Abstract:
An RF amplifier circuit includes a MOSFET connected to an RF output of the circuit via an impedance matching network including an inductor and a tuning capacitor connected in parallel with the inductor and the MOSFET. DC voltage is applied to the MOSFET via a series path through a radio frequency choke and the inductor of the impedance matching network.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority of U.S. Provisional Application No. 61/084,006, filed Jul. 28, 2008, the complete disclosure of which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates in general to radio frequency (RF) power supplies for driving diffusion-cooled, sealed-off, CO 2  gas-discharge. The invention relates in particular to solid-state RF amplifier stages used in such power supplies. 
       DISCUSSION OF BACKGROUND ART 
       [0003]    Solid-state RF power supplies (power supplies based on RF transistors) are preferred over RF vacuum tube power supplies for driving discharges in diffusion-cooled, sealed-off, CO 2  gas-discharge lasers used in material processing applications. One reason for this preference is that solid-state power supplies are typically smaller than corresponding vacuum tube supplies and do not need to be driven by potentially lethal voltages. A single mode diffusion cooled CO 2  laser typically has an efficiency of about 10%. This means that a RF solid-state power supply having a 10 kilowatt (kW) output is required to provide 1.0 kW output from the laser. 
         [0004]    A high-power RF power supply typically includes an oscillator or frequency source connected to a series of pre-amplifier stages followed by a final power amplifier stage. Each stage includes at least one power transistor. One of the most common types of RF power transistors used in an amplifier stage is a metal oxide semiconductor field-effect transistor (MOSFET). One commercially available power MOSFET is a model NXP model BLF175, form Koninklijke Philips Electronics N.V. (Philips) of Eindhoven, Holland. This MOSFET has a power gain of between about 13 and 14 dB with an output power of approximately 40 watts (W) when operated in a class C mode at a 100 megahertz (MHz) operating frequency. 
         [0005]    The number of transistors per amplifier stage increases as the power is increasingly amplified. By way of example, a final amplifier stage of an RF power supply having 10 kW RF output, would require about 25 of the above-mentioned Philips NXP/BLF 175 MOSFETS (transistors) connected in parallel. 
         [0006]    Manufacturers of MOSFETs do provide power combined RF MOSFETs arranged in a push-pull arrangement in a single package to a yield higher power output than that of a single MOSFET. One such MOSFET is a Philips Model BLF 278A capable of 300 W output. A number of such MOSFET packages can be connected in parallel to yield even higher output powers. 
         [0007]    Simply stated: as the output power of the RF power supply increases, the total number of transistors increases dramatically. Consequently, limiting the number of circuit elements, such as resistors, inductors and capacitors, per stage of amplification becomes increasingly important with increasing power output from the standpoint of size and cost of the RF power supply. 
         [0008]    Each amplifier stage in series thereof has an output impedance matching network for matching the impedances of that stage to a following stage in this series in order to maximize power transmission to the following stage. One example of a prior-art amplification stage is schematically illustrated by the circuit diagram  10  of  FIG. 1 . 
         [0009]    Here, the impedance matching network includes an inductor L 1 , connected in series with the drain (D) side of a MOSFET Q 1  (in n-channel configuration) plus a parallel tuning capacitor C 1 . The RF input to the amplified is connected to the gate G of MOSFET Q 1 , with source S of the MOSFET being connected to ground. The matching network takes the output impedance of the MOSFET and translates it to the, usually lower, input impedance of the transistor to which it delivers RF power. The impedance of that transistor is represented in  FIG. 1  by a 50 Ohm (50Ω) resistor R 1 . C 1  is usually adjusted consistent with the inductance of L 1  to obtain 50Ω impedance. The load impedance value of 50 is a common value in practice, however, the load impedance value can have some other value, with C 1  selected accordingly. 
         [0010]    An RF coupling capacitor C 2  serves to prevent DC power from being fed to the input of the following stage of amplification. The value of C 2  is selected to provide a small reactance, and accordingly low loss, at the operating frequency of the amplifier. Only one MOSFET transistor is depicted in  FIG. 1  for simplicity of illustration. Those skilled in the RF art will recognize that MOSFET Q 1  could be substituted by a higher wattage, power-combined transistor package such as the above-mentioned Philips BLF 278A, to obtain a higher output power. 
         [0011]    An L 1 -C 1  impedance matching network as depicted in  FIG. 1  usually has transmission roll-off at frequencies beyond the operating frequency of the amplifier between about minus 6 dB and minus 10 dB per frequency octave. This serves to suppress resonances above the operating frequency of the preamplifier for stabilizing power output. Such a minus 6 dB roll-off is often inadequate for such harmonic suppression. Consequently, additional circuitry (not shown in  FIG. 1 ) is normally added to the preamplifier for harmonic attenuation, at a cost of additional electronic components. 
