Abstract:
A distributed amplifier having an improved transimpedance and/or gain comprises an input transmission line, the input transmission line forming an input of the distributed amplifier and having a characteristic impedance associated therewith, and an output transmission line, the output transmission line forming an output of the distributed amplifier and having a characteristic impedance associated therewith. The distributed amplifier further comprises a plurality of amplifying stages, each of at least a subset of the amplifying stages including an input and an output, the input of each amplifying stage in the subset being operatively coupled to the input transmission line and the output of each amplifying stage in the subset being operatively coupled to the output transmission line. Each amplifying stage in the subset has a transconductance associated therewith which is operatively configured so as to produce a gain in the respective amplifying stage that substantially compensates for an input signal attenuation at the respective input of the amplifying stage. The present invention provides techniques for efficiently improving an output transimpedance and/or increasing an overall gain in a distributed amplifier without proportionally increasing the quiescent current and/or noticeably degrading the frequency response in the distributed amplifier. Moreover, such performance enhancements are achieved without a significant increase in the size of the distributed amplifier.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to distributed amplifiers, and more particularly relates to techniques for increasing an overall gain in a distributed amplifier without proportionally increasing a quiescent current in the distributed amplifier. 
     BACKGROUND OF THE INVENTION 
     Distributed amplifiers are well-known in the art. Such distributed amplifiers generally include a plurality of basic amplifying stages that are connected between input and output transmission lines such that the outputs of the basic amplifying stages are combined to produce a resultant amplified signal. 
     Each of the basic amplifying stages in a distributed amplifier contain reactances, mostly capacitive, which affect the input and output impedances of those stages. A properly designed distributed amplifier compensates for these reactances so as to minimize the effect upon the transfer of power in a desired frequency range of operation. Conventionally, this has been accomplished by including compensation networks coupled to the input and output transmission lines. These compensation networks typically include both inductances and capacitances along their lengths so as to appear as short lengths of transmission lines having specific characteristic impedances. 
     The input capacitance of each amplifying stage when separated by an inductance, whether lumped or distributed, determines the characteristic impedance of the overall transmission line. One problem with conventional distributed amplifier architectures is that as a signal propagates down the input transmission line it is attenuated, and the power input to each successive amplifying stage therefore becomes significantly reduced. This problem exists on the output transmission line as well, although it is generally less significant because the signals are amplified by the transconductance of each amplifying stage. For a given number of amplifying stages, the decay in input power degrades the total output power and gain available from the distributed amplifier. 
     Conventional techniques for reducing the input power decay include coupling additional amplifying stages to the distributed amplifier. However, because of the decaying input signal, there is a point of diminishing returns to this approach. Moreover, the use of additional amplifying stages increases both the size and current consumption of the distributed amplifier. Another conventional approach for increasing the overall gain of the distributed amplifier is to uniformly increase the transconductance of each of the amplifying stages to account for worst case loss on the transmission line. However, since uniformly increasing the transconductance of the amplifying stages generally involves increasing the size of one or more transistors in each of the amplifying stages of the distributed amplifier, this approach can significantly degrade the frequency response and undesirably increase the quiescent current in the distributed amplifier. 
     SUMMARY OF THE INVENTION 
     The present invention provides techniques for efficiently improving an output transimpedance and/or increasing an overall gain in a distributed amplifier without proportionally increasing the quiescent current and/or noticeably degrading the frequency response in the distributed amplifier. Moreover, such performance enhancements are achieved without a significant increase in the size of the distributed amplifier. 
     In accordance with one aspect of the invention, a distributed amplifier having an improved output transimpedance includes a plurality of amplifying stages operatively coupled between an input transmission line and an output transmission line. Each of at least a subset of the amplifying stages has a transconductance associated therewith which is operatively configured so as to produce a gain in the amplifying stage which substantially compensates for an input signal attenuation on the input transmission line and/or output transmission line. Thus, the transconductance of each successive amplifying stage in the subset is increased in accordance with the input line loss at the input of the respective amplifying stage. In this manner, each amplifying stage in the subset is individually configured to compensate for an input transmission line loss. In a preferred embodiment of the present invention, every amplifying stage in the distributed amplifier is configured so as to have a transconductance which substantially compensates for an input signal attenuation at a respective input and/or output on the input transmission line and/or output transmission line. 
