JP2018088575A - Class D power amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high quality class D audio power amplifier with simpler circuitry at a low cost, by bringing phase lag in the high range of audio frequency band.SOLUTION: A power amplifier performing power amplification of an audio input signal inputted to a signal input end includes a class D power amplifier performing power amplification of the audio input signal before being outputted, an LC low-pass filter for demodulating and outputting a pulse width modulation signal outputted from the class D power amplifier, and a first feedback circuit for feeding the output from the LC low-pass filter back to the input of the class D power amplifier. The class D power amplifier includes a preamplifier for inputting the audio input signal, a switching circuit performing class D operation with the output signal from the preamplifier, and a second feedback circuit from the output of the preamplifier to the input of the preamplifier, for bringing phase lag in the high range of audio frequency band close to zero.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a self-excited oscillation type class D power amplifier that amplifies an electric signal in an audio frequency band (20 to 20 kHz), and a power amplifying apparatus using the power amplifier.

In order to reduce the amplification error and obtain a high-quality audio output, a system configuration that feeds back the output is widespread and generalized. Looking at the details, there are two ways to construct a feedback system.
(Method 1)
As shown in FIG. 7, the method 1 inputs the audio input signal 11 to the class D power amplifier 13, a class D power amplifier 13 amplifies the class D power, and outputs a resistor 15 (Rf) from the exit of the class D power amplifier 13. Thus, a feedback is provided to the class D power amplifier 13. The audio signal amplified by class D by the class D power amplifier 13 is input to a low-pass filter including an inductor 16 and a capacitor 17, and is output as an audio output signal 12 from the speaker ZL through the low-pass filter.
(Method 2)
As shown in FIG. 8, the method 2 inputs an audio input signal 21 to a class D power amplifier 23, a class D power amplifier 23 amplifies the class D power, and outputs the output of the class D power amplifier 23 to an inductor 26 and a capacitor. 27 is input to the low-pass filter composed of 27 and fed back from the output of the low-pass filter. At the exit of the low-pass filter, that is, the point of the speaker ZL as a load, there is a feedback effect, and the characteristics are improved to some extent. See US Pat.

JP-A-61-21007

  Both of the above two methods are generalized, but there are insufficient points for high quality audio characteristics. It is the phase characteristic of the high frequency part of the audio frequency band.

(Method 1)
FIG. 9 is an example of the gain / phase frequency characteristics of the class D power amplifier shown in FIG. At the entrance point of the low-pass filter, there is a sufficient feedback effect and the characteristics are improved. However, since the signal is supplied to the load speaker ZL via the low-pass filter, the influence of the characteristics of the low-pass filter cannot be avoided.

  The cut-off frequency of the low-pass filter is set to a frequency of about 1/10 (30 kHz to 50 kHz) of the switching frequency (about 400 kHz) in order to sufficiently attenuate the high-frequency switching waveform of the class D power amplifier. As a result, the gain frequency characteristic can be substantially flat over the entire audio frequency. However, in the phase characteristics, the delay starts to be noticeable from several kHz, and at 20 kHz, the delay is 35 degrees or more. This is not sufficient to obtain a high-quality audio signal waveform.

(Method 2)
FIG. 10 is an example of the gain / phase frequency characteristics of the class D power amplifier of FIG. In this case, the feedback loop includes an LC low-pass filter. It is necessary to avoid oscillation at an undesirably low frequency (a frequency slightly higher than the cut-off frequency of the LC low-pass filter) caused by the phase delay of the low-pass filter. For this reason, the phase delay of the low-pass filter is compensated by the series circuit of the capacitor 29 (Cc) and the resistor 20 (Rc) in FIG. 8, and the frequency whose phase is delayed by 180 degrees is shifted to a predetermined high frequency (around 400 kHz). Oscillate (self-excited oscillation).

