Abstract:
An interface circuit has a circuit input. The circuit has a first controlled amplifier combined with an at least resistive element. The first amplifier provides at a first output a first output current dependent on a voltage of the circuit input. The circuit also has a second controlled current amplifier. The second amplifier provides at a second output a second output current dependent on a current of the circuit input. The second output is coupled to the first output. The circuit further comprises a third controlled current with a third input. The third input is coupled to the first and second outputs.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a telecommunication circuit having an input impedance and containing current sources. 
     The present invention also relates to a telephone circuit, telephone and telecommunication device comprising such a telecommunication circuit. 
     Such telecommunication circuits containing current sources are known in the art of telephone apparatus from U.S. Pat. No. 5,226,078 in which they are described as being a useful tool in controlling the line current in a wired connection between the telephone exchange and the subscriber. At the subscriber end, a communication device, such as a telephone or facsimile or the like, has to have line transmission circuitry which has to perform a number of functions. Examples of required functions, apart from the actual transmission and reception, are: 
     termination of the line with a correct definite impedance, 
     deriving of power from the line for feeding internal and/or peripheral devices and 
     modulating the line current with an AC-signal to be transmitted. Details of these requirements may vary from country to country. These requirements should be layed down so as to form standards. However, even the standards are changed frequently. One of the features varying per country is the value of the terminating line impedance. The above-mentioned US patent mentions off-chip components that are apparently necessary for performing the adjustments required by the varying standards. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a telecommunication circuit that can be used to cope with at least the varying line impedance in a way which allows smooth control thereof without jeopardizing the desired degree of integration of circuit components on the chip. To this end, the telecommunication circuit according to the invention is characterised in that at least two current sources are controlled current amplifiers, which are connected in such a way that their respective current transfer ratios determine the input impedance of the circuit. This enables the required input impedance to be adjusted simply by influencing the current transfer ratio of the controlled current amplifier(s). Influencing the transfer ratio of a controlled current amplifier takes place by means of controlling a control signal supplied to the controlled current amplifier. By means of this control signal, the input impedance for a particular country is set to the desired value, without any presettings or preadjustments in the telecommunication circuit being necessary. This even allows the input impedance to be controlled by programming it to the desired value. 
     A very simple embodiment of the telecommunication circuit in accordance with the invention is characterised in that the at least two controlled current amplifiers have outputs that are connected to one another. 
     A further embodiment of the telecommunication circuit according to the invention is characterised in that the telecommunication circuit comprises a further current amplifier having an input, and the two outputs of the at least two controlled current amplifiers and the input of the further current amplifier form a current summing node. Such a further embodiment is particularly suited for use in a telephone set and for controlling the telephone line current. 
     A still further embodiment is characterised in that at least the further current amplifier has two outputs, and, in particular, momentarily only one of the outputs conducts an output current. This still further embodiment of the telecommunication circuit according to the invention is particularly useful for providing a supply point as indicated in the aforementioned second function, which supply point can be used to electrically feed internal and peripheral circuitry, such as for hands free facilities, listening-in features, dialling features, loudspeaker features etcetera. 
     Another embodiment of the telecommunication circuit according to the invention is characterised in that the circuit comprises an AC-current source which is at least connected to the at least two controlled current amplifiers. This satisfies the requirements of the aforementioned third function as the AC-current source is capable of modulating the line current with the AC-signal to be transmitted, resulting in a flat frequency characteristic of the line voltage despite a complex line termination impedance. Desired replacement schemes of various telephone circuits in a variety of countries can be simulated in an embodiment of the telecommunication circuit according to the invention, which is characterised in that the controlled current amplifiers current transfer ratio of at least one of the controlled current amplifiers is frequency-dependent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 shows a first building block of a controlled current amplifier for use in the telecommunication circuit according to the invention, 
     FIG. 2 shows a second building block for use in the telecommunication circuit according to the invention, 
     FIG. 3 shows a first possible embodiment of the telecommunication circuit according to the invention, 
     FIG. 4 shows a well-known electric replacement scheme of the line termination impedance of a telephone, 
     FIG. 5 shows a telecommunication circuit according to a second possible embodiment for simulating the scheme of FIG. 4, 
     FIG. 6 shows a telecommunication circuit for realising a controlled current amplifier having a frequency-dependent current transfer ratio, 
     FIGS. 7 and 8 show third and fourth embodiments, respectively, of the telecommunication circuit according to the invention, 
     FIG. 9 schematically shows a telecommunication device in the form of a telephone having such a circuit, and 
     FIG. 10 shows a possible embodiment of a controlled current amplifier. 
     In the Figures, like reference numerals refer to like parts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 are schematic representations of controlled current amplifiers  1  and  2  of a first and a second type, respectively. Iin and Iout denote input and output currents, respectively, of the controlled current amplifier  1 . A controlled current amplifier will hereafter be denoted by CA. The relationship between input and output currents of the CA 1  of the first type can be given by the formula: 
     
