Abstract:
According to one embodiment, a circuit is disclosed. The circuit comprises a solid state power amplifying device, an input impedance matching circuit and an output impedance matching circuit coupled to the solid state amplifying device. The input impedance matching circuit includes an input pitchfork trace pattern. The output impedance matching circuit includes an output pitchfork trace pattern. The circuit further discloses an input bias circuit and an output bias circuit.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to the field of solid-state power amplifying devices. 
     BACKGROUND 
     It is widely known that improving output current balance of the die within solid-state, power amplifying devices results in performance improvement of gain, efficiency, peak output power and linearity. An area of amplifier performance enhancement that has heretofore been overlooked is the utilization and optimization of the amplifier circuit components to assist in balancing the output current distribution of the die of the amplifying device. Therefore, a method of balancing a solid state, power amplifying device is desired. 
     SUMMARY 
     According to one embodiment, a method of operating a power-amplifying device is disclosed. The method includes applying one or more circuit techniques in order to balance the output current of the solid state device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention can be obtained from the following detailed description in conjunction with the following drawings, in which: 
     FIG. 1 is a block diagram of one embodiment of a radio frequency amplification circuit; 
     FIG. 2A is a diagram of one embodiment of a radio frequency power BJT coupled to an input impedance matching circuit; 
     FIG. 2B is a diagram of one embodiment of a radio frequency power BJT coupled to an output impedance matching circuit; 
     FIG. 3A is a diagram of one embodiment of a base bias circuit coupled to a radio frequency power BJT; and 
     FIG. 3B is a diagram of one embodiment of a collector bias circuit coupled to a radio frequency power BJT. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
     FIG. 1 is a block diagram of one embodiment of a radio frequency amplification circuit  100 . Circuit  100  includes an input impedance matching circuit  110 , an output impedance matching circuit  120 , a radio frequency (RF) power bipolar junction transistor (BJT)  140 , a base bias circuit  170  and a collector bias circuit  180 . According to one embodiment, circuit  100  receives input RF signals at input impedance matching circuit  110 , amplifies the signal and transmits the amplified signal from output impedance matching circuit  120  to a load (not shown). In other embodiments, BJT  140  may comprise other solid state amplifying devices (e.g., HBT). 
     INPUT IMPEDANCE MATCHING CIRCUIT 
     As described above, input impedance matching circuit  110  is designed to receive RF signals. According to one embodiment, the impedance at the interface between the RF input and input impedance matching circuit  110  is 50 Ω. Input impedance matching circuit  110  transforms the impedance from the level of the RF input to the impedance of BJT  140 . FIG. 2A is a diagram of BJT  140  coupled to input impedance matching circuit  110 . 
     Referring to FIG. 2A, input impedance matching circuit  110  includes a multi-section “pitchfork feed”  220 . According to one embodiment, pitchfork feed  220  is a printed trace that is configured to provide a balanced current feed into an input lead  250  of BJT  140 . Typical printed traces are relatively wide single lines that feed input lead  250  of BJT  140 . However, whenever circuit  100  is operating at high frequency there is typically a higher current density towards the outside edges of the wide single trace. Such an occurrence results in an unbalanced current feed into input lead  250  of BJT  140 . Therefore, pitchfork feed  220  provides for balanced current flow into input lead  250  of BJT  140  by evenly dividing the current across multiple connected traces resulting in a more uniform current distribution into input lead  250  of BJT  140 . 
     Input impedance matching circuit  110  also includes series resistors  230  within branches of the pitchfork feed  220  traces. Resistors  230  further equalize the current paths into input lead  250  of BJT  140  so that the current will not prefer one side of the pitchfork feed  220  to the others. In addition, resistors  230  reduce the likelihood of low frequency oscillation of the high frequency BYT  140 . According to one embodiment, each resistor  230  has a 4.7 Ω resistance. Nevertheless, one of ordinary skill in the art will appreciate that other values for resistors  230  may be used. 
     Input impedance matching circuit  110  further includes resistors  235 . Resistors  235  are placed pairs of branches of pitchfork feed  220  to further equalize the current between any two branches of pitchfork feed  220 . For example, imbalances between the top two branches of pitchfork feed  220  are reduced by the resistor  235  between the two. According to one embodiment, each resistor  230  has a  100  resistance. Nevertheless, one of ordinary skill in the art will appreciate that other values for resistors  230  may be used. 
     OUTPUT IMPEDANCE MATCHING CIRCUIT 
     Output impedance matching circuit  120  is coupled to an output lead  270  of BJT  140 . Output impedance matching circuit  120  transforms the impedance from the level of output lead  270  of BJT  140  to the impedance level of load coupled to circuit  100 . According to one embodiment, the impedance at the interface between output impedance matching circuit and the load is  50 . FIG. 2B is a diagram illustrating output lead  270  of BJT  140  coupled to output impedance matching circuit  120 . 
     Referring to FIG. 2B, output impedance matching circuit  120  includes a multi-section pitchfork feed  260  similar to pitchfork feed  220  in input impedance matching circuit  110 . In addition to the advantages described above, the pitchfork feed  260  configuration in output impedance matching circuit  120  also presents a low impedance at the second and third harmonic frequencies to the output of BJT  140 . The low impedance at the harmonic frequencies minimizes the RF voltage peaks at the output of BJT  140 . 
     BASE BIAS CIRCUIT 
     Base bias circuit  170  connects a power supply voltage to BJT  140  without having an affect on the RF signal amplified by BJT  140 . According to one embodiment, base bias circuit  170  presents a low impedance, resistive load to the bases of BJT  140  at frequencies from 1 MHz to one-third of the operating RF frequency of BJT  140 . In addition, base bias circuit  170  delivers the appropriate amount of DC current to the base of BJT  140  to optimize RF performance. FIG. 3A is a diagram of one embodiment of base bias circuit  170  coupled to BJT  140 . According to one embodiment, bias circuit  170  includes a set of resistors. The resistors are coupled between a supply voltage (V BB ) and the base of BJT  140 . 
     COLLECTOR BIAS CIRCUIT 
     Collector bias circuit  180  connects a DC power supply voltage to BJT  140  without affecting the RF signal amplified by BJT  140 . According to one embodiment, bias circuit  180  results in uniform voltage across the entire lead  270  of BJT  140  coupled to the collectors of transistor  240 . FIG. 3B is a diagram of one embodiment of collector bias circuit  180  coupled to BJT  140 . Bias circuit  180  includes a transient voltage suppressor  310 , a capacitor (C) and an inductor (L). Transient voltage suppressor  310  is connected between a supply voltage (V cc ) and ground. 
     According to one embodiment, V cc  supplies 45-50 volts DC at  10 A to the collector of BJT  140 . Transient voltage suppressor  110  suppresses voltage spikes within circuit  100  caused during the switching between high and low current levels. In one embodiment, transient voltage suppressor  110  is implemented using a diode. However, one of ordinary skill in the art will appreciate that other fast voltage clipping devices may be used to implement transient voltage suppressor  110 . 
     Inductor L is coupled between the supply voltage and BJT  140 . Inductor L provides a predetermined impedance value that prevents RF current flow from BJT  140  through bias circuit  180 . However, according to one embodiment, inductor L is designed to be sufficiently small so as to minimize voltage spikes caused by transient currents that occur due to changing current through the circuit. For example, whenever the output power of circuit  100  is quickly switched from low to high (e.g., 50 ns rise/fall time), or vice versa, the current flow through inductor L changes, resulting in a transient voltage spike. The larger the inductance of inductor L, the higher the magnitude of the voltage spike. In cases where the voltage spike is sufficiently large, severe damage to BJT  140  may occur. Therefore, the small size of inductor L and the presence of transient voltage suppressor  110  permits BJT  140  to operate at higher voltages (e.g., 50 volts). 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that any particular embodiment shown and described by way of illustration is in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as the invention.