  Therefore, as a result of the series circuit of the capacitor 29 (Cc) and the resistor 20 (Rc) being connected in parallel with the feedback resistor (resistor 25 (Rf) in FIG. 8), the frequency characteristics of this system are such that the capacitor 29 (Cc) Cc) and the resistor 20 (Rc) are greatly affected by the impedance frequency characteristic of the series circuit. Moreover, the constant of the phase compensation circuit using the capacitor 29 (Cc) and the resistor 20 (Rc) needs to be a constant whose phase advances at a frequency slightly higher than the cutoff frequency of the low-pass filter. It cannot be determined regardless of the cut-off frequency.

  As a result, the frequency characteristic of the circuit configuration shown in FIG. 8 is slightly wider than that of the low-pass filter itself, and the gain frequency characteristic can be made almost flat over the entire audio frequency, but the phase characteristic is the circuit shown in FIG. Compared to the configuration, although there is some improvement (delay of about 20 degrees at 20 kHz), it is still not sufficient to obtain a high-quality audio signal waveform (see FIG. 10).

  Accordingly, an object of the present invention is to provide a high-quality class D audio power amplifier with a simpler circuit configuration and low cost by bringing the phase delay in the high frequency range of the audio frequency band close to zero.

  The above-described problem of the conventional technique is due to the fact that the phase frequency characteristic of the low-pass filter is compensated by the characteristic of the feedback network. Therefore, the present invention solves the above problem by configuring a compensation circuit having a function equivalent to the frequency characteristic in the class D audio power amplifier without giving the feedback circuit a frequency characteristic.

A class D power amplifier according to an embodiment of the present invention is a class D power amplifier that amplifies an audio input signal from a signal input terminal and outputs a pulse width modulation signal, and is output from the class D power amplifier. A low-pass filter including an inductor and a capacitor for demodulating and outputting the pulse width modulation signal, and a first feedback circuit for feeding back the output of the low-pass filter to the input of the class D power amplifier, The class D power amplifier is used in a power amplifying apparatus for amplifying the power of an audio input signal input to the audio input signal, the class D power amplifier including a preamplifier for inputting the audio input signal and the preamplifier. A switching circuit that performs a class D operation in response to an output signal from the output of the preamplifier, and an input of the class D power amplifier from the output of the preamplifier It is a D-class power amplifier, characterized in that it comprises a second feedback circuit for applying a feedback to the input of a width unit, the.
In another embodiment, the class D power amplifier includes a phase advance circuit composed of a resistor and a capacitor between the preamplifier and the switching circuit, and the second feedback circuit is composed of a resistor. .

  In the class D power amplifier according to another embodiment, the second feedback circuit includes a resistor connected to both the input terminal and the output terminal of the preamplifier, and the grounded branch is a series circuit of a resistor and a capacitor. A T-type circuit structure comprising:

  In the class D power amplifier of another embodiment, the second feedback circuit includes a series circuit of a resistor and a capacitor and a parallel circuit of a resistor, and the branch connected to the input terminal of the preamplifier. The branch connected to the output terminal is composed of a resistor, and the grounded branch is provided with a T-type circuit structure composed of a series circuit of a resistor and a capacitor.

In the class D power amplifier according to another embodiment, the second feedback circuit connects the output terminal of the preamplifier and the first node with a resistor, and the first node is grounded through a series circuit of a resistor and a capacitor. A high-pass T-type circuit in which the input terminal of the preamplifier and the second node are connected by a resistor, and the first node and the second node are configured by two capacitors and one resistor. A circuit structure connected by a bridged circuit is provided.
A class D power amplifier according to any one of the above embodiments, a low pass filter including an inductor and a capacitor for demodulating and outputting the pulse width modulation signal output from the class D power amplifier, and the low pass filter A power amplifying apparatus for amplifying the power of an audio input signal, comprising: a first feedback circuit that feeds back from an output to an input of the class D power amplifier.

  According to the present invention, it is possible to provide a high-quality class D audio power amplifier with a simpler circuit configuration and low cost by bringing the phase delay in the high frequency range of the audio frequency band close to zero.

