Abstract:
An improved line driver and method for increasing the power efficiency, signal accuracy, and stability of a transmit signal on a transmission line are disclosed. The improved line driver uses a negative feedback control loop, thereby enhancing operational stability and suppressing both amplifier imperfections and discrete component manufacturing variances. Furthermore, when the improved line driver output stage is integrated with a hybrid, the composite circuit provides a power efficient full duplex solution for line driver applications. In a preferred embodiment, the improved line driver may comprise an active line termination control loop with current sense feedback, a first amplifier, and a second amplifier. The present invention can also be viewed as providing a method for increasing the stability, power efficiency, and accuracy of a line driver in a duplex transmission system. In its broadest terms, the method can be described as: applying a transmit signal to an input stage of a line driver; amplifying the transmit signal; using an active termination feedback control loop to generate a feedback signal; amplifying the feedback signal; combining the feedback signal with a duplex signal on a transmission line to generate a scaled transmit signal; and combining the scaled transmit signal with the duplex signal to recover a remotely generated receive signal from the transmission line.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of co-pending U.S. provisional patent application, issued Ser. No. 60/166,057, and filed Nov. 17, 1999, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to high-speed data communications on a transmission line. More specifically, the invention relates to an improved line driver with active line termination, which solves problems associated with power efficiency, recovering receive signals from a duplex signal transmission, and manufacturing variances of circuit components. 
     BACKGROUND OF THE INVENTION 
     With the advancement of technology, and the need for instantaneous information, the ability to transfer digital information from one location to another, such as from a central office (CO) to a customer premise (CP) has become more and more important. 
     In a digital subscriber line (DSL) communication system, and more particularly an xDSL system where “x” indicates a plurality of various standards used in the data transfer, data is transmitted from a CO to a CP via a transmission line, such as a two-wire twisted pair, and is transmitted from the CP to the CO as well, either simultaneously or in different communication sessions. The same transmission line might be utilized for data transfer by both sites or the transmission to and from the CO might occur on two separate lines. In this regard reference is now directed to FIG. 1, which illustrates a prior art xDSL communication system  1 . Specifically, FIG. 1 illustrates communication between a central office (CO)  10  and a customer premise (CP)  20  by way of twisted-pair telephone line  30 . While the CP  20  may be a single dwelling residence, a small business, or other entity, it is generally characterized as having plain old telephone system (POTS) equipment, such as a telephone  22 , a public switched telephone network (PSTN) modem  25 , a facsimile machine  26 , etc. The CP  20  may also include an xDSL communication device, such as an xDSL modem  23  that may permit a computer  24  to communicate with one or more remote networks via the CO  10 . When a xDSL service is provided, a POTS filter  21  might be interposed between the POTS equipment  22  and the twisted-pair telephone line  30 . As is known, the POTS filter  21  includes a low-pass filter having a cut-off frequency of approximately 4 kilohertz to 10 kilohertz, in order to filter high frequency transmissions from the xDSL communication device  23  and to protect the POTS equipment from the higher frequency xDSL equipment (e.g., the phone  22  and the facsimile machine  26  ). 
     At the CO  10 , additional circuitry is typically provided. Generally, a line card (i.e., Line Card A)  18  containing line interface circuitry is provided to communicatively couple various xDSL service related signals along with PSTN voice signals to the twisted-pair telephone line  30 . In fact, multiple line cards  14 ,  18  may be provided to serve a plurality of copper telephone subscriber loops. In the same way, additional interface circuit cards are typically provided at the CO  10  to handle different types of services. For example, an integrated services digital network (ISDN) interface card  16 , a digital loop carrier line card  17 , and other circuit cards, for supporting similar and other communication services, may be provided. 
     A digital switch  12  is also provided at the CO  10  and is configured to communicate with each of the various line cards  14 ,  16 ,  17 , and  18 . At a PSTN interface side of the CO (i.e., the side opposite the various line cards  14 ,  16 ,  17 , and  18  supporting the telephone subscriber loops), a plurality of trunk cards  11 ,  13 , and  15  are typically provided. For example, an analog trunk card  11 , a digital trunk card  13 , and an optical trunk card  15  are illustrated in FIG.  1 . Typically, these circuit cards have outgoing lines that support numerous multiplexed xDSL service signal transmissions. 
