Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-10-09
2003-05-13
Mottola, Steven J. (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S059000, C330S109000, C330S20700P
Reexamination Certificate
active
06563377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to Class D switching audio amplifiers. More particularly, the present invention relates to a Class D switching audio amplifier making use of four state modulation, input-to-output drive and feedback signal isolation, a dual topology output filter, and a low inductance board layout.
2. Description of the Prior Art
It is often desirable to amplify audio signals using a Class D switching audio amplifier. Basic circuit layout of the Class D amplifier is substantially similar to that of linear amplifiers, such as Class A, B, and AB, with a major difference being in the signals provided to an output stage. Rather than feeding an audio waveform directly to the output stage, as is done in linear amplifiers, the Class D amplifier first feeds the audio waveform into a Pulse Width Modulator (PWM) circuit which feeds modulated pulses to the output stage. By quickly switching the output stage completely on and completely off with varying pulse widths, the Class D amplifier is able to recreate waveforms of almost any shape, and, by filtering the switching output, sound is produced by a loudspeaker connected thereto. In practice, the pulses are fed to the output stages at a frequency between 100 and 300 kHz, or 100 to 300 thousand pulses per second, which is required to produce a smooth waveform at the loudspeaker.
An advantage of the Class D amplifier is that the output stage transistors are switched either completely on or completely off. Amplifier topologies that operate in a partially on state, such as Class A and AB, act like resistors and produce heat, thereby wasting energy. Thus, Class D amplifiers are substantially more efficient than non-switching linear amplifiers. Higher efficiency and less waste heat allows the Class D amplifier to utilize a smaller power supply and to be offered in a more compact package than a comparable linear amplifier.
Unfortunately, existing Class D amplifier designs suffer several disadvantages, including disadvantages related to modulation, isolation, feedback, and board layout. Existing Class D amplifier designs incorporate a full H-bridge output stage and use a single PWM signal to derive four FET gate drive signals providing two H-bridge switch states. Both H-bridge switch states result in a differential voltage across the outputs leading to current flow through the load. These two-state Class D amplifiers typically compare a reference triangle waveform to an audio error waveform (audio feedback) using a single comparator. The output of the comparator is a single PWM signal with the same frequency as the reference triangle waveform. The PWM signal is then passed through a logic circuit that generates four drive signals used to drive the H-bridge, resulting in a 180° phase difference between output
NEG
and output
POS
. Thus, a differential voltage is always present at the output causing power to be lost via the loudspeaker or low pass filter even in the absence of an audio input to the amplifier.
Existing Class D amplifiers typically require large power transformers to accommodate a relatively inefficient output stage and to meet government regulations requiring high voltage isolation between AC mains and all user-accessible inputs and outputs. This isolation is typically achieved by incorporating one or more power transformers between the AC mains and the input and output stages. Unfortunately, such power transformers are large and expensive. Furthermore, because 99% of any incoming power is required to drive the output stage and the loudspeakers connected thereto, a power transformer isolating the output stage must be substantially larger than a power transformer isolating the input stage.
Even in applications where the outputs are not user-accessible, no effort is typically made to isolate the input stage from the output stage. Where input-to-output isolation is attempted, small-signal audio transformers are typically used. Unfortunately, these transformers suffer from limited frequency response, making implementation difficult.
Typical output efficiencies for prior art linear amplifiers are approximately 60%, with the remaining 40% of supplied power being dissipated as heat. Consequently, expensive heat-sinking is required, and large, expensive power transformers are needed to deliver 66% more power than the desired output power of the amplifier. With the development of Class D amplifiers, output efficiencies increased to 85%, thereby reducing power supply requirements and waste heat. Unfortunately, expected theoretical efficiencies of 90+% for the Class D amplifier have not been achieved, due primarily to the many problems and disadvantages set forth herein.
Existing high-power Class-D amplifier designs incorporate a control or feedback loop to minimize distortion. Conventional control theory requires filtering, attenuating, and summing the output signal with the input signal. This typically involves a feedback loop comprising a differential RC low pass filter, followed by an attenuating differential amplifier, and then a summing amplifier to combine the feedback signal with the input signal. For high power applications where common-mode voltages can exceed 70 Vdc, precision matching of feedback resistors is a critical concern. Resistor tolerances greater than 1% in the differential amplifier and the RC low pass filter sections result in reduced common-mode rejection, potentially damaging voltages at the differential amplifier, and degraded product reliability. The RC low pass filter is required to attenuate the PWM switching energy and to pass the audio signal to the differential amplifier. This can result in decreased efficiency as power is lost in the RC low pass filter even in the absence of an audio input signal. High power applications require the use of high power resistors (>1 W) that can effectively dissipate the switching energy. Unfortunately, precision matching and increased power handling requirements for the RC low pass filter resistors result in increased cost and size. For example, surface mount 1 W 1% resistors are 7.5 times larger and 18 times more expensive than standard ¼ W 5% surface mount resistors.
Existing Class-D amplifier designs incorporate pairs of multi-pole differential LC low pass filters to filter the ever-present differential switching output voltage. Typical multi-pole differential LC filter designs dissipate a majority of attenuated energy in the first LC low pass filter pair. No advantage is gained from common-mode filtering because the output of the H-bridge continues to be a differential voltage. As a result, high power designs are required to incorporate expensive high power inductors that can dissipate the switching energy even when no audio input signal is present.
Existing Class D amplifiers typically exhibit high harmonic distortion above 1 kHz as a result of pulse transient damping issues and poor triangle waveform damping generation. Excessive pulse undershoot and overshoot result from high inductance board layouts and power supplies. Some existing designs attempt to reduce pulse overshoot and undershoot on H-bridge outputs by incorporating large, expensive RC snubbers. Such undershoot and overshoot can degrade reliability for many standard FET driver ICs such as Harris' HIP4080A. Additionally, pulse transient damping issues also lead to increased EMI emissions that increase the cost of shielding the amplifier.
Triangle waveform generation has always been a source of distortion in Class D amplifier designs. Triangle waves are typically generated using RC oscillators made of operational amplifiers or logic gates. These Class D amplifier designs suffer from high frequency noise superimposed on the triangle waveform; in turn, the high frequency noise results in increased harmonic distortion. Thus, existing Class D amplifiers typically exhibit undesirable harmonic distortion much greater than 0.5%.
Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved
Evenstar, Inc.
Hovey & Williams, LLP
Mottola Steven J.
LandOfFree
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