Patent Publication Number: US-6714095-B2

Title: Tapered constant “R” network for use in distributed amplifiers

Description:
TECHNICAL FIELD 
     The present invention relates generally to constant “R” networks and, more particularly to a tapered constant “R” network for use in high power, high frequency distributed amplifiers. 
     BACKGROUND OF THE INVENTION 
     High powered, high frequency distributed amplifiers are well known in the art, having been around since the 1940&#39;s. Distributed or traveling wave techniques have been used to design distributed amplifiers comprising microwave GaAs FETs that operate from 2.0 to 20 GHZ. A discussion of distributed amplifier design is taught in the book entitled “Microwave Circuit Design Using Linear and Non-Linear Techniques” published by John Wiley &amp; Sons in 1990, pages 350-369. 
     The aforementioned prior art reference teaches the use of both constant K and constant R networks comprising series inductances and shunt capacitances, the latter of which is generally provided by the parasitic drain-to-source capacitance of a FET that is coupled between the series inductances of the network. Multiple sections of these networks are generally cascaded together and, by adjusting the individual phase shift therethrough, the respective gains of each FET stage will add along the associated transmission lines, as is well understood. 
     Prior art constant “R” distributed amplifiers as aforementioned have generally been fabricated on GaAs substrates. Because the GaAs substrate is formed of a single layer, the efficiency and bandwidth of these amplifiers has been limited. One reason for this is that mutual conductance coupling factor of the series inductances is limited since the series inductance is formed, for an example, by using interwoven spiral transmission lines formed on the surface of the single layer substrate. 
     Hence, a need exists for an improved, high efficiency, broadband power amplifier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the appended figures, wherein like numerals denote like elements, and in which: 
     FIG. 1 is an exploded perspective view of the LC structure of the present invention shown connected to parasitic capacitance of a FET device of distributed amplifier forms a novel constant “R” network; 
     FIG. 2 is a lumped-element schematic of the constant “R” network of the present invention; 
     FIG. 3 is an exploded perspective view of several layers of a multi-layer low temperature co fired ceramic structure on which the constant “R” network of a distributed amplifier is formed in accordance with the present invention; and 
     FIG. 4 is a schematic representation of a constant “R” FET distributed amplifier of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Turning now to the figures, in particular, FIGS. 1 and 3, the high frequency distributed amplifier of the present invention will now be described. An LC structure  10  is illustrated in FIG. 1 that is comprised of multiple transmission lines  16 ,  18 ,  20 ,  22 ,  24 ,  26 ,  28  and  30 . As will fully be explained hereinafter, these multiple transmission lines are spaced a predetermined vertical distance apart and are electrically connected by metallic connectors  32 ,  34 ,  36 ,  38 ,  40 , and  42  respectively. As illustrated in FIG. 3, metallic transmission line  16  is formed on upper planar surface of ceramic layer  52 . Similarly, transmission line  18  is formed on the upper planar surface of ceramic layer  54 . Ceramic layer  54  is shown having via  58  formed at the beginning end of transmission line  18  which directly overlays the distal end of transmission line  16 . As understood, during the fabrication of multi-layer ceramic structure  50 , metallic connector  32  is formed through via  58  to electrically connect transmission line  18  to transmission line  16 . Likewise, via  60  is formed through ceramic layer  56  while transmission line  20  is formed on the upper planar surface thereof. Metallic connector  34  is then formed through via  60  to electrically connect the distal end of transmission line  18  to the beginning end of transmission line  20 . In a continuing manner, each of the remaining transmission lines  22 ,  24 ,  26 , and  28  are formed on the upper planar surfaces of multiple ceramic layers (not shown) respectively. Vias are formed through the multi ceramic layers for connecting the distal end of the next lower transmission line to the beginning end of the next upper transmission line in the same manner as shown in FIG.  3 . Hence, as illustrated in FIG. 1, metallic connectors  36 ,  38 ,  40 , and  42  electrically connect transmission lines  20  to  22 ,  22  to  24 ,  24  to  26 , and  26  to  28  respectively. Thus, in the case of the LC network shown in FIG. 1, there would be at least seven ceramic layers, each having bottom and top planar surfaces the latter of which the aforementioned transmissions are formed respectively thereon. As further illustrated in FIG. 1, LC structure  10  is centered tapped at  30  to provide an output  44 . Output  44  is coupled at  46  to a capacitance C DS , the parasitic capacitance of a FET for instance, as will be described hereinafter. 
