Abstract:
A distributed amplifier may include an input transmission line for receiving on an input end an input signal, and an output transmission line for outputting on an output end an output signal. A plurality of amplifier stages may be coupled between intermediate positions on the input and output lines. Feedback impedance may negatively feed back a signal on the output end of the output line to a second end of the input line spaced from the first end of the input line.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/030,819, filed Feb. 22, 2008, which application is incorporated herein by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     Microwave circuits and more particularly distributed microwave circuits are disclosed. Such circuits and associated methods are applicable to telecommunications and other industries in which signals are processed. 
     Conventional circuits use lumped elements cascaded with isolated separate signal paths. Distributed integrated systems and circuits may rely on shared signal paths that may result in strong electromagnetic couplings between circuit components. A distributed amplifier includes a shared input transmission line and a shared output transmission line. A plurality of transistors, such as field-effect transistors (FETs), connect the input and output transmission lines at spaced locations and provide gain through multiple signal paths. A signal on the input transmission line, also referred to as a gate transmission line, is amplified by each transistor. An incident wave on the output transmission line, also referred to as a drain transmission line, travels toward the output in synchronization with the traveling wave on the input line. Each transistor adds power in-phase to the signal at each tap point on the output line. A forward-traveling wave on the gate line and any backward-traveling wave on the drain line are absorbed by terminations matched to the loaded characteristic impedance of the input line and output line, respectively, to avoid reflections. 
     Since FETs have intrinsic input capacitance and output capacitance, in general, the presence of these capacitances limits the bandwidth of operation of the FET when used in a conventional amplifier. However, with the distributed approach, the input and output capacitances of the FET become part of the propagation networks forming artificial transmission lines. In this manner, major band-limiting effects of the input and output capacitances of the transistors in reducing frequency bands of operation of the amplifier may be avoided. 
     SUMMARY 
     A distributed amplifier may include an input transmission line for receiving on an input end an input signal, and an output transmission line for outputting on an output end an output signal. Feedback impedance may couple the output end of the output line to a second end of the input line spaced from the first end of the input line. The feedback impedance may negatively feed back a signal on the output line to the input line. A plurality of amplifier stages may be coupled between intermediate positions on the input and output lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit schematic of a conventional distributed amplifier. 
         FIG. 2  is a circuit schematic of a first distributed amplifier having feedback. 
         FIG. 3  is a circuit schematic of a second distributed amplifier having feedback. 
         FIG. 4  is a plan view of an embodiment of the second distributed amplifier illustrated in  FIG. 3 . 
         FIG. 5  is a chart illustrating measured gain and noise factor for the distributed amplifier of  FIG. 1 . 
         FIG. 6  is a chart illustrating measured gain and input and output return-loss for the distributed amplifier of  FIG. 1 . 
         FIG. 7  is a chart illustrating measured gain and input and output return-loss for the distributed amplifier of  FIG. 3 . 
         FIG. 8  is a chart illustrating measured gain and noise factor for the distributed amplifier of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a conventional distributed amplifier  20 . Amplifier  20  includes an input transmission line  22 , an output transmission line  24 , and a plurality of active devices  26 . The active devices may be transistors, such as field-effect transistors (FETs)  28  and  30 . FETs  28  and  30  couple the input transmission line to the output transmission line at distributed locations along the transmission lines. Input transmission line  22  is represented by first gate inductance L G /2 connected to a signal input port  32 , intermediate second gate inductance L G , and third gate inductance L G /2. The capacitances of the inputs of active devices  28  and  30  are also part of this transmission line. The end of transmission line  22 , represented by third gate inductance L G /2, is terminated to ground by a terminating resistor R TERM  that has a value that is the same as the characteristic impedance of transmission line  22 . It is common for the characteristic impedance, and therefore terminating resistor R TERM , to have a value of 50 ohms. 
     Similarly, output transmission line  24  is represented by first drain inductance L D /2 connected to a signal output port  34 , intermediate second drain inductance L D , and third drain inductance L D /2. The end of transmission line  24 , represented by third drain inductance L D /2, is terminated to ground by a second terminating resistor R TERM  that has a value that is the same as the characteristic impedance of transmission line  24 , such as 50 ohms. The drains of the first and second FETs  28  and  30 , where they connect to transmission line  24 , are also coupled to ground by respective first and second shunt drain capacitors C ADD , as shown. The output capacitances of active devices  28  and  30  combined with the capacitances C ADD  to ground are also part of this transmission line. 
