Abstract:
A driver circuit for driving a transmit signal for a line includes a signal amplifier configured to deliver a transmit signal, present at a signal input of the signal amplifier, amplified with a gain to a signal output, a line interface circuit connected between the signal output of the signal amplifier and the line, and a hybrid circuit connected to the line interface circuit, the hybrid circuit configured to suppress the transmit signal and couple a received signal received via the line to a signal input of the signal amplifier to provide impedance synthesis.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation of U.S. application Ser. No. 11/474,636 filed on 26 Jun. 2006, which claims priority to German Patent Application No. 10 2005 030 049.9 filed on 27 Jun. 2005, the content of both applications incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a circuit and method for driving a transmit signal which, in particular, can be used in transceivers. 
       BACKGROUND 
       [0003]      FIG. 1  shows a conventional driver circuit for driving a transmit signal. The driver circuit contains an inverting amplifier, the signal output of which is fed back via a resistor R fb . The signal amplifier amplifies the transmit signal present at the signal input with a particular gain and delivers the amplified transmit signal via its signal output to a transformer connected thereto. The transformer couples the amplified transmit signal present at its primary winding into its secondary winding which is connected, in turn, to a twisted two-wire telephone line. The transformer is connected to a reference voltage potential via an external terminating impedance. The transformer is also fed back to the input via a feedback loop containing a synthesis resistor R syn  for generating a synthesized output impedance. 
         [0004]    In the driver circuit shown in  FIG. 1 , the signal output of the signal amplifier is directly connected to the transformer. 
         [0005]    From the point of view of the telephone line, the output impedance is: 
         [0000]        Z   Line   =n   2 ( Z   cosyn   +Z   coEXT ), 
         [0000]    where the actively synthesized impedance Z cosyn  is dependent on the synthesis resistor R syn  and the feedback resistor R fb : 
         [0000]    
       
         
           
             
               Z 
               cosyn 
             
             = 
             
               
                 Z 
                 coEXT 
               
                
               
                 ( 
                 
                   
                     R 
                     jb 
                   
                   
                     R 
                     syn 
                   
                 
                 ) 
               
             
           
         
       
     
         [0006]    The gain of the driver circuit according to the prior art, shown in  FIG. 1 , is 
         [0000]    
       
         
           
             
               
                 G 
                 tx 
               
               = 
               
                 
                   
                     U 
                     tx 
                   
                   
                     U 
                     in 
                   
                 
                 = 
                 
                   - 
                   
                     
                       1 
                       
                         R 
                         in 
                       
                     
                     
                       
                         1 
                         
                           R 
                           fb 
                         
                       
                       + 
                       
                         
                           1 
                           
                             R 
                             syn 
                           
                         
                         · 
                         
                           
                             Z 
                             t 
                           
                           
                             
                               Z 
                               t 
                             
                             + 
                             
                               Z 
                               L 
                             
                           
                         
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    Where Z L  is the impedance of the transformer as seen from the signal amplifier and wherein Z t  is equal to the external terminating impedance Z coEXT . 
         [0007]    If a class-D signal amplifier is used as signal amplifier which supplies an amplified pulse-width-modulated signal, an analogue low-pass filter is usually provided between the signal amplifier output and the transformer in order to filter out a high-frequency switching noise of the amplified transmit signal, produced during the amplification by the class-D signal amplifier. This low-pass filter is also called an OOB (out of band) filter. 
         [0008]      FIG. 2  shows a conventional driver circuit for driving a transmit signal which contains such an OOB filter. The out-of-band low-pass filter has a particular transfer function H cos  in order to meet the spectral power density requirements PSD of the transmission standard. 
         [0009]    In the driver circuit shown in  FIG. 2 , the impedance, as seen from the telephone line, is also: 
         [0000]        Z   Line   =n   2 ( Z   cosyn   +Z   coEXT ), 
         [0000]    where the synthesized impedance is dependent on the transfer function H cos  of the low-pass filter: 
         [0000]    
       
