Telephone subscriber circuit with galvanic isolating element for driving the on/off states of a telephone line

Abstract: A telephone subscriber circuit is coupled via a single galvanic isolation element (FD) to drive, from subscriber apparatus unconnected to the telephone line, the line hooking on/off and the transfer of signals thereto, and includes a speech circuit coupled to the line through a diode bridge circuit (D). Connected through a voltage-generating circuit means (Q5,R4) and level-translating circuit means (Q4,R5) to an output terminal of the galvanic isolation element, a photocoupler, are respectively an input terminal of the speech circuit and a control terminal of an electronic switch (Q1,Q2) connected between the speech circuit and the diode bridge circuit.

Description: This invention relates to telephone subscriber circuits, specifically circuits which enable subscriber apparatus drawing no power from a telephone line to be coupled to the line. Such equipment as telephone answering systems or computer terminals used in lieu of traditional phone sets require, for proper handling of all their usual functions, to be fully uncoupled electrically from the line. To enable the "decadic" functions of line hooking on/off, or pulse sending, in order to call another subscriber, and of voice signal transmitting in the hooked-on condition of the line -- using apparatus which are powered separately from the line -- to be performed, the electric uncoupling is customarily ensured by such elements as relays, transformers, or photocoupling devices. Prior approaches have used two discrete paths, quite simply implemented by conventional techniques, to provide the decadic line hook-on/off functions separately from the voice signal transmission function during the line hook-on step. Such is accomplished by means of two galvanic isolation elements of the aforementioned kinds. The two elements may be of two different types. A signal transmission circuit of a telephone system is described in the UK Patent Application GB-A-2 250 661. The underlying technical problem of this invention is to reduce the number of the components required in a telephone subscriber circuit for driving a telephone line hooking-on/off and transferring signals over the telephone line from subscriber apparatus not powered through said line, by omitting one of the two galvanic isolation elements employed heretofore. This reduction in the number of components is desirable to bring down cost and the occupation of integration area while improving reliability. This technical problem is solved by a telephone subscriber circuit as outlined above and defined in the characterizing parts of the appended claims to this specification. The features and advantages of a telephone subscriber circuit according to the invention will be apparent from the following description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. In the drawings: Figure 1 shows a diagram, partly in block form, of a telephone subscriber circuit according to the invention which may be implemented in a technology of the bipolar type; and Figure 2 shows a preferred embodiment of that same theoretical diagram which may be implemented in a technology of the MOS or mixed type. The circuit diagram shown in Figure 1 comprises a speech circuit for transmitting and receiving voice signals, SPEECH CIRCUIT, which may be a conventional type and is coupled to the two terminals, TIP and RING, of a telephone subscriber line through a bridge circuit, usually a diode bridge, D. This speech circuit has an input terminal coupled, through a voltage-generating circuit means, to a photocoupler element comprising a receiving phototransistor FQ and an emitting photodiode FD. The voltage-generating circuit means comprises a transistor Q5 connected at a circuit node C to the phototransistor FQ and connected to the input terminal of the speech circuit via a capacitor C2. Connected between a terminal of the bridge circuit D and the output terminal of the speech circuit is a switch, which may consist of a switching circuit means such as a Darlington arrangement, as shown in Figure 1. Such a circuit structure comprises a pair of transistors, Q1 and Q2, driven by a transistor Q4 connected, in series with a resistor R5, between said Darlington arrangement and the circuit node C. The control terminal of the transistor Q4 is connected to a balanced bias circuit means, to which the terminal of the transistor Q5 is also connected, this circuit means comprising a diode connected serially, in parallel with a capacitor C1, between the circuit node A and a reference potential VREF1 also having the phototransistor FQ connected thereto and constituting a virtual ground for the telephone circuit. The photodiode FD is incorporated to an input stage for connection to subscriber apparatus and connected between two terminals, +Vcc and -Vcc, of a voltage supply. Connected to said