Method and circuit for implementing an impedance, in particular for DC telephonic applications
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 08/120,604, filed Sep. 13, 1993, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to a circuit and method for implementing an impedance associated with a telephone subscriber circuit connected to a twisted-pair telephone line. 
     BACKGROUND OF THE INVENTION 
     Many applications require a predetermined DC or very low frequency resistive value be synthesized from another given resistance or impedance value. 
     The need for this is felt especially in telephone applications and speech circuits. 
     Such circuits are, in fact, required to have different resistances or impedances, depending on whether the signal to be processed is a DC or AC signal. 
     In this respect, each telephone company administration sets the values for these two different impedances, and the values change with the companies. For instance, for DC resistance, values in the 20 to 100 Ohms are generally provided by most telephone circuits, whilst values in the 200 to 1400 Ohms are provided for AC resistance. 
     Further, a frequent requirement is that the AC impedance be a complex type. 
     Heretofore, these two different impedances have been implemented by a complex structure external to the telephone integrated circuit using, moreover, comparatively expensive precision components. One prior approach uses a capacitor outside the telephone integrated circuit. This capacitor is DC operated in an open loop, and under these conditions, it has a low impedance value across it, set to approximately 30 Ohms. 
     Beyond a certain frequency, the capacitor becomes the equivalent of a short by completing a control and amplification loop internal of the integrated circuit. The completion of the control loop basically allows the previous impedance value to be stepped up to 600 Ohm. 
     While achieving its objective, this prior approach has a drawback in that, since the capacitor should be able to short out already at a low frequency, its value must be quite high, thereby making the integration of said capacitor to the circuit either inconvenient or impossible. 
     This prior system requires, however, pins be added to the integrated circuit to which the capacitor is connected, resulting in higher cost as well as lower reliability. 
     A second, more complicated prior approach is described in an article &#34;A software programmable CMOS telephone circuit&#34;, IEEE Journal of Solid-State Circuits, Vol. No. 7, July, 1991, wherein reference is made to a telephone circuit equipped with no less than six external components. 
     This second approach provides for two different impedances, DC and AC, to be implemented by two discrete control loops. The two signal components are separated by two external decoupling capacitors. 
     But even this prior approach is beset with the same drawbacks previously considered. 
     SUMMARY OF THE INVENTION 
     The underlying technical problem of this invention is to provide a method and a circuit for implementing an impedance, which overcomes the drawbacks mentioned above. 
     This technical problem is solved by a method as defined in the claims. The technical problem is also solved by a circuit described in detail herein and defined in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of a method according to the invention will become apparent from studying the following detailed description of an embodiment thereof, given by way of example and not of limitation in connection with an impedance synthesizing circuit implementing it, as illustrated in the accompanying drawings. 
     In the drawings: 
     FIG. 1 is a diagrammatic view of a circuit according to the invention for implementing an impedance in DC applications; and, 
     FIG. 2 is a diagrammatic detail view of a circuit in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENT 
     Referring to FIG. 1, a circuit 1 according to the invention for implementing an impedance is shown in association with a telephone speech circuit 2. The field of application of this invention is particularly, but not solely, related to telephone speech circuits incorporated to telephone subscriber apparatus, and reference will be made herein to that field of application for the sake of simplicity. 
     More particularly, the circuit of this invention is adapted to synthesize a complex impedance which can function both as a DC and an AC impedance. 
     The telephone circuit 2 is a monolithically integrated type and is connected to a telephone subscriber line 3 of the twisted pair type having a pair of terminals L+and L-. 
     Later in this description, the first of said terminals, designated L+, will be considered to be the telephone signal receiving terminal, and the second, designated L-, a signal reference or ground. 
     Advantageously, the circuit 1 of this invention is implemented as a feedback loop and comprises a single resistor Re, preferably a 250-Ohm precision resistor connected serially to the line, with one end being connected to the first terminal L+. A current I flows through this terminal L+to the resistor Re. 
     The other end of the resistor Re is connected to an input leg of a first, current-mirror circuit portion 4. 
     Specifically, this resistor is connected to the collector C1 of a first bipolar transistor T1 which has its emitter connected to the second line terminal L-. 
     The transistor T1, which is of the npn type, has its base B1 in common with the base B2 of a second, npn bipolar transistor T2 having its emitter E2 connected, in turn, to the terminal L-of the line 3. 
     The collector C2 of the second transistor T2 constitutes the output leg of the current mirror 4. 
     The area ratio of transistor T1 to T2 is arranged to be N:1 in order to allow a current N times lower than line current I to be transferred to the output leg. 
     Provided between the base B1 and the collector C1 of the first transistor T1 is also an amplifier driver block 5. The amplifier block 5 is not operated on DC, however under such conditions, its output to the collector C1 would be set to the desired DC value and have a very low output impedance. 
     The output leg of the mirror 4 is connected to an input 9 of a second current mirror 6 which is also constructed to function as a low-pass filter. The filter 6 will be described in greater detail with reference to FIG. 2. 
     The filter 6 receives a stabilized DC supply voltage VD to be explained. The filter 6 also has an output 7 connected to the input side of a third current mirror 8 being connected to the second line terminal L-. 
     This third mirror 8 also performs a two-factor multiplication of the input current. 