         [0012]    Each pre-amplifier in a series thereof is provided with an individual DC power input. In circuitry  10 , this is represented by a DC input port VCC+ to the amplifier. It is important to isolate the RF power output of the MOSFET from the DC power supply. This is usually accomplished with a large inductor, commonly called a RF “choke” (RFC), connected in series between the DC power supply and the MOSFET. This inductor is designated L 2  in  FIG. 1 . For additional isolation between the RF and DC, a large RF by-pass (RFBP) capacitor, C 3  is also provided. Capacitor C 3  is selected to have very low impedance over the RF frequency range of the amplifier. In practice several capacitors in parallel are usually required provide a low RF impedance path to ground. The combination of RFC L 2  and by-pass capacitor C 3  result in almost no RF energy entering the DC power supply circuitry. 
         [0013]    DC power from the DC supply passes through inductor (RFC) L 2  with only a very small loss, caused by the wire windings of the RFC. The RFC, however, presents a high impedance to the RF so only a very small amount of RF power is remaining after the RFC to enter the output port of the DC supply. To achieve a sufficiently high RF loss, the inductive reactance of the choke is usually chosen to be between about 10-times and 20-times the drain-impedance of the MOSFET. This requires that the RFC L 2  be very large. Large RFCs are known to exhibit poor high frequency characteristics which can contribute to amplifier instability. At high pulse repetition frequencies (PRF), pulse ringing, with high voltage peaks, is commonly encountered in the circuit of  FIG. 1 . The high voltage peaks of the pulse ringing arise because of the high value of the inductance in the RFC. Such high voltage ringing is undesirable because it deteriorates the reliability of the MOSFET. Because of this, additional circuitry (not shown) is usually added to suppress such ringing. 
         [0014]      FIG. 2  is a graph schematically illustrating computed transmission in decibels (dB) as a function of frequency in megahertz (MHz) for one example of a prior art, 40 W output preamplifier stage, constructed according to the circuit arrangement of  FIG. 1 , and having an operating frequency chosen as 81 MHz, indicated by circle  1 . Transmission at the second harmonic (162 MHz) is indicated by circle  2 . Transmission at the third harmonic (243 MHz) is indicated by circle  3 . It can be seen that second and third harmonics are attenuated by approximately −9 db and −16.25 dB respectively. The attenuation at 1 MHz is only approximately −18 dB which in most cases is considered insufficient. 
         [0015]    In summary, prior-art RF amplifier circuitry as represented schematically by circuitry  10  of  FIG. 1  has a relatively shallow roll-off at harmonic frequencies which leads to parasitic oscillation in the output. The circuitry also requires a large, heavy, and expensive RF choke in combination with a bank of low impedance capacitors to isolate a DC power supply for the amplifier from RF output of the amplifier MOSFET. In addition to size, weight, and expense, the large inductance RF choke contributes to pulse ringing and instability, a problem which increases with rapid rise and fall time of RF pulses as the pulse repetition frequency (PRF) increases. There are presently CO 2  laser material-processing applications that require PRFs as high as 200 kHz. To reduce or eliminate parasitic oscillation and pulse ringing in the circuit of  FIG. 1  additional circuitry is usually added. This additional circuitry further increases the size, complexity, and cost of a solid-state RF power supply that uses the prior-art circuitry. There is a need for circuitry that can mitigate if not eliminate shortcomings of prior-art circuitry, in order to facilitate development of CO 2  lasers having average power output of several kilowatts. 
       SUMMARY OF THE INVENTION 
       [0016]    In one aspect an RF amplifier circuit in accordance with the present invention comprises a transistor arranged to receive an RF input to be amplified. The circuit has an impedance matching network including a first inductor connected in series between the transistor and an RF output of the circuit, and a tuning capacitor connected in parallel with the transistor and the first inductor. A source of DC voltage is applied to the circuit between the first inductor and the RF output. A second inductor is connected between the DC voltage source and the amplifier circuit between the first inductor and the tuning capacitor and functions as an RF choke. 
         [0017]    Applying the DC voltage to the circuit between the first inductor and the tuning capacitor provides that the capacitance value of the tuning capacitor can be selected cooperative with the inductance value of the first inductor to maximize transmission of amplified RF from the transistor to the amplifier output at the RF input frequency, and also to resonate with the second inductor at that frequency. This allows the second inductor to be effective as an RF choke at an inductance value less than one-half of that which would be required in a prior-art amplifier of the same output power but wherein the DC voltage is applied via the second inductor directly to the transistor as described above with reference to  FIG. 1 . Other advantages of the inventive circuit are described in the detailed description of the present invention set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention. 
           [0019]      FIG. 1  schematically illustrates a prior-art RF amplifier circuit including a MOSFET connected to an RF output of the circuit via an impedance matching network including an inductor and a tuning capacitor connect in parallel with the inductor and the MOSFET, with DC voltage applied via an RFC directly to the MOSFET. 
           [0020]      FIG. 2  is a graph schematically illustrating computed transmission of the impedance matching network as a function of frequency for one example of the amplifier circuit of  FIG. 1 . 
