    Accordingly, HBT power amplifiers such as those used in radio frequency (RF) applications employ multiple small devices connected in parallel. As noted above, each of these small HBT devices require a ballast resistor to be connected to its base before being connected to the other HBTs forming the power amplifier (PA). The resistive ballasting of individual cells keeps parallel HBT fingers from entering thermal collapse. Additionally, a capacitor may be used to bypass the base resistor to preserve high frequency gain or the RF signal may be fed to the base connections through a separate capacitor. For purposes of illustration, the figures contained herein will illustrate the principal using capacitor bypassed ballast resistors although those skilled in the art will realize that this embodiment of the invention will work the same regardless of the connection of the RF capacitors feeding the base connection.  
           [0004]    [0004]FIG. 1 shows a typical multi-fingered base ballasted Power Amplifier (PA) circuit. A plurality of HBTs  110 , each ballasted with a resistor  130 /capacitor  120  are connected in parallel. For each small HBT device, first ends  130   a ,  120   a  of a resistor  130  and a capacitor  120  are connected to the base  185  of the individual HBT devices and the other ends  130   b ,  120   b  of the resistor and the capacitor become the input  180  of each base ballasted HBT device  190 . For purposes of simplicity, when two or more base ballasted HBT devices  190  are “connected in parallel”, their collectors  160  share a first common node, their emitters  170  share a second common node connected to ground, and the inputs  180  share a third common node. A radio frequency signal is received at the input  140  and connected to the commonly connected inputs  180  of the base ballasted HBT devices  190 . The commonly connected collectors  160  that are connected to a voltage source  155  produce an amplified RF output  150 .  
           [0005]    Due to the base current requirements, a biasing circuit  195  is usually included. Typical biasing circuits with RF decoupling components neglected for simplicity are shown in FIGS. 2 and 3. FIG. 2 shows a diode biasing circuit  200 . The base of an HBT device  210  is connected to the collector and the collective inputs  180  of the of the base ballasted HBT devices  190  of FIG. 1. The emitter is connected to ground. The first end  220   b  of a reference resistor  220  is connected to the collector and base while the second end  220   a  of the reference resistor  220  is connected to a reference voltage  230 .  
           [0006]    [0006]FIG. 3 shows a preferred current mirror biasing circuit  300 . A current mirror is formed by HBT devices  310  and  320 . The collector of the first HBT device  310  is connected to a voltage source  350 , its emitter is connected to the base of the second HBT device  320  and its base is connected to the collector of the second HBT device  320 . The emitter of the second device is connected to ground. And finally, the first end  330   b  of a reference resistor  330  is connected to the base of the first HBT device  310  and the collector of the second HBT device  320  while the second end  330   a  of the reference resistor is connected to a reference voltage  340 .  
           [0007]    Although not exhaustive, these biasing circuits are typical of those employed in the industry although other types of biasing circuits are contemplated and may be used with the invention. The biasing circuits try to keep the current through the power device constant with variations in temperature and reference voltages. Although either of these biasing circuits or others could be used, current mirror biasing is typically preferred and will be used in the discussion.  
           [0008]    Using the current mirror of FIG. 3 as the Bias of FIG. 1, the reference voltage  340  and the reference resistor  330  form a constant current source which is mirrored by the first HBT device  310  and the second HBT device  320 . If no ballast resistors  130  were required, the current mirror would be adequate up to the limits of the HBT devices  310  and  320 . However, with ballast resistors and during high power operation, the current mirror is unable to keep the voltage on the bases  185  of the individual HBT power device cells  110  constant because of the drop on the ballast resistor. In power operation when more HBTs are connected in parallel, increased base current is required from the current mirror. This strain on the current mirror results in increased voltage drops across the ballasting resistors  130  resulting in the voltage at the base of the individual HBT devices  110  to droop, limiting linearity and maximum output power.  
           [0009]    What is required is an improved HBT power amplifier circuit that doesn&#39;t effect the quiescent point at lower output powers, but comes into play when higher powers are being generated that effectively prevents this drooping from occurring. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a simplified electrical schematic of a prior art base ballasted HBT power amplifier;  
         [0011]    [0011]FIG. 2 is a simplified electrical schematic of a diode bias circuit occasionally used in HBT power amplifiers;  
         [0012]    [0012]FIG. 3 is a simplified electrical schematic of a current mirror bias circuit typically used in HBT power amplifiers;  
         [0013]    [0013]FIG. 4 are a simplified electrical schematic of a base ballasted HBT power amplifier including an HBT linearizer and power booster according to one embodiment of the invention; and  
         [0014]    [0014]FIG. 5 is a flow chart demonstrating a method of increasing linearity and boosting power of an HBT power amplifier according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    Referring to FIG. 4, one model of an embodiment of a base ballasted HBT power amplifier with an HBT linearizer and Power Booster is shown. Similar reference numerals are used throughout the figures to represent similar features when possible.  