     In accordance with another aspect of the present invention, a method is provided for forming a distributed amplifier including a plurality of amplifying stages. Each of the amplifying stages has a transconductance associated therewith and includes an input and an output, the inputs of the plurality of amplifying stages being operatively coupled to an input transmission line and the outputs of the amplifying stages being operatively coupled to an output transmission line. The method comprises the steps of: determining an input signal attenuation at the respective inputs of at least a subset of the plurality of amplifying stages; and selecting the transconductances of each of the amplifying stages in the subset such that a gain of each of the plurality of amplifying stages in the subset substantially compensates for an input signal attenuation on the input transmission line at the respective inputs of the amplifying stages. 
    
    
     These and other features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic diagram illustrating an exemplary distributed amplifier, formed in accordance with the present invention. 
     FIG. 2 is a graphical representation illustrating an input signal attenuation along a transmission line for each input tap in an exemplary six-stage distributed amplifier. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described herein in the context of an illustrative four-stage distributed amplifier circuit which may be used, for example, in an optical receiver front-end application. It should be appreciated, however, that the present invention is not limited to this or any particular distributed amplifier architecture or application. Rather, the invention is more generally applicable to any suitable distributed amplifier architecture in which it is desirable to improve a transimpedance and/or gain of the distributed amplifier without proportionally increasing a quiescent current in the distributed amplifier. Moreover, the present invention as described herein is not limited to a particular semiconductor fabrication process, but may be used with processes including, but not limited to, metal-oxide-semiconductor (MOS), bipolar, and metal-semiconductor field-effect-transistor (MESFET) technologies. The term “amplifier” as used herein essentially refers to a circuit for multiplying an input signal applied to the circuit by a predetermined gain which is greater than or equal to one. 
     FIG. 1 is a schematic diagram illustrating an exemplary distributed amplifier  100 , formed in accordance with one aspect of the present invention. The distributed amplifier  100  includes a plurality of amplifying stages  120 ,  130 ,  140  and  150 , each of the amplifying stages including an input and an output. It is to be appreciated that although the illustrative distributed amplifier  100  is shown having four amplifying stages, the present invention contemplates that any number of amplifying stages may be employed in accordance with the techniques described herein. 
     In the distributed amplifier  100 , an input signal is received through an input IN of the distributed amplifier. The distributed amplifier  100  has an input impedance Z IN  as measured at the input IN which enables the distributed amplifier to be coupled to an external input transmission line (not shown) having a characteristic impedance which is substantially equal to impedance Z IN  or to a preceding circuit or stage (not shown) having an output impedance substantially equal to impedance Z IN . The input impedance may be on the order of fifty ohms, for example, to allow the distributed amplifier  100  to be coupled to a standard fifty-ohm transmission line, although it is to be appreciated that the input impedance Z IN  may be of any desired value, as required by the transmission line or circuit to be coupled to the distributed amplifier  100 . 
     Likewise, in the distributed amplifier  100 , an output signal is generated at an output OUT of the distributed amplifier  100 . The distributed amplifier  100  has an output impedance Z OUT  associated therewith as measured at the output OUT which enables the distributed amplifier to be coupled to an external output transmission line (not shown) having a characteristic impedance which is substantially equal to impedance Z OUT  or to a subsequent circuit or stage (not shown) having an input impedance substantially equal to impedance Z OUT . For example, the output impedance may be about fifty ohms to allow the distributed amplifier  100  to be coupled to a standard fifty-ohm transmission line, although it is to be appreciated that the output impedance Z OUT  may be configured to be of any value as required by the transmission line or circuit to be coupled to the distributed amplifier  100 . 
     The amplifying stages  120 ,  130 ,  140 ,  150  of the distributed amplifier  100  are operatively coupled between respective nodes or taps on an input transmission line  102  and corresponding nodes on an output transmission line  104 . For example, in the illustrative distributed amplifier  100 , the input of amplifying stage  120  is coupled to the input transmission line at node TAP 1  and the output of amplifying stage  120  is coupled to the output transmission line at node  122 . Similarly, the input of amplifying stage  130  is coupled to the input transmission line at node TAP 2  and the output of amplifying stage  130  is coupled to the output transmission line at node  124 , the input of amplifying stage  140  is coupled to the input transmission line at node TAP 3  and the output of amplifying stage  140  is coupled to the output transmission line at node  126 , and the input of amplifying stage  150  is coupled to the in put transmission line at node TAP 4  and the output of amplifying stage  150  is coupled to the output transmission line at node  128 . 