         
           
             
               Z 
               cosyn 
             
             = 
             
               
                 Z 
                 coEXT 
               
                
               
                 ( 
                 
                   
                     H 
                     cob 
                   
                   · 
                   
                     
                       R 
                       fb 
                     
                     
                       R 
                       syn 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0010]    The gain of the driver circuit is obtained as: 
         [0000]    
       
         
           
             
               G 
               tx 
             
             = 
             
               
                 
                   U 
                   tx 
                 
                 
                   U 
                   in 
                 
               
               = 
               
                 
                   1 
                   
                     R 
                     In 
                   
                 
                 
                   
                     1 
                     
                       R 
                       fb 
                     
                   
                   + 
                   
                     
                       1 
                       
                         R 
                         sys 
                       
                     
                     · 
                     
                       H 
                       cos 
                     
                     · 
                     
                       
                         Z 
                         t 
                       
                       
                         
                           Z 
                           t 
                         
                         + 
                         
                           Z 
                           c 
                         
                       
                     
                   
                 
               
             
           
         
       
     
         [0011]    The driver circuit as shown in  FIG. 2  has the disadvantage that the gain G tx  is dependent on the transfer function H cos  of the analogue low-pass filter. The stability of the feedback loop for the impedance synthesis is thus dependent on the transfer characteristics of the OOB filter so that unstable feedback can occur. 
       SUMMARY 
       [0012]    The invention provides a driver circuit for driving a transmit signal for a line comprising a signal amplifier configured to deliver a transmit signal, present at a signal input of the signal amplifier, amplified with a gain at a signal output, a line interface circuit connected between the signal output of the signal amplifier and the line, and a hybrid circuit connected to the line interface circuit, the hybrid circuit configured to suppress the transmit signal and couple a received signal received via the line to a signal input of the signal amplifier to provide impedance synthesis. 
         [0013]    The invention also provides a driver circuit for driving a transmit signal for a line comprising a signal amplifier configured to deliver a transmit signal, present at a signal input of the signal amplifier, amplified with a gain at a signal output, a transformer having a secondary winding is connected to the line and a primary winding having a first terminal connected to the signal output of the signal amplifier and a second terminal connected to a terminating impedance, and at least one hybrid circuit having a first feedback impedance configured to feed back the first connection of the primary winding to the signal input of the signal amplifier and a second feedback impedance configured to feed back the second connection of the primary winding to the signal input of the signal amplifier. 
         [0014]    In one embodiment of the driver circuit according to the invention, the hybrid circuit couples a received signal received via the line. 
         [0015]    In one embodiment of the driver circuit according to the invention, the received signal coupled out by the hybrid circuit is added to the transmit signal at a summation node for the purpose of impedance synthesis. 
         [0016]    In a further embodiment of the driver circuit according to the invention, the synthesized impedance can be adjusted independently of the gain of the signal amplifier. 
         [0017]    In one embodiment of the driver circuit according to the invention, a filter for filtering high-frequency signal disturbances of the amplified transmit signal is provided between the signal amplifier and the transformer. 
         [0018]    In a further embodiment of the driver circuit according to the invention, the ratio of impedance values of the two feedback impedances contained in the hybrid circuit is such that the gain is independent of a filter transfer function of the filter. 
         [0019]    In one embodiment of the driver circuit according to the invention, the filter is an out-of-band filter. 
         [0020]    In one embodiment of the driver circuit according to the invention, the signal amplifier is a class-D signal amplifier which delivers a pulse-width-modulated transmit signal. 
         [0021]    In one embodiment of the driver circuit according to the invention, the filter is an analogue low-pass filter for filtering out a high-frequency switching noise of the pulse-width-modulated amplified transmit signal. 
         [0022]    In one embodiment of the driver circuit according to the invention, the feedback impedances are formed by resistors. 
         [0023]    In one embodiment of the driver circuit according to the invention, the feedback impedances are integrated in the signal amplifier. 
         [0024]    In a preferred embodiment of the driver circuit according to the invention, the second feedback loop generates a synthesized line terminating impedance. 
         [0025]    In one embodiment of the driver circuit according to the invention, it is of fully differential construction. 
