Field of the Invention

This invention relates to telephone line interface circuits.

Background of the Invention

Telephone apparatus connected to a telephone line must, in general, present certain DC characteristics and a defined AC impedance in the signal band to the telephone line. The DC characteristics and the AC impedance specifications differ between telephone systems, such that there are different requirements in different countries.

Voltages on a telephone line can be as much as 50 volts. This is too high for a CMOS integrated circuit, which can generally only handle a maximum of 5 volts. Therefore, if a CMOS integrated circuit is to be used in a telephone, external components are required to interface between the integrated circuit and the telephone line.

Furthermore, in CMOS circuits, if a relatively high current is to be passed, a relatively large transistor is required, which uses a large amount of silicon and hence increases the size and cost of the integrated circuit.

Summary of the Invention

Accordingly, the invention provides a telephone line interface circuit for interfacing between a telephone line and line powered telephone circuits, comprising a first portion for presenting predetermined impedance, current and voltage characteristics to the telephone line; and a serial regulator portion coupled to receive current from the line and having an output terminal for providing a regulated output voltage as a supply voltage for the first portion and for the line powered telephone circuits, whereby current drawn from the line is shared between the first portion and the serial regulator.

The serial regulator preferably comprises an operational amplifier coupled to receive a reference voltage at a first input and a voltage derived from the regulated output voltage at a second input, and having an output coupled to the output terminal of the serial regulator portion.

Preferably, a voltage divider defines the voltage at the second input of the operational amplifier. Preferably, the voltage divider is programmable to provide a selectable voltage at the second input of the operational amplifier to select the regulated output voltage in dependence on the DC characteristics of the line.

Brief Description of the Drawing

An exemplary embodiment of the invention will now be described with reference to the drawing of FIG.1 which shows a preferred embodiment of a telephone line interface circuit in accordance with the invention.

Detailed Description of a Preferred Embodiment

Referring to FIG.l, there is shown a telephone line interface circuit 10, comprising a rectifier circuit 15, a matching circuit 20, a power suβply circuit 30 and a programming circuit 40, all to be further described below.

The rectifier circuit 15 is coupled to line terminals 11 and 12, for receiving therefrom an AC telephone signal (indicated by arrow 13), from a typical two- wire telephone line of a telephone network. The rectifier circuit 15 provides a (DC) rectified output signal across first and second signal paths 16 and 17 respectively.

The first signal path 16 is coupled to a first output terminal 50 via a potential divider circuit comprising first and second resistors 52 and 53 respectively and capacitor 54, such that the first output terminal 50 is coupled to receive a voltage divided signal from the first signal path 16. The potential divider circuit thus arranged to extract a telephone signal from the first and second signal paths 16 and 17 respectively.

SUBSTrrUTE SHEET (RULE 26)

The second signal path 17 is directly coupled to a second output terminal 51. In this way the first and second output terminals 50 and 51 respectively provide an output voltage, as indicated by arrow 55.

The matching circuit 20 comprises a transconductance amplifier 21 having first and second input terminals and an output terminal. A potential divider circuit composed of resistors 22 and 23 and capacitor 24 is coupled across the first and second signal paths 16 and 17 respectively.

The first input terminal of the amplifier 21 is coupled to a point of divided potential provided by the potential divider circuit, so as to receive a divided portion of the rectified output signal from the rectifier circuit 15. The first terminal of the amplifier 21 is also coupled to a current source 25, to be further described below.

The second terminal of the amplifier 21 is coupled to a voltage reference 26, and the output terminal is coupled to a base terminal of a transistor 27. An emitter terminal of transistor 27 is coupled to the first signal path 16 from the rectifier circuit 15, and a collector terminal of transistor 27 is coupled to a ground node and to the second signal path 17 from the rectifier circuit 15 via a low value resistor 28.

The power supply circuit 30 comprises a first transistor 31, having an emitter terminal coupled to the first signal path 16, a collector terminal coupled to the ground node of the matching circuit 20 via a capacitor 32 and a gate terminal.

A potential divider circuit of resistors 33 and 34 is also coupled to the collector terminal of the first transistor 31, for providing a point of divided potential. A transconductance amplifier 35 has a first input terminal coupled to the point of divided potential between resistors 33 and 34, a second input terminal coupled to a voltage reference 36 and an output terminal coupled to a base terminal of a second transistor 37. A collector terminal of transistor 37 is coupled to the base terminal of transistor 31 and an emitter terminal of transistor 37 is coupled to ground.

