Subscriber line interface circuit

• US 4,540,854 A
• Filed: 07/28/1983
• Issued: 09/10/1985
• Est. Priority Date: 12/17/1982
• Status: Expired due to Term

• Current Assignee: Mitel Corporation (Microchip Technology Incorporated)
• Original Assignee: Mitel Corporation (Microchip Technology Incorporated)
• Inventors: Beirne, Patrick R.
• Primary Examiner(s): Rubinson, Gene Z.
• Assistant Examiner(s): Divito, Elio
• Application Number:
  US06/518,191
• Time in Patent Office: 775 Days
• Field of Search: 179/18 FA, 179/18 AD, 179/18 AH, 179/18 HB
• US Class Current: 379/345
• CPC Class Codes: H04B 1/586 using an electronic circuit
• CPC Class Codes: H04M 3/005 Interface circuits for subscriber lines current supply H04M19/00 and subgroups; supervisory, monitoring or testing arrangements H04M3/22 and subgroups; in key telephone systems H04M9/006
• CPC Class Codes: H04Q 1/38 using combinations of direct currents of different amplitudes or polarities over line conductors or combination of line conductors

Abstract

A transformerless, fully balanced, telephone line circuit is provided. Two differential amplifiers are employed, one for incoming signals and the other for outgoing signals. The latter drives the line with two out-of-phase, equal amplitude signals. A comparator detects grounding of either TIP or RING at the telephone to accommodate international applications.

Claims

1. A telephone line circuit for connection between a balanced two-terminal port and an unbalanced terminal comprising:

(a) first differential amplifier means for receiving incoming signals from said balanced two-terminal port and converting them to unbalanced signals;

(b) circuit means for applying said unbalanced signals to said unbalanced terminal;

(c) second differential amplifier means having a pair of differential input terminals coupled across said circuit means for receiving signals originating from said unbalanced terminal, and a pair of balanced output terminals for driving said balanced two-terminal port with a pair of balanced outgoing signals representative of said signals received from the unbalanced terminal;

(d) means for detecting an imbalance in dc-current flowing into and out of said balanced two-terminal port, and indicating grounding of either side of said balanced two terminal port in response thereto; and

(e) means for limiting said dc-current to a predetermined value in response to detection of said imbalance.

2. A telephone line circuit as defined in claim 1, said means for detecting an imbalance comprising a comparator for comparing the average voltage at said balanced two-terminal port and the average voltage at said pair of balanced output terminals of said second differential amplifier means, said comparator indicating a predetermined imbalance in said dc-current by a change in its output voltage.

3. A telephone line circuit as defined in claim 1 or 2, said first differential amplifier means comprising:

(a) an input amplifier having inverting and non-inverting inputs resistively coupled to said balanced two-terminal port; and

(b) an output amplifier, capacitively coupled to the output of said input amplifier, having an unbalanced output connected to one side of said circuit means.

4. A telephone line circuit as defined in claim 1 or 2, said circuit means being a resistor, and second differential amplifier means comprising:

(a) an input amplifier having inverting and non-inverting inputs coupled across said resistor;

(b) an output amplifier, dc-coupled to the output of said input amplifier; and

(c) the output of said input amplifier driving one side of said balanced two-terminal port, and the output of said output amplifier driving the other side of said balanced two-terminal port.

5. A telephone line circuit as defined in claim 2, wherein:

(a) said first differential amplifier means comprises a balanced input amplifier capacitively coupled to an output amplifier having an unbalanced output;

(b) said circuit means is a resistor, one side of which is connected to said unbalanced output of said output amplifier;

(c) said second differential amplifier means comprises a first amplifier having its differential input terminals coupled across said circuit means and having its output resistively coupled to a second amplifier; and

(d) the output of said first amplifier applies a first signal to one side of said balanced two-terminal port through a first resistor, and the output of said second amplifier applies a second signal to the other side of said balanced two terminal port through a second resistor, said first and second signals being of equal amplitude and out-of-phase.

6. A telephone line circuit as defined in claim 5, wherein said comparator is comprised of a differential amplifier having inverting and non-inverting inputs, said inverting input being connected to said balanced two-terminal port via a first pair of summing resistors and said non-inverting input being connected to said first and second resistors via a second pair of summing resistors.

7. A telephone line circuit as defined in claim 5 or 6, said first and second resistors being equal in value.

8. A telephone line circuit as defined in claim 6, said comparator having its output connected to an associated switching system to signal said change in the output voltage.

9. A telephone line circuit as defined in claims 5, 6 or 8, wherein said first and second resistors are equal, and further comprising:

(a) a comparator for indicating an off-hook condition of a telephone associated with said line circuit by comparing the dc-voltage of said output amplifier to a predetermined reference voltage.