Abstract:
Driver circuit for at least one subscriber terminal, comprising a first driver ( 14 ) for amplifying the power of a direct-voltage signal applied for supplying power to the subscriber terminal, the first driver ( 14 ) being supplied by a first supply voltage (V 1 ), and comprising a second driver ( 37; 37   a   , 37   b ), following the first driver, for modulating high-frequency signal currents onto the power-amplified direct-voltage signal.

Description:
RELATED APPLICATIONS 
     This application is a continuation of PCT patent application No. PCT/EP01/08668, filed Jul. 26, 2001, which claims priority to German patent application No. 10038374.2, filed Aug. 7, 2000, the disclosures of each of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to a driver circuit for subscriber terminals with very low power dissipation. 
     BACKGROUND ART 
     From DE 19740137 A1, a method and a circuit arrangement for generating AC ringing voltage for electronic subscriber circuits is known. In this arrangement, the operating voltage of a power amplifier is controlled in such a manner that the operating voltage follows the variation with time of an amplified AC ringing voltage in particular time intervals. 
     DE 19858963 A1 describes an audio signal amplifier with a controllable voltage supply for an amplifier stage and with an analyzing unit for analyzing the audio signal to be amplified. The audio signal amplifier also contains a control unit for controlling the voltage supply in dependence on the analyzed audio signal, and a delay unit  3  which delays the incoming audio signal relative to the audio signal supplied to the analyzing unit. 
     In conventional communication systems, direct voltage and low-frequency signals and high-frequency signals are frequently transmitted via one signal line or one transmission channel, respectively. The direct voltages are, for example, supply voltages for subscriber terminals. The low-frequency signals are, for example, analog voice signals or ringing tone signals for telephones. The high frequency signals to be transmitted by the communication systems are in most cases data signals such as, for example, xDSL data signals. 
       FIG. 1  diagrammatically shows an example of a communication system in which low-frequency and high-frequency signals are transmitted via the same line. From a voice and data network, low-frequency voice signals and high-frequency data signals are supplied to a switching center which has a switching device S, for example a multiplexer circuit. The switching device S is connected to a driver circuit T for a subscriber. Each subscriber connected to the switching center has his own driver circuit T. From the driver circuit T, the voice and data signals pass, for example, to a so-called splitter which consists of two filters, namely a high-pass filter HP and a low-pass filter TP. The low-pass filter TP is connected to a telephone which receives the low-frequency voice signals. The high-pass filter HP is connected to a data modem which demodulates and modulates the high-frequency data signals. 
     As a rule, the direct voltages which are transmitted from the driver circuit T to the circuit and are used, for example, to supply the telephone with voltage, and the low-frequency ringing tone have high signal amplitudes of more than 100 volts. This necessitates a very high supply voltage for the driver circuit T. The high-frequency signal currents which are used, for example, for transmitting data, have a much lower signal amplitude. Due to the lower-valued load impedance, the high-frequency signal currents produce large supply currents which are taken from the supply voltage of the driver circuit T, however. 
       FIG. 2  shows a driver circuit T of the prior art. The driver circuit T contains an operational amplifier OP which is supplied with voltage by an associated supply voltage source V B  via two supply voltage lines. The input E of the driver circuit T is connected to a low-frequency signal source S 1  and a high-frequency signal source S 2 , the signal currents of which are superimposed. The superposition of the various signal currents or signal components is indicated by a summing element in FIG.  2 . The low-frequency voltage source S 1  supplies the direct voltages for supplying the terminals and low-frequency signals, particularly low-frequency voice signals. The high-frequency signal source S 2  delivers high-frequency data signals with a relatively low signal amplitude. At the output A of the driver circuit T, the entire signal spectrum is available at a low impedance for driving the terminals connected to the output. 
     As a rule, the supply voltage V B  for the driver circuit T is supplied by a connected power supply. 
     The disadvantage of the driver stage T of the prior art as shown in  FIG. 2  consists in that the supply voltage V B  must be selected to be of such an amplitude that the maximum signal amplitudes of the two signal sources S 1  and S 2  are not limited in the operational amplifier OP. 
     The required supply voltage V B  for the operational amplifier OP within the driver stage T can be calculated from the following relation:
 
 V   B   =Ŝ   1   +   2   =V   Drop 
 
where
         Ŝ 1  . . . crest value of the low-frequency signal currents,               2  . . . crest value of the high-frequency signal currents,       

     V Drop  . . . design-related voltage drop in the driver circuit. 
     The power dissipation P V  of the driver circuit T, which is due to the load current is obtained from:
 
 P   V   =P   G   −P   L   =V   B   ·F   2   ·i   1,2   −P   L 
 
where
         P V  load-current-related power dissipation in the driver circuit T,   P G  load-current-related total power consumption of the driver circuit T,   P L  power delivered to the load,   V B  supply voltage of the driver circuit T,   i 1,2  rms value of the total signal current,   F 2  the form factor, i.e. the signal-shape-dependent ratio between rectified value and rms value of a signal.       