       FIG. 2  illustrates an example of a distributed amplifier  40  having negative feedback. Amplifier  40  may include an input transmission line  42 , an output transmission line  44 , and a plurality of active devices  46 . The active devices may be transistors, such as FETs  48  and  50 . FETs  48  and  50  may couple the input transmission line  42  to the output transmission line  44  at distributed locations along the transmission lines. In this example, input transmission line  42  is represented by the series connection of first gate inductance L G /2 connected to or extending from an input end of the transmission line as represented by a signal input port  52 , intermediate second gate inductance L G , and third gate inductance L G /2. The capacitances of the inputs of active devices  48  and  50  may also be part of this transmission line. 
     Similarly, output transmission line  44  may be represented by the series connection of a first drain inductance L D /2 connected to the output end of transmission line  44  represented by a signal output port  54 , intermediate second drain inductance L D , and third drain inductance L D /2. The end of transmission line  44 , represented by third drain inductance L D /2, may be terminated to ground by a terminating resistor R TERM  that has a value that may be the same as the characteristic impedance of transmission line  44 , such as 50-ohms. The drains of the first and second FETs  48  and  50 , where they connect to transmission line  44 , may also be coupled to ground by respective first and second shunt drain capacitors C ADD , as shown. The output capacitances of active devices  48  and  50  combined with the capacitances C ADD  to ground are also part of this transmission line. 
     The end of transmission line  42  opposite the input end, represented by the distal end of third gate inductance L G /2, may not be terminated to ground, but rather may be connected to a feedback resistor R FB . Resistor R FB  may have a value that is substantially more than the characteristic impedance of transmission line  42 . For example, the value of feedback resistor R FB  may be more than twice the characteristic impedance of transmission line  42 . A value that has been found to be effective is about 500-ohms, which is about ten times the value of the characteristic impedance of 50-ohms of transmission line  42 . The other end of feedback resistor R FB  may be connected to the output end of output transmission line  44  and output port  54 . 
     Distributed amplifier  40  may have more than two active devices  46 , and transmission lines  42  and  44  may be formed in various configurations. A transmission line may be simple (formed of a single element) or compound (formed of plural elements). As used herein, a simple or real transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves, and that may be characterized by characteristic impedance, transmission-time delay, phase shift, and/or other parameter(s). A compound transmission line, also referred to as an artificial transmission line, may be a four-terminal electrical network that may have the characteristic impedance, transmission-time delay, phase shift, and/or other parameter(s) similar to a real transmission line and therefore can be used to emulate a real transmission line in one or more of these respects. Accordingly, transmission lines  42  and  44  may be simple or compound transmission lines. 
     There are various ways that transmission lines may be implemented. Transmission lines may be a network of one or more sections of each of a simple transmission line (T), an inductor (L), and/or a capacitor (C). A few non-exclusive examples of transmission lines include series (in signal line) T; series L-shunt (to ground) C-series L; shunt C-series L-shunt C; series T-shunt C-series T; shunt C-series T-shunt C; and series L-shunt T-series L. Other networks may also be used. 
       FIG. 3  illustrates a distributed amplifier  60  as a further example of amplifier  40  shown in  FIG. 2 . As seen, amplifier  60  may be very similar to amplifier  40 , and elements that are the same are given the same reference numbers or names. Equivalent elements include input transmission line  42  with gate inductance L G  and gate inductances L G /2; output transmission line  44  with drain inductance L D  and drain inductances L D /2; active devices  46  including FETs  48  and  50 ; input port  52 ; output port  54 ; drain capacitors C ADD ; and feedback resistor R FB . Terminating resistor R TERM , terminating the end of output transmission line  44  distal of output port  54 , may also be referred to as R DRAIN . 
     Amplifier  60  may differ from amplifier  40  in that it may have a feedback inductor L FB  in series with feedback resistor R FB . Feedback inductor L FB  may extend between feedback resistor R FB  and the output end of output transmission line  44  proximate output port  54 . Inductor L FB  may be a discrete inductor, a transmission line, or other equivalent device. Also, an in-line or series output capacitor  62  may couple output transmission line  44  to output port  54  and a blocking capacitor  63  may couple R TERM  to ground. 
