Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2001-11-14
2003-07-01
Choe, Henry (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P
Reexamination Certificate
active
06586991
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an audio power amplifier for amplifying an audio signal to drive a speaker, and particularly to a digital power amplifier capable of amplifying an audio signal by driving a switching means with a signal modulated by a suitable modulation system such as a PWM (pulse width modulation) system.
2. Description of the Related Art
Heretofore, various power amplifiers for amplifying an audio signal to drive a speaker have become commercially available on the market. As one system of these power amplifiers, there have been developed a power amplifier called a digital power amplifier capable of amplifying an audio signal by directly switching a stabilized power-supply with a pulse signal modulated by an inputted digital audio signal. This digital power amplifier generates pulse-width-modulated (PWM) PWM wave (hereinafter this PWM wave will be referred to as a “PWM signal”) based upon the inputted digital audio signal, for example, and switches a stabilized DC (direct current) power-supply at a high speed by using this PWM signal to thereby obtain a speaker drive signal by extracting an audio signal component from the power-supply thus switched.
FIG. 1
of the accompanying drawings is a schematic block diagram showing an example of an arrangement of a digital power amplifier according to the related art.
FIG. 1
shows a circuit arrangement of a related-art digital power amplifier capable of switching a power switch by using a PWM signal. As shown in
FIG. 1
, this digital power amplifier generates a PWM signal, which is a pulse-width-modulated pulse signal, based upon an audio signal supplied to a PWM signal generating circuit
91
. A PWM signal outputted from the PWM signal generating circuit
91
is supplied to a switching module
92
which serves as a means for switching a power-supply.
As shown in
FIG. 1
, within the switching module
92
, the supplied PWM signal is supplied through a buffer amplifier
93
to the gate of a first switching element
95
. Further, the supplied PWM signal is supplied through an inverter gate
94
to the gate of a second switching element
96
. The first and second switching elements
95
and
96
are each formed of a MOS (metal oxide semiconductor) field-effect transistor in which the source-drain path is conducted under control of the PWM signal developed at the gate thereof.
The first and second switching elements
95
and
96
are formed as means for switching a DC power-supply output from the power-supply circuit
97
. Specifically, there is prepared the power-supply circuit
97
which generates a stabilized DC voltage by rectifying and smoothing a commercially-available power source. An output end of a positive (+) power source obtained from this power-supply circuit
97
is connected to the source of the first switching element
95
, and an output end of a negative (−) power source obtained from this power-supply circuit
97
is connected to the drain of the second switching element
96
. Then, the drain of the first switching element
95
and the source of the second switching element
96
are connected together to provide a junction
100
. A signal developed at this junction
100
is supplied to a low-pass filter (LPF)
98
as an output from the switching module
92
.
The low-pass filter
98
removes high-frequency components from the signals obtained at the switching of the first and second switching elements
95
and
96
to extract an audio signal component. The audio signal thus extracted is supplied to a speaker apparatus
99
as an output audio signal of an audio amplifier and thereby outputted from the speaker apparatus
99
to the outside.
The first and second switching elements
95
and
96
respectively function as switches which are driven in a push-pull circuit fashion. Specifically, the first and second switching elements
95
and
96
are each switching means in which the source-drain path conducts when a pulse waveform of a PWM signal supplied to the gate is held at high level and in which the source-drain path does not conduct when the above pulse waveform of the PWM signal is held at low level. The pulse waveform of the PWM signal supplied to the gate of the first switching element
95
and the pulse waveform of the PWM signal supplied to the second switching element
96
are in 180° phase relationship with each other.
Therefore, when the pulse waveform supplied to the gate of the first switching element
95
, for example, is held at high level, the pulse waveform supplied to the gate of the second switching element
96
goes to low level. As a consequence, any one of the positive power source and the negative power source is connected to and supplied to the side of the low-pass filter
98
connected to the junction
100
between the first and second switching elements
95
and
96
in response to the switching state obtained at that time. The switching is controlled as described above, whereby an audio signal waveform amplified by the voltage is outputted as the output of the low-pass filter
98
. At that very moment, the central level of the audio signal waveform is set to 0 V.
The processing for generating a PWM signal, which is a pulse-width-modulated signal, from the audio signal is executed based upon a principle shown in
FIGS. 2A
to
2
C, for example. When there is an analog audio signal having a sine wave shown in
FIG. 2A
, this analog audio signal is converted into digital data of one bit system. According to this embodiment, the analog audio signal is converted into digital data of one bit system, shown in
FIG. 2B
, in which the levels of the signal waveforms are expressed in the form of a pulse waveform density in a so-called DSD (direct stream digital) fashion. Then, based upon the digital data of one bit system, there is executed a processing for generating a PWM signal which is pulse-width-modulated as shown in FIG.
2
C. Then, the switching means which switches the power-supply is turned on and off under control of the PWM signal thus generated with the result that there can be generated a waveform equal to the waveform of the analog audio signal amplified by the power-supply voltage.
When the power amplifier has the arrangement in which the two switching elements
95
and
96
are driven in a push-pull fashion as shown in
FIG. 1
, the inverted signal of the PWM signal which drives one switching element is generated by using the inverter gate
94
within the switching module
92
. However, when the inverted signal is generated by using the inverter gate as described above, a very small difference occurs between a timing of the control signal supplied to the gate of one switching element and a timing of the control signal supplied to the gate of the other switching element. As a consequence, there arises a problem that noise will be generated from the power-supply due to the above difference between the timings.
FIGS. 3A through 3F
are diagrams of waveforms of respective signals and respective differential components obtained when the two switching elements
95
and
96
are driven in a push-pull fashion in the circuit arrangement shown in FIG.
1
. For example, when the PWM signal supplied from the PWM signal generating circuit
91
to the switching module
92
has a waveform shown in
FIG. 3A
, this pulse waveform is supplied to and amplified by the buffer amplifier
93
so that this waveform is slightly delayed from the waveform of the inputted PWM signal as shown in FIG.
3
B. Further, the waveform of the inverted pulse which results from inverting the PWM signal inputted to the switching module
92
by the inverter gate
94
becomes a waveform which is slightly delayed from the inputted PWM waveform in timing as shown in FIG.
3
C.
Since the buffer amplifier
93
and the inverter gate
94
are the circuits whose characteristics are different from a principle standpoint, a time difference occurs in the timings between the leading edge and the trailing edge of the control signal waveform (see
FIG.
Masuda Toshihiko
Ohkuri Kazunobu
Choe Henry
Maioli Jay H.
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