Amplifiers – With semiconductor amplifying device – Including particular power supply circuitry
Reexamination Certificate
2000-03-30
2001-11-27
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular power supply circuitry
C330S276000
Reexamination Certificate
active
06323733
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This is the first application filed for the present invention.
MICROFICHE APPENDIX
Not Applicable.
TECHNICAL FIELD
The present invention relates in general to power amplifier circuits, and in particular to a high efficiency power amplifier useable as a line driver within a modem for transmitting modulated analog signals.
BACKGROUND OF THE INVENTION
Most users of data communications services access data communications networks (e.g. the Internet) using dial-up connections established through the Public Switched Telephone Network (PSTN). The PSTN is still substantially an analog communications network, designed to transmit sounds in the audible range of the human voice.
Digital data is transported across the PSTN by converting the data into an analog signal that is transmitted by varying, or modulating, the frequency, phase, amplitude or other characteristic of a carrier signal. The analog signal is then transmitted over a standard telephone line (referred to as a “local loop”) using a power amplifier (e.g. a line driver), that operates to amplify the power of the analog signal.
For high speed data transmission, the use of multi-carrier (e.g., Discrete Multitone (DMT)) modulation schemes in Digital Subscriber Line (DSL) modems is well known. In DMT systems, an input bit stream is first serial-to-parallel converted. The parallel output is then grouped into N groups of bits corresponding to the number of bits per symbol. Portions of bits are allocated to each DMT carrier or sub-channel. The power transmitted over each sub-channel is preferably approximately the same.
In typical multi-carrier modulation systems, the modulation output approximates a normal distribution, with the result that the peak-to-Root Mean Squared (RMS) ratio of the output analog signal is relatively high. Because of this high ratio, the line driver must have high supply voltages (+Vc and −Vc) in order to adequately transmit the occasional high signal peaks without “clipping” or other distortion. However, such high supply voltages result in substantial power dissipation in the line driver. In fact, in a typical Digital Subscriber Line (DSL) modem, the efficiency of the line driver may be as low as 10%.
The efficiency of the line driver can be improved by dynamically changing the level of the supply voltages between a high level (+/−VH) and a low level (+/−VL) in response to changes in the level of the input analog signal. The goal of this strategy is to minimize the voltages supplied to the line diver when the level of the input signal is at a low (approximately RMS) level. Most of the time, therefore, the line driver will be supplied by the low level (+/−VL) voltage. When a signal peak must be transmitted, the supply voltages are switched to their respective high levels (+/−VH) to minimize clipping or other distortion of the signal peak.
U.S. Pat. No. 6,028,486, which issued to André on Feb. 22, 2000, teaches a system in which a dual supply amplifier is connected to a high voltage power supply and a low voltage power supply. A switch is provided to enable the dual supply amplifier to operate on the basis of a selected one of the high voltage and lower voltage power supplies. In one embodiment, a pair of amplifier circuits are provided, each of which is supplied by a respective one of the high and low voltage power supplies. The switch is then connected between respective outputs of each of the amplifier circuits and the output of the dual supply amplifier, and operates to connect the output of a selected one of the amplifier circuits to the dual supply amplifier output. In another embodiment, a single amplifier circuit is provided and the switch is connected between the high and low power supplies and the single amplifier. In this case, the switch operates to selectively connect one of the high and low power supplies to the amplifier.
In each of the above embodiments, the switch is controlled by a control signal which is generated on the basis of a comparison between the input signal level and a predetermined threshold. As a result, when the input analog signal is at a low (RMS) value, the control signal operates the switch so that the dual supply amplifier is powered by the low voltage power supply. However, when a signal peak arrives at the dual supply amplifier, the control signal operates the switch so that the dual supply amplifier is powered by the high voltage power supply.
A disadvantage of known systems, as exemplified by the system of U.S. Pat. No. 6,028,486, is that two supply rails (one each for positive and negative voltages) must be used to connect each power supply to the line driver. In a dual supply amplifier system, this means that four supply rails are needed. Typically, a line card (e.g. such as may be used at a central office) will be provided with multiple modems, each having at least one respective line driver. In order to minimize crosstalk between the modems, separate supply rails must be furnished for each line driver. Supplying four separate supply rails to each line driver consumes limited space on the line card, which in turn reduces the number of modems that can be located on a single card, and also tends to complicate the design of the card.
Accordingly, a dual supply line driver which can be conveniently fabricated on a line card having multiple modems remains highly desirable.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a dual supply power amplifier circuit which requires only two power supply rails.
Accordingly, an aspect of the invention provides a power amplifier for receiving an input analog signal and generating a corresponding output analog signal. The power amplifier includes first and second power supply rails, an amplifier circuit, and a s witch circuit. The first and second power supply rails are capable of supplying power having respective first and second predetermined levels, and a single predetermined polarity. The amplifier circuit is connected to receive the input analog signal and adapted to generate the output analog signal. Finally, the switch circuit is adapted to selectively connect one of the first and second power supply rails to the amplifier circuit.
A further aspect of the invention provides a modem including a modulator for generating a modulated analog signal; and a line driver responsive to the modulated analog signal to generate a transmission signal Tx for transmission over a communications network. The line driver is a power amplifier which includes first and second power supply rails, an amplifier circuit, and a switch circuit. The first and second power supply rails are capable of supplying power having respective first and second predetermined levels, and a single predetermined polarity. The amplifier circuit is connected to receive the input analog signal and adapted to generate the output analog signal. Finally, the switch circuit is adapted to selectively connect one of the first and second power supply rails to the amplifier circuit.
Preferably, the first predetermined level corresponds to a low voltage, and the second predetermined level corresponds to a high voltage.
The amplifier circuit preferably includes a push-pull circuit connected to receive the input analog signal; and an output transformer comprising a primary winding and a secondary winding. The primary winding is connected to the push-pull circuit, and selectively connectable to one of the first and second power supply rails via the switch circuit. The secondary winding is connected to the signal line for generating the output analog signal.
The push-pull circuit may comprise an input amplifier connected to receive the input analog signal, and a pair of output transistors. The input amplifier may include an inverting output and a non-inverting output for generating respective inverted and non-inverted analog signals. A first one of the output transistors may be connected to the non-inverting output of the input amplifier, and the other one of the outpu
Foster David W.
Gorcea Dan V.
Parker Kevin R
Daniels Kent
Mottola Steven J.
Nortel Networks Limited
Swabey Ogilvy Renault
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