It is a figure explaining 1st Embodiment of the class D power amplifier by this invention. It is a figure explaining 2nd Embodiment of the class D power amplifier by this invention. It is a figure explaining 3rd Embodiment of the class D power amplifier by this invention. It is a figure explaining 4th Embodiment of the class D power amplifier by this invention. It is a figure which shows the basic composition of a switching circuit. It is a figure which shows an example of the gain and phase frequency characteristic of the class D power amplifier by this invention. It is a figure which shows an example of the conventional class D power amplifier. It is a figure which shows an example of the conventional class D power amplifier. It is a figure which shows an example of the gain and phase frequency characteristic of the class D power amplifier of FIG. It is a figure which shows an example of the gain and phase frequency characteristic of the class D power amplifier of FIG.

  Embodiments 1 to 4 of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining a first embodiment, FIG. 2 is a diagram for explaining a second embodiment, FIG. 3 is a diagram for explaining a third embodiment, and FIG. First, a configuration common to FIGS. 1 to 4 will be described.

  The preamplifiers (512, 601, 701, 801) are preamplifiers common to the drawings and having the same characteristics. Any so-called high-speed OP amplifier having a bandwidth of 10 MHz or more can be used as a preamplifier.

  What is indicated by reference numerals 513, 602, 702, and 802 is a switching circuit that performs class D operation. FIG. 5 is a diagram showing the basic configuration inside. The input signals of the switching circuits 513, 602, 702, and 802 are output after the level of the signal is determined by the comparator 91. The output is added with a delay time (about 100 to 200 ns) by a delay circuit 92 associated with predetermined signal processing to control the switch unit 93. The switches connected in series output positive (+ Vcc) or negative (-Vcc) voltage by alternately turning on and off according to the control signal Vi. A circuit block having this function is a general one and can be easily obtained.

(Embodiment 1)
FIG. 1 is a diagram for explaining a first embodiment of a class D power amplifier according to the present invention. The class D power amplifier 33 includes a preamplifier 512 and a switching circuit 513. The class D power amplifier 33 receives the audio input signal 501 and outputs a pulse width modulation signal. A pulse width modulation signal that is an output signal of the class D power amplifier 33 is input to the speaker ZL through an LC low-pass filter including an inductor 510 and a capacitor 511. An audio output signal 502 is output from the speaker ZL.

  In the embodiment of the present invention shown in FIG. 1, the input of the class D power amplifier 33 (that is, the negative input terminal of the preamplifier 512) is between the LC low-pass filter composed of the inductor 510 and the capacitor 511 and the speaker ZL. A feedback circuit including a resistor 505 is fed back. This feedback circuit is referred to as a first feedback circuit.

  The audio input signal 501 is input to the signal input terminal (positive input terminal) of the preamplifier 512 of the class D power amplifier 33. In the class D power amplifier 33, a phase advance circuit having characteristics equivalent to those of the feedback network shown in FIG. 8 is formed between the output of the preamplifier 512 and the switching circuit 513 by the resistors 506, 507, and 508 and the capacitor 509. insert. Thereby, a loop characteristic equivalent to the circuit configuration shown in FIG. 8 can be realized. A circuit including the resistor 504 is referred to as a second feedback circuit.

  Since the gain frequency characteristic of the audio frequency band as a whole is substantially determined by (resistor 503 + resistor 505) / resistor 503, it does not include an inductor and a capacitor that are related to the frequency, and therefore a phase lag that causes a problem is generated. Improved. However, although it is theoretically feasible, in practice, a large output amplitude is required for the preamplifier 512 in order to compensate for attenuation due to signal voltage division by the resistors 507 and 508.

(Embodiment 2)
FIG. 2 is a diagram for explaining a second embodiment of the class D power amplifier according to the present invention. The class D power amplifier 43 includes a preamplifier 601 and a switching circuit 602. The class D power amplifier 43 receives the audio input signal 61 and outputs a pulse width modulation signal. The audio input signal 61 is input to the positive signal input terminal of the preamplifier 601 of the class D power amplifier 43.