           [0021]      FIG. 3  schematically illustrates an RF amplifier circuit in accordance with the present invention similar to the circuit of  FIG. 1  but wherein the DC voltage is applied to the MOSFET via the RFC and the inductor of the impedance matching network. 
           [0022]      FIG. 4  is a graph schematically illustrating computed transmission as a function of frequency of RF power from the MOSFET to the RF output of one example of the amplifier circuit of  FIG. 3 , wherein the capacitance value of the tuning capacitor is selected cooperative with the inductance of the inductor to maximize transmission of the impedance matching network and to resonate with the RFC. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring again to the drawings, wherein like features are designated by like reference characters,  FIG. 3  is a circuit diagram schematically illustrating a preferred embodiment  20  of RF amplifier circuitry in accordance with the present invention. Circuitry  20  is similar to circuitry  10  of  FIG. 1  with a major difference being that DC power is connected at the high impedance (50Ω) output side of the impedance matching network formed by inductor L 1  and capacitor C 1  instead of being connected directly to the low impedance (typically about 12Ω) output (drain D) of MOSFET Q 1 . 
         [0024]    Inserting the DC power at the 50 Ohm output side of the L 1 -C 1  matching network has several benefits. One benefit is that the DC power supply is now rendered relatively insensitive to the RF output of the MOSFET by decoupling RFC L 2  and by-pass capacitor C 3  from the drain of the MOSFET. Another advantage is that inductor C 1  can function as a composite capacitor which can serve both to tune the impedance matching network to the required value, here, nominally 50Ω, and to resonate the RFC inductor L 2  to the operating frequency of the amplifier. By resonating the RFC to the amplifier frequency, the inductance of the RFC can be less than that required in the prior-art circuitry, which means that the choke can be correspondingly smaller, lighter and less expensive, in addition to providing performance advantages such as reduced pulse ringing and spurious oscillation. 
         [0025]    Normal convention is to select an inductance reactance for the RFC to be, say 20 times 50Ω, i.e., 1000Ω. At a frequency of 81 MHz (see  FIG. 2 ), for example, the inductance required to obtain an impedance of 1000 Ohms is approximately 2 micro-Henrys (μHy). There is a high probability that this value of inductance would be self resonant at a frequency other than the desired operating frequency, wherever the MOSFET has gain. Each undesired resonance can lead to spurious oscillation or amplifier instability. At 81 MHz an inductor of 100 nano Henrys (nHy) has a reactance of 50Ω which is equal to the design impedance of the preamplifier. The desired impedance of the matching network is obtained by adjusting the value of the composite capacitor C 1  once the inductance of inductor L 1  is selected. The capacitor C 1  is tuned to be in resonance with the choke at the RF frequency which helps to minimize any unwanted resonances. 
         [0026]    By way of example at 81 MHz, a 69.7 picofarad (pF) capacitor is required to resonate with a value of 42.0 nHy selected for inductor L 1  of the impedance matching network. Optionally, an additional capacitance can be added in parallel to C 1  so that the composite C 1  capacitor can also resonate with the RFC at the amplifier operating frequency while still providing variable matching for the L 1 -C 1  impedance matching circuit. By way of example, for a value of L 2  equal to 100 nHy, the amount of added capacitance is 38.8 pF for a total capacitance of 108.5 pF for C 1 . 
         [0027]      FIG. 4  is a graph schematically illustrating transmission in dB as a function of frequency in MHz for one example (bold curve) of a, 40W-output preamplifier stage constructed according to the inventive circuit arrangement of  FIG. 3  and having an operating frequency chosen as 81 MHz represented by circle  1 . A dashed curve shows, for comparison, the performance of the prior art example of  FIG. 2 . 
         [0028]    The transmissions at the second harmonic (162 MHz-circle  2 ) and third harmonic (243 MHz-circle  3 ) are attenuated by approximately −11.5 and −19.7 dB respectively. These vales are an appreciable improvement over the −9 dB and −16.75 dB values obtained for the prior-art example. At 1MHz the transmission for the inventive circuitry was found to be −25.8 dB compared with −18 dB for the prior-art circuitry. The bandwidth of the inventive circuitry is somewhat less than that of the prior-art circuitry but is still more than adequate for most applications, such as dual frequency discharge ignition and maintenance, where some limited tunability of the output frequency is desirable. 
         [0029]    In TABLE 1 are listed assumed values for components and parameters used to generate the graphs of  FIGS. 2 and 4 . Of particular note is the 2.5 times reduction in inductance of the RFC for the inventive circuitry. 
         [0000]                            TABLE 1               Component/Parameter   Prior-Art Value   Inventive Value                   L2 (RFC)   250 nHy   100 nHy       L1   42.1 nHy   42.1 nHy       C1   74.9965 pF   108.5 pF       MOSFET Output Impedance   12 Ohms   12 Ohms       R1 (Load Impedance)   50 Ohms   50 Ohms       Operating Frequency   81 MHz   81 MHz                    
The value of the tuning capacitor (C 1 ) is approximately 45% larger when compared with the value for the prior-art invention circuitry. The advantages incurred by C 1  resonating both with L 1  and the RFC at the operating frequency of the amplifier, however, are an excellent trade-off with the increased value of C 1  in the inventive circuitry.
 