     Having introduced a conventional xDSL communication system  1  as illustrated and described in relation to FIG. 1, reference is now directed to FIG. 2, which is a prior art functional block diagram further illustrating the various functional elements in a xDSL communications link  40  between a line card  18  located within a CO  10  and a xDSL modem  23  located at a CP  20  as introduced in FIG.  1 . In this regard, the xDSL communications link  40  of FIG. 2 illustrates data transmission from a CO  10  to a CP  20  via a transmission line  30 , such as, a twisted-pair telephone transmission line as may be provided by a POTS service provider to complete a designated link between a CO  10  and a CP  20 . In addition, FIG. 2 further illustrates data transmission from the CP  20  to the CO  10  via the same twisted-pair telephone transmission line  30 . With regard to the present illustration, transmission of data may be directed from the CP  20  to the CO  10 , from the CO  10  to the CP  20  or in both directions simultaneously. Furthermore, data transmissions can flow on the same twisted-pair telephone transmission line  30  in both directions, or alternatively on separate transmission lines (one shown for simplicity of illustration). Each of the separate transmission lines may be designated to carry data transfers in a particular direction either to or from the CP  20 . 
     The CO  10  may include a printed circuit line card  18  (see FIG. 1) that includes a CO-digital signal processor (DSP)  43 , which receives digital information from one or more data sources (not shown) and sends the digital information to a CO-analog front end (AFE)  45 . The CO-AFE  45  interposed between the twisted-pair telephone transmission line  30  and the CO-DSP  43  may convert digital data, from the CO-DSP  43 , into a continuous time analog signal for transmission to the CP  20  via the one or more twisted-pair telephone transmission lines  30 . 
     One or more analog signal representations of digital data streams supplied by one or more data sources (not shown) may be converted in the CO-AFE  45  and further amplified and processed via a CO-line driver  47  before transmission by a CO-hybrid  49 , in accordance with the amount of power required to drive an amplified analog signal through the twisted-pair telephone transmission line  30  to the CP  20 . 
     As is also illustrated in FIG. 2, the xDSL modem  23  located at the CP  20  may comprise a CP-hybrid  48 . The CP-hybrid  48  may be used to de-couple a received signal from the transmitted signal in accordance with the data modulation scheme implemented by the particular xDSL data transmission standard in use. A CP-AFE  44 , also located at the CP  20 , may be configured to receive the de-coupled received signal from the CP-hybrid  48 . The CP-AFE  44  may be configured to convert the received analog signal into a digital signal, which may then be transmitted to a CP-DSP  42  located at the CP  20 . Finally, the digital information may be further transmitted to one or more specified data sources such as the computer  24  (see FIG.  1 ). 
     In the opposite data transmission direction, one or more digital data streams supplied by one or more devices in communication with the CP-DSP  42  at the CP  20  may be converted by the CP-AFE  44  and further amplified via CP-line driver  46 . As will be appreciated by those skilled in the art, the CP-line driver  46  may amplify and forward the transmit signal with the power required to drive an amplified analog signal through the twisted-pair telephone transmission line  30  to the CO  10 . It is significant to note that the CP-hybrid  48  is used to regenerate the transmit signal so it may be subtracted from the receive signal when the DSL communication system  1  is receiving at the CP  20 . As a result, the CP-hybrid  48  does not affect the transmitted signal in any way. The CO-AFE  45  may receive the data from the CO-hybrid  49 , located at the CO  10 , which may de-couple the signal received from the CP  20  from the signal transmitted by the CO  10 . The CO-AFE  45  may then convert the received analog signal into one or more digital signals, which may then be forwarded to the CO-DSP  43  located at the CO  10 . Finally, the digital information may be further distributed to one or more specified data sources (not shown) by the CO-DSP  43 . 
     Having briefly described a xDSL communications link  40  between the line card  18  located within the CO  10  and the xDSL modem  23  located at the CP  20  as illustrated in FIG. 2, reference is now directed to FIG.  3 . In this regard, FIG. 3 is a prior art circuit schematic for a conventional hybrid  49 . 
     As illustrated in FIG. 3, a transmit signal, TX, may be provided from the CO-Line Driver  47  (FIG. 2) and applied across a back-matching resistor  57 , herein labeled, “R b .” As is further illustrated in FIG. 3, impedance and voltage scaling may be performed by coupling the transmit signal, TX′, to a two-wire transmission line, herein labeled, “TIP” and “RING” via a transformer  59 . 
     As is also illustrated in FIG. 3, the transmit signal, TX, may be applied to a scaled voltage divider consisting of a first filter  53 , labeled, “Z b ” and a second filter  55 , labeled, “Z m .” The first filter  53  may be configured such that it emulates a scaled version of the back-matching resistor  57 . For example, if the back-matching resistor is implemented with a resistor having a resistance of X Ohms, the first filter  53 , Z b , may be implemented such that its equivalent impedance is nX Ohms. Similarly, the second filter  55 , Z m , may be configured such that it emulates the sum of the line and load impedances, multiplied by the same scale factor, n. In a manner well known in the art, the transmit signal, TX′, may be echoed across the second filter  55  and may be subtracted from a duplex signal, V DUPLEX , comprising the combined receive and transmit signals, RX′ and TX′, respectively, appearing at the primary of the transformer  59  by a hybrid amplifier  61 . As further illustrated in FIG. 3, the output of the hybrid amplifier  61 , should comprise the received signal, RX″, from a remotely located transmitter after the transmit signal, TX′, has been subtracted. 