input stage are a generator G of alternating signals and a pulse-generating switch I, as shown in the drawings. Such generators represent the subscriber apparatus not powered from the telephone line. In Figure 1, all the transistors in the diagram are bipolar transistors. The circuit diagram depicted in Figure 2 illustrates a different embodiment of a telephone subscriber circuit according to the invention. It also includes a voltage supply means comprising a transistor Q5 connected between the photodiode FD and the input terminal of the speech circuit. However, the switch may be a field-effect transistor M1, specifically a P-channel transistor, although a Darlington arrangement may also be used. The level translation circuit means, that in the scheme of Figure 1 was implemented by the balanced bias transistor Q4, should now be implemented, however, by a transistor M2 of the D-MOS depletion type. As mentioned, the invention is directed to using a single galvanic isolation element (photocoupler), capable of transferring the telephone line closing and opening functions, the decadic function, and transfer of the transmitted voice signal from an apparatus powered from an external source of the telephone line. The opening, closing, and decadic functions will now be discussed. The Darlington Q1, Q2 acts as a line electronic switch. Upon a voltage equal to 2Vbe appearing across R1, Q1 and Q2 saturate and close the line terminals TIP and RING over the diode bridge D in the speech circuit, which synthesizes the appropriate line closure impedances AC and DC. With Q1 and Q2 "making", at the node B there will appear a positive DC potential relative to the reference potential VREF which is dependent on the value of the line current, and this potential holds a bias of 4Vbe at the node A, via D1 and R3. With Q1 and Q2 "breaking", on the other hand, the potential at the node B will be cancelled relative to the reference, and the bias of the node A held by the charge of the capacitor C1 and the small current (on the order of a few microamperes) drawn by R2 from the line, which has a large value. The line closing/opening function and the decadic function (similar to the previous one but having definite open/close timing sequences) are controlled from the switch I, being electrically isolated from the line by the photocoupler F. With the switch I open, the resistors RA and RB are connected to a positive DC reference potential (+Vcc) and cause a DC biasing current to flow through the photodiode FD which is transferred into the transistor FQ at a multiplication factor K. Thus, a current will flow through the phototransistor which is given by the following relation: Ifq = [(Vcc - Vfd)/(RA + RB)] * K where, Vdf is the voltage drop across the photodiode. With the switch I closed, the photodiode would be off, and the current value Ifq cancelled. The current Ifq will flow from the collector of the phototransistor FQ and split at the node C to follow two paths: a portion of it goes through R5, and the remainder through Q5 to R4, namely IFQ=IR5+IR4 . The two emitter followers associated with the transistors Q4 and Q5 set the potential values at the nodes C and D to 1Vbe and 3Vbe, respectively, where Vbe is the base-emitter voltage, thereby determining the value of IR5, namely IR5=2Vbe/R5. . This current, when injected from the collector of Q4 into the base terminal of the Darlington arrangement including the transistors Q1 and Q2, brings the latter to a saturation range, thereby completing the line close function. For proper operation, the value of IR5 should be selected to ensure saturation of the Darlington under all the operating conditions of the line current. Of course, the value of IFQ should be selected higher than IR5 at all times. As for the voice signal transfer, the current IFQ in excess of the constant value of IR5 is transferred into the resistor through the emitter follower Q5, the resistor being connected to a reference potential VREF1 which may be incorporated to the speech circuit. Thus, a DC bias is established at the node E as follows: VE = Vref1 - (R4 * IR4) If, with the switch I open (line hooked on), a voltage signal (Vs) is applied to the node linking RA and RB together, a current is established through the photodiode FD which is proportional to the value of Vs divided by the resistive value of the RA and RB parallel. This current, as transferred at a factor K of the photocoupler, is injected into the node C from the phototransistor. Since IR5 is a constant current, all the signal current will be transferred to R4, and at the node E will appear a voltage signal VsE which, when the splitting between the emitter resistance of the transistor Q5 and R5 is neglected, is Vs E = [Vs/(RA//RB)] * K * R4 This signal is applied, through the capacitor C2, to the input terminal of the speech circuit, which will suitably amplify and transfer it to the telephone line. The