         [0030]    A comparison of the transmission versus frequency improvements between the prior-art circuitry and the inventive circuitry of TABLE 1 is provided in TABLE 2. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Transmission 
                   
                 Improvement 
                   
               
             
          
           
               
                 Frequency 
                 Prior Art 
                 Inventive 
                 Δ 
                 % 
               
               
                   
               
               
                 162 MHz 
                   −9 dB 
                 −11.5 dB 
                  −2.5 dB 
                 27.8 
               
               
                 243 MHz 
                 −16.5 dB 
                 −19.7 dB 
                 −2.95 dB 
                 17.6 
               
               
                  1 MHz 
                   −18 dB 
                 −25.8 dB 
                   −7 dB 
                 43.3 
               
               
                   
               
             
          
         
       
     
         [0031]    It can be seen that the inventive circuitry provides a 27.8% improvement in transmission reduction at the second harmonic of 81 MHz, namely 162 MHz; a 17.6% improvement at the third harmonic (243 MHz); and a 43.3% improvement at 1 MHz. These performance improvements are obtained while also providing lower costs and smaller size for the RFC and more stable amplifier characteristics attendant on that smaller size. 
         [0032]    Those skilled in the art to which the present invention pertains will recognize that while the circuitry in accordance with the present invention is discussed in the context of a pre-amplifier stage of an RF power supply, the circuitry can also be used as a stand alone RF power stage to drive a laser having sufficiently low output power. Those skilled in the art will also recognize that any single electronic component of the above-described inventive circuitry may be replaced with a combination of two or more like components to provide a particular value or function. 
         [0033]    In summary present invention is described above in terms of a preferred embodiment. The invention is not limited, however, to the embodiment described and depicted. Rather, the invention is limited only by the claims appended hereto.