     The power dissipation of the driver circuit T increases with increasing supply voltage V B  of the driver circuit as can be seen from the equation. Since the supply voltage V B  of the driver circuit T of the prior art is greater than the sum of the two crest values              1 , Ŝ 2  of the low-frequency and high-frequency signal currents, the necessary supply voltage V B  of the driver circuit T, and thus the power dissipation P V  of the driver circuit T are very high. In the conventional driver circuit T shown in  FIG. 2 , the power dissipation P V  produced is of such a magnitude that it has hitherto not been possible to implement integrated driver circuits for full-rate ADSL in connection with an analog voice signal transmission without an elaborate housing with very large heat sinks.
     SUMMARY OF THE INVENTION 
     It is, therefore, the object of the present invention to create a driver circuit for transmitting low-frequency and high-frequency signal currents to subscriber terminals, the power dissipation of which is minimum. 
     According to the invention, this object is achieved by a driver circuit to subscriber terminals, having the features specified in claim  1 . 
     The invention creates a driver circuit for at least one subscriber terminal comprising a first driver for power amplification of a direct voltage signal applied for supplying power to the subscriber terminal, the first driver being supplied by a first supply voltage (V 1 ), and comprising a second driver following the first driver, for modulating high-frequency signal currents onto the power-amplifier direct-voltage signal. 
     The first driver is preferably followed by a low-pass filter. 
     In a preferred embodiment, this low-pass filter is a first-order low-pass filter. 
     The first-order low-pass filter is preferably an RC low-pass filter which contains a resistor and a capacitor. 
     The resistor is preferably a passive component. 
     In an alternative embodiment, the resistor is constructed as an active impedance. 
     The cut-off frequency of the low-pass filter can preferably be adjusted. 
     This provides the advantage that the driver circuit can be used for various communication standards, for example different DSL standards. 
     The cut-off frequency of the low-pass filter is preferably between the upper cut-off frequency of the low-frequency signal currents and the lower cut-off frequency of the high-frequency signal currents. 
     In a preferred embodiment of the driver circuit according to the invention, a first voltage source is provided which has a fixed reference potential for generating the first supply voltage. 
     The reference potential is preferably the ground potential. 
     In a further preferred embodiment of the driver circuit according to the invention, a second voltage source is provided for generating the second supply voltage which is connected free of reference potential to the output of the low-pass filter. 
     In a preferred embodiment of the driver circuit according to the invention, the second supply voltage for the second driver, delivered by the second voltage source, is much lower than the first supply voltage for the first driver, delivered by the first voltage source. 
     The first driver is preferably an operational amplifier. 
     In an alternative embodiment, the first driver is an AC/DC converter with current feedback. 
     In a further alternative embodiment, the first driver is a DC/DC converter with current feedback. 
     The second driver is preferably an operational amplifier with a linear amplification. 
     In a further preferred embodiment of the driver circuit according to the invention, a low-frequency signal source can be connected to a signal input of the first driver. 
     In a further preferred embodiment, a signal input of the second driver is connected to the output of a summing circuit. 
     In this arrangement, the low-frequency signal source can be preferably connected to a first input of the summing circuit. 
     The high-frequency signal source can be preferably connected to a second input of the summing circuit. 
     In an especially preferred embodiment of the driver circuit according to the invention, a third supply voltage source for increasing the signal level at the signal input of the second driver is connected to a third input of the summing circuit. 
     The third supply voltage source preferably delivers a supply voltage which has half the amplitude of the second supply voltage delivered by the second supply voltage source. 
     The subscriber terminal can be preferably connected to a signal output of the second driver. 
     The low-frequency signal currents which are amplified by the first driver preferably comprise direct currents for supplying the subscriber terminal, voice signal currents and ringing tone currents. 
     The high-frequency signal currents which are amplified by the second driver preferably comprise high-frequency data signal currents. 
     These high-frequency data signal currents are preferably xDSL data signal currents. 
     In an especially preferred embodiment of the driver circuit according to the invention, this driver circuit is of differential construction. 
     In this arrangement, the second driver exhibits two differentially interconnected operational amplifiers which are in each case supplied by a supply voltage free of reference potential. 
     The two supply voltages free of reference potential are preferably in each case generated by flyback converters, the secondary winding of which is not grounded. 