     In systems designated for data transmission over metallic transmission lines  30 , the line driver amplifier  47  is the power amplifier which delivers the necessary energy to transmit a signal through the transmission line  30  through the back-matching resistor  57 . The back-matching resistor  57  serves two purposes. First, the back-matching resistor  57  serves to match the impedance at the end of the transmission line  30 . In order to provide a sufficient return loss, a resistor approximately equal to the transmission line&#39;s  30  characteristic impedance must terminate the line. Second, the back-matching resistor  57  permits the hybrid  49  to simultaneously receive signals generated from a remote transmitter coupled to the transmission line  30  at the same time the line driver  47  is transmitting. The line driver  47  cannot terminate the transmission line  30  alone because the line driver  47  presents a low load impedance to the remotely transmitted signal, RX. As a result, using a line driver  47  alone would be the equivalent of shunting the remote signal to ground, thus making the receive signal, RX, unrecoverable. The remotely transmitted signal, RX, is recovered by subtracting from the voltage on the transmission line  30  (i.e., the duplex signal) the voltage introduced on the transmission line  30  by the local transmitter, TX′. As shown, the hybrid amplifier  61  performs the task of separating and recovering the remotely transmitted signal (i.e., the received signal) from the transmission line  30 . 
     For simplicity of illustration and description the prior art hybrid circuit of FIG. 3 is depicted in a single-ended configuration. Those skilled in the art will appreciate that in practice a differential and balanced version of the hybrid  49  may be implemented. The hybrid  49  functions properly if the line driver  47  has a very low output impedance. From a data transmission viewpoint, the output of the line driver  47  is an amplified version of the transmit signal. This amplified version of the transmit signal, TX, is applied across a voltage divider comprising the back-matching resistor  57  and the primary winding of the transformer  59 . As a result, a voltage corresponding to the amplified transmit signal is present on the primary of the transformer  59 . 
     From a data receive viewpoint, a receive signal, RX, originating at a CP  20  may arrive at the secondary winding of the transformer  59 . As is known, a corresponding receive signal voltage, RX′, is created via inductance on the primary winding of the transformer  59  and results in a current flowing into the back-matching resistor  57 . Since the line driver  47  has a low output impedance, no component of the receive signal, RX′, is present at the output of the line driver  47 , which leaves only the amplified transmit signal, TX, at the output of the line driver  47 . Since the xDSL communication system  1  operates in a substantially linear fashion, superposition applies and the voltage across the primary winding of the transformer  59 , V DUPLEX  consists of both the receive, RX′, and the transmit signals, TX′. 
     If the first and second filters  53 ,  55  replicate the voltage divider formed by the back-matching resistor  57  and the primary winding of the transformer  59 , then the voltage at the junction between the filter  53 ,  55  is equivalent to the voltage that would be applied across the transformer primary in the absence of a far end generated receive signal, TX′. As a result, the receive signal, RX, can be recovered by simply taking the difference between the voltage at the primary winding of the transformer  59  and the voltage at the junction between the first and second filters  53 ,  55 . Hence, it is possible to simultaneously transmit and receive. 
     The prior art hybrid  49  circuit of FIG. 3 has the additional characteristic that components introduced by the line driver  47  are removed by the hybrid  49 . In particular, transmit signal components due to imperfections in the line driver  47 , such as noise and distortion, are removed by the hybrid and do not get forwarded to the CO-AFE  45  (FIG. 2) with the remotely generated receive signal. This functional aspect of the hybrid  49  is important because typically a high power amplifier, such as the line driver  47  amplifier, which provides the transmit signal will not be characterized by negligible noise and distortion at the required xDSL data transmission power levels. 
     The prior art hybrid  49  circuit of FIG. 3 suffers from the disadvantage that it is relatively inefficient. The voltage swing and power ultimately delivered to the primary winding of the transformer  59  and hence the secondary winding and the transmission line  30 , is lower than the voltage swing and power sourced by the line driver  47 . Assuming that the first and second filters  53 ,  55  have a sufficiently large and relatively matched impedance so that the power consumed within the filters  53 ,  55  is relatively negligible, a portion of the power delivered by the line driver  47  is dissipated in the back-matching resistor  57  with the remaining portion available at the primary winding of the transformer  59 . That portion of the transmit signal dissipated in the back-matching resistor  57  can be reduced by reducing the magnitude of the resistance. However, the back-matching resistor  57  cannot be made arbitrarily small because the transmission line  30  would not be properly terminated at the primary winding of the transformer  59 . Since the line driver  47  has a very small output impedance there would be no way of recovering the remotely generated receive signal, RX. 