       
         Iout=T 1 *Iin 
       
     
     wherein T 1  is the current transfer ratio of CA 1 . CA 2  is of a second type and has two outputs carrying potentials V 1  and V 2  for conveying currents Iout 1  and Iout 2  in a vertically drawn sense and a horizontally drawn sense as indicated in FIG.  2 . The actual direction of the output current depends on the voltages V 1  and V 2 . If: 
     V 1  &gt;V 2  than Iout 2 =0, otherwise Iout 1 =0 
     For CA 2  the formula is: 
     
       
         (Iout 1  or Iout 2 )=T 2 *Iin 
       
     
     wherein T 2  is the current transfer ratio of CA 2 . 
     Generally controlled current amplifiers such as CA 1 , CA 2 , CA 6 , CA 7 , and CA 12  have a very low input impedance and a very high output impedance. Their current transfer ratio can range from finite, such as CA 6  and CA 7 , to practically infinite, such as CA 12 , in which case CA 12  will not be controlled. The current transfer ratio, which may be larger or smaller than 1, of any CA can be influenced by means of any control signal C S , such as a current control signal, a voltage control signal etcetera. 
     A typical outline of a CA is shown in FIG.  10 . The CA shown is in the form of a current controlled current amplifier, and this amplifier per se is also known as “current gain cell”, or “current controlled differential current mode amplifier”. The CA comprises two emitter coupled controllable semiconductors S 1  and S 2  having respective diode connected control inputs, whereto respective ingoing currents I in +/−ΔI in  flow. To the respective main current streams of S 1  and S 2  currents I out +/−ΔI out  flow, whereas the control signal in this case emanates from a current source I control  which is connected to both emitters of S 1  and S 2 . The relationship for the current transfer ratio T is given by is: 
     
       
         T=ΔI out /ΔI in =I out /I in =I control /2*I in   
       
     
     FIG. 3 denotes a first possible embodiment of a telecommunication circuit  3  for a situation in which only the outputs  4  and  5  of CA 6  and CA 7 , both being of the first type, are connected in series with one another. Input  8  of CA 6  is generally connected to line terminals  9  through an impedance, but specifically through a resistor R. On the exchange side connected to line terminals  9 , there is schematically shown a line impedance Z_line connected in series with a line voltage source V_exch. Input  10  of CA 7  is connected to line terminal  9  and to an internal line terminal  11 . In this case it can simply be derived that, if T 12  is very large (practically infinite), the input resistance of the circuit  3  is: 
     R*(T7/T6) 
     wherein T 6  and T 7  are the current transfer ratios of CA 6  and CA 7  respectively. 
     In a second embodiment of the telecommunication circuit  3  CA 12 , being of he second type, is connected to the node of CA 6  and CA 7 . In particular input  13  of CA 12  is connected in parallel to output  5  of CA 7 . CA 12  has one output  14 , which is connected to lower terminal  9  and the other output  15  is connected to supply terminal  16  connected to a schematically shown load-supply RC combination for supplying peripheral devices (not shown). This RC combination is only provided with an output current through output  15  if the internal line voltage V 11  on terminal  11  is larger than the supply voltage on terminal  16 , otherwise output current flows to lower terminal  9 . 
     In a third embodiment of the telecommunication circuit  3 , a modulating AC current source  17  is connected to the node of CA 6 , CA 7  and possibly CA 12  for providing a modulating current J_send. It can be demonstrated that for AC-signals, such as speech signals, the impedance synthesized by the circuit  3  between terminals  9  is equal to Z line , the line voltage V_line on terminal  9  can be approximated by: 
     R*J_send/(2*T7) 
     FIG. 4 shows a well-known electric replacement scheme of the line termination impedance of a generally known telephone. In the case of a complex termination impedance, it comprises a parallel combination of a series-connected coil L and resistor R_ 1 , and a resistor R_ 2  which is series-connected with a parallel arrangement of a resistor R_ 3  and capacitor C, which combination is connected to line terminals  9 . If the termination impedance is real, R_ 2  and C are omitted. It can be demonstrated that this electric replacement scheme can be simulated by a proper choice of the current transfer ratios of each of the CAs of FIG.  5 . The embodiment shown therein is an extension of the one shown in FIG. 3 in that CA 18  and CA 19  are added. Contrary to CA 6 , CA 7  and CA 12 , both CA 18  and CA 19  have a frequency-dependent current transfer ratio, which will be explained hereinbelow. Inputs  20  and  21  of CA 18  and CA 19 , respectively, are connected in series with inputs  8  and  10  of CA  6  and CA 7 , respectively. Both outputs  22  and  23  are connected to the above-mentioned node. For reasons of clarity, the various control inputs C S  are omitted in the figures. The complex form T 18  and T 19  of CA 18  and CA 19 , respectively, can, in terms of the frequency w (with w=2*π*f), be denoted as: 
     