     In the further text, preferred embodiments of the driver circuit according to the invention are described with reference to the attached figures, for explaining features essential to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures: 
         FIG. 1  shows the structure of a communication system for transmitting high-frequency and low-frequency signals on a line according to the prior art; 
         FIG. 2  shows a driver circuit T according to the prior art; 
         FIG. 3  shows a first embodiment of the driver circuit according to the invention; 
         FIG. 4  shows a second embodiment of the driver circuit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment shown in  FIG. 3 , the driver circuit  1  for subscriber terminals according to the invention has two signal inputs  2 ,  3  for connecting a low-frequency signal source  4  and a high-frequency signal source  5 . The low-frequency signal currents delivered by the low-frequency signal source S 1  pass from the signal input  2  via a line  6  to a first input  7  of a summing circuit  8 . The high-frequency signal currents delivered by the high-frequency signal source  5  are applied to a second input  10  of the summing circuit  8  via a line  9 . The summing circuit  8  has a third input  11  to which a third voltage source  13  is connected via a line  12 . The summation or superposition of the high-frequency and low-frequency signal currents delivered by the low-frequency signal current source  4  to the high-frequency signal current source  5  can also take place outside the driver circuit  1 . 
     The driver circuit  1  contains a first driver  14  for the low-frequency signal currents, the input  15  of which is connected via an internal line  16  to a branching node  17  with the line  6 . The first driver  14  has two supply voltage terminals  18 ,  19  for applying a first supply voltage V 1 . The first supply voltage terminal  18  of the driver  14  is connected to a first terminal  21  of a supply voltage source  22  via a supply voltage line  20 . The second supply voltage terminal  19  of the driver  14  is connected to a branching node  24  via a line  23 . The branching node  24  is located on a line  25  which applies a second terminal  26  of the supply voltage source  22  to the reference potential ground. The signal output  27  of the driver  14  is connected to an input terminal  29  of a low-pass filter  30  via a line  28 . The low-pass filter  30  contains a resistor  31  and a capacitor  32 . The capacitor  32  of the low-pass filter  30  is connected to a node  33  with the resistor  31  and is connected to the reference potential ground at its opposite terminal. 
     The low-pass filter  30  shown in  FIG. 3  is a first-order low-pass filter, the circuit of which can be produced with very little expenditure. The resistor  31  is either a passive component or an active impedance which is implemented by impedance synthesis. This can be, for example, an active output impedance of the driver  14 . The cut-off frequency f g  of the low-pass filter  30  is preferably adjustable. For this purpose, the resistance of the resistor  31  or the capacity of the capacitor  32  is preferably changed. In an alternative embodiment, the resistance value of the resistor  31  can be programmed. The cut-off frequency f g  of the low-pass filter  30  is between the upper cut-off frequency of the low-frequency signal currents, for example 4 kHz for voice signals, and the lower cut-off frequency of the high-frequency signal currents, for example 138 kHz for DSL data signal currents. An output  34  of the low-pass filter  30  is connected to a node  36  via a line  35 . 
     The driver circuit  1  according to the invention contains a second driver  37 , the signal input  38  of which is connected to the output  40  of the summing circuit  8  via a line  39 . The second driver  37  exhibits a first supply voltage terminal  41  and a second supply voltage terminal  42 . The first supply voltage terminal  41  of the driver  37  is connected to a first terminal  44  of a supply voltage source  45  via a supply voltage line  43 . The supply voltage source  45  exhibits a second supply voltage terminal  46  which is connected to the node  34  via a line  47 . The second supply voltage terminal  42  of the driver  37  is also connected to the node  36  via a supply voltage line  48 . The second voltage source  45  for generating the second supply voltage for the driver  37  is connected free of reference potential to the output  34  of the low-pass filter  30  as can be seen from FIG.  3 . The amplitude of the supply voltage V 2  free of reference potential of the supply voltage source  45  is adapted to the signal amplitudes of the high-frequency signal currents and high-frequency signals, respectively. 
     At the signal input  38  of the driver  37 , the supply voltage source  13  produces a constant increase in signal level by a voltage which is delivered by the supply voltage source  13 . The supply voltage delivered by the supply voltage source  13  is preferably one half of the supply voltage V 2  delivered by the supply voltage source  45 . This makes it possible to modulate the driver  37  fully within its supply voltage range. 
     The output  49  is connected to an output  51  of the driver circuit  1  via a line  50 . The subscriber device can be connected to the output  51  of the driver circuit  1  via protective circuits. The subscriber terminals connected and the line between the driver circuit  1  and the subscriber terminals have a complex impedance Z L . 
     The supply voltage V 1  of the first supply voltage source  22  for supplying voltage to the first driver  14  is obtained as follows:
 
