Abstract:
Driver circuit for driving a useful signal having: (a) at least one amplifier circuit ( 3   a   , 3   b ) with low output impedance for the signal amplification of the useful signal; (b) a protection impedance ( 9   a   , 9   b ) respectively connected downstream of the amplifier circuit ( 3   a   , 3   b ) and serving to protect the amplifier circuit ( 3   a   , 3   b ); characterized in that (c) provision is respectively made of a feedback circuit ( 14   a   , 14   b ) for the frequency-dependent signal feedback of the useful signal amplified by the amplifier circuit ( 3   a   , 3   b ).

Description:
TECHNICAL FIELD 
     The invention relates to a driver circuit for driving a useful signal in accordance with the preamble of Patent claim  1 . 
     BACKGROUND ART 
       FIG. 1  shows a driver circuit according to the prior art for driving a useful signal. 
     The driver circuit illustrated in  FIG. 1  is of differential construction and contains two operational amplifiers OPA, OPB for signal amplification. The operational amplifiers amplify a useful signal arriving from a signal source and output the amplified useful signal via protection impedances Z a , Z b  to a connected terminal, for example a telephone T. The operational amplifiers are integrated for example in an SLIC circuit situated on a line card. The operational amplifiers each have a low output impedance for the signal amplification of the useful signal. The impedances connected downstream of the two operational amplifiers serve to protect the amplifier circuits and for electromagnetic compatibility (EMC). The output impedances Z a , Z b  preferably protect the amplifier circuit from overvoltages, which may be caused by a flash of lightning, for example, and for the suppression of interference signals, for example radio signals. 
       FIGS. 2   a  to  2   c  show practical realizations of the protection impedances Z a , Z b  according to the prior art. 
     The driver circuit illustrated in  FIG. 1  is of differential construction, the components being symmetrical, i.e. in particular the two protection impedances Z a , Z b  are as far as possible of identical construction in order to have a maximum longitudinal conversion loss. In communication systems, for example appertaining to voice telephony, the driver circuit for driving the useful signal must satisfy very stringent circuitry requirements with regard to the longitudinal conversion loss LCL. The standards relevant to the longitudinal conversion loss are the TR57 standard in the USA and the Q552 and G712 standards in Europe. 
       FIG. 3  shows a measuring circuit for determining the longitudinal conversion loss LCL. 
       FIG. 4  shows an associated equivalent circuit diagram. The output impedance Z out  illustrated in the equivalent circuit diagram is the output impedance of the overall circuit to the left of the output pads Outa, Outb for the SLIC circuit in  FIG. 1 . The measuring circuit illustrated in  FIG. 3  serves for measuring the longitudinal conversion loss LCL. A signal source feeds a sinusoidal measurement signal into the two output pads A, B of the line card via feed-in resistors R L . The two measuring resistors R L  are high-precision resistors with matching resistances. The voltage between the two output pads A, B is measured. 
     The following holds true for the longitudinal conversion loss: 
             LCL   =       20   ·   log     ⁢            V   L       V   T                      (   1   )             
 
The following relationship holds true between the longitudinal conversion loss LCL and the impedances illustrated in the equivalent circuit diagram according to  FIG. 4 : 
             LCL   =       20   ·   log     ⁢          1           Z   out     +     Δ   ⁢           ⁢   Z           R   L     +     Z   out     +     Δ   ⁢           ⁢   Z         -       Z   out         R   L     +     Z   out                            (   2   )             
 
The high-precision feed-in resistors R L  have a value of 300 ohms, for example. The impedance difference ΔZ between the output impedances exists because of manufacturing tolerances and inaccuracies of the protection impedances Z a , Z b  in the conventional driver circuit as illustrated in  FIG. 1 .
 
     With the applicable condition ΔZ&lt;&lt;R L , equation (2) can be greatly simplified to: 
             LCL   ≅       20   ·   log     ⁢              R   L     +     Z   out         Δ   ⁢           ⁢   Z                      (   3   )             
 