     The circuit 1 includes a fourth current mirror having an input 11 connected directly to the output of the third mirror 8. The current mirrors 8 and 10 are of a type generally known. Those skilled in the art may select from any such current mirrors and use currently available designs which are suitable for use in the present invention. 
     A second input 12 of the fourth mirror 10 is connected to the first terminal L+of the line 3 to directly pick up the line current I. It should be also noted that a high multiplication factor, preferably 80, is selected for the fourth current mirror 10. 
     Further, the output 13 of the fourth current mirror 10 is connected to one end of a capacitor Ce external to the integrated circuit 2 which has the other end connected to the second line terminal L-. 
     Picked up across the capacitor Ce is the supply voltage VD as suitably stabilized and supplied to both the second mirror 6 and the other internal components of the circuit 2. 
     The circuit 1 includes a shunt regulator block 14 in parallel with the capacitor Ce, which functions to fetch current for stabilizing the voltage value VD. 
     The construction of the second current mirror 6 with low-pass filter functions will now be described with specific reference to the example in FIG. 2. 
     The input 9 of the filter 6 comprises the drain terminal D1 of a P-channel MOS transistor M1 which is connected into a diode configuration with the respective gate G1 and drain D1 terminals connected together. 
     The source S1 of this transistor M1 is connected to the voltage supply VD. 
     A resistor R has one end connected to the gate terminal G1 of the transistor M1 and the other end connected to a first low-pass filter 16. The first low-pass filter 16 has antialiasing functions to suppress any intermodulation noise. The filter 16 comprises essentially an operational amplifier A1 having its output fed back by being connected to the input side through a capacitor Ca1. 
     Another filter 15, preferably of the switched capacitor type, is downstream connected to this first filter 16. The filter 15 comprises a pair of MOS transistors Mc1 and Mc2 and a capacitor Cj connected between such transistors and the voltage supply VD. 
     This switched-capacitor filter 15 introduces a so-called pole at a very low frequency, below one Hertz, into the frequency response of the mirror 6. Switched-capacitor filters are well known to those skilled in the art, and such filters may be selected from any currently available which are suitable for use in the present invention. 
     The structure of the mirror 6 is completed by a second low-pass filter 17 which comprises an amplifier A2 feedback connected through a capacitor Ca2. Lastly, a second MOS transistor M2 is incorporated to the mirror 6 with its gate terminal G2 being connected after the last filter. The second transistor M2 has its source terminal S2 connected to the voltage supply VD and its drain D2 arranged to constitute the output 7 of the mirror 6. 
     The transistors M1 and M2 are the same size, and accordingly, the current mirror 6 has unity DC gain. The method of implementing impedance according to the invention will now be described. 
     The line current I flowing through the resistor Re is first mirrored to a reduction ratio of N:1, to thereby reduce the power consumption of the telephone circuit 2. The transistors T1 and T2 in the mirror 4 perform this scaling down of the current I. 
     The current Iin at the input 9 of the current mirror 6 is, therefore, equal to I/N and voltage-wise transformed by the MOS transistor M1, which is connected in diode configuration. 
     This voltage is filtered by the first low-pass filter 16 with antialiasing functions, and further filtered by the switched-capacitor filter 15. 
     This voltage is converted back to a current Iout by the MOS transistor M2 located at the output of the current mirror 6. 
     Presently, the current Iout at the output 7 flows first through the current mirror 8, where it is multiplied by a factor of two, and then to the current mirror 10 where it is multiplied by a factor of eighty. 
     Summarizing, the value &#34;N&#34; of the first current mirror 4 is selected so as to provide an appropriate impedance depending upon the impedances required for a particular telephone circuit design, and in one embodiment N is selected as 40. The value N can be selected over a range of values to provide the required impedance for a particular telephone circuit as will be apparent to those of skill in the art. If a current I flows through the resistor Re, then the feedback effect enacted by the circuit 1 will draw from the terminal L+of the line 3 a current equal to, 
     (80 * 2/40) * I=4 * I 
     The overall current draw of the circuit will, therefore, be equal to 5*I and be the equivalent of dividing the resistance Re by a factor of five. 
     Since a value of 250 Ohms has been selected for the resistance Re, the DC resistance as seen from the line terminal L+is one fifth that value, i.e. 50 Ohms. 
     In essence, the method of this invention arranges for the resistance Re, that is the single external component of the integrated circuit 2, to be divided by a factor of five through the control loop represented by the circuit 1 feedback. 
     The values taken by the impedance as seen from the line terminal L+conform, both DC and AC-wise, with the requirements of the various telephone companies. 
     In addition, the DC impedance value may be altered to suit individual circuit requirements by acting on the multiplication ratii of the current mirrors 8 and 10. This can be simply arranged by changing the area ratio of the MOS transistors M1, M2 at the input and the output of the mirror 6. 
     The circuit of this invention solves the technical problem discussed above and affords a number of advantages, including the use of a single external component of a discrete type, namely the resistor Re. This resistor Re is also utilized to synthesize the AC impedance. 
     Another advantage of the present system is that its speed may be easily increased. This may be accomplished by shorting the gate terminals of the transistors M1, M2 in the current mirror 6. 
     Finally, by having the time constant of the low frequency circuit portion within current mirror 6 set by the switched capacitor filter 15, it can be compensated for more easily and accurately where its effect on the other performance features of a telephone speech circuit is to be attenuated.