Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2002-07-18
2003-11-25
Callahan, Timothy P. (Department: 2816)
Amplifiers
Modulator-demodulator-type amplifier
C330S197000
Reexamination Certificate
active
06653897
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power amplifier apparatus.
2. Description of the Related Art
In some audio power amplifier apparatuses, power amplification is performed by switching. Such a switching-type power amplifier apparatus is configured, for example, as shown in FIG.
1
.
In
FIG. 1
, a broadcast wave, such as a frequency modulation (FM) broadcast wave, is received by an antenna
11
and is supplied to a receiving circuit
12
where an analog audio signal of a target broadcast is extracted. The analog audio signal is then supplied to a switching-type power amplifier apparatus
20
.
In the power amplifier apparatus
20
, the audio signal output from the receiving circuit
12
is supplied to a converter circuit
21
, where it is converted into a digital audio signal. Simultaneously, the digital audio signal is converted into a pulse modulation signal, for example, a pulse width modulation (PWM) signal P
21
, in which the integrated value of one sample is equivalent to the level of the original analog audio signal. The carrier frequency of the PWM signal P
21
is, in general, from a few hundred of kilohertz to a few megahertz.
The PWM signal P
21
is then supplied to a drive amplifier
22
where it is amplified to a pair of PWM signals +P
22
and −P
22
which are 180° out of phase with each other, and the PWM signals +P
22
and −P
22
are supplied to power amplifiers
23
A and
23
B, respectively. The power amplifiers
23
A and
23
B are class D amplifiers, and the PWM signals +P
22
and −P
22
switch a predetermined DC voltage, thereby amplifying the power of the PWM signals +P
22
and −P
22
.
The output terminals of the power amplifiers
23
A and
23
B are connected to the corresponding ends of a speaker
14
via coils
24
A and
24
B, respectively, and a filter
25
, which is described below. In this case, a capacitor
24
C is connected to the output terminals of the coils
24
A and
24
B. With this configuration, the coils
24
A and
24
B, and the capacitor
24
C form a low-pass filter
24
.
Accordingly, the power amplifiers
23
A and
23
B operate as a single balanced-transformerless (BTL)-type amplifier with respect to the speaker
14
. Then, amplified PWM signals +P
23
and −P
23
are output from the power amplifiers
23
A and
23
B, respectively, and are integrated by the low-pass filter
24
into the original analog audio signal. The analog audio signal is then supplied to the speaker
14
via the filter
25
, which is formed of, for example, a choke coil.
According to the above-configured switching-type power amplifier apparatus
20
, the power amplifiers
23
A and
23
B perform power amplification by turning on or off the voltage and the current in accordance with the PWM signals +P
22
and −P
22
, thereby obtaining a high-power output with high efficiency.
The PWM signals +P
23
and −P
23
are pulses obtained by PWM-modulating the audio signal. Accordingly, the presence of many spectra can be observed, as shown in
FIG. 2
, at the frequency positions of the fundamental wave and the higher harmonics, and the spectral components are distributed over a wide frequency range from a few hundred kilohertz to a few hundred megahertz. In the example shown in
FIG. 2
, the carrier frequency of the PWM signals +P
23
and −P
23
is 240 kHz. Additionally, the PWM signals +P
23
and −P
23
are amplified by the power amplifiers
23
A and
23
B, respectively.
As a result, part of the higher-harmonic components of the PWM signals +P
23
and −P
23
interferes with the reception of amplitude modulation (AM) broadcasts, FM broadcasts, or television broadcasts, and in some cases, a broadcast of a desired frequency cannot be received. Such higher-harmonic components may adversely influence peripheral electronic circuits or devices.
Accordingly, the low-pass filter
24
is provided for eliminating the interfering noise components (higher-harmonic components). That is, among the noise components contained in the PWM signals +
23
P and −
23
P, noise components which are 180° out of phase with each other (normal mode noise) cancel each other out.
However, the noise components contained in the PWM signals +P
23
and −P
23
also include in-phase noise components (common mode noise), which cannot be canceled out in the low-pass filter
24
, and are disadvantageously output from the low-pass filter
24
. Thus, the filter
25
is provided for the power amplifier apparatus
20
shown in
FIG. 1
for eliminating the common mode noise.
Accordingly, in this power amplifier apparatus
20
, two filters, such as the filters
24
and
25
, are required for eliminating the 180°-out-of-phase noise components and the in-phase noise components, thereby increasing the cost.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to solve the above-described problem.
In order to achieve the above object, the present invention provides a power amplifier apparatus including:
a pair of power amplifiers, which form a BTL-type amplifier, for performing switching-type power amplification; a transformer including a pair of coils connected in series between the output terminals of the pair of power amplifiers and the corresponding ends of a speaker; and a capacitor connected in parallel with the output terminals of the pair of coils. The pair of coils include a mutual inductance and a leakage inductance.
With this configuration, the 180°-out-of-phase and in-phase noise components contained in the output of the BTL-type power amplifier can be eliminated by the mutual inductance and the leakage inductance simply by using one transformer and one capacitor. This is effective for space saving, and the cost can be reduced. The number of assembly steps can also be decreased.
REFERENCES:
patent: 4447791 (1984-05-01), Noro et al.
patent: 5115205 (1992-05-01), Holmes, Jr.
patent: 5973569 (1999-10-01), Nguyen
Callahan Timothy P.
Maioli Jay H.
Nguyen Linh M
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