  A pulse width modulation signal that is an output signal of the class D power amplifier 43 is input to the speaker ZL through an LC low-pass filter including an inductor 65 and a capacitor 60. An audio output signal 62 is output from the speaker ZL.

  The embodiment of the present invention shown in FIG. 2 feeds back from the filter constituted by the inductor 65 and the capacitor 60 and the speaker ZL to the input of the class D power amplifier 43 (that is, the negative input terminal of the preamplifier 601). A feedback circuit including a resistor 68 is provided. This feedback circuit is referred to as a first feedback circuit.

The audio input signal 61 is input to the preamplifier 601 of the class D power amplifier 43. In the class D power amplifier 43, the preamplifier 601 includes a partial feedback circuit including resistors 64, 66, and 69 and a capacitor 67. This feedback circuit is referred to as a second feedback circuit.
In the second feedback circuit, the branch connected to the input terminal of the preamplifier 601 is composed of a resistor 64, the branch connected to the output terminal of the preamplifier 601 is composed of a resistor 66, and the branch grounded is a resistor 69. A T-type circuit structure including a series circuit of capacitors 67 is provided.

  The preamplifier 601 is a delayed phase network composed of a resistor 66, a capacitor 67, and the like, and is partially fed back. As a result, the frequency characteristic from the input terminal to the output terminal of the preamplifier 601 mainly increases from the frequency determined by the resistor 66 and the capacitor 67, and the phase advances at the same time as the frequency increases. The resistor 69 has a characteristic in which the gain approaches a constant value and the phase returns to the original value.

  Since this circuit does not attenuate the signal of the circuit shown in FIG. 1, the preamplifier 601 does not require an excessive output amplitude. Further, by compensating the phase lag of the low-pass filter composed of the inductor 65 and the capacitor 60 with the phase lead characteristic of this circuit, the frequency at which the phase lag of the feedback loop including the low-pass filter becomes 180 degrees is set to around 400 kHz. Shift to

  As a result, the self-excited oscillation at around 400 kHz causes the oscillation rectangular wave to pass through the low-pass filter and the preamplifier 601 to be attenuated, and the signal having a waveform close to a sine wave is the audio signal of the audio input signal 61. At the same time, by being input to the switching circuit 602, a class D power amplifier is obtained in which the PWM modulation is performed and the audio signal is demodulated and output through the LC low-pass filter (65, 60).

  The audio frequency band gain of this amplifier is substantially determined by (resistor 63 + resistor 68) / resistor 63, and does not include a frequency-related element, so that the phase delay in question is improved. As an example, the characteristics shown in FIG. 6 are obtained, and the phase delay in the audio frequency band is negligible.

(Embodiment 3)
FIG. 3 is a diagram for explaining a third embodiment of a class D power amplifier according to the present invention. The class D power amplifier 53 includes a preamplifier 701 and a switching circuit 702. The class D power amplifier 53 receives the audio input signal 703 and outputs a pulse width modulation signal. The audio input signal 703 is input to the positive signal input terminal of the preamplifier 701 of the class D power amplifier 53.

  The embodiment of the present invention shown in FIG. 3 feeds back from the filter constituted by the inductor 713 and the capacitor 714 and the speaker ZL to the input of the class D power amplifier 53 (that is, the negative input terminal of the preamplifier 701). A feedback circuit including a resistor 712 is provided. This feedback circuit is referred to as a first feedback circuit.

  An audio input signal 703 is input to the preamplifier 701 of the class D power amplifier 53. In the preamplifier 701, as in FIG. 2, a partial feedback circuit is configured by a network of resistors and capacitors. The frequency characteristics of this circuit also increase from the frequency mainly determined by the resistor 710 and the capacitor 709, and the phase advances at the same time as the frequency increases.

Then, the resistor 711 has a characteristic that the gain approaches a constant value and the phase returns to the original value. By compensating for the phase delay of the LC low-pass filter composed of the inductor 713 and the capacitor 714 with this phase advance characteristic, the frequency at which the phase delay of the feedback loop including the low-pass filter becomes 180 degrees is shifted to around 400 kHz. . This feedback loop is referred to as a second feedback circuit.
In the second feedback circuit of the second feedback circuit of the second embodiment, the branch connected to the input terminal of the preamplifier 601 is replaced with a series circuit of a resistor 706 and a capacitor 707 and a parallel circuit of a resistor 708.