embodiment shown in Figure 2 is advantageous over that in Figure 1 on account of it using a smaller number of components, and the fact that with the line in the open condition there will occur no drawing on the telephone line. The voice signal is transferred similar to the previous embodiment. The electronic switch M1, R1 is formed by a P-channel MOS transistor of the high-voltage type which, as mentioned, may replace the Darlington arrangement including Q1, Q2, R1 in Figure 1 to perform the same function. The level translator consisting of Q4 in Figure 1 is replaced by an N-channel MOS transistor of the depletion type having a gate terminal connected to the potential VREF. With the switch I open, as explained above, the phototransistor FQ goes into its conduction range to establish a flow of current through R5 and M2, thereby providing the appropriate gate voltage across R1 to bring the transistor M1 to its conduction range. This gate voltage is tied to the values of R1, R5 and the threshold voltage of the transistor M2. With the switch I closed, no current flows through the phototransistor FQ, and hence through M2, so that the transistor M1 will also be "open" to the line. The advantage of the proposed circuit solution is made apparent by that the possibility of using a single galvanic isolation photocoupling element involves neither designing problems nor complicated circuitry. It will be appreciated that modifications, integrations, and substitutions may be made unto the embodiment form described hereinabove by way of non-limitative example without departing from the protection scope of the appended claims.

Claims: A telephone subscriber circuit coupled to the terminals (TIP, RING) of a telephone subscriber line through a bridge circuit (D) and comprising a speech circuit for transmitting and receiving voice signals which has at least one input terminal adapted to be coupled to subscriber apparatus not powered from the telephone line and an output terminal coupled to a terminal of the bridge circuit, characterized in that it comprises a switch (I) connected between said terminal of the bridge circuit and said output terminal of the speech circuit, and a galvanic isolation element (F) having at least one input terminal and one output terminal, the input terminal of said element being coupled to the subscriber apparatus and the output terminal coupled both to the input terminal of the speech circuit and the control terminal of the switch. A telephone subscriber circuit according to Claim 1, characterized in that the switch comprises a transistor element having first and second terminals wherethrough it is connected between said terminal of the bridge circuit and the output terminal of the speech circuit, and a control terminal forming the control terminal of the switch. A telephone subscriber circuit according to Claim 2, characterized in that the galvanic isolation element comprises an emitting photodiode (FD) and a receiving phototransistor (FQ), the photodiode (FD) being incorporated to an input stage (FD,RA,RB) for connection to subscriber apparatus which is connected between two terminals (+Vcc,-Vcc) of a separate voltage supply from the telephone line, the phototransistor being connected to a circuit node (C), having the control terminal of the switch (Q1,Q2) and the input terminal of the speech circuit coupled thereto, and to a reference voltage (VREF) of the circuit. A telephone subscriber circuit according to Claim 3, characterized in that, connected between the control terminal of the switch and the circuit node is a level-translating circuit means, and that the input terminal of the speech circuit is connected to a first terminal of a capacitive element, a second terminal whereof is connected to the circuit node through a voltage-generating circuit means, said level-translating circuit means and said voltage-generating circuit means being driven by the phototransistor through said circuit node. A telephone circuit according to Claim 4, characterized in that the second terminal of the capacitive element is connected to a reference voltage through a first resistive element and to the circuit node through a first transistor having a control terminal connected to a potential reference. A telephone circuit according to either Claim 4 or 5, characterized in that the transistor element has a Darlington type of structure and the level-translating circuit means comprises a second transistor being connected, by first and second terminals thereof and in series with a second resistive element, between the control terminal of the switch and the circuit node, and having a control terminal connected to a reference voltage. A circuit according to any of Claims 4, 5 and 6, characterized in that the second transistor is a D-MOS depletion type.

Inventors: TORAZZINA ALDO, TORAZZINA, ALDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H04M1
IPC codes: H04M1, H04M11