Abstract:
A line driver for connection to a transmission line having a characteristic impedance. The line driver can be arranged in a voltage-mode or a current-mode configuration. In the voltage-mode configuration, the line driver comprises an amplifier, a transformer, a reference resistor and a feedback circuit. The first winding of the transformer has a first end connected to the output of the amplifier and the second winding is connectable to the transmission line. The reference resistor has an end connected to the second end of the first winding at a junction point and the feedback circuit is connected to the input and output of the amplifier and also to the junction point. The reference resistor has a resistance equal to {fraction (1/k)} times the characteristic impedance of the transmission line. The feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to −(K−1) times the voltage at the junction point, for a predetermined value of K. This results in a synthesized output impedance equal to the characteristic impedance of the transmission line, while losses in the reference resistor are reduced. At the same time, the gain is easily set by the specifying the resistance of a source resistor placed at the input to the amplifier.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to line drivers and more particularly to line drivers having a desired output impedance and required to have a high efficiency. 
     BACKGROUND OF THE INVENTION 
     A line driver is typically used for sending an electronic signal onto a transmission line type medium, such as a copper twisted pair. To avoid unwanted reflections at the far end of the transmission line, the latter is usually matched, i.e., terminated by a far-end impedance element which presents to the transmission line a resistance equal to the characteristic impedance of the line. 
     In many situations, the transmission medium is bidirectional, meaning that there is a second transmitter at the far end of the transmission line. In order for the transmission line to be properly matched in the reverse direction, the line must “see” a near-end impedance equal to its own characteristic impedance. The required near-end impedance, known as the output impedance of the line driver, is usually provided by a reference resistor having a resistance equal to the characteristic impedance of the transmission line. In a voltage-mode configuration, the reference resistor is usually placed in series with the line driver, while in a current-mode configuration, one usually opts for a parallel arrangement. 
     For example, referring to FIG. 1, there is shown a conventional voltage-mode line driver comprising a voltage source/amplifier combination, shown in dotted outline at  102 , connected in series with a reference resistor  106  having a resistance R e . The reference resistor  106  is connected to ground via a transformer  108 . The transformer  108  generally has two windings with an equal number of turns, one of which is connected to the reference resistor  106  and the other being used for interfacing with a transmission line  110 . The transmission line  110  has a characteristic impedance Z c , which is typically in the range of 50 Ω to 600 Ω. 
     The output resistance of the line driver (denoted R out ) is defined as the ratio of the voltage V 0  appearing across the line driver side of the transformer  108  to the current I 0  caused by the voltage V 0  when the voltage source/amplifier combination  102  is short-circuited. Upon applying the short circuit, it is seen that the voltage V 0  appears in its entirety across the reference resistor  106 , from which it follows that R out  equals R e . Therefore, in order for the line driver to be matched to the transmission line  110 , R out  should equal the characteristic impedance Z c , which means that R e  should be set equal to Z c . 
     Unfortunately, this conventional arrangement results in a wastage of power, as half of the energy output by the voltage source/amplifier combination  102  is lost in the form of heat dissipated in the reference resistor  106 . This prevents high-speed modems and other devices that use line drivers from meeting strict power efficiency guidelines. Furthermore, since the amount of circuit card real estate required for a line driver depends on the amount of power that is dissipated, it follows that only a few such drivers can be placed on a circuit card. 
     A similar scenario occurs in the current-mode dual configuration, now briefly described with reference to FIG.  2 . The current-mode line driver comprises a current source/amplifier combination (shown in dotted outline at  202 ) and placed in parallel with the first winding of a transformer  206 . The transformer  206  has a second winding for interfacing with a transmission line  208  having a characteristic impedance Z c . Proper termination at the line driver end is provided by a reference resistor  210  having a resistance R e  and also placed in parallel with the transformer  206 . 
     The output resistance P out  of the line driver in FIG. 2 is defined as the voltage V 0  appearing across the line driver side of the transformer  206 , divided by the current I 0  caused by the voltage V 0  with the current source/amplifier combination  202  open-circuited. By applying this open circuit condition, it is seen that the voltage V 0  appears in its entirety across the reference resistor  210 . Therefore, R out  is simply equal to the resistance R e  of the reference resistor  210 . To achieve proper termination at the line driver end, R out  should be equal to Z c  and thus R e  is usually set equal to Z c . 