  In the second feedback circuit, a circuit in which a resistor 706 and a capacitor 707 are connected in series is connected to the resistor 708 in parallel. The difference from the circuit configuration shown in FIG. 2 in the second embodiment is the functional difference between the resistors 706 and 708, the capacitor 707, and the resistor 64 of the circuit in FIG.

  In FIG. 3, the loop gain in the low frequency range of the audio frequency band is sufficiently increased, and the aim is to secure a feedback amount that reduces the influence on power supply voltage fluctuation and the like. Therefore, it is necessary to determine the constants of the resistor and the capacitor so that the circuit operation equivalent to that in FIG. 2 is performed in the phase compensation band of the resistor 710 and the capacitor 709.

  That is, in the phase compensation band, the capacitor 707 has a capacity that can be regarded as a short circuit. (ΩC707 · R706 >> 1) The resistance 706 has substantially the same value as the resistance 64 of FIG. The resistor 708 can be as large as possible. In general, this characteristic is close to that of a first-order integration circuit.

  As a result, the obtained characteristics are as shown in FIG. 6, and the phase delay in the audio frequency band is negligible. Further, the characteristics are greatly improved in the low frequency band of the audio frequency band, and a high quality audio output can be obtained as the audio output signal 704 from the speaker ZL.

(Embodiment 4)
FIG. 4 is a diagram for explaining a fourth embodiment of the class D power amplifier according to the present invention. The class D power amplifier 73 includes a preamplifier 801 and a switching circuit 802. The class D power amplifier 73 receives the audio input signal 803 and outputs a pulse width modulation signal.

  In the embodiment of the present invention shown in FIG. 4, the input of the class D power amplifier 73 (that is, the negative input terminal of the preamplifier 801) is between the LC low-pass filter composed of the inductor 815 and the capacitor 816 and the speaker ZL. A feedback circuit including a resistor 810 that feeds back is provided. This feedback circuit is referred to as a first feedback circuit.

  The audio input signal 803 is input to the positive signal input terminal of the preamplifier 801 of the class D power amplifier 73. In the preamplifier 801, a partial feedback circuit is configured by a network of resistors and capacitors as in FIG. The frequency characteristic of this circuit also increases from the frequency mainly determined by the resistor 812 and the capacitor 813 as the frequency increases, and the phase advances simultaneously.

  The resistor 814 has a characteristic in which the gain approaches a constant value and the phase returns to the original value. By compensating for the phase delay of the low-pass filter (815, 816) with this phase advance characteristic, the frequency at which the phase delay of the feedback loop including the low-pass filter becomes 180 degrees is shifted to around 400 kHz. This feedback loop is referred to as a second feedback circuit.

  The second feedback circuit in the second feedback circuit of the second embodiment is replaced with a high-pass type T-type circuit composed of two capacitors and one resistor bridged by a resistor, and the preamplifier 801 is connected via the resistor. It is structured to return to the input terminal. In other words, in the second feedback circuit, the output terminal of the preamplifier 801 and the first node 821 are connected by the resistor 812, the first node 821 is grounded via a series circuit of the resistor 814 and the capacitor 813, and the A high-pass T-type circuit in which the input terminal of the amplifier 801 and the second node 822 are connected by a resistor 806, and the first node 821 and the second node 822 are constituted by two capacitors 808 and 811 and one resistor 809. A circuit structure connected by a circuit bridged by a resistor 807 is provided.

  In the feedback loop of the fourth embodiment, a resistor 807 is connected in parallel to a circuit in which a capacitor 808 and a capacitor 811 are connected in series. 2 is a functional difference between the resistors (806, 807, 809), the capacitors (808, 811), and the resistor 64 in FIG.