       
         T 18 , 19 =T 18 , 19 (jw)=T 18 , 19 /(1+jwτ 18 , 19 ) 
       
     
     which means physically that T 18  and T 19  have a low-pass character having time constants τ 18  and τ 19 , respectively. 
     FIG. 6 shows a possible embodiment of a CA having such a low-pass character. It comprises a main input  24  and a main output  25 , which are interconnected by means of encircled current-splitting means  26  and  27 , such as well-known current mirror means. A current flowing to the current splitting means  26 ,  27  is split into substantially equal outgoing currents. The CA shown in FIG. 6, itself comprises CA 28  and an OTA 29 , having respective inputs  30 ,  31 , and outputs  32  and  33 . The OTA 29  is an Operational Transconductance Amplifier, which is in fact a voltage-current converter having a conductance G. Input  30  is connected to main input  24 , and output  32  is connected to input  31  and to a capacitor  34  connected parallel thereto. Output  33  is connected to the main output  25  through the current-splitting means  26 ,  27 . It can be shown that theoretically the current transfer ratio T of the CA as a whole correspnds to: 
     T(jw)=1/(1+jwτ) 
     wherein τ can be expressed in terms of a capacitance value C, current transfer ratio T of CA 28 , and the voltage transfer ratio G (being Iout 25  divided by the voltage across input  31 ). Thus, a variation of T leads to a shift of the tilting frequency of the frequency-dependent current transfer ratio T(jw) of the CA shown. Of course, by a variation or a replacement of the capacitor C by a coil L (not shown) other frequency dependencies can, if desired be simulated in order to comply with different requirements imposed on the circuit  3  by, for example, local telecommunication authorities. 
     FIG. 7 shows an alternative design of the telecommunication circuit  3  of FIG. 5, except that a current subtraction means  35  connected to output  22  of CA 18  is added in the path from line terminal  9  to supply terminal  16 . In the current-subtraction means  35 , the DC line current is subtracted from the AC-line current before entering CA 7  and CA 19  in order to separate AC and DC line current handling in the circuit  3 . Theoretically, this circuit embodiment shows simplified expressions if it is used for simulating the telephone replacement scheme of FIG.  6 . 
     FIG. 8 shows a further alternative embodiment of the telecommunication circuit  3  of FIG.  7 . The circuit  3  comprises a line voltage sensing CA  36  whose output is provided with current splitting means  37 ,  38  to feed CA 6  and CA 18 , a line current sensing circuit  39  whose output is provided with current splitting means  40  and  41  to feed CA 7  and CA 19 , and an output CA 42  which now achieves the required loop gain, whereas CA 12  now only serves as a purpose of voltage regulator to maintain a stable supply voltage on supply terminal  16 . The result is a separate, easily controllable DC-loop in the circuit  3 . 
     FIG. 9 shows a telecommunication device  43  in the form of a telephone having a telecommunication circuit  3 , which telephone is connected to an exchange means (not shown). Generally, the telephone  43  will be controlled by a microprocessor  44  and provided with the basic features  45  for providing transmission and reception possibilities, which additional features  46  such as listening-in, loudspeaker facilities, hands-free facilities, dialling facilities etcetera are added.