     Because R e  is equal to the characteristic impedance of the transmission line  208 , half the energy output by the source/amplifier combination  202  is dissipated in the reference resistor  210 . This causes the above-mentioned disadvantages, namely the inability of a conventional line driver to meet power efficiency requirements and the imposition of an undesirably low limit on the number of devices employing line drivers that may be placed on a circuit card. 
     A known solution is the use of a smaller reference impedance between the output of the line driver and the transmission line and to use a combination of positive and negative feedback around this reference impedance to achieve the desired output impedance. However, one limitation of this method is that the gain of the line driver is low and an additional stage is required at the expense of efficiency and noise performance. Higher gains are achievable but the output impedance is severely affected by tolerances of the components in the positive and negative feedback loops and in the reference impedance. 
     The above background has shown that there is a need in the industry to provide a line driver which can have the same output resistance as a conventional line driver while reducing the amount of energy or power dissipated in the reference resistor. Furthermore, it would be advantageous to provide a line driver which would also have an independently specifiable gain. 
     SUMMARY OF THE INVENTION 
     The invention may be summarized according to a first broad aspect as a line driver equipped with an amplifier, a transformer, a reference resistor and a feedback circuit. The amplifier has an input for connection to a voltage source and having an output. The transformer has a first winding and a second winding, the first winding having a first end connected to the output of the amplifier and having a second end and the second winding being connectable to a transmission line having a characteristic impedance. The reference resistor has an end connected to the second end of the first winding at a junction point. The feedback circuit is connected to the input of the amplifier, to the output of the amplifier and to the junction point. 
     In accordance with this first broad aspect of the invention, the reference resistor has a resistance equal to {fraction (1/K)} times the characteristic impedance of the transmission line and the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to −(K−1) times the voltage at the junction point, for a predetermined value of K. 
     The resulting output impedance will be equal to K times the reference impedance. At the same time, the voltage across the reference impedance will be reduced by a factor of K, which advantageously reduces the power lost in the reference resistor by a factor of K. 
     Preferably, the amplifier is an operational amplifier connected in an inverting configuration and the feedback circuit is a resistive network consisting of a first feedback resistor having a first end connected to the output of the amplifier and having a second end connected to the input of the amplifier and also having a second feedback resistor of which a first end connected both to the input of the amplifier and to the second end of the first feedback resistor and of which a second end connected to the junction point. The first feedback resistor preferably has a resistance equal to (K−1) times the resistance of the second feedback resistor. 
     According to a second broad aspect, the invention may be summarized as a line driver equipped with a transformer, a reference resistor and a feedback circuit. Again, the transformer has a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance. However, in this case, the reference resistor has a first end connected to a first end of the first winding and the feedback circuit has a first port connected both to the first end of the reference resistor and to the first end of the first winding and having a second port connected to the second end of the reference resistor. 
     The feedback circuit is arranged to draw, through the first port, K−1 times the amount of current flowing through the reference resistor. Hence, the current flowing through the reference resistor will be K times less than the current flowing through the first winding of the transformer, which reduces the losses in the reference resistor while continuing to present a desired output impedance to the transmission line. 
     Preferably, the feedback circuit is equipped with a first current-to-voltage converter connected to the second port of the feedback circuit, for measuring the current flowing in the reference resistor and a second current-to-voltage converter connected to the first port of the feedback circuit, for measuring the current drawn by the feedback circuit. The feedback also preferably employs a divider circuit and a differential circuit connected to the first and second measuring current-to-voltage converters, for determining the difference between the measured amount of current flowing in the reference resistor and        1     K   -   1                            
     times the measured amount of current drawn by the feedback circuit. The feedback circuit then generates a compensatory current flowing out the first port, the compensatory current being proportional to the difference so determined. 
