Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
1999-11-10
2001-06-19
Shingleton, Michael B (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P, C330S251000
Reexamination Certificate
active
06249182
ABSTRACT:
BACKGROUND OF INVENTION
Class D amplifiers use a low pass filter before the load (loudspeaker) to convert the amplified digital signal into an audio signal. The load may include speakers with impedance ranging from 2-16 &OHgr;. A typical low pass filter of a class D amplifier is strongly dependent upon load impedance. An example of the typical low pass filter is shown in
FIGS. 1 and 4
. It includes the inductor L and the capacitor C
LP
. The low pass filter has a Laplace transfer function as follows:
H
(
s
)
=
1
s
2
L
C
+
L
s
R
+
1
In conventional Butterworth, Bessel and linear phase filters, the terms s
2
LC and Ls/R are on the same order of magnitude. Thus, the transfer function depends strongly on the load impedance. The first step for creating a transfer function that is independent of load impedance is to make sure that the term s
2
LC>>Ls/R. Unfortunately, this approach results in a high quality (Q) factor filter which exhibits peaking at a resonant frequency
FRES
=
1
2
π
f
C
.
as shown in FIG.
2
. Such peaking is unacceptable in audio amplifiers where the gain must be flat throughout the audio bandwidth.
One prior art solution to the problem places an impedance balancing filter, also known as a Zobel filter, at the output. An example of a Zobel filter is shown in FIG.
3
A. Its frequency response is shown in FIG.
3
B. Although the Zobel filter reduces the Q of the low pass output filter and results in less peaking, it is costly and inefficient because the resistor in the Zobel network dissipates a significant fraction of the carrier as heat. This dissipation increases the cost of the Zobel network because the components must be chosen to handle this power. Since the Zobel network has a low impedance at ultrasonic frequencies, it limits the power bandwidth of the class D amplifier. If the input signal is not band limited, the Zobel may overheat and fail. The amplifier may still function, but without the Zobel, the peaking at high frequency will be audible.
SUMMARY
This invention solves the functional and economic problems of the prior art by providing a filter that places a pole-zero combination in the feedback loop of the amplifier to effectively compensate for the high Q output filter. In the Laplace transform of a system, zeros are the roots of the numerator of the transfer function and poles are roots of the denominator. In the preferred embodiment, the feedback network comprises two resistors and a capacitor in parallel with one of the resistors. The feedback network reduces the gain of the amplifier to compensate for the high Q of the output filter.
The invention provides a class D amplifier that has a bandwidth that is almost independent of the load impedance. The amplifier includes a pulse width modulator that comprises an integrator with feedback from the output of the amplifier and a comparator coupled to the output of the integrator for converting an input analog audio signal into a digital audio signal. The output of the amplifier is coupled to a bridge driver circuit which in turn controls the gate terminals of the mosfets of a half bridge. The half bridge output is connected to a low pass high Q output filter. The output filter is connected to a load impedance such as a speaker. This load may vary between 2-16 ohms. A feedback compensation network for reducing the Q of the output filter comprises two resistors with a capacitor in parallel with one of the resistors.
REFERENCES:
patent: 4458208 (1984-07-01), Abe
patent: 5410592 (1995-04-01), Wagner et al.
patent: 5805020 (1998-09-01), Danz et al.
patent: 5949282 (1999-09-01), Nguyen et al.
patent: 6107875 (2000-08-01), Pullen et al.
patent: 1575291 (1990-06-01), None
Intersil Corporation
Jaeckle Fleischmann & Mugel LLP
Shingleton Michael B
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