GB2087199A - Amplifiers for driving balanced lines - Google Patents

Abstract

In a telephone subscriber's line interface unit a circuit which transmits to the subscriber's line includes a differential output amplifier A2,A3 with current feedback A5,R5-R8 to stabilise the output signal current with respect to the input voltage. A second feedback loop is provided which includes a pair of equal value resistors connecting each of the two outputs A,B of the differential amplifier to the summing junction of an amplifier A6 connected as an integrator. This monitors the voltages on the outputs A,B and if they are not dynamically balanced about earth potential produces an output which is fed to non-inverting inputs of the output stages A2,A3 of the differential amplifier so as to offset the voltages at the outputs of the output stages in such senses as to tend to restore the balance. <IMAGE>

Description

SPECIFICATION Improvement in or relating to amplifiers This invention relates to amplifiers and in particular amplifiers for driving a balanced line, for example a telephone subscriber's line.

In a telephone exchange each subscriber's line is terminated by subscriber's line interface unit which receives signals from and transmits singals to the subscriber's line. This circuit conventionally includes a hybrid transformer. Transformers are, however, bulky and expensive and attention recently has focussed on the use of transistor circuitry.

In order to ensure that an adequte line current is received by a subscriber's telephone instrument irrespective of the series resistance of the line, it is becoming common practice for subscriber's line interface units to feed the subscriber's line with a constant current. At the same time to ensure efficient signal transmission it is desirable that the output impedance of the subscriber's line interface unit for the speech signal be matched to the characteristics of the line. Moreover in the interests of reducing electrical interference, crosstalk and the detrimental effect of stray electrical leakages to earth it is important to balance this output impedance about earth. To achieve satisfactory performance the output impedance of the subscriber's line interface unit must typically be balanced about earth to within 1 part in 1000.

Using conventional techniques this requires many very high precision components, some of which may in lightning strikes have to withstand extreme thermal stress.

It is an object of the present invention to provide a differential output circuit capable of being used to transmit to a subscriber's line in which the problems outlined above are at least to some extent alleviated.

According to the first aspect of the present invention there is provided a differential amplifier circuit for driving a balanced line including at least one inputterminal; a pair of outputterminals; main amplifier means coupled between said at least one input terminal and said output terminals for producing a differential output signal at said output terminals in response to a signal applied to said at least one input terminal; monitoring means for monitoring the instantaneous voltage at each output terminal and deriving therefrom by additive combination an error signal representing the instantaneous voltage imbalance between the output terminals with respect to a fixed potential and for feeding back said error signal to the main amplifier means to cause a voltage correction to appear at both of the output terminal of such a sense and magnitude to minimise the error signal and thus balance the voltage on the output terminals about fixed potential.

According to a second aspect of the present invention there is provided a method of balancing about a fixed potential the outputs of a differential output circuit for driving a balanced line and having at least one input terminal, a pair of output terminals and main amplifier means coupled between said at least one input terminal and said output terminals, including the steps of monitoring the instantaneous voltage at each termiinal; and deriving therefrom by additive combination an error signal representing the instantaneous voltage imbalance about the fixed potential of the voltages on the output terminals; so applying the error signal to the main amplifier means as to produce a voltage correction to appear at both of the output terminals of such a sense and magnitude to reduce the error signal and thus balance the voltages on the output terminals about the fixed potential.

According to a third aspect of the present invention there is provided a method of driving one end of a balanced line including the steps of causing a signal current to flow in one wire of the balanced line and a complementary signal current to flow in the other wire; maintaining the signal voltages on the wires of the balanced line such site as to be of equal and opposite polarity with respect to earth potential.

In order that the invention be readily understood and put into practice an embodiment of it wiil now be described by way of example only making reference to the accompanying drawings in which: Figure 1 shows a block diagram of a subscriber's line interface unit; and Figure 2 shows the transmit circuit of the subscriber's line interface unit of Figure 1.

Referring to Figure 1, a subscriber's line interface unit consists of a transmit circuit 10 connected to a subscriber's line terminals A,B for transmitting speech signals overthe line to a subcriber's telephone instrument in response to a speech signal on an input terminal 72. Also connected to the subscriber's line terminals A,B is a receive circuit 11 which receives any speech signal originating from the subscriber's instrument S and produces a corresponding speech signal on an output terminal 13. The receive circuit 11 also takes a signal from the input terminal 12 of the transmit circuit 10 to enabie it to eliminate or reduce side tone.

