Amplifiers – With semiconductor amplifying device – Including class d amplifier
Reexamination Certificate
1999-09-01
2002-03-12
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including class d amplifier
C330S010000
Reexamination Certificate
active
06356151
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to class D power amplifiers and more particularly relates to class D power amplifiers for use in low power applications such as in battery powered systems where thermal dissipation is a factor.
2. Description of the Prior Art
Often, audiophile applications use class A or class AB amplifiers to minimize distortion, notwithstanding the high inefficiency of class A and class AB amplifiers. Thus, such amplifiers consume a relatively large amount of power for the amount of power provided to the output. Typically, such amplifiers will dissipate heat energy at least equal to the amount of energy being provided to the load. Thus, cooling of such class A or class AB amplifiers is often an important consideration and requires large (and generally heavy) heat sinks and/or cooling fans. Nonetheless where AC power is available and where weight and size are not crucial factors, high fidelity audio amplifiers use such class A and class AB amplifiers for audio amplification.
Due to their high fidelity, such class A and class AB amplifiers are also used in conventional portable applications such as portable compact disc (CD) players, portable tape players and notebook and subnotebook computers. While such class A and class AB amplifiers will provide output signals with relatively high fidelity, such class A and class AB amplifiers can provide only limited output power. Otherwise, due to their inefficiency, they draw too much power for long term battery operation. Still further, such class A and class AB amplifiers dissipate too much power as heat at high power applications. Heat dissipation in many portable applications such as portable CD players and portable computers creates severe problems in such small units. Therefore, in such portable applications, the power amplifiers are typically limited to less than one-half a watt of output power. As a result, any sound produced by speakers directly driven by such amplifiers will have a relatively low loudness.
However, there has been a recent trend towards using portable computers for multimedia presentations. Since the audio output of the computer is limited due to the restrictions on the power output of the amplifiers, such computers can generally only be used for presentations to a few people. Alternatively, separate powered speakers can be used, but such speakers are bulky and heavy. Therefore, they are often undesirable, not only for laptop and other portable computers, but for desktop and other nonportable computers.
While it might appear that a class D amplifier might be used in lieu of a class A or a class AB amplifier, such class D amplifiers suffer from a number of drawbacks that have limited their utility in audio applications.
FIG. 1
shows a typical class D amplifier
10
. The input signal
12
, which may be an audio signal, is provided to the non-inverting input of a comparator
14
. The inverting input of the comparator
14
is coupled to a triangle wave generator
13
that generates a triangle wave symmetric to ground. The output of the class ED amplifier is used to drive a switching bridge circuit such as the bridge circuit
16
shown in
FIG. 2
where a first transistor pair Q
1
and Q
4
are controlled by a driver circuit
15
to conduct simultaneously and a second, complementary transistor pair Q
2
and Q
3
are controlled to conduct simultaneously. When transistor pair Q
1
and Q
4
conduct, current from the power supply voltage flows through the loudspeaker
18
in a first direction and when transistor pair Q
2
and Q
3
conduct, the current through the loudspeaker reverses. Further the transistor pairs are complementary in that when transistor pair Q
1
and Q
4
conduct, transistor pair Q
2
and Q
3
do not conduct and vice versa. Thus, the two transistor pairs comprise a full bridge switching circuit. A filter circuit
19
comprised of inductors and capacitors (not shown) serves to transform the current into a substantially sinusoidal signal that varies with the input audio signal, and thereby substantially reproduces the input signal, theoretically.
However, in practice, there are a number of problems associated with such prior art class D amplifiers that prevent the class D amplifier from accurately reproducing the input waveform except for low fidelity applications. In particular, as a practical matter, it is difficult to generate a symmetrical and linear triangle wave. Any lack of symmetry or non-linearity in the triangle wave's waveshape introduces distortion into the output signal. Further, the comparators
14
are subject to switching transients at about the crossover points for the amplitude of the input signal versus the triangle wave. For example, noise and other fluctuations on the signal may cause the output of the comparator to switch back to the prior state temporarily, injecting noise into the output signal.
Still further, the class D amplifier is also subject to power supply perturbations. As the power supply voltage varies, the gain of the amplifier varies. This causes potential frequency instabilities in the performance of the class D amplifier such as in the filters necessary to reproduce audio.
As a result, class D amplifiers are typically not used in applications where good fidelity is demanded such as in portable compact disc players or on notebook computers used for multimedia presentations where total harmonic distortion should be preferably less than 1%. Rather, class D amplifiers are typically used in other applications such as hearing aids where fidelity is not a concern or for driving woofers or subwoofers where the low frequencies mean that typical class D amplifiers will have better performance.
A second prior art audio class D amplifier is shown in
FIGS. 2A
,
2
B, and
2
C which is described partially in each of Harris Corporation Application Note No. AN9525.2 dated March 1996 for the Harris Class D Audio II Evaluation Board (HIP4080A EVAL2) and the Application Note AN9404.1 dated March 1995.
FIG. 2A
shows an audio input
51
that is coupled through an analog summing network
52
to provide an input signal to the pulse width modulation (PWM) comparator
72
(FIG.
2
C). The summing network
52
level shifts the input signal to be centered about 6 volts, and sums the audio input signal with the feedback current and a current limit set to limit the output drive current through the bridge
62
in (FIG.
2
B). In addition, a symmetrical triangle wave signal is applied to the +input of the PWM comparator
72
to provide pulse width modulation of a symmetric triangle wave. Referring to
FIG. 2C
, the PWM comparator
72
is applied to delay circuitry
63
a
and
63
b
that controls the switching of the bridge transistors so that when transistors Q
2
and Q
5
are conducting, transistors Q
3
and Q
4
are not conducting and vice versa. The level shift circuitry
64
is used for shifting the signaling voltages for controlling the high MOSFET's Q
2
and Q
4
and further “latching” circuitry is used to keep the high transistors Q
2
and Q
4
conducting even though the output pulse from the level translation circuitry is short for purposes of power reduction.
However, circuits such as those described above, have a number of disadvantages. First, in portable applications such as laptop computers, AC and DC power supply variations cause significant problems in the stability of class D amplifiers. For example, in typical class D amplifiers, the class D amplifier is subject to gain variations as the power supply varies. As the power applied to the sources of the high transistors increases such as transistors Q
1
and Q
3
above, the gain of the amplifier increases. This can also cause frequency instability in the output filter.
Therefore, the usage of class D audio amplifiers in battery applications has generally been limited to cases where the battery output is stable such as in automobile applications. Further, such class D amplifiers are typically used for high power subwoofers for automobile stereos rather th
Knobbe Martens Olson & Bear LLP
Linfinity Microelectronics
Nguyen Khanh V.
Pascal Robert
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