         [0026]    The invention also creates a hybrid circuit for a transceiver which couples out a received signal received via a line and adds it to a transmit signal of the transceiver at a summation node for the purpose of impedance synthesis of an output impedance of the transceiver. 
         [0027]    The invention also creates a method for driving a transmit signal, in which a transmit signal is amplified and coupled into a line, wherein a received signal received via the line is coupled out and added to the transmit signal for the purpose of impedance synthesis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    In the text which follows, embodiments of the driver circuit according to the invention are described with reference to the attached figures, for explaining features essential to the invention. 
           [0029]      FIG. 1  shows a conventional driver circuit for driving a transmit signal; 
           [0030]      FIG. 2  shows a further conventional driver circuit with an OOB filter; 
           [0031]      FIG. 3  shows a circuit diagram of a first embodiment of the driver circuit according to the invention for driving a transmit signal; 
           [0032]      FIG. 4  shows a fully differential embodiment of the driver circuit according to the invention; 
           [0033]      FIG. 5  shows transfer functions of a conventional driver circuit without impedance synthesis; 
           [0034]      FIG. 6  shows transfer functions of a conventional driver circuit with impedance synthesis; 
           [0035]      FIG. 7  shows transfer functions of a possible embodiment of a driver circuit according to the invention for driving a transmit signal. 
           [0036]      FIG. 8  shows a further embodiment of the driver circuit according to the invention; 
           [0037]      FIG. 9  shows a further embodiment of the driver circuit according to the invention; 
           [0038]      FIG. 10  shows a further embodiment of the driver circuit according to the invention; 
           [0039]      FIG. 11  shows a further embodiment of the driver circuit according to the invention; 
           [0040]      FIG. 12  shows a further embodiment of the driver circuit according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0041]    As can be seen from  FIG. 3 , the embodiment shown there contains a driver circuit  1  according to the invention for driving a transmit signal, a signal amplifier  2  with a signal input  3  and a signal output  4 . In the embodiment shown in  FIG. 3 , the signal amplifier  2  is an inverting amplifier with an input resistor  5  which is connected to a first input  6  of the operational amplifier  7 , the second input  8  of which is connected to a reference potential. At the signal output  4 , the operational amplifier  7  is fed back to the first signal input  6  via a feedback resistor  9 . The signal amplifier  2  amplifies the transmit signal present at the signal input  3  with a particular gain which delivers the amplified transmit signal at its signal output  4  via a line  10  to an OOB low-pass filter  11 . The filter  11  has a particular transfer function H OOB  and filters out high-frequency signal disturbances of the amplified transmit signal. 
         [0042]    The driver circuit  1  also contains a transformer  12  with a primary winding  12 - 1  and a secondary winding  12 - 2 . The primary winding  12 - 1  has a first terminal  13  and a second terminal  14 . The first terminal  13  of the primary winding  12 - 1  of the transformer  12  is connected to the OOB filter  11 . The second terminal  14  of the primary winding  12 - 1  is connected to a terminating impedance  15 . At the secondary winding  12 - 2  of the transformer  12 , a line  16 , particularly a twisted two-wire telephone line  16 , is connected. 
         [0043]    The embodiment of the driver circuit  1  shown in  FIG. 3  contains two feedback loops. The first feedback loop feeds the first connection  13  of the primary winding  12 - 1  back to the first signal input  6  of the operational amplifier  7  via a first feedback impedance  17 . The signal fed back is preferably inverted as shown in  FIG. 3 . 
         [0044]    In addition, the driver circuit  1  contains a second feedback loop which feeds the second connection  14  of the primary winding  12 - 1  back to the first signal input  8  of the operational amplifier  7  via a second feedback impedance  18 . In the driver circuit  1  according to the invention, the ratio of the resistance values of the two feedback impedances  17 ,  18  is dimensioned in such a manner that the gain is independent of the filter transfer function of the OOB filter  11 . 
         [0045]    As can be seen by comparing  FIG. 2 , which shows a conventional driver circuit, and  FIG. 3 , which shows the driver circuit according to the invention, the driver circuit according to the invention has, apart from the usual feedback loop containing the synthesis impedance  18 , an additional feedback loop which is formed by the feedback impedance  17 . By providing this additional feedback loop or this additional feedback signal path, only the received signal is fed back for impedance synthesis so that the transmit signal gain is independent of the transfer characteristics of the OOB filter  11 . 
         [0046]    The signal amplifier  2  is preferably formed by a class-D signal amplifier which delivers an amplified pulse-width-modulator transmit signal to the OOB filter  11 . The OOB filter  11  is an analogue low-pass filter which filters out the high-frequency disturbances produced during the signal amplification or, respectively, the high-frequency switching noise of the pulse-width-modulated amplified transmit signal in order to ensure, in particular, the spectral power density of the transmit signal demanded by the standard. 
         [0047]    The feedback impedances  17 ,  18  are preferably formed by resistors. These feedback resistors are preferably integrated in the signal amplifier  2 . The output impedance, seen by the telephone line  16 , of the driver circuit  1  according to the invention is: 
         [0000]        Z line= n   2 ·( Z co_syn+ Z co_ext),
 