     One way to avoid the power inefficiency inherent in the hybrid  49  presented in FIG. 3 is to construct a feedback circuit around the line driver  47  amplifier. Such a feedback circuit is presented in the circuit of FIG.  4 . The circuit schematic presented in FIG.  4  and generally identified with reference numeral  60  is an example of a combination of a line driver  47  in cooperation with a positive feedback network and the transformer  59  of FIG.  3 . 
     As illustrated in FIG. 4, a line driver amplifier with active termination  65  may be coupled with the transformer  59  of FIG. 3 to provide a transmit signal, TX′, at the primary winding of the transformer  59 . This configuration may further provide an inductively coupled transmit signal, TX″, on a transmission line  30  that is electrically coupled to the secondary of the transformer  59 . In this way, the line driver with active termination  65  appears as a voltage source at its output terminal with a low output impedance in series with a finite impedance. The apparent impedance may be adjusted such that the impedance matches the resistance of the back-matching resistor  57  of FIG.  3 . The procedure of using feedback with an amplifier to generate an apparent impedance is generally known as active termination. The circuit schematic presented in FIG. 4 illustrates a relatively simple single-ended version of a line driver amplifier  47  with a positive feedback resistive network. For simplicity of illustration and description a single-ended version of the line driver with active termination  65  is presented. This presentation is by way of example only. Those skilled in the art will appreciate that a differential circuit implementation is typically selected to provide a line driver with active termination  65 . 
     As illustrated in the exemplary circuit architecture of FIG. 4, the feedback network may comprise a plurality of individual components, typically resistors, generally configured as follows. A first resistor  71 , herein labeled, “R 1 ,” maybe interposed between an input or transmit signal terminal and the positive input terminal of the line driver power amplifier  47 . A second resistor  73 , labeled, “R 2 ,” may be placed between the positive input terminal of the line driver power amplifier  47  and an output terminal of the line driver with active termination  65 . A third resistor  75 , labeled, “R G ,” may be applied between signal ground and a negative input terminal of the line driver power amplifier  47 . A fourth resistor  79 , herein labeled, “R′ B ,” may be interposed between the output of the line driver power amplifier  47  and the output terminal of the line driver with active termination  65 . A fifth resistor  77 , labeled, “RF,” may be placed between the negative input terminal of the line driver power amplifier  47  and the output terminal of the line driver amplifier  47  as shown. 
     It can be further shown that as viewed from the primary winding of the transformer  59 , the resistive network surrounding the line driver power amplifier  47  may cause the voltage across the primary winding to vary as a function of the current, I L , flowing through the primary winding, so that the primary winding appears to be driven by a voltage source through a finite impedance. With a suitable choice of resistance values for the various resistors  71 ,  73 ,  75 , and  77 , the apparent finite impedance can be shown to be the resistance value of the fourth resistor  79 , (i.e., R′ B ) multiplied by a factor given by the resistance values of the other resistors  71 ,  73 ,  75 , and  77 . Similarly, the equivalent line driver transmit gain of the line driver with active termination  65  (assuming an unloaded condition) may be determined in accordance with equation  1  using the resistance values for the various resistors  71 ,  73 ,  75 , and  77 . 
     More specifically, the equivalent line driver gain may be determined as follows:                A   0     =         (     A   -   K     )       (     1   -   K     )       .             Eq   .              1                                
     where,        A   =       (       R   G     +     R   F       )       R   G                              
     and        K   =         A   ×     R   1         (       R   1     +     R   2       )       .                            
     Similarly, the apparent back-matching resistance may be determined from the following function:                R   OUT     =         R   B   ′       (     1   -   K     )       .             Eq   .              2                                
     Since the apparent back-matching resistance is not implemented as a physical resistor, but rather by controlling the output voltage as a function of the output current, little power is dissipated and little signal swing is lost. In the limit, if the fourth resistor  79 , R′ B , is implemented with a very low resistance value and the other resistors  71 ,  73 ,  75 , and  77  are implemented to give the desired apparent resistance, virtually all the power from the line driver power amplifier  47  may be delivered to the primary winding of the transformer  59 . In this case, the remotely generated receive signal, RX, sees the appropriate back-matching resistance (i.e., impedance) and the receive signal, RX′, can be recovered from the primary winding of the transformer  59 . 