     To ensure that the longitudinal conversion loss is as high as possible and LCL thus assumes a maximum value, conventional driver circuits have hitherto used expensive output impedances with high accuracy, i.e. impedances which have very small tolerances during production. Such components which have to satisfy very high accuracy requirements can only be produced with considerable technical outlay and high costs. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide a driver circuit which, with the use of components with relatively large manufacturing tolerances, nevertheless ensures a very high longitudinal conversion loss LCL. 
     This object is achieved according to the invention by means of a driver circuit having the features specified in Patent claim  1 . 
     The invention provides a driver circuit for driving a useful signal having at least one amplifier circuit with low output impedance for the signal amplification of the useful signal, a protection impedance respectively connected downstream of the amplifier circuit and serving to protect the amplifier circuit, provision respectively being made of a feedback circuit for the frequency-dependent signal feedback of the useful signal amplified by the amplifier circuit. 
     The amplifier circuit is preferably an operational amplifier having an inverting signal input, a noninverting signal input and a signal output. 
     The protection impedance is preferably connected between the signal output of the operational amplifier and a signal line connection for the connection of a signal line. 
     The signal line is preferably a telephone line for connecting a telephone to the driver circuit. 
     The driver circuit is preferably of differential construction and has two symmetrically constructed amplifier circuits, two symmetrical protection impedances and two symmetrically constructed signal feedback circuits being provided. 
     The signal feedback circuits preferably respectively contain a capacitor, which is connected between the signal output of the operational amplifier and a signal input of the operational amplifier, and also a resistor, which is connected between the signal line connection and a further signal input of the operational amplifier. 
     The signal feedback circuit feeds back high-frequency signal components of the useful signal amplified by the amplifier circuit to the signal input of the amplifier circuit to a greater extent than low-frequency signal components of the useful signal amplified by the amplifier circuit, so that the output impedance of the driver circuit is reduced in a specific first frequency range up to a first predetermined limiting frequency (f G1 ) which lies above the second limiting frequency (f G2 ) of the useful signal. 
     In this case, the first frequency range comprises a second frequency range preferably provided for the transmission of a useful signal. 
     The second frequency range is preferably the voice signal band for the transmission of a telephone voice signal. 
     In this case, the limiting frequency (f g2 ) of the useful signal is preferably about 4 kHz. 
    
    
     
       Preferred embodiments of the driver circuit according to the invention are described below with reference to the accompanying drawings in order to elucidate features that are essential to the invention. 
       In the figures: 
         FIG. 1  shows a driver circuit for driving a useful signal according to the prior art; 
         FIG. 2  shows various embodiments of protection impedances for protecting the amplifier circuits according to the prior art; 
         FIG. 3  shows a measuring circuit for measuring the longitudinal conversion loss of a driver circuit according to the prior art; 
         FIG. 4  shows an equivalent circuit diagram for the measuring circuit illustrated in  FIG. 3 ; 
         FIG. 5  shows a preferred embodiment of the driver circuit according to the invention for driving a useful signal; 
         FIG. 6  [sic] shows a diagram illustrating the output impedance of the driver circuit according to the invention as a function of the frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the driver circuit according to the invention for driving a useful signal is explained in detail below with reference to  FIG. 5 . The driver circuit  1  as illustrated in  FIG. 5  serves for driving a useful signal, for example a voice signal for the transmission of an analog voice signal to a telephone terminal. 
       FIG. 5  shows a measuring circuit for measuring the longitudinal conversion loss LCL in the driver circuit  1  according to the invention. The driver circuit  1  illustrated in  FIG. 5 , which is of differential construction, contains two signal inputs  2   a ,  2   b  for receiving a useful signal to be driven. The driver circuit  1  furthermore contains two amplifier circuits  3   a ,  3   b  in the form of operational amplifiers each having a noninverting input  4   a ,  4   b  and an inverting signal input  5   a ,  5   b . The noninverting signal input  4   a ,  4   b  is connected to the associated signal input  2   a ,  2   b  via a signal line  6   a ,  6   b . The operational amplifier  3   a ,  3   b  respectively has a signal output  7   a ,  7   b . The signal output  7   a ,  7   b  of the operational amplifier is connected via a line  8   a ,  8   b  to a protection impedance  9   a ,  9   b  connected downstream. The protection impedance  9   a ,  9   b  respectively serves to protect the amplifier circuit  3   a ,  3   b . The protection impedances  9   a ,  9   b  are constructed for example as illustrated in  FIGS. 2   a  to  2   c . The protection impedances  9   a ,  9   b  are connected to the output pads  11   a ,  11   b  of the line card  12  via lines  10   a ,  10   b.    
     In the measuring circuit for measuring the longitudinal conversion loss as illustrated in  FIG. 5 , the output pads  11   a ,  11   b  of the line card  12  are connected to a feed-in signal source  13  via measuring resistors  12   a ,  12   b . The signal source  13  is a voltage source which feeds a sinusoidal measurement signal into the output connections  11   a ,  11   b  of the line card  12  via the measuring resistors  12   a ,  12   b.    
     The driver circuit  1  according to the invention as illustrated in  FIG. 5  additionally has electrically constructed feedback circuits  14   a ,  14   b . The feedback circuits  14   a ,  14   b  carry out a frequency-dependent signal feedback of the useful signal amplified by the associated amplifier circuit  3   a ,  23   b  [sic] to a signal input of the amplifier circuit  3   a ,  3   b . The signal feedback circuits  14   a ,  14   b  respectively contain a capacitor  15   a ,  15   b , which is connected between the signal output  7   a ,  7   b  of the associated operational amplifier  3   a ,  3   b  and a signal input  5   a ,  5   b  of the operational amplifier. In the embodiment illustrated in  FIG. 5 , the useful signal is fed back to the inverting input  5   a ,  5   b  of the associated operational amplifier  3   a ,  3   b . In addition, the signal feedback circuits  14   a ,  14   b  respectively contain a resistor  16   a ,  16   b , which is connected between the signal line connection  11   a ,  11   b  and the inverting signal input  5   a ,  5   b  of the operational amplifier  3   a ,  3   b . The signal feedback circuits  14   a ,  14   b  of the driver circuit  1  of differential construction each have two inputs  17   a ,  17   b ,  18   a ,  18   b . The first input  17   a ,  17   b  of the signal feedback circuit  14  is connected to the connecting line  10  between the protection impedance  9  and the signal line connection or pad  11 . The second signal input  18  of the signal feedback circuit  14   a ,  14   b  is connected to the connecting line  8  between the operational amplifier  3  and the protection impedance  9 . The signal feedback circuits  14   a ,  14   b  respectively have a signal output  19   a ,  19   b , which is connected to the inverting input  5   a ,  5   b  of the associated operational amplifier  3   a ,  3   b  via a feedback line  20   a ,  20   b.    
     The signal feedback circuit  14   a ,  14   b  in each case carries out a frequency-dependent signal feedback of the useful signal to the inverting input  5   a ,  5   b  of the operational amplifier  3   a ,  3   b . In this case, high-frequency signal components of the useful signal amplified by the amplifier circuit  3   a ,  3   b  are fed back to the inverting input  5   a ,  5   b  of the operational amplifier  3   a ,  3   b  to a greater extent than the low-frequency signal components of the amplified useful signal. As a result of this, the output impedance is reduced in a frequency range up to a limiting frequency f g1 . The protection impedances  9   a ,  9   b  are included in the feedback loop or control loop for low signal frequencies, in particular in a specific signal band, so that the output impedance of the driver circuit is greatly reduced in a low frequency range up to the limiting frequency f g1 . For high signal frequencies, the feedback loop is closed by means of the capacitors  15   a ,  15   b , so that the feedback is stable. 
       FIG. 6  shows the differential output impedance of the driver circuit  1  as a function of the frequency. In this case, Z 9   a , Z 9   b  are the output impedance values of the protection impedances  9   a ,  9   b  and A is the open signal gain of the operational amplifiers  3   a ,  3   b . Up to a first limiting frequency f g1 , the output impedance of the driver circuit according to the invention is reduced by the gain factor A. 
     The limiting frequency f g1  is determined by the capacitance of the capacitor  15  and by the resistance of the resistor  16  of the feedback circuit  14 . 
               f   g1     =     1     2   ⁢   π   ⁢           ⁢       R   16     ·     C   15                   (   4   )             
 