  In FIG. 4, the loop gain in the middle and low frequencies of the audio frequency band is sufficiently increased, and the aim is to secure a feedback amount that reduces the influence on power supply voltage fluctuations and the like. Therefore, it is necessary to determine the constants of the resistor and the capacitor so that the circuit operation equivalent to that in FIG. 2 is performed in the phase compensation band of the resistor 812 and the capacitor 813. That is, in this band, the capacitors (808, 811) have a capacity that can be regarded as a short circuit. (ΩC808 / R806 >> 1, ωC811 / R806 >> 1)

  The resistor 806 has substantially the same value as the resistor 64 in FIG. The resistor 807 may be as large as possible. The resistor 809 is generally intended to be close to that of the secondary integration circuit, and is determined so that its characteristics do not affect the phase compensation band. As a result, the obtained characteristics are as shown in FIG. 6, and the phase delay in the audio frequency band is negligible. Further, the characteristics are greatly improved in the middle and low frequencies of the audio frequency band, and a high-quality audio output can be obtained as the audio output signal 804 from the speaker ZL.

13, 23, 33, 43, 53, 73 Class D power amplifiers 11, 21, 501, 61, 703, 803 Audio input signals 12, 22, 502, 62, 704, 804 Audio output signals 512, 601, 701, 801 Preamplifiers 513, 602, 702, 802 switching circuits 14, 15, 20, 24, 63, 64, 66, 68, 69, 503, 504, 505, 506, 507, 508, 706, 708, 710, 711 712, 805, 806, 807, 809, 810, 812 814 Resistor 16, 26, 510, 65, 713, 815 Inductor 17, 27, 29, 60, 67, 509, 511, 707, 711, 714, 816 Capacitor 821 First node 822 Second node 91 Comparator 92 Delay circuit 93 Switch unit V Control signal ZL speaker

Claims (6)

A class-D power amplifier that amplifies the audio input signal from the signal input terminal and outputs a pulse width modulation signal; an inductor and a capacitor that demodulates and outputs the pulse width modulation signal output from the class D power amplifier; And a first feedback circuit that feeds back an output of the low-pass filter to an input of the class D power amplifier, and amplifies the power of an audio input signal input to the signal input terminal. Said class D power amplifier used in
The class D power amplifier is:
A preamplifier for inputting the audio input signal;
A switching circuit that performs a class D operation by an output signal from the preamplifier;
A second feedback circuit for applying feedback from the output of the preamplifier to the input of the preamplifier, which is the input of the class D power amplifier;
A class-D power amplifier comprising: Between the preamplifier and the switching circuit, comprising a phase advance circuit composed of a resistor and a capacitor,
The class D power amplifier according to claim 1, wherein the second feedback circuit comprises a resistor.   The second feedback circuit has a T-type circuit structure in which the branches connected to the input terminal and the output terminal of the preamplifier are both composed of resistors, and the grounded branch is composed of a series circuit of a resistor and a capacitor. The class D power amplifier according to claim 1.   In the second feedback circuit, the branch connected to the input terminal of the preamplifier comprises a series circuit of a resistor and a capacitor and a parallel circuit of the resistor, and the branch connected to the output terminal of the preamplifier comprises a resistor. 2. The class D power amplifier according to claim 1, wherein the branch to be grounded has a T-type circuit structure comprising a series circuit of a resistor and a capacitor.   The second feedback circuit connects the output terminal of the preamplifier and the first node with a resistor, grounds the first node through a series circuit of a resistor and a capacitor, and connects the input terminal of the preamplifier with the first node. A circuit structure is provided in which two nodes are connected by a resistor, and the first node and the second node are connected by a circuit in which a high-pass T-type circuit configured by two capacitors and one resistor is bridged by a resistor. The class D power amplifier according to claim 1. The class D power amplifier according to any one of claims 1 to 5,
A low pass filter including an inductor and a capacitor for demodulating and outputting the pulse width modulation signal output from the class D power amplifier;
And a first feedback circuit that feeds back an output of the low-pass filter to an input of the class D power amplifier.

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