     The line driver with active termination  65  illustrated in FIG. 4 has several drawbacks. First, the feedback network in cooperation with the line driver power amplifier  47  uses positive feedback. This can be determined by examining the expressions for determining the equivalent line driver gain, A 0 , and the apparent back-matching resistance, R OUT . Observe that as the various resistance values are changed so that K approaches and then exceeds  1 , the behavior of the circuit will qualitatively change as the signs (not only the magnitude) of the equivalent line driver gain, A 0 , and the apparent back-matching resistance, R OUT , change. These breaks or critical points in the functions defining both variables are characteristic of positive feedback systems. Positive feedback, in addition to introducing the qualitative changes noted above, also tends to emphasize component imperfections, system noise, and signal distortion. 
     Furthermore, a hybrid circuit cannot be connected to the line driver with active termination  65  illustrated in FIG. 4, because there is no node at which the voltage is due solely to the transmit signal other than the unamplified transmit signal input. The amplified transmit signal can be used to power the divider formed by the first and second filters  53 ,  55  (FIG.  3 ), or equivalent filters for that matter, which would lead to a recovery of the remotely generated receive signal at the primary winding of the transformer  59  (FIG.  3 ). However, in contrast to the conventional hybrid of FIG. 2, imperfections in the line driver amplifier  47  (FIG. 3) in the form of noise and distortion would be introduced only onto the primary winding of the transformer  59  (FIG. 3) and not onto the divider formed by the filters  53 ,  55  (FIG.  3 ). As a result, noise and distortion introduced by the line driver amplifier  47  would not be canceled out by the hybrid amplifier  61  (FIG.  3 ). 
     Finally, if it were desired to change or adjust the apparent back-matching impedance, R OUT , in order to compensate for variance in the manufacture of R′ B  while attempting to maintain the equivalent line driver gain, A 0 , the various resistors  71 ,  73 ,  75 , and  77  must be adjusted in a complicated way because the gain and back-matching impedance are not independent of one another. 
     Accordingly, there is a need for a line driver with improved power efficiency that can be used in cooperation with a hybrid to remove line driver generated signal imperfections and to recover a remotely generated receive signal from a duplex signal transmission on a transmission line. 
     SUMMARY OF THE INVENTION 
     In light of the foregoing, the invention is a circuit and a method for constructing a line driver having increased power efficiency capable of driving a data transmission line with a transmitted signal having no line driver amplifier generated signal imperfections and capable of cooperation with a hybrid to recover a remotely generated receive signal from a duplex signal transmission. The improved line driver architecture of the present invention uses a negative feedback control loop, thereby enhancing operational stability and suppressing both amplifier introduced imperfections and discrete component manufacturing variances. Furthermore, the improved line driver of the present invention provides a power efficient full duplex solution for line driver applications. 
     In a preferred embodiment, an improved line driver may comprise an active line termination control loop with a current sense feedback, a first amplifier, and a second amplifier. By integrating the improved line driver with a hybrid the composite circuit provides a scaled version of the transmit signal which is free from remotely generated or receive signal effects, as well as, imperfections due to noise and distortion. In addition, the composite circuit provides a power efficient solution through the use of a finite and independently adjustable output impedance that may be used to avoid some of the loss in signal power that is typically dissipated within the line termination (i.e., the back-matching) resistance. 
     The present invention can also be viewed as providing a method for increasing the stability, power efficiency, and accuracy of a line driver. In its broadest terms, the method can be practiced by performing the following steps: applying a transmit signal to an input of a line driver; amplifying the transmit signal; applying the amplified transmit signal to a transmission line load to generate a load current; sensing the load current; and applying the sensed load current in a negative feedback control loop to generate a feedback signal responsive to the load current such that an output impedance that emulates a back-matching resistor is generated. 
     The present invention can be further viewed as providing a method for recovering a remotely generated signal from a transmission line in a duplex communication system. In its broadest terms the method can be practiced by performing the following steps: applying a transmit signal to an input stage of a line driver; amplifying the transmit signal; using an active termination feedback control loop to generate a feedback signal; amplifying the feedback signal; combining the feedback signal with a duplex signal on a transmission line to generate a scaled transmit signal; and combining the scaled transmit signal with the duplex signal to recover a remotely generated receive signal from the transmission line. 
     Other features and advantages of the present invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention, as defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully understood from the detailed description given below and from the accompanying drawings of the preferred embodiment of the invention, which however, should not be taken to limit the invention to the specific embodiments enumerated, but are for explanation and for better understanding only. Furthermore, the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Finally, like reference numerals in the figures designate corresponding parts throughout the several drawings. 
