Method and circuit for implementing an impedance, in particular for D.C. telephonic applications

Abstract: The invention relates to a method for implementing an impedance associated with a monolithically integrated telephone subscriber circuit (2) which is connected to a telephone line (3) having at least one terminal pair (L+,L-). The method consists of providing, a single resistor (Re) connected serially to one terminal (L+) of the telephone line (3), and circuit means (1) connected in a closed loop to the terminal (L+) to divide, by a predetermined factor, the value of the resistor (Re) on the occurrence of DC or very low frequency signals.

Description: This invention relates to a method for implementing an impedance associated with a telephone subscriber circuit connected to a twisted-pair telephone line. The invention also relates to a circuit for implementing an impedance in accordance with said method. 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 hereinafter to that field of application for the sake of simplicity. As it is well-recognized, many applications require that 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 the aforesaid 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, whilst values in the 500 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 of the telephone integrated circuit using, moreover, comparatively expensive precision components. Mention can be made of a known approach which provides for the use of a capacitor outside the telephone integrated circuit. That capacitor is DC operated in an open loop, and under these conditions, it has a low impedance value across it, set to aapproximately 30 Ohms. Upward of 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 altogether impossible. All this requires, however, that some pins be added to the integrated circuit for connection to the capacitor, which results in higher end cost as well as lower reliability of the respective holder plate. A second, more complicated prior approach is described in an article "A software programmable CMOS telephone circuit", IEEE Journal of Solid-State Circuits, Vol. No. 7, July, 1991, wherein reference is made to a telephone circuits equipped with no less than six external components. This second approach provides for the two different, DC and AC, impedances to be implemented by two discrete control loops with the two signal components being separated by two external decoupling capacitors. But even this prior approach is beset with the same drawbacks as that previously considered. The U.S. patent No. 4,485,341 discloses a current limiter circuit for limiting the magnitude of a current sourced from a first load circuit to a second load circuit. This document teaches of using several resistors in providing both the DC and the AC impedances. However, according to this solution, a DC current can flow through these resistors even with no input signal applied to the input terminal of the current limiter circuit. Another U.S. patent No. 4,300,023 relates to a SLIC circuit providing DC and AC impedance matching by using a negative feedback wherein a pair of large sensing resistors are made to appear much smaller. However, the loop impedance presented between this kind of SLIC circuit and the telephone may vary a lot depending on the distance between the telephone and the Centrale Office. The underlying technical problem of this invention is to provide a method and a circuit for implementing an impedance, which have such functional and structural characteristics, respectively, as to overcome the drawbacks mentioned hereinabove in connection with the prior art approaches. This technical problem is solved by a method as indicated in the preamble and defined in the characterizing part of Claim 1. The technical problem is also solved by a circuit as indicated in the preamble and defined in the characterizing part of Claim 6. The features and advantages of a method according to the invention will become apparent from 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: Figure 1 is a diagramatic view of a circuit according to the invention for implementing an impedance in DC applications; Figure 2 is a diagramatic detail view of the circuit in Figure 1. With reference to the drawing views, generally and schematically shown at 1 is a circuit according to the invention for implementing an impedance associated with a telephone speech circuit 2. 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, 250-Ohm precision resistor Re connected serially to the line, with one end being connected to the first terminal L+. A current I is flowed 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 this, the second, transistor T2 constitutes the output leg of the current mirror 4. Advantageously, 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. However, this component 5 is not operated on DC, and under such conditions, its output 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 and will be described in greater detail with reference to Figure 2. That filter 6 is supplied 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. Advantageously, the circuit 1 includes a further current mirror 10 having an input 11 connected directly to the output of the third mirror 8. A second input 12 of the last-mentioned 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 peculiar high multiplication factor, e.g. of 80, is selected for the mirror 10. Further, the output 13 of the mirror 10 is connected to one end of a capacitor Ce external of 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 for 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 Figure 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 with antialiasing functions to suppress any intermodulation noise. The filter 16 comprises essentially an operational amplifier A1 having its output feedback connected to the input side through a capacitor Ca1. Another filter 15, 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. The mirror 6 structure 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. This, 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 impedance implementing 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, thereby the power consumption of the telephone circuit 2 can be lowered. The transistors T1 and T2 in the mirror 4 are, in fact, assigned to scaling down 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 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 is flowed first through the current mirror 8, where it is multiplied by a faactor of two, and then to the current mirror 10 where it is multiplied by a factor of eighty. Summarizing, if a current I is flowed 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, it is to be concluded that 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 demands 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 does solve the technical problem and affords a number of advantages, outstanding among which is 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. Not to be neglected are, moreover, the advantages offered by easier speeding up, in that to speed up the system. it will suffice to short the gate terminals of the transistors M1, M2 in the current mirror 6. Finally, by having the time constant of the low frequency circuit 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.

Claims: A method for implementing an impedance associated with a monolithically integrated telephone subscriber circuit (2) associated with a telephone line (3) having at least a first and a second line terminals (L+,L-), characterized by the provision of a single resistor (Re) connected serially to the first terminal (L+) of the telephone line (3) and by a series of circuit means (1) connected in a closed loop to said terminal (L+) for filtering the signal applied to said resistor and dividing, by a predetermined factor, the value of said resistor (Re) on the occurrence of DC or very low frequency signals. A method according to Claim 1, characterized in that said predetermined factor is a factor of five. A method according to Claim 1, characterized in that the division operation is performed by successive current-mirror transformations of the current (I) flowed through said resistor (Re). A method according to Claim 3, characterized in that the first of said current-mirror transformations provides a proportional reduction of said current (I) by a predetermined factor of N. A method according to Claim 3, characterized in that the value of said predetermined division factor can be controlled by acting on the current-mirror transformation ratii. A circuit for implementing an impedance associated with a monolithically integrated telephone subscriber circuit (2) connected to a telephone line (3) having a first and a second line terminals (L+,L-), characterized in that it comprises: a single resistor (Re) connected serially to the first terminal (L+) of said line; and a loop circuit (1) incorporating a series of current mirrors (4,6,8,10) connected serially together and feedback connected to said one terminal (L+); one (6) of said current mirrors being also designed to function as a low-pass filter. A circuit according to Claim 6, characterized in that said low-pass filter (6) comprises a switched capacitor filter. A circuit according to Claim 6, characterized in that one (4) of said current mirrors, being connected directly to the resistor (Re), has respective input and output legs provided with corresponding transistors (T1,T2) showing an area ratio of N:1 to allow for transfer, to the output leg, of a current which is N times less than the current (I) present at the input leg and being flowed through said resistor (Re). A circuit according to Claim 6, characterized in that the overall gain of said current mirrors (4,6,8,10) leads to multiplication by a factor of four of the current being flowed through said resistor (Re). A circuit according to Claim 6, characterized in that the second mirror (6) of said series of current mirrors (4,6,8,10) functions as a low-pass filter.

Inventors: CASTELLO RINALDO, TOMASINI LUCIANO, CASTELLO, RINALDO, TOMASINI, LUCIANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H04M1
IPC codes: H04B1, H03F3, H03H11, H04M1