High-efficiency line driver

• US 6,665,399 B1
• Filed: 03/25/1999
• Issued: 12/16/2003
• Est. Priority Date: 03/25/1999
• Status: Expired due to Fees

• Current Assignee: Northern Telecom Limited (Nortel Networks Corporation)
• Original Assignee: Nortel Networks Limited (Nortel Networks Corporation)
• Inventors: Anderson, Carl, Gorcea, Dan, Jakab, Gyula
• Primary Examiner(s): Iser, Forester W.
• Assistant Examiner(s): HAROLD, JEFFEREY F
• Application Number:
  US09/276,143
• Time in Patent Office: 1,727 Days
• Field of Search: 379/394, 379/398, 379/399.01, 379/402, 379/403, 379/404, 375/219, 375/214, 375/221, 375/242
• US Class Current: 379/399.01
• CPC Class Codes: H03F 1/565 using inductive elements
• CPC Class Codes: H04L 25/0266 Arrangements for providing Galvanic isolation, e.g. by means of magnetic or capacitive coupling

Abstract

A line driver for connection to a transmission line having a characteristic impedance. The line driver can be arranged in a voltage-mode or a current-mode configuration. In the voltage-mode configuration, the line driver comprises an amplifier, a transformer, a reference resistor and a feedback circuit. The first winding of the transformer has a first end connected to the output of the amplifier and the second winding is connectable to the transmission line. The reference resistor has an end connected to the second end of the first winding at a junction point and the feedback circuit is connected to the input and output of the amplifier and also to the junction point. The reference resistor has a resistance equal to {fraction (1/k)} times the characteristic impedance of the transmission line. The feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to −(K−1) times the voltage at the junction point, for a predetermined value of K. This results in a synthesized output impedance equal to the characteristic impedance of the transmission line, while losses in the reference resistor are reduced. At the same time, the gain is easily set by the specifying the resistance of a source resistor placed at the input to the amplifier.

Claims

1. A line driver, comprising:

an amplifier having an input for connection to a voltage source and having an output;

a transformer having a first winding and a second winding, the first winding having a first end connected to the output of the amplifier and having a second end, the second winding being connectable to a transmission line having a characteristic impedance;

a reference resistor having an end connected to the second end of the first winding at a junction point; and

a feedback circuit connected to the input of the amplifier, to the output of the amplifier and to the junction point;

wherein the reference resistor has a resistance equal to {fraction (1/K)} times the characteristic impedance of the transmission line and wherein the feedback circuit is arranged to produce a voltage at the output of the amplifier substantially equal to −

(K−

1) times the voltage at the junction point, for a predetermined value of K.

2. A line driver as claimed in claim 1, wherein the amplifier is an operational amplifier connected in an inverting configuration.

3. A line driver as claimed in claim 1, wherein the feedback circuit comprises:

a first feedback resistor having a first end connected to the output of the amplifier and having a second end connected to the input of the amplifier; and

a second feedback resistor having a first end connected both to the input of the amplifier and to the second end of the first feedback resistor and having a second end connected to the junction point;

wherein the first feedback resistor has a resistance equal to (K−

1) times the resistance of the second feedback resistor.

4. A line driver as claimed in claim 3, further comprising a source resistor connected to the input of the amplifier, whereby the line driver produces a gain equal to the negative of the ratio of the resistance of the first feedback resistor to the resistance of the source resistor.

5. A line driver as claimed in claim 1, wherein the transformer is a 1:

1 transformer and wherein the first and second windings have an equal number of turns.

6. A line driver as claimed in claim 1, wherein another end of the reference resistor is connected to a ground reference.

7. A line driver, comprising:

a transformer having a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance;

a reference resistor having a first end connected to a first end of the first winding;

a feedback circuit having a first port connected both to the first end of the reference resistor and to the first end of the first winding and having a second port connected to the second end of the reference resistor;

wherein the feedback circuit is arranged to draw, through the first port, K−

1 times the amount of current flowing through the reference resistor.

8. A line driver as claimed in claim 7, wherein the feedback circuit comprises:

first means connected to the second port of the feedback circuit, for measuring the current flowing in the reference resistor;

second means connected to the first port of the feedback circuit, for measuring the current drawn by the feedback circuit; and

means connected to the first and second measuring means, for determining the difference between the measured amount of current flowing in the reference resistor and $\frac{1}{K-1}$

times the measured amount of current drawn by the feedback circuit and generating a compensatory current flowing out the first port of the feedback circuit, said compensatory current being proportional to said difference.

9. A line driver as claimed in claim 8, wherein the first measuring means comprises:

an operational amplifier having an inverting input connected to the second port of the feedback circuit, a non-inverting input connectable to a ground reference and an output; and

a resistor connected between the input and output of said operational amplifier.

10. A line driver as claimed in claim 8, wherein the second measuring means comprises a current transformer.

11. A line driver as claimed in claim 10, wherein the second measuring means further comprises a resistor placed in parallel with the current transformer.

12. A line driver as claimed in claim 8, wherein the means for determining comprises:

a divider circuit connected to the second measuring means, for dividing the measured current current drawn by the feedback circuit by (K−

1); and

a differential circuit having a first input connected to the divider circuit, a second input connected to the first measuring means and an output, for producing said compensatory current at the output of the differential circuit as a function of the difference between the voltage at the first and second inputs of the differential circuit.

13. A line driver according to claim 7, wherein the reference resistor has a resistance equal to K times the characteristic impedance of the transmission line.

14. A line driver, comprising:

a transformer having a first winding and a second winding, the second winding being connectable to a transmission line having a characteristic impedance;

a feedback circuit having a first port connected to an end of the first winding and having a second port, a third port and a fourth port; and

a reference resistor having a first end connected to the second and fourth ports of the feedback circuit and having a second end connected to the third port of the feedback circuit;

wherein the feedback circuit is arranged to draw, through the fourth port, K times the amount of current flowing through the reference resistor.

15. A line driver as claimed in claim 14, wherein the feedback circuit comprises:

first means connected to the third port of the feedback circuit, for measuring the current flowing in the reference resistor;

second means connected to the first and second ports of the feedback circuit, for measuring the current flowing in the first winding of the transformer; and

means connected to the first and second measuring means, for determining the difference between the measured amount of current flowing in the reference resistor and {fraction (1/K)} times the measured amount of current flowing in the first winding of the transformer and generating a compensatory current flowing out of the fourth port of the feedback circuit, said compensatory current being proportional to said difference.

16. A line driver as claimed in claim 14, wherein the first measuring means comprises:

an operational amplifier having an inverting input connected to the second port of the feedback circuit, a non-inverting input connectable to a ground reference and an output; and

a resistor connected between the input and output of said operational amplifier.

17. A line driver as claimed in claim 14, wherein the second measuring means comprises a current transformer.

18. A line driver as claimed in claim 17, wherein the second measuring means further comprises a resistor placed in parallel with the current transformer.

19. A line driver as claimed in claim 14, wherein the means for determining comprises:

a divider circuit connected to the second measuring means, for dividing the measured current flowing in the first winding of the transformer by K; and

a differential circuit having a first input connected to the divider circuit, a second input connected to the first measuring means and an output, for producing said compensatory current at the output of the differential circuit as a function of the difference between the voltage at the first and second inputs of the differential circuit.

20. A line driver according to claim 14, wherein the reference resistor has a resistance equal to K times the characteristic impedance of the transmission line.