     FIG. 1 is a prior art block diagram illustrating a xDSL communications system between a central office (CO) and a customer premise (CP). 
     FIG. 2 is a prior art functional block diagram illustrating a xDSL communication link used in the xDSL communication system of FIG. 1 between a line card and a xDSL modem. 
     FIG. 3 is a prior art circuit schematic of a conventional hybrid that may be used to implement the xDSL communication link of FIG.  2 . 
     FIG. 4 is a prior art circuit schematic of a line driver having active termination as may be applied to the line driver of FIG.  2 . 
     FIG. 5 is a circuit schematic of an improved combination line driver—hybrid in accordance with the present invention. 
     FIG. 6 is a flowchart highlighting a method of performing active termination that may be used by the circuit of FIG. 5 to provide a power efficient solution for systems designed to output a signal on a metallic transmission line. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It is significant to note that the description presented herein will focus on the line driver  47  and the hybrid  49  (FIG. 2) located within the line card A  18  at the CO  10  within a xDSL communication system  1  (FIG.  1 ). This explanation and description are by way of example only. Those skilled in the art will appreciate that the concepts and teachings of the present invention may be applied to various line drivers as may be applied in a plethora of systems. 
     Turning now to the drawings illustrating the present invention, wherein like reference numerals designate corresponding parts throughout the drawings, FIG. 5 illustrates a circuit schematic of an improved combination line driver—hybrid circuit  100  in accordance with the present invention. As illustrated in FIG. 5, the improved combination line driver—hybrid  100  may comprise a back-matching impedance emulator  110  interposed generally in the position of the back-matching resistor  57  of the hybrid circuit of FIG.  3 . The back-matching impedance emulator  110  may comprise an active termination loop  120 , a first amplifier  111 , labeled, “Amplifier  1 ,” and a second amplifier  113 , labeled, “Amplifier  2 .” As shown in FIG. 5, the back-matching impedance emulator  110  may receive a transmit signal input, V TX , at a transmit signal input port. The transmit signal, V TX , may be applied at an input of a line driver amplifier  147  having a transmit gain of A and configured to form a portion of the active termination loop  120 . As is further illustrated in FIG. 5, the line driver amplifier  147  may be configured to source a load current, I L , that may be applied through a current sensing means  122 . The active termination loop  120  may be configured to apply the sensed current at the input of a feedback amplifier  125  having a gain of −H. It is significant to note that the feedback amplifier  125  gain may be expressed as the transfer function formed by the relationship between the output current and the output voltage from the feedback amplifier  125  which in operation has the dimension of resistance. 
     As illustrated in FIG. 5, the output of the line driver amplifier  147  may serve as a first output of the active termination loop  120  and may be electrically coupled to a first output of the back-matching impedance emulator  110 . The amplified and current sensed transmit signal, V TX ′, may then be applied at the primary winding of the transformer  59  or at any other suitable isolation device capable of coupling the amplified transmit signal on the transmission line  30 . 
     It will be appreciated that the amplified and current sensed transmit signal, V TX ′, may be inductively coupled from the primary winding of the transformer  59  to the secondary winding of the transformer  59 , where in accordance with the turns ratio of 1:N a scaled version of the amplified and current sensed transmit signal, V TX ″, may be electrically coupled to a subscriber loop. It will be further appreciated that a remotely generated receive signal, V RX , may also be applied at the secondary of the transformer  59 . The receive signal, V RX , may be inductively coupled from the secondary winding of the transformer  59  to the primary winding of the transformer  59 , where in accordance with the turns ratio of 1:N a scaled version of the receive signal, V RX ′, may be electrically coupled to the back-matching impedance emulator  110 . 
     As is also illustrated in FIG. 5, the output of the feedback amplifier  125  may be applied at a second output of the line driver amplifier  147 , completing the active termination loop  120 , as well as, at the input to the first amplifier  111 , thus providing a second output from the active termination loop  120 . It is significant to note that the first amplifier  111 , coupled to the second output of the active termination loop  120 , may be configured such that it has an equivalent transmit gain of −A. As further illustrated in the circuit of FIG. 5, the output of the first amplifier may be applied at a first input of a second amplifier  113 . A second input to the second amplifier  113  may be coupled to the primary winding of the transformer  59  such that both the amplified transmit signal, V TX ′, and a remotely generated receive signal, V RX ′, may be present at node V L . The output of the second amplifier  113  may form a second output of the back-matching impedance emulator  110  which may provide a scaled (e.g., amplified) version, V′ TX , of the transmit signal, V TX ′. The scaled version of the transmit signal, V′ TX , may be applied to a divider formed by the series combination of a first filter  53  and a second filter  55  as previously described with relation to the hybrid circuit of FIG. 3. A node interposed between the series combination formed by the first and second filters  53 ,  55  may be applied to a negative input terminal of a hybrid amplifier  161  as previously illustrated and described with relation to the hybrid of FIG.  3 . As is also illustrated in the circuit of FIG. 5, a positive input terminal of the hybrid amplifier  161  may be coupled to the primary winding of the transformer  59 . 
