Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-11-04
2001-10-23
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S251000
Reexamination Certificate
active
06307431
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to low frequency/high performance amplifiers, commonly referred to as Class-D amplifiers and particularly, though not exclusively, to audio amplifiers.
BACKGROUND OF THE INVENTION
The efforts of consumer device manufacturers to reduce energy consumption, weight and size of heat dissipaters, for example in the field of automobile entertainment devices, have generated a demand for power amplifiers with a greater efficiency than Class-AB amplifiers. Class-D amplifiers have been proposed to respond to these demands. Substantially, Class-D amplifiers include a DC-AC converter circuit which produces a pulse width modulated (PWM) output signal. This pulse width modulated output signal drives power switches, which in turn switch a load which may implement a passive lowpass filter for reconstructing the amplified audio signal, of which the actual load (e.g. a speaker) may be a part. The analysis of the behavior of a single ended amplifier with an analog input and a PWM output (a Class-D amplifier) is described in the paper “Analysis of a quality Class-D amplifier”, F. A. Himmelstoss, et al., I.E.E.E. Transactions on Consumer Electronics, Vol. 42, No. 3, August 1996.
On the other hand, the increasing interest in digital audio signal processing, will make it more convenient to use digital amplifiers rather than analog amplifiers. Therefore, many manufacturers of audio equipment require Class-D power amplifiers capable of handling analog audio signals and digital audio signals. According to a common technique, to process an analog input signal requires the generation of a reference waveform, commonly a triangular or a saw-tooth signal, whose generation is relatively critical and at any rate requires a certain circuit complexity.
An example of a Class-D amplifier capable of selectively handling analog audio signals and digital audio signals, is described in U.S. Pat. No. 5,672,998. The amplifier uses a circuit for generating a PWM signal from an analog audio signal, separated from the feedback loop of the power amplifier itself. Even in this case, in order to generate a PWM signal, a triangular reference signal, produced by a dedicated generator, is employed.
SUMMARY OF THE INVENTION
An object of the invention is to provide a Class-D power amplifier capable of handling analog and/or digital input signals (PWM), without requiring generation of a reference (triangular) wave, according to a traditional analog/PWM conversion scheme, or any selfoscillating structures (such as of the so-called BOSE type).
According to an embodiment of the amplifier of the invention, which is simple to adapt to particular user conditions, an input network configurable through a pair of four-position switches, allows for the selection of the input signal, analog or digital (PWM), and a further selection between a PWM-standard digital input or a phase shift PWM digital input, as well as a selection between a standard PWM output or a phase shift PWM output. Of course, the configuration of the appropriate input network may be carried out in a permanent manner or even by the use of external networks without employing any selection switch. Even according to these alternative embodiments, the core of the amplifier of the invention remains the same.
The advantage of a phase shift PWM modulation, wherein the inverted signal is shifted by a half clock period in respect of the noninverted signal, is that the differential output carrier frequency of the PWM power amplifier is twice the clock frequency which is therefore more easily filtered. Moreover, under null modulation conditions, ideally, there are no residues of the carrier signal on the load terminals.
Basically, the amplifier of the invention is composed of two identical amplifying modules, one amplifying channel for the direct (noninverted) PWM signal and the other for the inverted PWM signal. Each module includes a switching output operational amplifier, having a first noninverting voltage input, a second inverting current input and a loop filter forming with the amplifier a single slope or a two-slope integrator. Each module also includes a logic inverter (or cascade of logic inverters) coupled in cascade to the output of the integrator, and an output power stage converting the logic level PWM signal output by the inverter or cascade of logic inverters to an analog signal switching between the potentials of the two supply rails. Also, each module includes a negative feedback resistor coupled between the output of the power stage and the inverting input of the operational amplifier.
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Botti Edoardo
Grosso Antonio
Masini Marco
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Choe Henry
Galanthay Theodore E.
Pascal Robert
STMicroelectronics S.r.l.
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