         [0016]    A plurality of base ballasted HBT devices  190  are connected in parallel. Each of the ballasted HBT devices  190  have a ballast resistor  130  connected at one end to the base of the HBT cell  110  and the other producing an input  180  of the base ballasted HBT device  190 . A ballast capacitor  120  may be connected across the ballast resistor as shown. Although the embodiment shown includes a resistor biasing scheme with a bypassing capacitor, other ballasting schemes are possible such as bringing in the RF signal to each base node through an individual capacitor, often called split ballasting. As split ballasting is well know in the industry, a detailed description is not included for simplicity. A plurality of base ballasted HBT devices  190  are connected in parallel such that their inputs  180  share a common node and receive an RF input signal, their collectors  160  are connected to a voltage source  155  and produce an RF output signal  150 , and their emitters  170  share a node and are connected to ground.  
         [0017]    At least one base  185   a  of one of the HBT devices is connected through a resistor  430  to a base  412  of an HBT transistor  410 . The emitter  411  of this HBT transistor  410  is connected to ground  360  while the collector  413  is connected to a base  422  of another HBT transistor  420 . The emitter  421  of the HBT transistor  420  is connected to the commonly connected inputs  180  of the base ballasted HBT devices  190  while the collector is connected to a voltage source  350 . A resistor  450  is connected such that one end connects with the base  422  of HBT transistor  420  and collector  413  of HBT transistor  410  and the end of the resistor connects to the voltage source  350 . A capacitor  440  is connected between the base  412  of the HBT transistor  410  and ground  360  such that resistor  430  and capacitor  440  form a low pass filter for the signal detected from at least one base  185   a  of one of the HBT devices  110  forming the power amplifier.  
         [0018]    A bias circuit  195  of the current mirror type is connected such that the collector of the first HBT device  310  is connected to a voltage source  350 , its emitter is connected to the base of the second HBT device  320  and its base is connected to the collector of the second HBT device  320 . The emitter of the second device is connected to ground  360 . And finally, the first end  330   b  of a reference resistor  330  is connected to the base of the first HBT device  310  and the collector of the second HBT device  320  while the second end  330   a  of the reference resistor is connected to a reference voltage  340 .  
         [0019]    In operation, the HBT device  410  senses the voltage appearing on one of the bases  185   a  of at least one of the HBT device cells  110  through the low pass filter formed by resistor  430  and capacitor  440 . Resistor  450  is set such that at quiescent or low power operation, HBT device  420  supplies negligible current to the commonly connected inputs  180  of the base ballasted HBT devices  190  allowing the power device to be entirely controlled by the current mirror bias formed by HBT devices  310  and  320 .  
         [0020]    During high power operation, if the voltage on the base of the power HBT device cell  100  sags, it is detected across the low pass filter by HBT device  410 , which begins to turn off. As HBT device  410  begins to turn off it caused the base voltage appearing on the base  422  of HBT transistor  420  to increase. An increase voltage at the base  422  of HBT transistor  420  causes an increased voltage to appear on the emitter of HBT transistor  420  which drives the commonly connected inputs  180  of the base ballasted HBT devices  190 . Thus, at high power operation, the voltage boost circuit created by HBT devices  410  and  420  takes over the power amplifier from the normal current mirror biasing circuit. Since this added circuitry supplies a higher voltage supply than the current mirror alone, it keeps the base bias at needed values further into compression and improves linearity and boosts power.  
         [0021]    [0021]FIG. 5 is a flowchart demonstrating a method of linearizing the HBT power amplifier and boosting power during drooping of high power operations according to one embodiment of the invention. When the circuit is on, in step  520 , the power booster and linearizer constantly monitors the voltage appearing directly at the base of one of a plurality of base ballasted HBT devices connected in parallel where the base ballast of the individual HBT devices may cause a voltage difference to occur between the input to the plurality of base ballasted devices and the bases of the HBT cells. If the voltage appearing directly at the base of at least one of the HBT cells begins to droop in step  530 , the circuit provides supplemental power to the plurality of parallel connected base ballasted HBT devices in step  540 , otherwise, the circuit passively provides negligible quiescent current to the plurality of base ballasted HBT devices. This allows the circuit to predictably run according to any well known bias that has been implemented to control the power amplifier.  
         [0022]    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.