Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-06-18
2001-02-13
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S251000, C375S238000
Reexamination Certificate
active
06188276
ABSTRACT:
FIELD OF INVENTION
The invention relates to power amplifiers, and more particularly to a push-pull switching power amplifier and converter.
BACKGROUND OF THE INVENTION
In a conventional telephone system, a central office provides a ring signal to a telephone set to indicate an incoming call. In response to the ring signal, the telephone set produces intermittent, audible tones. A typical ring signal is powered by a −48 VDC central office power supply, and is delivered as a sequence of 90 Vrms, 20 Hz, signals across the two electrical connections, a “ring” and a “tip,” extending from a central office to a telephone handset. This local ring signal must be generated even where digital call signaling is used because telephone sets in a local loop may be designed to respond to the characteristic ring signal. Further, the ring signal is used by some telephone sets to directly drive an electro-mechanical ringing device, which places load requirements on the ring signal generator.
One approach to providing a ring signal is to employ a conventional class D switching power amplifier. These amplifiers are efficient; however, they typically require four switching transistors selectively driven by a pulse-width modulated error signal and two transformers. They are thus relatively large and expensive. Additionally, these amplifiers generally require significant output ripple filtering which adds to cost and complexity. The output filter prevents these amplifiers from passing direct current (DC) signals, which presents a further disadvantage where DC signals are desired.
More specialized circuits have been developed specifically for the generation of analog ring signals from digital inputs. For example, U.S. Pat. No. 5,555,301 describes a circuit for converting digital call signaling. However, this circuit is limited to a uni-polar, approximately square ring signal. Another ring signal generator is described in the data sheet for Lucent Technologies' L7590 Telephone Ringing Driver. This circuit receives as an input a 100 kHz pulse-width modulated signal, which is converted to a ring signal. The L7590 requires two power supplies and an external pulse-width modulator. Further, the maximum output excursion from ground is limited by the negative voltage supplied to the circuit.
There is thus a continuing need for a simple ring signal generator which operates from a single power supply and provides voltage gain and power amplification for an analog ring signal.
SUMMARY OF THE INVENTION
In view of the foregoing, the applicant has devised an improved ring signal generator However, as will be clear from the following description, the invention is more generally an efficient, linear power amplifier with broad potential applications.
The amplifier comprises two pulse-width modulators, two switches, and an inductive element, all of which cooperate to maintain an output voltage in fixed proportion to an input signal. An error signal is generated in a resistive divider formed between the input voltage and the output voltage. The error signal is compared to two fixed voltage references, and the differences are used by the pulse-width modulators to generate two pulse-width modulated signals. One pulse-width modulated signal is generated when the error signal is below one voltage reference, and it drives a switch to inductively increase the output voltage. The other pulse-width modulated signal is generated when the error signal is above another voltage reference, and it drives another switch to inductively decrease the output voltage. By continuously feeding back the output voltage to the pulse-width modulators in this manner, a simple amplifier is realized.
In one embodiment of the invention, the inductive element is a two-terminal inductor
In another embodiment, the inductive element is a power transformer having a primary and a secondary winding. One switch is connected to each winding of the transformer such that current pulses may be used to increase or decrease the voltage of the output.
In another aspect of this embodiment, the inductive element is used as an isolation barrier between an input power supply and the voltage output. This topology further employs isolation of the feedback error signal to completely isolate the input stage of the amplifier from the output stage of the amplifier. This embodiment can be further adapted to provide bi-directional control of the output voltage, that is, the control voltage may be provided from either side of the isolation barrier, or from both sides in combination.
The amplifier can be used as a DC/DC converter by supplying a DC input signal. The amplifier can also be used as a ring signal generator to drive a Ringer Equivalent Number load with large signal gain and a characteristic ring signal.
In another embodiment, one of the pulse-width modulators is a variable frequency pulse-width modulator.
REFERENCES:
patent: 4217531 (1980-08-01), Minalga
patent: 5218315 (1993-06-01), Turner
patent: 5311548 (1994-05-01), Nikolaus
patent: 5555301 (1996-09-01), Boike et al.
patent: 6038265 (2000-03-01), Pan et al.
Cuk, et al., “A Conceptually New High-Frequency Switched-Mode Power Amplifier Technique Eliminates Current Ripple” (1978).
Lucent Technologies, “L7590 Telephone Ringing Driver” Advance Data Sheet (Sep. 1997).
Motorola, “High Performance Dual Channel Current Mode Controllers” Motorola Analog IC Device Data (1996).
Cesari and McKenna
Nguyen Patricia T.
Pascal Robert
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