     As already specified in equation (3), the following holds true for the longitudinal conversion loss LCL: 
             LCL   ≅       20   ·   log     ⁢              R   L     +     Z   out         Δ   ⁢           ⁢   Z                      (   3   )             
 
     The frequency-dependent signal feedback by the signal feedback circuit  14  greatly reduces the output impedance Z out  and the impedance difference ΔZ within the voice band, up to the limiting frequency f g1 . As emerges from equation (3), this leads to a great increase in the longitudinal conversion loss LCL. Typical values are 80 ohms for the output impedance and 0.5 ohm for the impedance difference. The longitudinal conversion loss LCL is 57.6 dB for this case. If the output impedance Z out  and the impedance difference ΔZ are reduced by the factor A=100 by the feedback circuit  14  according to the invention, a value of 95.6 dB results for the longitudinal conversion loss LCL. 
     Consequently, in the case of the driver circuit according to the invention, the longitudinal conversion loss LCL can be considerably increased even with the use of inexpensive and inaccurate components on account of frequency-dependent feedback. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 List of reference symbols 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  1 
                 Driver circuit 
               
               
                   
                  2 
                 Signal input 
               
               
                   
                  3 
                 Amplifier circuit 
               
               
                   
                  4 
                 Input 
               
               
                   
                  5 
                 Input 
               
               
                   
                  6 
                 Line 
               
               
                   
                  7 
                 Output 
               
               
                   
                  8 
                 Line 
               
               
                   
                  9 
                 Protection impedance 
               
               
                   
                 10 
                 Line 
               
               
                   
                 11 
                 Signal line connection 
               
               
                   
                 12 
                 Measuring resistor 
               
               
                   
                 13 
                 Measurement signal source 
               
               
                   
                 14 
                 Signal feedback circuit 
               
               
                   
                 15 
                 Capacitor 
               
               
                   
                 16 
                 Resistor 
               
               
                   
                 17 
                 Input 
               
               
                   
                 18 
                 Input 
               
               
                   
                 19 
                 Output 
               
               
                   
                 20 
                 Feedback line