     In accordance with this second broad aspect, the first current-to-voltage converter may be an operational amplifier having an inverting input connected to the second port of the feedback circuit, a non-inverting input connectable to a ground reference and an output and a resistor connected between the input and output of the operational amplifier. The second current-to-voltage converter may be a current transformer connected to the first port of the feedback circuit and having an output. 
     According to a third broad aspect, the invention may be summarized as a line driver having a transformer, a reference resistor and a feedback circuit. As usual, the transformer has a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance. The feedback circuit has a first port connected to an end of the first winding and also has a second port, a third port and a fourth port. The reference resistor having a first end connected to the second and fourth ports of the feedback circuit and has a second end connected to the third port of the feedback circuit. 
     In this case, the feedback circuit is arranged to draw, through the fourth port, K times the amount of current flowing through the reference resistor. The current through the reference resistor will be reduced by a factor of K relative to the current flowing through the first winding of the transformer, thereby reducing the energy lost in the reference resistor. Advantageously, the output impedance seen by the transmission line is K times the resistance of the reference resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures, in which: 
     FIG. 1 is a circuit diagram of a conventional voltage-mode line driver; 
     FIG. 2 is a circuit diagram of a conventional current-mode line driver; 
     FIG. 3 is a circuit diagram of an inventive voltage-mode line driver, comprising a line driver similar to that of FIG. 1 in addition to a negative feedback circuit; 
     FIG. 4 shows in greater detail the feedback circuit of 
     FIG. 3 in accordance with an embodiment of the invention: 
     FIG. 5 is a circuit diagram of an inventive current-mode line driver, comprising a line driver similar to that of FIG. 2 in addition to a feedback circuit; 
     FIG. 6 shows in greater detail the feedback circuit of FIG. 5 in accordance with an embodiment of the invention; and 
     FIG. 7 shows a current-mode line driver in accordance with an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     I. VOLTAGE-MODE LINE DRIVER 
     In order to reduce the amount of energy dissipated in the reference resistor while keeping the same output resistance from the point of view of the transmission line, it is proposed to change the position of the reference resistor, to reduce its resistance and to reduce the voltage across the reference resistor relative to the voltage across the transformer on the line driver side. 
     To this end, FIG. 3 shows a line driver comprising an amplifier  302  driven by a voltage source/amplifier combination (shown in dotted outline at  304 ) in series with a source resistor  303  having a resistance R B . The amplifier  302  is preferably an operational amplifier (opamp) having an inverting input terminal  302 A, a non-inverting input terminal  302 B and an output terminal  302 C. The non-inverting input terminal  302 B of the opamp  302  is connected to a ground reference  303 , Assuming the opamp  302  to be ideal, a virtual ground exists at the inverting input terminal  302 A. 
     The inventive voltage-mode line driver of FIG. 3 also  30  comprises a transformer  306 , preferably having first and second windings. The first winding has one end connected to the output terminal  302 C of the opamp  302  and another end electrically connected to one end of a reference resistor  308  at a junction point  308 A. The other end of the reference resistor is connected either to ground or to a virtual ground (e g., the input terminal of an operational amplifier). The second transformer winding is connected across a transmission line  310 , which has a characteristic impedance Z c . According to a first preferred embodiment of the invention, the reference resistor  308  has a resistance          R   e     =         Z   c     K     .                            
     The factor        K   =       Z   c       R   e                              
     is a real number that preferably ranges from 1 to 10, although higher values may be used while remaining within the scope of the invention. 
     The line driver of FIG. 3 also comprises a feedback circuit  312  connected to the output terminal  302 C of the opamp  302 , the inverting input terminal  302 A of the opamp  302  and the junction point  308 A between the transformer  306  and the reference resistor  308 . As will be shown in detail hereinafter, for a given voltage V 0  across the line driver side of the transformer  306 , the feedback circuit  312  is arranged so that the voltage V 308A  at the junction  308 A equals          -                1   K            V   0                            
     and the voltage V 302C  at the output terminal  302 C of the opamp  302  equals            K   -   1     K            V   0     .                            
     The output resistance R out  of the circuit in FIG. 3 is calculated as the voltage V 0  divided by the current I 0  generated as a result of V 0 . Clearly, if the voltage V 308A  at the junction point  308 A is equal to            -                1   K            V   0       ,                          
     and if the feedback circuit  312  generates (or absorbs) a negligible amount of current, then one has:          I   0     ≃         V   0       KR   e       .                            
     Hence: 
     
