Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-06-24
2001-10-02
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P
Reexamination Certificate
active
06297692
ABSTRACT:
TECHNICAL FIELD
This invention relates to a power amplifier for the audio frequency range, comprising a pulse modulator, a power amplifier stage for amplifying the modulated signal, the output of which is low pass filtered in a demodulation filter for obtaining an analog output to feed to a consumer.
1. Background
Nearly all commercially available power amplifiers for frequencies in the above range are of the linear analog type: class A, AB, and B. Since the output transistors of such amplifiers operate in the linear region, they have a low efficiency and dissipate a considerable amount of heat. The basic pulse modulation (“digital”) switching class D power amplifier method in theory provides a much higher efficiency, which lowers the amplifier volume and heat development. Despite this efficiency advantage, prior art in the field has not provided solutions with an acceptable audio quality, that could make them generally useful and direct replacements for analog amplifiers. Accordingly, the use of digital power amplification has been limited to applications where the demands for output quality are low. The reasons for this will be presented in the following, when the general prior art principles are discussed.
2. Prior Art
There is a fundamental requirement for efficient control systems to eliminate errors generated in the various blocks of the digital switching power amplifier. In the basic digital power amplifier the input signal is modulated into a pulse modulated signal. A switching power stage performs amplification of the signal, and a low pass filter regenrates the modulated but now amplified signal. The basic method form the basis for a range of prior art arrangements. However, there are numerous non-ideal features with this method:
Any modulator errors are fed directly to the load.
Any power stage errors are fed directly to the load.
There is no rejection of power supply perturbations. Since the power stage output is largely proportional to the supply voltage, any supply ripple will intermodulate with the audio signal.
The post filter errors will introduce further distortion, since magnetic core materials are not ideal.
The total output impedance is high, especially at high frequencies, due to the filter.
The sensitivity to load variations is high due- to the passive post filter. Accordingly, changes in load impedance will distort the frequency response of the amplifier.
The sensitivity to temperature drift, component tolerances, and aging effects is high. The non-controlled digital switching amplifier is therefore not robust and reliable.
Compensation for these major problems areas is absolutely vital, if very high fidelity is to be obtained. Prior art methods are based on two different control methods, characterized by having a single feedback loop.
One basic principle is to feed back the amplifier output. However, the poles of the low pass filter cause a large phase shift which puts strong restrictions on loop design. Consequently, reasonable loop bandwidths requires high switching frequencies. This causes several problems, such as lower efficiency, and poor performance due to the first two errors mentioned above.
Another basic principle of feedback in prior art, is single loop feedback before the filter network, whereby the phase lag of the post filter is avoided. However, the high frequency content at the power stage output makes the feedback source potentially very noisy. Furthermore, a number of the above errors are not (or only partially) compensated
The fundamental problem of both methods are the conflicting desires for a low carrier frequency, high gain-bandwidth product and good stability characteristics in all situations. A further problem with prior art arrangements is the apparent complexity of the system, partially caused by inefficient control.
The following publications are relevant as background material and illustration of the methods and the problematic issues of three prior art arrangements:
[1] Suzuki, T.: Pulse Width Modulated Signal Amplifier. U.S. Pat. No. 4021745 (1977)
[2] Yokoyama, K.: Pulse-Width Modulation Circuit. U.S. Pat. No. 4531096 (1986)
[3] Attwood, B. E.: Design Parameters Important for the Optimization of Very High-Fidelity PWM (Class D) Audio Amplifiers , Journal of the AES, Nov. 1983. p. 842-853.
[4] Taylor, W. E. Digital Audio Amplifier. U.S. Pat. No. 4724396 (1988).
[5] Hancook, J.: A class D Amplifier Using MosFET's with Reduced MinorityCarrier Lifetime , 89
th
Convention of the AES. Los Angeles. Calif. Sep. 21-25. 1991.
[6] Solomon, E. E : Digital Power Amplifier. U.S. Pat. No. 5126684 (1992).
[7] McCorkle, D. P. Class D amplifier. European Patent. Publ. No. 557032A2 (1993).
[8] Nakajima, Y. Pulse-Width Modulation amplifier. European Patent. Publ. No.503571A1 (1993).
[9] Leigh, S. P et al. Distortion analysis and reduction in a completely digital PWM class D power amplifier, International Journal of Modeling & Simulation, Vol. 14, No. 2, 1994.
OBJECTIVES
According to the above stated problems that exist with prior art arrangements, the primary objective of the present invention is to provide a pulse modulation amplifier which can deliver very high power outputs, and still provide ultra low distortion (less than 0.01% and noise (less than 100 &mgr;V RMS), and yet a very high efficiency (90-95%) and low idle losses.
Another objective of the invention is to maintain low complexity by avoiding the use of advanced but complex and hence potentially unreliable circuitry, and also to eliminate the requirement for tuning in production.
Another important objective of the invention is to eliminate the need for a stabilized supply, meaning that a simple non-regulated bridge rectifier with a stabilizing capacitor is sufficient. In this way, a minimal complexity and maximal efficiency is secured from the mains input to the amplifier output terminals.
The final objective of the invention is to obtain minimal sensitivity to load variations, and furthermore to provide robustness and reliability.
SUMMARY OF TEE INVENTION
The above objectives are obtained with the present invention. In the first preferred embodiment, an amplifier according to the invention is particular in that negative feedback is introduced from the switching power stage output to one or several loops feeding into one or several pre-amplifier stages preceding the modulator. This offers a range of advantages that are new to the art, in terms of performance and stability control. A further embodiment of the invention is particular in that the local feedback has a enhanced cascaded structure with a single feedback path, that comprises a filter with a phase characteristic such that a pole in the demodulation filter is compensated.
Another embodiment of the invention is particular in that further feedback is established from the output of the demodulation filter to one or several pre-amplifier stages, so that the pulse modulation (“digital”) switching power amplifier circuit elements are enclosed by an enhanced cascade structure of feedback loops, providing further improved performance and stability control.
A further embodiment of the invention is particular in that the pulse modulator-is a controlled self-oscillating modulator comprising a non-hysteresis comparator for pulse modulation, and a higher order oscillating loop realized by means of two poles, preferably a pole in the first (local) forward path and feedback path. This provides en extremely simple and stable configuration, obviating the need for a separate carrier frequency generator.
A further embodiment of the invention is particular in that the pulse modulator is a carrier based modulator. This means that well-known design techniques for carrier based modulators may be used in the configuration according to the invention.
A further advantageous embodiment of the invention when carrier based modulation is used, is particular in that a notch filter is provided in the single feedback path block between the amplifier and the
Bang & Olufsen A/S
Nguyen Khanh Van
Nixon & Peabody LLP
Pascal Robert
Safran David S.
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