         [0000]    where the actively synthesized impedance Z cosyn  is obtained as follows: 
         [0000]    
       
         
           
             Zco_syn 
             = 
             
               
                 ZCo_ext 
                 · 
                 
                   ( 
                   
                     
                       1 
                       Rsyn 
                     
                     
                       
                         1 
                         
                           Rfb 
                           · 
                           Hoob 
                         
                       
                        
                       
                         1 
                         
                           k 
                           · 
                           Rsyn 
                         
                       
                     
                   
                   ) 
                 
               
               ≈ 
               
                 Zco_ext 
                 · 
                 
                   
                     ( 
                     
                       Hoob 
                       · 
                       
                         Rfb 
                         Rsyn 
                       
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    Where R syn , is the resistance value of the feedback impedance  18 , k·R syn  is the resistance value of the feedback impedance  17 , R fb  is the resistance value of the feedback resistor  9 , and Zco EXT  is the impedance of the external terminating impedance. 
         [0048]    The gain is then obtained as follows: 
         [0000]    
       
         
           
             Gtx 
             = 
             
               
                 Utx 
                 Utn 
               
               = 
               
                 
                   1 
                   Rin 
                 
                 
                   
                     1 
                     Rfb 
                   
                   + 
                   
                     
                       1 
                       Rsyn 
                     
                     · 
                     Hoob 
                     · 
                     
                       Zt 
                       
                         Zt 
                         + 
                         Zl 
                       
                     
                     · 
                     
                       ( 
                       
                         1 
                         - 
                         
                           
                             
                               Zl 
                               + 
                               Zt 
                             
                             Zt 
                           
                           · 
                           
                             1 
                             k 
                           
                         
                       
                       ) 
                     
                   
                 
               
             
           
         
       
     
         [0000]    if the ratio k between the resistance values of the two feedback impedances  17 ,  18  is selected in such a manner that 
         [0000]    
       
         
           
             
               ( 
               
                 1 
                 - 
                 
                   
                     
                       Zl 
                       + 
                       Zt 
                     
                     Zt 
                   
                   · 
                   
                     1 
                     k 
                   
                 
               
               ) 
             
             = 
             0 
           
         
       
       
         
           
             
               i 
               . 
               e 
               . 
               
                 
 
               
                
               k 
             
             = 
             
               
                 Zl 
                 + 
                 Zt 
               
               Zt 
             
           
         
       
     
         [0000]    then the gain G is independent of the transfer function H OOB  of the filter  11 : 
         [0000]    
       
         
           
             Gtx 
             = 
             
               
                 Utx 
                 Uin 
               
               = 
               
                 - 
                 
                   
                     Rfb 
                     Rin 
                   
                   . 
                 