       
         
           
             
               
                 R 
                 out 
               
               = 
               
                 
                   
                     
                       V 
                       0 
                     
                     
                       I 
                       0 
                     
                   
                   == 
                   
                     
                       V 
                       0 
                     
                     
                       
                         V 
                         0 
                       
                       
                         KR 
                         E 
                       
                     
                   
                 
                 = 
                 
                   
                     KR 
                     E 
                   
                   = 
                   
                     
                       K 
                        
                       
                           
                       
                        
                       
                         
                           Z 
                           C 
                         
                         K 
                       
                     
                     = 
                     
                       Z 
                       C 
                     
                   
                 
               
             
             , 
           
         
                 
         
             
         
      
     
     which is the desired output resistance. Thus, the inventive line driver acts as a transimpedance amplifier. 
     The power P 308  dissipated in the reference resistor  308  is given by:            P   308     =         V     308      A     2       R   E       =           (       V   0     K     )     2         Z   C     K       =       1     K                                    V   0   2       Z   C               ,                          
     demonstrating that there is K times less power wastage than is the case with conventional voltage-mode line drivers. 
     FIG. 4 shows one possible implementation of the feedback circuit  312 , comprising a first feedback resistor  314  (with a resistance R A ) connected between the inverting input terminal  302 A and the output terminal  302 C of the opamp  302  and a second feedback resistor  316  (with a resistance R B ) connected between the inverting input terminal  302 A of the opamp  302  and the junction point  308 A between the transformer  306  and the reference resistor  308 . In this way, the two feedback resistors  314 ,  316  each have one end electrically connected to the inverting input terminal  302 A of the opamp  302 . 
     Due to the output of the opamp  302  being partially fed back to its inverting input, and by virtue of the voltage division effected by the resistors  314 ,  316 , the voltage V 302C  at the output terminal  302 C will be equal to        -                  R   B       R   A                              
     times the voltage V 308A  at the junction point  308 A. By selecting R B  to equal (K−1) times R A , it follows that V 302C  is equal to −(K−1) times V 308A . As a result, the voltage V 0 , which is the difference between V 302C  and V 308A , is equal to −K times V 308A  and hence            V     308      A       =       -                1   K            V   0         ,                          
     as desired. 
     Furthermore, recalling that a virtual ground exists at the inverting input terminal  302 A of the opamp  302 , and applying Kirchhoff&#39;s voltage law around loop  318 , it is seen that the voltage V B  across the second feedback resistor  316  will be equal to the negative of the voltage V 308A  at the junction point  308 A. Therefore, V 302C  (which is the difference between V 0  and V B ) is equal to              K   -   1     K          V   0       ,                          
     as desired. 
     Those skilled in the art will appreciate that the resistances R A  and R B , which are selected to have the desired ratio of (K−1):1, should also be sufficiently large so that the current flowing through the two feedback resistors  314 ,  316  is small (e.g., two or more orders of magnitude smaller than the current flowing through the reference resistor  308 ). 
     Furthermore, the gain of the line driver is equal to          -                  R   A       R   B         ,                          
     which can be specified to have any value by appropriately selecting the resistance R B  of the source resistor  303 . 
     II. CURRENT-MODE LINE DRIVER 