In addition to producing a speech signal the transmit circuit 10 also feeds a constant current to the line to power the subscriber's instrument S.

Referring to Figure 2 the transmit circuit of the subscriber's line interface unit includes output amplifiers A2 and A3, differential and common mode feedback amplifiers A5 and A6 and an inverting amplifer A4. The amplifiers A2 and A3 are arranged to produce a signal in a pair of output terminals A,B connected to respective wires of a subscriber's line, in response to a speech signal voltage applied to an input terminal X.

The forward path of the transmit circuit includes the output amplifiers A2 and A3. The amplifier A3 is driven directly at its inverting input from an input summer 1 and the amplifier A2 is driven at its inverting input by a signal derived from the summer 1 via the unity gain inverting amplifierA4.

Each of the output amplifiers A2 and A3 drives one of the output terminals A,B via a resistor R1 ,R2. In a practical circuit R1 and R2 are of equal value and of the order of 100 ohms. The current flowing through these resistors is sensed by monitoring the potential difference across them using the differential feedback amplifier A5 and its associated resistors R5 to R8. A voltage proportional to this current is fed back to the input summer 1 to complete a negative feedback current control loop. The bias conditions of the closed loop configuration of amplifiers A2, A3, A4 and A5 are arranged so that in the quiescent state (no speech signal) the configuration feeds a constant current to the subscriber's line to power the subscri ber's instrument.A reactive network 2 is connected between the output of the feedback amplifier A5 and the summer 1 to cause the transmit circuit to present a complex output impedance at the output terminals A,B to match the reactive characteristics of a subscri ber's line.

Each output terminal A,B of the transmit unit is connected via a respective one of a pair of accurately matched resistors R3,R4 to the inverting input of the common-mode feedback amplifier A6. The non inverting input of the amplifier A6 is connected to a potential divider formed of equal value resistors R9,R10 respectively connected to earth and a termin al -VsArr which in turn is connected to the tele phone exchange battery (not shown). The junction of the resistors R9 and R10 is decoupled by a capacitor C2 to earth to ensure that it is held at earth potential as far as speech signals are connected. The output of the amplifier A6 is connected to the non-inverting inputs of both of the output amplifiers A2 and A3.A capacitor C1 connected between the inverting input and the output of amplifier A6 ensures the stability of the feedback loops formed by amplifier A6 around the amplifiers A2 and A3 and improves the accuracy of the loop. Each of the amplifiers A2 to A6 may be any standard high gain amplifier, for example a 741, and is provided with feedback in the conventional manner to give the desired response. In operation the amplifier A6 monitors the voltage on the junction of resistors R3 and R4. This is compared with the voltage on the junction of R9 and R10. As mentioned above, the junction of R9 and R10 is decoupled and it is at earth potential as far as speech signals are concerned, but it has a d.c. potential of VBA/2.

Any difference between the voltages applied to the amplifierA6 results in the production of an output voltage which is applied to the non-inverting inputs of the output amplifiers A2 and A3 as a commonmode error correction signal superimposed on the signal appearing at the output terminals A,B. The effect of this correction signal is to adjust the voltage on the junction of R3 and R4 so that it equals the voltage on the junction of R9 and R10. Thus the amplifier A6 ensures that any signal appearing on the output terminal A is matched by an equal and opposite signal on the terminal B, and vice versa.

This causes the transmit circuit as a whole to have an output impedance which is accurately balanced about earth as far as speech signals are concerned.

The provision of the amplifier A6 enables the transmit unit to reject common-mode signals and to compensate for unequal signals on the output terminals. In the case of a common-mode signal the input impedance at the output terminals A,B is the sum of the resistance of resistors R1 and R2 divided by the loop gain of the output amplifier A2, A3 and the amplifier A6. As the gain of amplifier A6 is typically very high and the resistors R1 and R2 typically 100 ohms the impedance presented to common-mode signals is considerably less than an ohm. Common-mode signals are therefore in effect short-circuited to earth.