 V   1   =Ŝ   1   +V   Drop1 
 
where
                     1  is the sum of the crest values of the direct-voltage signal, the ringing tone signal and possibly of the voice signal,   V Drop1  is the design-related voltage drop in the driver circuit  14 .       

     The supply voltage V 2  delivered by the second supply voltage source  45  for supplying voltage to the second driver  37  is obtained as follows:
 
 V   2   =Ŝ   2   +V   Drop2 
 
where
         Ŝ 2  is the crest value of the high-frequency signal currents and possibly of the voice signal,   V Drop2  is the design-related voltage drop in the driver circuit  37 .       

     This provides the load-current-related power dissipation of the overall driver circuit  1  from:
 
 P   V   =P   G   −P   L   =P   V1   +P   V2   −P   L   =V   1   ·F   1   ·i   1   +V   2   ·F   2   ·   1,2   −P   L 
 
where:
         P V  is the load-current-related power dissipation in the overall driver circuit   P G  is the load-current-related total power consumption of the overall driver circuit   P L  is the power delivered to the load Z L     V 1  is the supply voltage of the driver circuit  14     V 2  is the supply voltage of the driver circuit  37     i 1,2  is the rms value of the total signal current   i 1  is the rms value of the DC signal current and of the low-frequency signal currents   F 1  is the form factor, i.e. the signal-shape-dependent ratio between the rectified value and the rms value of the high-frequency signal   F 2  is the form factor, i.e. the signal-shape-dependent ratio between the rectified value and the rms value of the total output signal.       

     As a comparison between the two equations  2 , 6  shows, the power dissipation of the driver circuit according to the invention is much less than in the conventional driver circuit T shown in FIG.  2 . 
       FIG. 4  shows an especially preferred embodiment of the driver circuit  1  according to the invention. The preferred embodiment of the driver circuit  1  shown in  FIG. 4  is of differential construction. The second driver  37  consists of two differentially interconnected operational amplifiers  37   a ,  37   b  which are in each case supplied by an associated supply voltage V 2a , V 2b  free of reference potential. The two supply voltages V 2a , V 2b  free of reference potential are preferably in each case generated by flyback converters, the secondary winding of which is not grounded. 
     In comparison with the conventional driver circuit T as shown in  FIG. 2 , the power dissipation of the driver circuit  1  according to the invention is very low as is proven by the following example. 
     Assuming a ringing tone signal of 65 Vrms/20 Hz as low-frequency current component and a full-rate ADSL signal with 20 dBm transmit power as high-frequency signal current on the line, the following values are obtained: 
     Voltage crest value              1  of the low-frequency signals
 
 Ŝ   1 =110V;

     Voltage crest value Ŝ 2  of the high-frequency signals
 
 Ŝ   2 =35V;
 
     Design-related voltage drop of the driver circuit T of the prior art V Drop =15V; 
     Thus, the supply voltage of the driver circuit T of the prior art is:
 
 V   B   =Ŝ   1   +Ŝ   2   +V   Drop =160V
     P L =950 mW   i 1,2 =32 mA   F 1 =0.85   F 2 =0.9   

     The load-current-related power dissipation P V  in the driver circuit T of the prior art is:
 
 P   V   =P   G   −P   L   =V   B   ·F·i   1,2   −P   L =160V·0.85·32 mA−950 mW=3.4 W
 
     By comparison, the following is obtained for the driver circuit  1  according to the invention as shown in FIG.  4 : 
     with Ŝ 1 =110V 
     
         
         
