FR2549325A1 - SUBSCRIBER TELEPHONE SET AND OUTPUT CIRCUIT FOR SAME - Google Patents

Abstract

THE INVENTION RELATES TO A SUBSCRIBER DELEPHONIC TELEPHONE STATION INCLUDING AN OUTPUT TRANSISTOR TR1 FOR THE TRANSMISSION OF SIGNALS ON THE LINE, AND A RESISTOR R1 IN SERIES WITH THE LINE AND THROUGH WHICH A DC POWER SUPPLY FOR THE STATION IS OBTAINED FROM THE LINE, IN OPERATION. THIS STATION INCLUDES A DUDIT TRANSISTOR TR1 CONTROL AMPLIFIER, A FIRST DIRECT CURRENT FEEDBACK LOOP 212, 211 AND A SECOND ALTERNATIVE CURRENT FEEDBACK LOOP C3, R4, 213, 211, BOTH INCORPORATING THE ITS AMPLIFIER. COMMAND 211, THANKS TO WHICH THE TR1 OUTPUT TRANSISTOR HAS A NEGATIVE IMPEDANCE IN PARALLEL WITH THE SAID RESISTANCE, SO THAT THE COMBINATION OF THESE TWO IMPEDANCES IS ADAPTED TO THAT OF THE LINE.

Description

The present invention relates to a subscriber telephone set and, more

  in particular, a circuit making it possible to adapt the impedance of

  output of the station to that of the line to which it is connected.

  With the introduction of electronic telephone sets replacing conventional electro-mechanical sets, problems have arisen concerning the adaptation of impedance between the set and the line All telephone administrations determine the impedance that the set

  must present to the line: this impedance is commonly 600 ohms.

  This brings difficulties for certain types of fully electronic telephone sets since all the direct current necessary for the operation of the station circuits must be obtained from the line by means of this impedance. A conventional telephone station circuit is represented by simplified by Figure 1 The line is connected to the substation by a transmission impedance Zt whose end 15 m c 8 t substation circuits is grounded through a capacitor C 11 l The substation circuits are powered by current line through the impedance Zt and, in general, require a minimum operating voltage Vmin The voltage VL appearing at the terminals of the substation is thus provided by the expression: v = V in + IZ L min to I is the current obtained of the line In order to meet the specifications

  of the various administrations, it is necessary to reduce VL to the minimum.

  The problem could, of course, be eliminated by the use of a battery

  local, but this increases the cost and complexity of the station.

  The object of the present invention is to propose a solution to this problem. According to the invention, a subscriber telephone set comprising two output terminals for connection to a line and a first impedance in series with the line contains means by which a second impedance, of opposite sign to that of the first impedance, is connected in parallel with the first impedance, the combination of the two impedances

  providing suitable termination of the position on the line.

  The various objects and characteristics of the invention will be

  now detailed in the description which follows, made under

  non-limiting example, referring to FIGS. 2 and 3 appended which represent: FIG. 2, the diagram of the output stages of a telephone set,

  FIG. 3, the diagram of the feedback loop in alter-

  native to the output stage of figure 2.

  By first considering FIG. 2, the output stages of a telephone set include a line output transistor TRI, its control stage 211 and a direct current amplifier 212 The control amplifier 211 is itself controlled by a transmission amplifier 213, for the transmission of speech signals and, optionally, by a voice frequency signal amplifier, for signaling needs This circuit includes a feedback loop

  in direct current and in alternating current.

  The positive line wire is coupled to terminal 22 via a resistor R 1 whose value is normally 300 ohms This terminal 22 is coupled to the negative line wire (the ground of the circuit) by a

  capacitor C 1 which eliminates an AC component of the line current.