     The voltage-mode line driver of FIG. 4 has a current-mode dual, which is now described with reference to FIG.  5 . In the inventive current-mode configuration, the goal is to reduce the power dissipated in the reference resistor by increasing its resistance, thereby to reduce the amount of current passing through it for the same voltage, while keeping the same output resistance from the point of view of the transmission line. 
     The current-mode line driver of FIG. 5 comprises a feedback circuit  502  having two ports  502 A,  502 B. Port  502 B is connected to one end of a reference resistor  510 , while port  502 A is connected to the other end of the reference resistor  510 , to a current source/amplifier combination (shown in dotted outline at  504 ), and to one end of a first winding of a transformer  506 . The other end of the first transformer winding is grounded. A second transformer winding interfaces with a transmission line  508  having a characteristic impedance Z c . The resistance R e  of the reference resistor  510  is selected to equal K times Z c , where K preferably ranges from 1 to 10 but may be larger if desired. 
     It is noted that although the first transformer winding is grounded, the reference resistor  510  is not connected to a physical ground. Nevertheless, as will be shown in further detail hereinafter, port  502 B still lies at zero potential due to a virtual ground provided by the feedback circuit  502 . 
     In the preferred current-source embodiment, the feedback circuit  502  is designed to draw a current I 502  which is (K−1) times larger than the current I 510  passing through the reference resistor  510 . Thus, for a given current I 0  exiting the line driver side of the transformer, one will have          I   510     =         1   K          I   0                   and                                  I   502       =         K   -   1     K            I   0     .                                
     Naturally, the feedback circuit  502  should be equipped with a sink (e.g., a physical ground connection) for absorbing the currents entering it via ports  502 A and  502 B. 
     The output resistance of the line driver R out  is defined as the voltage V 0  across the line driver side of the transformer  506  divided by I 0 , where I 0  is the current resulting from V 0  with the current source/amplifier combination  504  open-circuited. Since the reference resistor  510  is connected to a virtual ground at port  502 B of the feedback circuit  502 , the reference resistor  510  will be submitted to V 0  in its entirety. Furthermore, since            I   510     =       1   K          I   0         ,                          
     one has:          V   0     =         I   510     ·     R   e       =         1   K            I   0     ·     R   e         =         1   K          I   0          KZ   c       =       I   0            Z   c     .                                    
     Therefore,            R   our     =         V   0       I   0       =     Z   c         ,                          
     as desired. The power P 510  dissipated in the reference resistor  510  is given by:            P   510     =         V   0   2       R   E       =         V   0   2       KZ   C       =       1   K                       V   0   2       Z   C               ,                          
     which, as before, is K times less than what is dissipated in the reference resistor of a conventional line driver. 
     An embodiment of the feedback circuit  502  with the desired properties is now described with reference to FIG.  6 . The feedback circuit  502  comprises a first current-to-voltage converter  602  for measuring the current I 510  and a second current-to-voltage converter  604  for measuring the current I 502 . 
     The first current-to-voltage converter  602  is connected to port  502 B of the feedback circuit and preferably comprises an opamp  606  with a feedback resistor  608  connected between its output terminal  606 C and its inverting input terminal  606 A. The non-inverting terminal  606 B of the opamp  606  is preferably grounded. When arranged as shown in FIG. 6, the current-to-voltage converter  602  will provide a voltage V 606C  at the output terminal  606 C of the opamp  606  which is proportional to the current I 510 . It is noted that use of the opamp  606  provides the desired virtual ground connection at the inverting terminal  606 A, which is electrically connected to port  502 B and to the reference resistor  510 . 
     The second current-to-voltage converter  604  consists of a current transformer  608  connected to port  502 A of the feedback circuit. The current transformer  608  is of standard design, having a first set of windings  608 A through which the current I 502  flows and a second set of windings  608 B coupled to the first set of windings, which produces a voltage V 604  proportional to the current I 502 . To limit the amount of current coupled by the second set of windings  608 B, a resistor  610  may be placed in parallel therewith. 