     The input transmission line  102  and output transmission line  104  are preferably formed as co-planar structures which are suitable for high-frequency operation (e.g., 75 gigahertz (GHz)). Both the input transmission line  102  and output transmission line  104  have a respective line impedance associated therewith. It is to be understood that alternative structures for implementing the transmission lines are similarly contemplated by the present invention, including, for example, spiral inductors and conductor buses, each having a certain characteristic impedance associated therewith that is selectively adjustable as desired. As apparent from FIG. 1, the transmission lines  102 ,  104  may include distributed series inductors coupled between corresponding inputs or corresponding outputs of the plurality of amplifying stages, with inductors L 1 , L 2  and L 3  corresponding to the output transmission line  104 , and inductors L 4 , L 5  and L 6  corresponding to the input transmission line  102 . Specifically, in the illustrative distributed amplifier  100 , inductor L 4  is shown coupled between the inputs of amplifying stages  120  and  130  at nodes TAP 1  and TAP 2 , respectively, and inductor L 1  is coupled between the outputs of amplifying stages  120  and  130  at nodes  122  and  124 , respectively. Likewise, inductor L 5  is coupled between the inputs of amplifying stages  130  and  140  at nodes TAP 2  and TAP 3 , respectively, and inductor L 2  is coupled between the outputs of amplifying stages  130  and  140  at nodes  124  and  126 , respectively. Inductor L 6  is coupled between the inputs of amplifying stages  140  and  150  at nodes TAP 3  and TAP 4 , respectively, and inductor L 3  is coupled between the outputs of amplifying stages  140  and  150  at nodes  126  and  128 , respectively. 
     Although the series inductors L 1  through L 6  are depicted as purely inductive impedances, each of these impedances more accurately includes a line resistance R (not shown), connected in series with a corresponding inductor L, and a shunt capacitance C (not shown), connected between the inductor and ground, which contribute to the attenuation of the signal along the transmission lines  102 ,  104 . The shunt line capacitance will be negligible, however, in comparison to an input capacitance and an output capacitance associated with each of the plurality of amplifying stages. The series R-L, shunt C representation closely approximates a typical transmission line model and is thus treated as such for the purposes of the present invention. By varying the series inductance in the transmission lines  102 ,  104 , the characteristic impedance of the lines can be operatively adjusted as desired, such as, for example, to compensate for the input and/or output capacitance of the amplifying stages. The series resistance associated with the transmission lines, however, cannot be easily eliminated, and therefore the distributed amplifier  100  must be configured to compensate for the signal loss corresponding to this series resistance. 
     The input transmission line  102  preferably includes an input termination impedance  160  which is coupled to the input transmission line. Specifically, the input termination impedance  160  is coupled between the input to the final amplifying stage  150  at node TAP 4  and a negative voltage supply, which may be ground. The value of the input termination impedance  160  is preferably selected to substantially match an impedance of the input transmission line  102 , typically about fifty ohms. Similarly, the output transmission line  104  preferably includes a back termination impedance  110  which is coupled to the output transmission line. Specifically, the back termination impedance  110  is coupled between the output of the first amplifying stage at node  122  and ground. The value of the back termination impedance is preferably selected to substantially match an impedance of the output transmission line  104 , again typically about fifty ohms. As understood by those skilled in the art, the input and back termination impedances may be purely reactive (inductive and/or capacitive), purely resistive, or a combination of resistive and reactive, as required to achieve a desired characteristic impedance of the respective transmission lines at the respective termination points. A load impedance represented as RLOAD is also shown in FIG. 1 coupled between the output OUT of the distributed amplifier  100  and ground. 
     The amplifying stages  120 ,  130 ,  140 ,  150  comprising the distributed amplifier  100  are preferably implemented using transconductance stages, each transconductance stage having a predetermined transconductance g m1 , g m2 , g m3  and g m4 , respectively, associated therewith. A more detailed discussion of amplifying stages that may be suitable for use with the present invention can be found, for example, in the texts by Paul R. Gray et al.,  Analysis and Design of Analog Integrated Circuits , John Wiley &amp; Sons (2001) and Alan B. Grebene,  Bipolar and MOS Analog Integrated Circuit Design , John Wiley &amp; Sons (1984), which are incorporated herein by reference. Accordingly, a detailed description of the amplifying stages will not be presented herein. 
     Typically, each amplifying stage in an ideal distributed amplifier is the same, and consequently the transconductance (g m ) associated with each of the amplifying stages in the ideal distributed amplifier will be equal. The transconductance for each amplifying stage is preferably selected so as to make the gain of each amplifying stage equal to a desired gain A for the distributed amplifier divided by the number of amplifying stages n, where n is an integer greater than or equal to one. The output signal generated by a conventional distributed amplifier would thus be approximated as: 
     
       
         Signal out= g   m  ×no. of amplifying stages×RLOAD×Signal in 
       
     
     As previously stated, there is a distributed signal loss associated with the transmission lines  102 ,  104  in the distributed amplifier  100 . This loss increases as the input signal propagates along the transmission lines from the input IN toward the output OUT of the distributed amplifier. Consequently, the actual gain of each amplifying stage will be less than the ideal gain A/n per stage, assuming no compensation is used. The input signal attenuation for an exemplary six-stage distributed amplifier is shown in FIG.  2 . Waveform  210  represents the input signal at tap T 1 , which is the input to the first amplifying stage, waveform  220  represents the input signal at tap T 2 , which is the input to the second stage, waveform  230  represents the input signal at tap T 3 , which is the input to the third stage, waveform  240  represents the input signal at tap T 4 , which is the input to the fourth stage, waveform  250  represents the input signal at tap T 5 , which is the input to the fifth stage, and waveform  260  represents the input signal at tap T 6 , which is the input to the sixth stage. Relative amplitudes of the input signal at the respective taps T 1  through T 6  along the input transmission line are also shown in FIG. 2, with respect to tap T 1 . For example, at tap T 2 , the input signal is attenuated by 0.8612. At tap T 6 , the input signal is attenuated by almost half, namely, 0.506. 
     Referring again to FIG. 1, in accordance with the present invention, the transconductance of each of the amplifying stages  120 ,  130 ,  140 ,  150  in distributed amplifier  100  is substantially matched to the attenuation of the input signal on the input transmission line  102 , as measured at the inputs of the respective amplifying stages, to compensate for the respective losses. In this manner, a successive amplifying stage further down the input transmission line  102 , with respect to the input IN of the distributed amplifier  100 , will preferably have a transconductance which is greater than the transconductance of a preceding amplifying stage, such that g m1 &lt;g m2 &lt;g m3 &lt;g m4 . In accordance with the present invention, the distributed amplifier  100  can thus be considered as having a tapered transconductance architecture. 
     Although the present invention contemplates that the transconductance of each of the amplifying stages may also be adjusted to compensate for the signal attenuation on the output transmission line  104 , the loss associated with the output transmission line will generally be significantly less as compared to the input transmission line  102 , since the output signal loss, when referred back to the input of the particular amplifying stage, will be divided by the gain of the amplifying stage. Thus, for simplicity of explanation, the transconductances of the amplifying stages in the distributed amplifier  100  will only be adjusted to compensate for input signal attenuation. 
     Since the transconductances g m1 , g m2 , g m3 , g m4  corresponding to the amplifying stages  120 ,  130 ,  140 ,  150 , respectively, will most likely not be equal to one another, the output signal gain can be determined using a variation of the above equation. Specifically, the output signal generated by the distributed amplifier  100  at output OUT can be approximated as: 
     
       
         Signal out= g   m1   ×g   m2   ×g   m3   ×g   m4 ×RLOAD×Signal in 
       
     
     It is to be appreciated that if fewer amplifying stages are employed in the distributed amplifier  100 , the transconductances of these additional stages would not appear in the above equation. Similarly, if more amplifying stages are used, the transconductances of these additional stages would be included in the above equation, as understood by those skilled in the art. 