The programming circuit 40 comprises a first amplifier 41, having an input terminal coupled to the second signal path 17 and an output terminal. A second amplifier 42 has a first input terminal coupled to the output terminal of the first amplifier 41, a second input terminal and an output terminal, which are coupled to each other so as to provide a feedback path. In this configuration the second amplifier 42 performs the function of a first order filter, with a cut off frequency of 10Hz to eliminate AC signals higher than 10Hz.

An analogue-to-digital converter (ADC) 43 has an input terminal coupled to the output terminal of the second amplifier 42, a first output terminal 45 and a second output terminal. A digital-to-analogue converter (DAC) 44 has a first input terminal 46, a second input terminal coupled to the second output terminal of the ADC 43, and an output terminal coupled to the first input terminal of the amplifier 21 of the matching circuit via the current source 25.

In operation, the telephone line interface circuit 10 performs a number of functions. A first function is to present a programmable DC characteristic to the telephone line, which is performed by the matching circuit 20. The DC characteristic is the relation between DC line voltage VL measured across line terminals 11 and 12 and fine current IL. In the telephone line interface circuit 10, the fine current IL is split between transistors 27 and 31 and then flows through the low value resistor 28, back to the rectifier circuit 15. -

The DC characteristic is given by:

VL = 2VD + VREFl + ZOxIK + RElxIL

where 2VD is the voltage drop through the rectifier circuit 15, Z0 is the impedance of resistor 22, IK is the value of the current source 25 and VREFl the voltage reference 26. IK is the programmable current source generated by the programming circuit 40.

The first amplifier 41 of the programming circuit 40 measures the voltage through the low value resistor 28 and shifts it over the ground to get a positive voltage over the ground (the voltage being negative under the low value resistor 28). The second amplifier 42 is arranged with a cut-off frequency of

substantially lOKHz, for filtering the voltage at the output of 41 to eliminate high frequency components of the AC signal (frequencies higher than approximately lOHz).

The ADC 43 converts the voltage at the output of the second amplifier 42 into a 5 bit data block. As the DC fine current flows through the low value resistor 28, the voltage at the output of the second amplifier 42 is an image of the line current and the 5 bits of data represents the value of the line current IL digitally.

This digital value of the line current IL can be read by a microcontroller which can program the DC characteristic by programming IK. This digital value of the line current IL also goes to the DAC 44 which generates IK:

IK = KxIL + IS

where K and IS are parameters programmed in two 4 bit data blocks from the microcontroller through input 46. So the DAC 44 receives the digital value of the line current IL, and two 4 bit data blocks which program K and IS , and generates an output which is an analogue current source IK

The DC characteristic becomes :

VL = 2VD + VREFl + ZOxIS + (RE1 + Z0xK)xIL

Therefore the DC characteristic VL(IL) is fully programmable through IS and K. IS programs the starting point for IL=0, namely 2VD + VREFl + ZOxIS and K programs the slope RE1 + ZOxK.

The AC impedance is also fixed through the matching circuit 20 according to the formula :

ZAC = RSxd + Z0/Z1)

where RS is the value of resistor 22 and Zl is the impedance of resistor 23 and capacitor 24.

SUBSTITirrE. SHEET (RULE 26)

The second function of the circuit, which is to provide a regulated supply for the line powered telephone circuit is performed by the power supply circuit 30. The transconductance amplifier 35 which drives the external transistors 37 and 31 receives the voltage reference 36 called VREF2 at the first input. On the second input, the amplifier 35 receives a portion of the output voltage from the collector of the first transistor 31 through the potential divider circuit of resistors 33 and 34. The regulated voltage on the collector of the first transistor 31 is filtered by capacitor 32 and it's value is given by :

VREG = VREF2 x (Rl + R2)/R2

where Rl and R2 are the impedances of resistors 33 and 34 respectively.

The value of R2 can be programmed by a microcontroller, so the regulated voltage is also adaptable to be optimised to the DC voltage/current characteristic fixed by the matching circuit 20.

The line current is split between the matching circuit 20 and the power supply circuit 30 such that the power supply circuit 30 only takes what is required by the telephone set to operate. The rest of the line current passes through the matching circuit 20.

It will be appreciated by a person skilled in the art that alternative embodiments to the one described above are possible.