       FIG. 4  illustrates an embodiment of distributed amplifier  60 , with components labeled with the same reference numbers and names as used in  FIG. 3 . It is seen in this example that the transmission lines  42  and  44  are formed as combinations of continuous conductor sections of varying impedance, bond wires, and inductor coils to provide the desired balance between series inductance and shunt capacitance characteristics of transmission lines. For example, each of inductances L G  and L D , as well as L D /2 connected to terminating resistor R TERM , include inductance coils and bond wires as well as lengths of continuous conductor. Further, drain capacitors C ADD  are provided by stubs or lateral extensions of portions of transmission line  44 , as shown. 
     Specifically, input transmission line  42  may be connected to input port  52  and include inductances L G /2, including microstrip lines  66  and  68 , and inductance L G , including microstrip line  70 , lead line  72  and inductor coil  74 . Output transmission line  44  may extend from output port  54  and output capacitor  62  to a terminating resistor R TERM  in series with capacitor  63  coupled to ground. The output transmission line may include inductances L D /2 and L D . Active devices  46  include FETs  48  and  50 . 
     Feedback is provided by feedback resistor R FB  and feedback inductor L FB . In this example, feedback resistor R FB  includes a first feedback resistor  76  in series with a second feedback resistor  78 . Feedback inductor L FB  includes lead line  80  connecting resistors  76  and  78  and lead line  82  connecting resistor  78  to output port  54 . Additionally, FET  48  includes two source terminals coupled to ground via respective capacitors  84  and  86 . Similarly, FET  50  includes two source terminals coupled to ground via respective capacitors  86  and  88 . 
       FIG. 5  is a chart illustrating measured power level at 1 dB compression and noise factor for a constructed embodiment of the distributed amplifier of  FIG. 1  over a frequency band of about 1 GHz to about 9 GHz. It is seen that the power level is above 13 dBm, but the noise figure varies from about 4 dB at the lower frequencies to just under 2 dB at the higher frequencies. 
       FIG. 6  is a chart illustrating measured gain and input and output return-loss for the constructed embodiment of the distributed amplifier of  FIG. 1 . It is seen that the gain is above 12 dB for the frequency band of about 1 GHz to about 9 GHz. The input return-loss is seen to be below about −12 dB for this frequency band, and the output return-loss is seen to be generally below about −10 dB. 
       FIG. 7  is a chart illustrating measured gain and input and output return-loss for the distributed amplifier of  FIG. 4 . It is seen that the gain is 13 Db+/−1 dB for the frequency band of about 1 GHz to about 9 GHz. The input return-loss is seen to be below about −12 dB, and the output return-loss is seen to be generally below −5 dB for the same frequency band. 
       FIG. 8  is a chart illustrating measured power level at 1 dB compression and noise factor for the distributed amplifier of  FIG. 4  designed for operation in a frequency band of 1 to 9 GHz. It is seen in this example that the power level is above about 12 dB, and the noise figure (NF) is a maximum of about 2.2 dB. 
     It will be appreciated that a distributed amplifier as described having input-transmission-line terminating feedback may have a noise figure that is less than that of a distributed amplifier without feedback. Feedback is provided by terminating the input transmission line with a higher-value resistor connected to the output of the amplifier. Due to the gain and phase reversal of the amplifier, the feedback resistor looks like a 50-ohm load when viewed from the input line, even though its value is much higher. This effect is due to the feedback the resistor provides from the output. However, the feedback does not enter into the noise calculations, and the effect of the higher-value resistor on the noise figure is considerably less than that of a 50-ohm resistor. 
     The above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. Accordingly, while embodiments of a distributed amplifier and associated methods of signal processing have been particularly shown and described, many variations may be made therein. Such variations, whether they are directed to different combinations or directed to the same combinations, whether different, broader, narrower, or equal in scope, are also included. This disclosure may include one or more independent or interdependent inventions directed to various combinations of features, functions, elements, and/or properties. Thus, any given invention disclosed by example in the disclosure does not necessarily encompass all or any particular features, characteristics or combinations, except as specifically claimed. 
     Where “a” or “a first” element or the equivalent thereof is recited, such usage includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second, or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated.