GB2075305A - Combined amplifier-limiter for a telephone system - Google Patents

Abstract

In telephone systems the length of the lines to the exchanges varies, dependent, inter alia, on the distance to the exchange. To overcome these difficulties a combined amplifier-limiter has been produced for use as an interface for a dialler (2), e.g. an electronic circuit for deriving break impulses from a key-pad (1), or the speech network (3). This interface includes a FET (6) connected across the line in series with a resistor the operation of which is controlled by a reference amplifier (7). The latter is also across the line, and it is connected to the FET (6) to regulate the impedance thereof in accordance with line conditions so that it acts as a line current limiter. The "dialling" or speech is applied via a switch (4) to the gate of the FET (6), so that the latter also acts as an amplifier. The basic circuit arrangement can also be used in other so-called "telephone-related" applications, e.g. Viewdata, modems. <IMAGE>

Description

SPECIFICATION Combined amplifier-limiter This invention relates to a combined amplifierlimiter circuit, which when in use is connected to a line to a remote point, over which line the circuit receives its direct current power and which line has an impedance which may vary.

One application of such a circuit is in a telephone subscriber's instrument, which is basically a standard item which has to be connected to a line to the telephone exchange. Since the distance between the instrument and the exchange varies from subscriber to subscriber, so does the line impedance. As the instruments are usually line-powered from the exchange, the line direct current therefore also var ies.tThis can have inconvenient results since the current on very short lines is too high, which causes power wastage.

An object of the invention is to produce a combined amplifier-limiter circuit usable, inter-alia, in a telephone subscriber's instrument, but usable elsewhere if similar problems exist.

According to the present invention there is provided a combined amplifier-limiter circuit for connection to a two-wire line over which the circuit is to be power fed and which line may have a variable impedance, which circuit includes a field-effect transistor (FET) whose source-drain path is connected across the line, an input connection to the gate of the FET via which signals for transmission to the line are received, a further transistor whose emitter-collector path is connected across the line in series with a first resistive impedance, a connection including a second resistive impedance from the junction of the collector of the further transistor and the first resistive impedance to the gate of the FET, a connection including a third resistive impedance from the base of the further transistor to the source of the FET, and a fourth resistive impedance connected between the source of the FET and one of the line wires.

An embodiment of the invention will now be described with reference to the accompanying drawing in which Fig. 1 is a block diagram of a telephone subscriber's instrument using a circuit embodying the present invention, while Fig. 2 shows in detail the circuit embodying the invention, which forms the dashed line rectangle of Fig. 1.

In Fig. 1, the telephone set is of the so-called electronic type, and has a key pad 1 connected to a socalled dialler 2, which converts the key operation into 10 i.p.s. impulses as required by most cu rrently-used telephone exchanges. A circuit of this type is described in our British Patent No. 1,369,829 (M. J. Debenham -- 1). Also included in the set is the speech network 3. The dialler and the speech network are connected to a switch 4, which could be an electronic switch, to voltage-controlled current source 5, which is shown in more detail in Fig. 2.

However, it will be seen that this includes a field effect transistor 6, actually a VFET in the embodiment described, which is connected across the line in series with a resistive impedance. The block 5 also includes a reference amplifier 7.

When the subscriber lifts his handset, see Fig. 1, the speech network 3 is connected to the voltage controlled source 5 via the switch 4, and the dialier 2 receives direct current from the line over connection (not shown). When in this condition, the "dialling" impulses are applied to the block 5, which acts as an interface in that is passes them to the line. Then in the break condition, the line current through the VFET 6 is at a very low value, while in the make condition it is at the feed line value. The circuit across the line via the VFET, when in the make condition acts as a current limiter. In the present embodiment, the parameters are such that not more than 50 mA of line direct current flows in the make condition.

In the case of longer lines, the line current is reduced progressively as line length increases to a minimum of about 25 mA.

During speech transmission, when the switch 5 is set to connect the speech network to the block 5, the letter maintains the line current at a lower level, e.g.

25 mA, and this is modulated with the speech. The speech network is preferably of the so-called electronic type, in which there is no outpulsing through the microphone. Thus we dispense with the carbon microphone, which needs a polarizing current, for use, for example an electret microphone.

Such an arrangement has the advantage that only a single line current control circuit is needed, which gives a useful simplification of the circuit. In addition, when a relatively large number of the subscribers' sets are close to the exchange, the current limiting provides a useful economy in power. This follows since the line current in such cases, which is usually of the order of 110 mA, is reduced to a maximum of 50 mA.

We now turn to Fig. 2, which shows the contents of the block 5 in detail. Note that additional circuit elements (not shown) are needed for speech reception.

Also, although specifically designed for use in a system using Strowger-type impulsing, the circuit can be used in a telephone set using MF signalling.

When the telephone set goes off hook, the circuit receives its line direct current, perhaps via a bridge rectifier or its equivalent to make sure that the polarity is correct as seen by the electronic circuitry. The transistor T, in series with resistor R1 is now connected across the line, as is field effect transistor FET, which is a VFET. FET is in series with a zener diode ZD, which is included to prevent the voltage across the terminals A and B from falling below a certain preset minimum value when the voltage drop across the FET is low, as may occur with long lines.

The current flow in R1-T when the line is looped produces a voltage at the base of the transistor T, which is applied via a resistor R2 to the source of the FET, which latter is connected to the line wire B via another resistor R3. The collector of T is also con nected to the gate of FET via another resistor R4 so that the voltage conditions on the electrodes of T determine the operating parameters of the FET. This has a current limiting effect, as mentioned above.

The input, from the switch 4 in Fig. 1, is to the gate of FET, and during the transmission of break impulses the line current is turned off, and it is turned on during the make periods. Thus for each break impulse the input to the gate of FET is connected to line B for the duration of that impulse, the disconnection of the gate from line B at the end of each impulse allowing the circuit to re-establish the line current.

During speech transmission, the circuit maintains an average current of NmA, modulated by the speech information, which latter is applied to the gate of the FET. The diode D is included so that under long line conditions the FET cannot saturate.

This is desirabie since, if saturation does occur, speech modulation ofthe line current is not possible.

Although the circuit shown in Fig. 2 was developed specifically for telephone application, it is applicable in other "telephone related" applications, e.g. Viewdata, modems, and wherever a high input impedance voltage-controlled current generator is required. It is also usable to combine analogue and digital operations in a single stage.

In the arrangements described herein, the field effect transistor used is a VFET, but it will be appreciated that other forms of field-effect transistors can also be used.

Claims (5)

1. Acombined amplifier-limiter circuitfor connection to a two-wire line over which the circuit is to be power fed and which line may have a variable impedance, which circuit includes a field-effect transistor (FET) whose source-drain path is connected across the line, an input connection to the gate of the FET via which signals for transmission to the line are received, a further transistor whose emitter-collector path is connected across the line in series with a first resistive impedance, a connection including a second resistive impedance from the junction of the collector of the further transistor and the first resistive impedance to the gate of the FET, a connection including a third resistive impedance from the base of the further transistor to the source of the FET, and a fourth resistive impedance connected between the source of the FET and one of the line wires.

2. An interface circuit for connecting the speech circuitry or the circuitry for sending impulse trains to a telephone line, the circuit being supplied with direct current over the line, wherein the line impedance varies dependent on the distance of the circuit from the telephone exchange, and wherein the interface circuit includes a field-effect transistor (FET) with its source-drain path connected across the line, an input connection to the gate of the FETvia which a signal, which may be wanted, number identifying data or speech, fortransmission to the line, is received, a further transistor whose emitter-collector path is connected across the line in series with a first resistive impedance, a connection including a second resistive impedance from the junction of the collector of the furthertransistor and the first resistive impedance to the gate of the FET, a connection including a third resistive impedance from the base of the further transistor to the source of the FET, and a fourth resistive impedance connected between the source of the fet and one of the line wires.

3. A circuit as claimed in claim 1 or 2, and which includes a zenerdiode connected between the drain of the FET and the other line wire.

4. A circuit as claimed in claim 1,2 or 3, and which includes a diode connected between the gate and the chain of the FET and so poled as to ensure that the FET does not saturate.

5. An interface circuit for use, for instance, in a telephone subscriber's instrument, substantially as described with reference to the accompanying draw- ing.