Circuit and method for active termination of a transmission line

• US 6,788,745 B1
• Filed: 11/17/2000
• Issued: 09/07/2004
• Est. Priority Date: 11/17/1999
• Status: Expired due to Term

• Current Assignee: Ikanos Communications Incorporated (Qualcomm, Inc.)
• Original Assignee: Fujitsu Limited
• Inventors: Lim, Drahoslav, Muralt, Arnold
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): Perilla, Jason M.
• Application Number:
  US09/715,293
• Time in Patent Office: 1,390 Days
• Field of Search: 375/258, 375/222, 375/220, 375/377, 375/285, 379/402, 379/405, 379/345, 379/391, 379/414, 330/260, 330/293, 330/85, 330/165-167, 370/286, 370/290, 370/282
• US Class Current: 375/297
• CPC Class Codes: H04L 25/0298 Arrangement for terminating transmission lines
• CPC Class Codes: H04M 3/007 Access interface units for simultaneous transmission of speech and data, e.g. digital subscriber line [DSL] access interface units DSL access multiplexers H04Q11/0478, and H04L12/2856

Abstract

An improved line driver and method for increasing the power efficiency, signal accuracy, and stability of a transmit signal on a transmission line are disclosed. The improved line driver uses a negative feedback control loop, thereby enhancing operational stability and suppressing both amplifier imperfections and discrete component manufacturing variances. Furthermore, when the improved line driver output stage is integrated with a hybrid, the composite circuit provides a power efficient full duplex solution for line driver applications. In a preferred embodiment, the improved line driver may comprise an active line termination control loop with current sense feedback, a first amplifier, and a second amplifier. The present invention can also be viewed as providing a method for increasing the stability, power efficiency, and accuracy of a line driver in a duplex transmission system. In its broadest terms, the method can be described as: applying a transmit signal to an input stage of a line driver; amplifying the transmit signal; using an active termination feedback control loop to generate a feedback signal; amplifying the feedback signal; combining the feedback signal with a duplex signal on a transmission line to generate a scaled transmit signal; and combining the scaled transmit signal with the duplex signal to recover a remotely generated receive signal from the transmission line.

Claims

1. A line driver with back-matching impedance emulation having a transmission line termination control loop, comprising:

a power amplifier having a power amplifier transmit gain configured with a first input for receiving a transmit signal, a second input for receiving an amplified feedback signal, and an output configured to provide an amplified transmit signal to the transmission line;

a current sensing means electrically coupled to the power amplifier output and configured to generate a feedback input signal;

a feedback amplifier having a negative transresistance forward transfer function configured to receive the feedback input signal and provide an amplified feedback signal to the second input of the power amplifier;

a first amplifier configured to receive the amplified feedback signal and apply a first amplifier transmit gain equivalent to the inverse of the power amplifier transmit gain to provide a first amplifier output signal; and

a second amplifier configured to receive a first input comprising at least one signal from the transmission line and a second input comprising the first amplifier output signal, the second amplifier configured to mathematically combine the input signals to generate a scaled version of the transmit signal.

2. The line driver of claim 1, wherein the current sensing means comprises a resistor coupled in series with the amplified transmit signal.

3. The line driver of claim 1, wherein from the perspective of the transmission line, the transmission line termination control loop is the equivalent of an amplifier in series with an emulated back-matching resistor.

4. The line driver of claim 3, wherein the power amplifier transmit gain and the back-matching resistor have values that maintain a constant sign.

5. The line driver of claim 1, wherein the amplified transmit signal is suitable for driving a hybrid circuit.

6. The line driver of claim 3, wherein the emulated back-matching resistor has an equivalent resistance value less than the transmission line impedance.

7. The line driver of claim 3, wherein the resistance of the emulated back-matching resistor is a function of the transmit gain of both the power amplifier and the feedback amplifier.

8. The line driver of claim 7, wherein the transmit gain of the feedback amplifier is controllably adjusted to modify an output impedance as seen by a transmission line.

9. The line driver of claim 7, wherein the transmit gain of the feedback amplifier is controllably adjusted in response to scaling errors generated by the current sensing means.