     Having described the architecture of the composite line driver—hybrid circuit  100  of FIG. 5 hereinabove, the following discussion will focus on circuit  100  operation in order to highlight the various functional aspects. In this regard, the composite line driver—hybrid circuit  100  functions as follows. The line driver amplifier  147  receives a signal to be transmitted, V TX , and may be configured to amplify the transmit signal in accordance with a desired transmit signal power. The amplified transmit signal, V TX ′, may then be passed through a current sensing means  122  capable of sensing the load current, I L , sourced by the line driver amplifier  147 . The current sensing function may be implemented by any convenient means, for example a resistor having a small resistance value. The passage of the amplified transmit signal, V TX ′, through the current sensing means  122  (e.g., the small resistor) will generate a corresponding small voltage in accordance with the well known relationship, V=I×R. where V is the voltage generated in Volts, I is the load current in Amperes sourced by the line driver amplifier  147 , and R is the resistance value in Ohms of the small resistor. With suitable amplification, as may be provided by the feedback amplifier  125 , the equivalent of a current sensing resistor with a current-to-voltage conversion ratio of −H can be implemented. In other words, the sensed value of the load current, I L , may be multiplied by a factor of −H (which has the dimension of resistance) by the feedback amplifier  125  in order to generate an output voltage, herein labeled, “VFB.” Assuming both the line driver amplifier  147  and the feedback amplifier  125  of the active termination loop  120  have high input impedances, the output of the feedback amplifier  125  can be described as follows: 
     
       
           V   FB   =−H×I   L .  Eq. 3 
       
     
     As a result of the active termination loop  120 , the load voltage at the primary winding of the transformer  59  can be determined by the following relationship: 
     
       
           V   L   =A ×( V   TX   +V   FB ),  Eq. 4 
       
     
     or in simpler terms, V L =AV TX −AHI L . Thus the voltage, V L , applied at the primary winding of the transformer  59  and the current through the winding, I L , are related in a way which corresponds to a voltage controlled voltage source having a gain, A, in series with an impedance having an equivalent resistance of A×H Ohms. As a result, the active termination loop  120  functions as the equivalent of a line driver  47  (FIG. 2) with a transmit gain of A and a back-matching resistor  57  (FIG. 3) of A×H Ohms. 
     It is significant to note that the active termination loop  120  circuit configuration is that of a negative feedback circuit, where, assuming ideal components, any positive values may be used for both A and H, and the gain can be increased without loss of stability or a change in the sign of the output voltage. In other words, the equivalent line driver gain, A, and the back-matching impedance, A×H, may vary in magnitude but not in their sign (i.e., A and A×H will never go negative). 
     In order for a hybrid circuit to provide full duplex signal transmission (i.e., simultaneously transmit and receive separate and distinct signals on a transmission line  30 ), a voltage which comprises only the amplified transmit signal is required. Note that the output of the line driver amplifier  147  will have a component originating with a remotely generated receive signal, V RX . To generate a voltage which is solely reflective of the transmit signal, the composite line driver—hybrid circuit  100  takes the voltage present on the primary winding of the transformer  59 , V L , and a voltage, V FB , scaled by the gain of the first amplifier  111  to derive V′ TX  as shown by the following relationship: 
     
       
           V′   TX   =V   L   −A×V   FB   =V   L   +AHI   L   =AV   TX ,  Eq. 5 
       
     
     which indicates that V′ TX  is the transmit signal, V TX , multiplied (scaled) by the line driver amplifier  147  transmit gain. As further illustrated in FIG.  5  and in accordance with the hybrid circuit of FIG. 3, V′ TX , can be used by the hybrid filters  53 ,  55  (Z B  and Z M ) to recover the remotely generated receive signal at node V RX ″. It is significant to note that the composite line driver—hybrid circuit  100  architecture illustrated in FIG. 5, permits the hybrid circuit components to remove line driver amplifier  147  noise and distortion. 
     It is of further significance to note that the combined line driver—hybrid circuit  100  of FIG. 5 was presented by way of example only. In alternative implementations, the various amplifiers may be merged together and equivalent signal gains could be obtained by appropriate signal scaling techniques well known and appreciated by those skilled in the art. The relative signal strength to and through the hybrid amplifier  161 , however, must remain as described above in order to properly recover a remotely generated receive signal free of the effects of a local line driver amplifier responsible for generating a transmit signal in a duplex data transmission scheme. 