  The resulting DC signal is supplied to one of the inputs of the DC amplifier 212, the other input of which is connected to a reference voltage source Vref The output of the amplifier 212 corresponds to the difference between the two voltages It controls the line transistor TRI via the control amplifier 211 The amplifier loop comprising the direct current amplifier 20 212, the control amplifier 211 and the line output transistor TR 1 maintains the potential of terminal 22 at a constant value, for example at 2.6 V. The positive line wire is also coupled, by a terminal 23 to the collector circuit of the transistor TRI and to a terminal 24, via of a capacitor C 2 This capacitor stops the DC component of the line current and supplies the AC component by the resistors R 2 and R 4 to an input of the emission amplifier 213 The AC voltage appearing at the terminals of the resistor R 3 in the emitter circuit of the transistor TR 1 is coupled to the emission amplifier 213 via the intermediate 30 of the capacitor C 3 The value of this resistor will be for example 12 ohms The other input of the transmission amplifier is connected to the signal input VIN The circuit includes both a positive and a negative feedback loop, of which Figure 3 gives an example of an embodiment The positive feedback path on the amplifier AMP includes capacitor C 31 and resistance Rf The negative feedback path borrows capacitor C 32 and resistance R By a Theve analysis of this circuit, we can demonstrate that the impedance Ro presented to the line is defined by the expression: RL. Rf RR = Re RRRR efg L Thus, the output impedance of the circuit of figure 2 in the vocal band is equal to the combination in parallel of the impedance R 1 and the internal impedance of the circuit In the case of the circuit here described, we can demonstrate by analogy with the circuit of figure 3, that the output impedance Z of the line output stage is given by: 10 R 1 x R 2 x R 3 Z = R 3 x R 2 R 4 x R 1 With R 3 fixed at 12 ohms, the values of R 2 and R 4 are chosen so as to give the line output stage an impedance of -600 ohms in the vocal band Thus, if RI is 300 ohms, the combination

  in parallel with R 1 and Z provides an output impedance of 600 ohms.

  O Preferably, the resistors R 1, R 2, and the capacitors C 1,

  C 2 and C 3 are discrete components, all the other components being able to be included in an integrated circuit.

  Since the DC power supply for the circuit and the auxiliary circuits is obtained from the line by the resistor R 1, the arrangement described has the advantage that the voltage drop across the resistor Pl is only half that is produced by a conventional circuit. It is clear that the technique described here is not limited to the particular circuit of Figure t and that other methods for producing the parallel combination of positive and negative impedances can

also be applied.

  In general, it is obvious that the foregoing descriptions have been given by way of non-limiting example and that

  many variations can be considered without going out

of the scope of the invention.

Claims (6)

  1 subscriber telephone set comprising two output terminals for connection to a line and a first impedance (R 1) connected in series with the line, characterized in that it contains means by which a second impedance, of opposite sign to that of the first impedance, is connected in parallel with the first impedance, the combination of these two impedances providing a suitable termination of the post on the line.   2 Subscriber telephone set comprising an output transistor (TR 1) for the transmission of signals on the line and a resistor (RI) 10 in series with the line and through which a DC power supply for the set is obtained from the line, in operation, characterized in that it contains an amplifier for controlling said transistor (TR 1), a first feedback loop in direct current and a second feedback loop in alternating current, both incorporating said control amplifier, by means of which the output transistor (TR 1) has a negative impedance in parallel with said resistance, so that the combination of these two impedances is adapted to that of the line.   3 telephone set according to claim 2, characterized in that said direct current feedback loop comprises a direct current amplifier with two inputs (212), one input of which is   connected to a reference voltage source.   4 telephone set according to claim 2 or 3, characterized in   that said alternating current feedback loop comprises a positive feedback loop (Rf, C 31) and a negative one (R, C 32).   Line output circuit for a subscriber telephone set comprising an output stage (213,211, TR 1) by which signals are transmitted over the line, a resistive impedance (R 1) in series with the line by which, in operation , a direct current supplying the circuit is obtained from the line characterized in that means are provided so that the output stage has a negative impedance in parallel with the resistive impedance in such a way that the impedance of the circuit is adapted to that of the line.   6 Circuit according to claim 5, characterized in that said negative impedance comprises an amplifier (213,211) provided with loops   direct and alternating current feedback.   7 Circuit according to claim 6, characterized in that said alternating current feedback loop comprises a loop of   positive feedback (Rf, C 31) and negative (Rg, C 32).