If signals of uneuqal amplitude appear on the A and B terminals, for example due to mismatch between the amplifiers A2 and A3 or unequal parasitic components to earth on the wires of the subscriber's line, the transmit unit will compensate by offsetting the voltage on both output terminals to restore an exact dynamic balance about earth poten- tial. This balancing of the output voltages about signal earth potential compensates not only for imbalances in the amplifier itself but also for asymmetric loading of the subscriber's line.

The invention has appiication whenever it is necessary to ensure that a balanced line is accurately balanced about any datum potential.

A similar technique can be used to ensure balance of a line with three or more wires.

Claims (16)

1. A differential amplifier circuit for driving a balanced line including at least one input terminal; a pair of output terminals; main amplifier means coupled between said at least one input terminal and said output terminals for producing a differential output signal at said output terminals in response to a signal applied to said at least one input terminal;; monitoring means for monitoring the instantaneous voltage at each output terminal and deriving therefrom by additive combination an error signal representing the instantaneous voltage imbalance between the output terminals with respect to a fixed potential and for feeding back said error signal to the main amplifier means to cause a voltage correction to appear at both of the output terminals of such a sense and magnitude to minimise the error signal and thus balance the voltage on the output terminals about fixed potential.

2. A circuit according to claim 1 wherein the monitoring means includes a potential divider connected between the output terminals and comparing means coupled to a centre tap of said potential divider and the fixed potential for comparing the potential on said centre tap with the fixed potential and producing the error signal in response thereto.

3. A circuit according to claim 2 wherein the comparing means includes a high gain differential input amplifier with one input connected to the centre tap of the potential divider and the other to the fixed potential and the error signal is derived from the output of the high gain differential input amplifier.

4. A circuit according to claim 3 wherein the high gain differential input amplifier is an integrating amplifier.

5. A circuit according to claim 1,3 or 4 wherein said potential divider consists of a pair of accurately matched resistors connected in series.

6. A circuit according to any preceding claim wherein the main amplfier means includes two output stages each one consisting of a differential input amplifier connected to drive a respective one of said output terminals and wherein the monitoring means is connected to feed said error signal to the same polarity input of the differential input amplifier of each of the output stages.

7. A circuit according to claim 6 wherein the main amplifier means includes an input stage connected to apply complementary signals to the other inputs of the differential input amplifiers of the output stages respectively.

8. A circuit according to any preceding claim wherein the main amplifier means has internal feedback to stabilise the signal at the output terminals with respect to the signal applied to the at least one input terminal.

9. A circuit according to claim 8 wherein each output terminal is connected to a respective resistor within the main amplifier means so that, in operation, the current flowing in the output terminal flows through the respective resistor and the potential differences across the resistors are monitored to produce a feedbck signal representative of the currents flowing through the output terminals, which feedback signal is fed back within the main amplifier means to stabilise the current flowing in the output terminal means with respect to the signal applied to the output terminals.

10. A circuit according to claim 9 including a filter network for modifying the feedback signal to produce a partially reactive output impedance at the output terminals.

11. A method of balancing about a fixed potential the outputs of a differential output circuit for driving a balanced line and having at least one input terminal, a pair of output terminals and main amplifier means coupled between said at least one input terminal and said output terminals, including the steps of monitoring the instantaneous voltage at each output terminal; and deriving therefrom by additive combination an error signal representing the instantaneous voltage imbalance about the fixed potential of the voltages on the output terminals; so applying the error signal to the main amplifier means as to produce a voltage correction to appear at both of the output terminals of such a sense and magnitude to reduce the error signal and thus balance the voltages on the output terminals about the fixed potential.

12. A method of driving one end of a balanced line including the steps of causing a signal current to flow in one wire of the balanced line and a complementary signal current to flow in the other wire; maintaining the signal voltages on the wires of the balanced line such as to be of equal and opposite polarity with respect to earth potential.

13. Amethod according to claim 12 wherein the step of maintaining the voltages includes monitoring and additively combining the voltages, comparing the combination with the fixed potential, and so reducing the current in one wire and increasing the current in the other as to make the additive combination of the voltages equal to the fixed potential.

14. A differential output circuit for driving a balanced line and substantially as herein described with reference to the accompanying drawings.

15. A method of balancing about earth potential the outputs of a differential output circuit for driving a balanced line, the method being substantially as herein described with reference to the acccompanying drawings.

16. A telephone subscriber's line interface unit including a differential output circuit according to any one of ciaims 1 to 10.