     Turning to FIG. 2, the ideal high frequency equivalent of LC structure  10  is shown at  46 , which, when connected to the drain of FET  48  at  44 , functions as a constant “R” network as is understood. Thus, inductance Ld/2 established between end  12  and node  44  (the center tap point  30 ) at the frequency of operation is equal to the inductance created by transmission lines  16 ,  18 ,  20 , and one-half of transmission line  22 . Similarly, the inductance Ld/2 established between node  44  and end  14  is equal to the inductance created by transmission lines  24 ,  26 ,  28 , and the latter one-half of transmission line  22 . The total capacitance, C S , established between end  12  and end  14  is the sum of the individual capacitances created between adjacent transmission lines and the thickness of the ceramic layer therebetween. The value of C S  can be tailored by, among other things, varying the thickness of the ceramic layers and the widths of the transmission lines. By tightly wrapping overlaying transmission lines of LC structure  10 , the mutual inductance M can be maximized. LC transmission line structure  10  is illustrated as being coupled to the drain of FET  48  the source of which is returned to ground potential. C DS  is the parasitic drain to source capacitance of FET  48  and varies with the size thereof. 
     Hence, what has been described above is a novel constant “R” network  46  formed using multiple low temperature co fired ceramic layers that form a complete ceramic structure. The inductances and capacitances associated with network  46  are balanced and if necessary can be adjusted by varying ceramic layer thickness, transmission line widths and the tightness of the inductance wrap. Although LC transmission line structure  10  is shown as being rectangular in shape it is not conclusive. LC transmission line structure  10  could be any numbered of geometric shapes such as a spiral and a square for instance. 
     Turning to FIG. 4, simplified high frequency distributed amplifier  70  is shown that incorporates constant “R” networks described above. Amplifier  70  is formed of low temperature co fired ceramic (LTTC) structure  50 . Distributed amplifier  70  includes multiple cascaded constant “R” networks  77   a ,  77   b  through  77   n  with their associated FETs  78   a ,  78   b  through  78   n . The cascaded constant “R” networks form a “transmission line” for coupling an input wave signal across outputs  80  and  82 . The drains of the FETs comprising distributed amplifier  70  are terminated by drain termination  72 . An input signal is applied across input terminals  74  and  76 , the latter of which is coupled to ground reference. The series inductances consisting of L g /2 form an artificial transmission line between input terminal  74  and gate termination  84 . 
     In operation, an input signal applied across inputs  74  and  76  will travel down the transmission line and be proportionally coupled to each of the gate electrodes of respective FETs  78   a - 78   n . Each of the FETs of a respective cascaded constant “R” network provides gain from its gate to drain and propagates the amplified signal down the drain transmission line formed by the constant “R” network as understood. Each FET gain stage provides a predetermined phase (φ) delay from gate to drain. By using drain and gate tapering techniques at each FET gain stage, the phase delayed signals can be added to provide overall amplification of the input signal that appears at outputs  80  and  82 . Additionally, tapering each constant “R” network, each individual FET gain stage will have the same load impedance to the traveling input wave signal to provide maximum efficiency and amplification through the distributed amplifier. The constant “R” networks are tapered for loading the input signal applied thereto by, among other techniques, changing the lengths and widths of the transmission lines forming the inductance, L, as well as the individual capacitance of CS. 
     Hence, what has been described above is a novel tapered constant “R” network distributed amplifier incorporated into a multi-layer low temperature co fired ceramic structure. By using gate and drain tapering along with the cascaded constant “R” networks the amplifier exhibits a wide bandwidth while using large periphery semiconductor power devices. In addition, by fabricating the tapered constant “R” network distributed amplifier in a multi-layer low temperature co fired ceramic structure, the tight coupling coefficients, which are required to realize the constant “R” networks make the aforedescribed novel amplifier practical to make. Thus, a low cost high efficiency broadband power amplifier is achieved using the teaching of the present invention, which can be used in software defined radio applications for example.