     The feedback circuit  502  also comprises a voltage divider circuit  612  connected to the second current-to-voltage converter  604 . In the preferred current-mode embodiment of the invention, the voltage divider circuit  612  divides the voltage V 604  by (K−1), producing a voltage V 612 . Those skilled in the art will be familiar with the design of such a voltage divider circuit. 
     Finally, the feedback circuit  502  comprises a differential circuit (e.g., an opamp)  614  connected to the output of the first current-to-voltage converter  602  and to the output of the voltage divider circuit  612 . The opamp  614  has an output terminal  614 C at which is produced a voltage V 614  that is proportional to the difference between V 612  and V 606C . The output terminal  614 C of the opamp  614  is connected to port  502 A of the feedback circuit via the first set of windings  608 B of the current transformer  608 . 
     In operation, if the difference between V 612  and V 606C  is positive, i.e., if the current I 502  is too large, then V 614C  is positive, which causes the opamp  614  a small error current to flow in the direction opposite the direction of the current I 502  indicated in FIG.  6 . The effect is compensatory, as less current is then sensed by the second current-to-voltage converter  604  and more current is sensed by the first current-to-voltage converter  602 . This feedback mechanism ensured that error currents are so generated until V 612  is equal to V 606C , i.e., until the current sensed by the first current-to-voltage converter  602  is equal to        1     K   -   1                            
     times the current sensed by the second current-to-voltage converter  604 , thus achieving the desired current relationship. 
     In an alternative embodiment of the current-mode line driver, shown in FIG. 7, the feedback circuit  702  has four ports, identified as ports  702 A,B,C,D. Port  702 A is connected to one end of the first winding of the transformer  506 , port  702 B is connected to a first end of the reference resistor  510 , port  702 C is connected to a second end of the reference resistor  510  and port  702 D is connected to the first end of the resistor and to port  702 B of the feedback circuit itself. 
     The feedback circuit  702  comprises a second current-to-voltage converter  704  which is connected between ports  702 A and  702 B and which is also connected to a “divide by K” voltage divider  712 . This is in contrast to the “divide by (K−1)” voltage divider  612  of FIG.  6 . The second current-to-voltage converter  704  may be similar to the second current-to-voltage converter  604  in that it may also consist of a current transformer placed in parallel with a resistor. 
     The remainder of the feedback circuit  702  is similar to the feedback circuit  502  of FIG. 6, with the first current-to-voltage converter  602  being connected to the second end of the reference resistor  510  (at port  702 C) and the opamp  614  having inputs connected to the voltage divider circuit  712  and to the second current-to-voltage converter  602  and having an output connected to the first end of the reference resistor  510  (via port  702 D). 
     Operation of the feedback circuit in FIG. 7 is virtually identical to that of FIG.  6 . The first current-to-voltage converter  602  produces a voltage V 602  which is proportional to the current I 510  through the reference resistor  510 , while the second current-to-voltage converter  704  produces a voltage V 704  proportional to the current I 0  through the first winding of the transformer  506 . The “divide by K” voltage divider  712  produces a voltage V 712  which is proportional to {fraction (1/K)} times the current I 0  through the first winding of the transformer  506  and the opamp  614  generates an error current proportional to the difference between the voltages V 602  and V 712 . 
     Regardless of whether the implementation of FIG. 6 or FIG. 7 is chosen, the current I 510  drawn by the reference resistor  510  will be {fraction (1/K)} times the current I 0  exiting the line driver side of the transformer. However, depending on the value of K, it may be more practical to implement the voltage divider circuit as a “divide by K” circuit (at  712  in FIG. 7) rather than a “divide by (K−1)” circuit (at  612  in FIG.  6 ). 
     While the preferred embodiment of the present invention has been described and illustrated, it will be apparent to persons skilled in the art that numerous modifications and variations are possible. The scope of the invention, therefore, is only to be limited by the claims appended hereto.