               
             
           
         
       
     
         [0049]    In the embodiment of the driver circuit  1  as shown in  FIG. 3 , a change in the transfer function of the OOB filter  11  has no effect whatsoever on the gain of the transmit signal. This has the advantage that the OOB filter  11  can be designed independently of the signal amplifier  2 . The impedance ratio is, for example about 7, the resistance value of the feedback impedance  18  being about 20 kΩ. 
         [0050]    The external terminating impedance  15 , shown in  FIG. 3 , can consist of a conventional resistor or of a complex switching network containing coils, capacitors and resistors. 
         [0051]    In one embodiment, the embodiment is of fully differential construction as is shown in  FIG. 4 . 
         [0052]    In the fully differential embodiment according to  FIG. 4 , the OOB filter  11  is formed by an in-phase reactor  11 - 1  and by a number of capacitors  11 - 2 ,  11 - 3 ,  11 - 4 ,  11 - 5 . In this arrangement, a first pair of capacitors  11 - 2 ,  11 - 3  is connected to the primary windings and a second pair of capacitors  11 - 4 ,  11 - 5  is connected to the secondary windings of the two transformers  12 A,  12 B. The secondary windings of the transformers  12 A,  12 B are also connected to one another via a capacitor  19 . 
         [0053]      FIG. 5  shows transfer functions of conventional driver circuits without impedance synthesis. The upper line shows the transfer function between the driver input and the telephone line and the lower line shows the transfer function between the driver input and the driver output. 
         [0054]      FIG. 6  shows transfer functions of conventional driver circuits with standard impedance synthesis. As can be seen from  FIG. 6 , the transfer functions have peaks at about 2 MHz which illustrate the impaired stability of the signal amplifier  1  due to the presence of the OOB filter  11 . 
         [0055]      FIG. 7  shows transfer functions of the driver circuit  1  as shown in  FIGS. 3 ,  4 . The upper line again shows transfer functions between the driver input  3  and the telephone line  16 . The lower line shows the transfer function between the signal amplifier input  3  and the signal amplifier output  4 . As can be seen clearly from  FIG. 7 , the transfer peaks in the driver circuit  1  are flattened distinctly in comparison with the variation shown in  FIG. 6 . 
         [0056]    By providing an additional feedback loop, the driver circuit  1  thus achieves a situation where the gain is independent of the transfer characteristics of the OBB filter  11  and where no instabilities can occur in the feedback loop. 
         [0057]      FIG. 8  shows a further embodiment of the driver circuit  1  according to the invention. In the driver circuit  1  shown in  FIG. 8 , a signal amplifier  20  comprises only one feedback impedance  9  whereas the feedback impedance  18 , together with the feedback impedance  17  and the preceding inverter, form a hybrid circuit  21 . The hybrid circuit  21 A couples out a received signal received via the line  16  for further signal processing. The received signal is fed into the transmit signal path at a summation node  22 , i.e. is added to the transmit signal to be amplified as shown in  FIG. 8 . Using the hybrid circuit  21 , an impedance synthesis is produced, the synthesized impedance being adjustable independently of the gain of the signal amplifier  20 . In an alternative embodiment, two hybrid circuits can also be provided, a first hybrid circuit being provided for coupling out the received signal and the other hybrid circuit being provided for the impedance synthesis. In the embodiment shown in  FIG. 8 , the hybrid circuit  21 A is used both for impedance synthesis and for coupling out the received signal. In the embodiment shown in  FIG. 8 , a filter  11  is provided between the signal amplifier  20  and a line interface circuit  23 A which is formed by the transformer  12  and the terminating impedance  15 . The filter  11  is preferably an out-of-band filter and is used for filtering high-frequency signal disturbances of the amplified transmit signal. In a preferred embodiment, the ratio of the impedance values of the two feedback impedances  17 ,  18  contained in the hybrid circuit  21 A is set in such a manner that the gain of the signal amplifier  20  is independent of the filter transfer function H(f) of the filter  11 . The signal amplifier  20  is preferably a class-D signal amplifier which delivers a pulse-width-modulator transmit signal, the filter  11  filtering out a high-frequency switching noise of the pulse-width-modulated amplifier transmit signal. 
         [0058]      FIG. 9  shows an alternative embodiment of the driver circuit  1  according to the invention, in which the filter  11  is not provided, and the signal output  4  of the signal amplifier  20  is connected directly to the first terminal  13  of the primary winding  12 - 1  of the transformer  12 . 
         [0059]      FIG. 10  shows a further embodiment of the driver circuit  1  according to the invention with an alternative hybrid circuit  21 B. As seen from the primary winding of the transformer  12 , there is a complex impedance Z L+T (Z Line+Transformer ) composed of the line  16  and the transformer  12 . The impedance of the terminating impedance  15  is 
         [0000]    
       