     It is also significant to note that the active termination loop  120  of the back-matching impedance emulator  110  is quite different from the prior art positive feedback resistive network of FIG.  4 . First, the feedback voltage, V FB , a quantity proportional to the load current, I L , is made available. Second, the feedback circuit is inherently stable in that the sign of the gain and the transfer function of the feedback amplifier can never change. Furthermore, an output signal, V′ TX , suitable for driving a hybrid is readily available. Last, by varying H or its equivalent, only the effective output impedance is changed not the equivalent transmit gain, A, of the line driver amplifier  147 . As a result, H can be adjusted in order to compensate for scaling errors introduced by the current sensing means  122 . In this way, the composite line driver—hybrid circuit  100  of FIG. 5, provides full duplex operation while removing many of the difficulties of the simple active termination of FIG.  4 . 
     Having introduced and described an exemplary embodiment of an improved line driver—hybrid circuit  100  in accordance with the present invention with regard to FIG. 5, reference is now directed to FIG.  6 . In this regard, FIG. 6 illustrates a flowchart highlighting a method for performing active termination that may be used by the circuit of FIG. 5 to provide a power efficient solution for systems designed to output a signal on a metallic transmission line. In this regard the method for performing active termination  200  begins with step  202  herein designated as “start.” The method for performing active termination  200  may be configured to receive a previously generated transmit signal as indicated in step  204 . Once the transmit signal is available, the composite line driver-hybrid circuit  100  of FIG. 5, or another suitable circuit or system, may amplify the transmit signal as illustrated in step  206 . As further illustrated in step  208  of FIG. 6, the method for performing active termination  200  may proceed by using a current sensing means in cooperation with a suitably configured circuit having a negative transfer function to create a feedback signal. The amplified transmit signal may be applied to the transmission line as shown in step  210 . 
     The method for performing active termination  200  may continue with step  212  where the feedback signal is received. Having generated and received the feedback signal, the method for performing active termination  200  may proceed by amplifying the feedback signal with the inverse gain of the line driver amplifier  147  (FIG. 5) (i.e., multiply the feedback signal with −A as illustrated in step  214 . Next, the method for performing active termination  200 , may acquire a duplex signal transmission from a transmission line as indicated in step  216 . Having adjusted the feedback signal and acquired the duplex signal transmission as described above, a scaled version of the transmit signal may be generated through a mathematical combination of the signals as illustrated in step  218 . The method for active termination  200  may then proceed to recover a remotely generated receive signal by performing a mathematical combination of the scaled transmit signal and the acquired duplex signal as illustrated in step  220 . Those skilled in the art will appreciate that steps  204  through  220  may be repeated as desired in order to perform actively terminate a transmission line in a duplex transmission system. Any suitable method for aborting and or ending the method herein described may be used as illustrated in step  222 , labeled, “stop.” 
     It is significant to note that the sequence presented in FIG. 6 is by way of example only. Those skilled in the art will appreciate that particular steps may in fact be performed out of sequence or substantially simultaneously. For example, once the transmit signal is received and amplified it may be applied to the transmission line at any time prior to acquiring the duplex signal transmission from the transmission line. As a result step  210  may be performed as early or substantially simultaneously across a range of steps generally defined from after step  206  to before step  216 . 
     In the preferred embodiment of the present invention, which is intended to be a non-limiting example, each of the functions herein introduced and described may be implemented through a combination of an improved line driver  47  with a hybrid  49  in a circuit configuration. Furthermore, the method for performing active termination  200  as illustrated in FIG. 6 may comprise a set of processing steps that may be implemented in software and executed by a computing device in communication or integrated within the aforementioned devices. For example, each of the aforementioned devices may be in communication with but not limited to, a personal computer, a workstation, minicomputer, a controller, or a mainframe computer. The software based system, which comprises an ordered list of executable instructions for implementing logical functions, can be embodied in any computer readable medium for use by, or in connection with, an instruction execution system, apparatus, or device such as a computer based system, processor containing system, or other systems that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer readable medium” can be any means that can contain, store, communicate, propagate or transport the program for use by or in connection with the instruction execution system, apparatus or device. The computer readable medium can be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read only memory (ROM) (magnetic), an erasable program read only memory (EPROM or flash memory) (magnetic), an optical fiber (optical), and a portable compact disk read only memory (CDROM) (optical). Note that the computer readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance, optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. 
     It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of the present invention and protected by the following claims.