           
             V Drop1 =10V
 
a first supply voltage obtained for the first driver  14  is
 
 V   1   =Ŝ   1   +V   Drop1 =120V
 
with Ŝ 2 =35V and
 
             V Drop2 =5V
 
the supply voltage for the second driver is
 
 V   2a   , V   2b   =Ŝ   2   +V   Drop2 =40V
 
with P L =950 mW
 
             i 1 =11 mA 
             i 1,2 =32 mA 
             F 1 =0.85 
             F 2 =0.9 
           
         
       
    
     The power dissipation P V  of the driver circuit  1  according to the invention is thus: 
               P   V     =         P   G     -     P   L       =       P   V1     +     P   V2     -     P   L                     =         V   1     ·     F   1     ·     i   1       +       V   2     ·     F   2     ·     i     1   ,   2         -     P   L                   =       120   ⁢           ⁢     V   ·   0.85   ·   11     ⁢           ⁢   mA     +     40   ⁢           ⁢     V   ·   0.9   ·   32     ⁢           ⁢   mA     -     950   ⁢           ⁢   mW                   =     1.324   ⁢           ⁢   W               
 
     Accordingly, the saving in power dissipation ΔP of the driver circuit  1  according to the invention as shown in  FIG. 4  compared with the conventional driver circuit T as shown in  FIG. 2  is:
 
Δ P= 3.4W−1.324W=2.076W
 
     The driver circuit  1  according to the invention separates the frequencies of the signal circuits into two signal paths. Due to the novel design of the supply voltage concept which provides for dividing the signal current paths with respect to frequency, considerable savings in power dissipation can be achieved by the driver circuit  1  according to the invention. In conventional ADSL systems, the ADSL signals are generated by special modems and looped into the telephone line via elaborate and cost-intensive analog filters or splitters. The voice and ringing tone signals are regenerated by analog so-called line cards. The splitter provides for a parallel connection of the driver for low-frequency signals from the analog line card and the driver for the high-frequency signals from the ADSL modem. The two signal current paths are separated at signal level. As a result, the splitters must consist of analog filters with very high filter orders. Filters having such high filter orders can only be produced at very high costs. 
     In the driver circuit  1  according to the invention, by comparison, the signal current paths are separated at the supply voltage level as a result of which it is sufficient to use a very simple first-order low-pass filter  30 . The driver circuit  1  according to the invention can, therefore, be produced with very little cost expenditure. 
     The reduction in power dissipation by the circuit configuration of the driver circuit  1  according to the invention allows the driver circuit to be integrated into a simple housing having very small heat sinks. 
     The driver circuit  1  according to the invention also has the following further advantages. 
     The quiescent current flowing in the driver circuit  37  is very high due to the necessary frequency bandwidth of the circuit. This quiescent current now exclusively flows via the supply voltage V 2  which is much lower than the supply voltage V 1 . This ensures further considerable saving in power dissipation. 
     A further advantage consists in that for the low-frequency signal currents in the first driver  14 , lower current limiting values can now be specified than for the total signal current in the driver  37 . This results in higher overcurrent endurance of the driver circuit  1  according to the invention. 
     A further advantage consists in that, due to the much lower supply voltage V 2  for the second driver  37 , this driver  37  can be constructed with faster transistors having a lower dielectric strength. This considerably facilitates the achievement of the necessary wide bandwidth. 
     LIST OF REFERENCE DESIGNATIONS: 
     
         
           1  Driver circuit 
           2  Signal input 
           3  Signal input 
           4  Low-frequency signal source 
           5  High-frequency signal source 
           6  Line 
           7  Summing element input 
           8  Summing element 
           9  Line 
           10  Summing element input 
           11  Summing element input 
           12  Line 
           13  Voltage source 
           14  First driver 
           15  Driver input 
           16  Line 
           17  Branching node 
           18  Supply voltage terminal 
           19  Supply voltage terminal 
           20  Supply voltage line 
           21  Voltage source terminal 
           22  Voltage source 
           23  Supply voltage line 
           24  Node 
           25  Line 
           26  Voltage source terminal 
           27  Driver output 
           28  Line 
           29  Filter input 
           30  Low-pass filter 
           31  Resistor 
           32  Capacitor 
           33  Node 
           34  Filter output 
           35  Line 
           36  Node 
           37  Second driver 
           38  Driver input 
           39  Line 
           40  Summing element output 
           41  Supply voltage terminal 
           42  Supply voltage terminal 
           43  Supply voltage line 
           44  Voltage source terminal 
           45  Voltage source 
           46  Voltage source terminal 
           47  Line 
           48  Supply voltage line 
           49  Driver output 
           50  Line 
           51  Driver output