         
           
             
               Z 
               
                 
                   LT 
                   + 
                   T 
                 
                 k 
               
             
             , 
           
         
       
     
         [0000]    in the embodiment shown in  FIG. 10 , where k is a real number. In a preferred range of values, the real number k is 3.2 k6.0. The hybrid circuit  21 B is connected to the first terminal  13  and to the second terminal  14  of the primary winding  12 - 1  of the transformer  12 . The hybrid circuit  21 B has an inverting amplifier, the input of which is connected to the first terminal  13 , and a second amplifier which is connected to the second terminal  14  and the gain of which is 1+k. The output signals of the two signal amplifiers contained in the hybrid circuit  21 B are added and supplied to a further signal amplifier which delivers the signal amplified with a gain factor G. The signal coupled out of the hybrid circuit  21 B is also supplied to a summation node  22  and added to the transmit signal before the signal input of the signal amplifier  20 . 
         [0060]      FIG. 11  shows a further embodiment of the driver circuit  1  according to the invention. In the embodiment shown in  FIG. 11 , the complex terminating impedance  15 , the impedance of which is 
         [0000]    
       
         
           
             
               Z 
               
                 
                   LT 
                   + 
                   T 
                 
                 k 
               
             
             , 
           
         
       
     
         [0000]    is connected between the signal amplifier  20  and the primary winding  12 - 1  of the transformer  12 . The filter  11  shown in  FIG. 11  is not provided in an alternative embodiment so that the signal output of the signal amplifier  20  is connected directly to the terminating impedance  15 . The tap for a hybrid circuit  21 C is made between the first terminal  13  of the primary winding  12 - 1  of the transformer  12  and a node  25 . The signal picked up at the node  13  is delivered by a signal amplifier with the gain 
         [0000]    
       
         
           
             - 
             
               
                 k 
                 + 
                 1 
               
               k 
             
           
         
       
     
         [0000]    to a summation node provided in the hybrid circuit  21 C and added to the signal branched off at the branching node  25 . The sum signal is amplified at a further signal amplifier G and applied to the summation node  22 . 
         [0061]      FIG. 12  shows a further embodiment of the driver circuit  1  according to the invention. The embodiment shown in  FIG. 12  is of fully differential construction and has a fully differential hybrid circuit  21 D for coupling out the received signal received via the line  16  to a signal output  24   a ,  24   b . In addition, the hybrid circuit  21 D is used for producing an impedance synthesis, wherein the gain of the fully differential signal amplifier  200  can be adjusted independently of the synthesized impedance. This is achieved by feeding back the signal coupled out to the input side of the signal amplifier  200  via resistors  26   a ,  26   b  at summation nodes  22   a ,  22   b . Furthermore,  FIG. 12  shows the coupling capacitors  27   a ,  27   b ,  28   a ,  28   b  present at the signal input  3   a ,  3   b  and at a signal output  24   a ,  24   b.