10. The line driver of claim 1, wherein the scaled version of the transmit signal is applied at a voltage divider comprising a first filter configured to emulate the back-matching resistance and a second filter configured to emulate the transmission line impedance, wherein the first input of a hybrid amplifier is configured to receive the voltage at a node disposed between the first and second filters and wherein the at least one signal from the transmission line is applied at a second input of the hybrid amplifier, the hybrid amplifier configured to mathematically combine the scaled version of the transmit signal with the at least one signal from the transmission line to recover a receive signal.

11. The line driver of claim 10, wherein the scaled version of the transmit signal is subtracted by the hybrid amplifier in order to recover the receive signal.

12. The line driver of claim 10, wherein the scaled version of the transmit signal comprises transmit signal imperfections.

13. The line driver of claim 12, wherein the transmit signal imperfections comprise noise and distortion introduced by the power amplifier.

14. An improved line driver having an output load current, comprising:

means for amplifying a transmit signal; and

means for emulating a back-matching resistance using a negative feedback circuit configuration wherein the means for emulating a back-matching resistance is accomplished by amplifying a feedback signal responsive to the output load current such that the emulated back-matching resistance appears to have a resistance value equivalent to the product of the transmit signal amplification gain and the feedback signal amplification gain.

15. The line driver of claim 14, wherein the output load current sensing means comprises a resistor.

16. A signal transmission unit having a transmit signal amplification gain, comprising:

a line driver in cooperation with a negative feedback transmission line termination control loop having a feedback signal amplification gain that emulates a back-matching resistor having a resistance value equivalent to the product of the transmit signal amplification gain and the feedback signal amplification gain.

17. A communications system having a transmit signal amplification gain, comprising:

a transmission unit having a line driver, the line driver having a negative feedback transmission line termination control loop having a feedback signal amplification gain that emulates a back-matching resistor having a resistance value equivalent to the product of the transmit signal amplification gain and the feedback signal amplification gain.

18. A method for increasing the power efficiency, signal accuracy, and stability of a transmit signal on a transmission line, comprising:

applying a transmit signal to an input of a line driver;

amplifying the transmit signal;

applying the amplified transmit signal to a transmission line load to generate a load current;

sensing the load current;

applying the sensed load current in a negative feedback transmission line termination control loop having a feedback amplifier to generate an amplified feedback signal responsive to the load current such that an output impedance emulates a back-matching resistor wherein the transmit gain of the feedback amplifier is controllably adjusted in response to scaling errors associated with the sensed load current.

19. The method of claim 18, wherein the load current is sensed with a resistor having a resistance value less than the expected transmission line load impedance.

20. The method of claim 18, wherein the impedance of the emulated back-matching resistor is a function of the transmit gain of both a line driver power amplifier and a feedback amplifier.

21. The method of claim 20, wherein the transmit gain of the feedback amplifier is controllably adjusted to modify the output impedance as seen by a transmission line.

22. A method for recovering a remotely generated signal from a transmission line in a duplex communication system, comprising:

applying a transmit signal to an input stage of a line driver;

amplifying the transmit signal with a line driver amplifier;

using an active termination feedback control loop in cooperation with the line driver amplifier to generate a feedback signal responsive to a load current;

applying the amplified transmit signal on the transmission line;

amplifying the feedback signal to generate a line driver output impedance that emulates a back-matching resistor;

further amplifying the feedback signal with an inverted transmit gain substantially equivalent in magnitude with the gain of the line driver amplifier;

combining the further amplified feedback signal with a duplex signal on the transmission line to generate a scaled transmit signal; and

combining the scaled transmit signal with the duplex signal to recover a remotely generated receive signal from the transmission line.

23. The method of claim 22, wherein the active termination feedback control loop senses the load current with a resistor having a resistance value less than the transmission line load impedance.

24. The method of claim 23, wherein the impedance of the emulated back-matching resistor is a function of the transmit gain of both the line driver power amplifier and a feedback amplifier.

25. The method of claim 24, wherein the transmit gain of the feedback amplifier is controllably adjusted to modify the output impedance as seen by the transmission line.

26. The method of claim 24, wherein the transmit gain of the feedback amplifier is controllably adjusted in response to scaling errors associated with the sensed load current.