     As previously stated, the transconductance of each amplifying stage is selected so as to produce a gain in the respective amplifying stage which substantially compensates for an input signal attenuation measured at the input of the amplifying stage. Preferably, the transconductance g m1  of the first amplifying stage  120  nearest the input IN of the distributed amplifier  100  is used as a reference for calculating the transconductances g m2 , g m3 , g m4  of the succeeding amplifying stages  130 ,  140 ,  150 , respectively. The transconductance g m1  of the first amplifying stage  120  is preferably selected so that the gain of the first amplifying stage is substantially equal to a desired gain A divided by the total number of amplifying stages, which is four in the illustrative distributed amplifier  100 , although any predetermined transconductance may be chosen. Since there will be essentially no input signal loss at the input TAP 1  to the first amplifying stage  120 , the transconductance of the first stage does not need to be adjusted to compensate for input signal loss. Thus, since the gain of the amplifying stage is directly proportional to the transconductance, the transconductance of the first amplifying stage  120  is preferably determined as          g   m1     ∝     A   4                            
     The input signal attenuation at the input TAP 2  of the second amplifying stage  130  can be determined as a ratio of the magnitude of the signal at node TAP 2  divided by the magnitude of the signal at node TAP 1 . This attenuation value will be less than one. It is to be appreciated that the relative magnitudes of the input signals at each respective node along the input transmission line  102  can be determined using, for example, conventional network analysis techniques or network simulation results, as understood by those skilled in the art. A solution may be determined in a manner consistent with a typical voltage division problem. Once this attenuation value is determined, the transconductance of the second amplifying stage  130  is preferably increased accordingly to substantially compensate for the input signal attenuation. Thus, the transconductance g m2  of the second amplifying stage  130  in the distributed amplifier  100  is preferably determined as            g   m2     =       g   m1     T2       ,                          
     where T 2  is the input signal attenuation at node TAP 2 . The transconductances of the remaining amplifying stages  140 ,  150  may be similarly determined as          g   m3     =           g   m1     T3                     g   m4       =       g   m1     T4                              
     where T 3  and T 4  are the input signal attenuations at nodes TAP 3  and TAP 4 , respectively. 
     Assuming metal-oxide semiconductor (MOS) devices are employed in the distributed amplifier  100 , the transconductance of a particular amplifying stage can be selectively varied, for example, by changing a width W of one or more input transistors comprising the amplifying stage while leaving a length L of the input transistors constant, thereby increasing a W/L ratio of the particular transistor. Thus, by increasing the width of the input transistors in a given amplifying stage, the transconductance of that stage will increase proportionally. The transconductance of an amplifying stage may also be changed using other suitable techniques, for example, by varying a quiescent current in the input transistors comprising the amplifying stage. As previously stated, the techniques of the present invention are not limited to a MOS fabrication process. Rather, the present invention may be employed with other semiconductor processes including, but not limited to, bipolar and MESFET process technologies. In a bipolar process, for example, the transconductance of the input transistors is generally approximated as            I   C       V   T       ,                          
     where I C  is the collector current in the transistors and V T  is the thermal voltage of the transistors, which is typically about 26 millivolts (mV) at 300 degrees Kelvin. Therefore, the bipolar transconductance may be selectively varied, for example, by changing a collector current I C  in the transistors or an emitter area of the transistors, as understood by those skilled in the art. 
     In accordance with the techniques of the present invention, the transconductance of each of the amplifying stages is preferably ratioed in accordance with the relative attenuations of the input signal at the inputs to the respective amplifying stages. As previously stated, since the relative input signal attenuation increases as the signal propagates along the input transmission line  102  from the input IN to the output OUT of the distributed amplifier  100 , the transconductance of each successive amplifying stage down the input transmission line is operatively increased to compensate for the respective signal loss, such that the effective gain at the output of each amplifying stage substantially matches the ideal gain        A   n                          
     if transmission line losses were not present,where A is the desired gain of the distributed amplifier and n is the number of amplifying stages in the distributed amplifier. 
     It is to be appreciated that every amplifying stage in the distributed amplifier  100  need not be configured in the manner described above. Instead, a subset comprising one or more amplifying stages in the distributed amplifier may be configured in the manner thus described and still achieve at least some of the objectives and advantages of the present invention. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made therein by one skilled in the art without departing from the scope or spirit of the invention.