JP4393216B2 - Class D amplifier - Google Patents

Description

  The present invention relates to reduction of electromagnetic interference (hereinafter referred to as EMI (electromagnetic interference)) generated by a class D amplifier whose main purpose is power amplification of an audio signal.

Conventionally, class D amplification has been used as a method that enables downsizing of a device by performing power amplification of an audio signal with high efficiency and low loss. The modulation method of the class D amplifier is roughly divided into a pulse width modulation (PWM) type and a delta sigma (ΔΣ) modulation type.
When the audio PCM signal is converted into a PWM signal (see, for example, Patent Document 1), the rounding error caused by the requantization means is reduced by using ΔΣ modulation in the PCM-PWM converter, so that ΔΣ modulation type PWM is used together. Often a type.
In addition, there is one that converts an analog audio signal into a PWM signal by comparing it with a triangular wave signal having a PWM fundamental frequency and leads it to a power switch (see, for example, Patent Document 2).

  In the class D amplifier, the switching operation of the power switch is controlled by the PWM signal. However, each time the switching is performed, a large current is passed or stopped, which causes a problem of generating a large electromagnetic interference. As an improvement measure, only a high frequency component is extracted from the analog signal demodulated by the output LC filter, an inverted signal thereof is created, and a means for canceling and removing the high frequency component from the original analog signal via the choke transformer is used. Some devices reduce electromagnetic interference (for example, see Patent Document 3).

JP 2001-292040 A (paragraph 0003, FIG. 7) Japanese Examined Patent Publication No. 07-28181 (page (1), page 2, column 2, line 8 to page (2), page 3, column 3, line 11, FIG. 5)) JP 2002-118430 A (paragraphs 0020 to 0022, FIG. 1)

  However, in the electromagnetic interference reduction method of the class D amplifier described above, the electromagnetic interference radiated from the analog signal output line can be reduced, but the electromagnetic interference radiated from the power supply line due to the high frequency current derived from the switching operation in the power switch. On the other hand, there was no reduction effect. That is, the source of electromagnetic interference that appears in both the analog signal output line and the power supply line is high-frequency noise that is generated along with the switching operation of the power switch, but this is not an improvement measure for this source.

  Further, it is known that the characteristic of electromagnetic interference appears mainly with the harmonics of the PWM carrier frequency, and the level of the center frequency tends to appear higher as the modulation degree of the audio input signal decreases. In particular, this is a phenomenon that is maximized when the duty ratio of the PWM signal is 50%, that is, when the audio input signal is not modulated, and may cause the electromagnetic interference (EMI) standard reference value not to be satisfied.

  An object of the present invention is to solve the above-described problems. A class D amplifier that reduces the peak level of harmonic noise resulting from switching operation in a power switch and suppresses the occurrence of electromagnetic interference is provided. Is to provide.

A class D amplifier according to the present invention includes a PWM signal generation unit that receives a voice out-of-band signal and an analog voice signal, generates a PWM signal that is frequency-modulated by the voice out-of-band signal and is based on the analog voice signal, Power switch means for performing power amplification with the output of the PWM signal generation means as input, and filter means for performing PWM demodulation with the output of the power switch means as input , wherein the PWM signal generation means has a reference frequency of the PWM signal. On the other hand, frequency modulation means for performing frequency modulation based on the out-of-speech signal, triangular wave generation means for generating a triangular wave signal based on the output of the frequency modulation means, output of the analog voice signal and the triangular wave generation means Comparing means for generating a PWM signal based on the frequency modulation means, the triangular wave generating means An integrating means for converting the force into a triangular wave signal, characterized in that the amplitude of the output of said integration means constituted by the amplitude correction means for correcting, based on the voice-band signal.

  Since the class D amplifier of the present invention is configured such that the PWM signal is frequency-modulated by the out-of-band signal, the harmonic noise derived from the operation of the power switch that amplifies the PWM signal is also frequency-modulated. Since the frequency component is dispersed, the peak level of the harmonic noise is reduced, and a class D amplifier with less electromagnetic interference can be provided.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration example of a class D amplifier according to Embodiment 1 of the present invention, and FIG. 2 is a diagram for explaining the PWM signal generation operation. The operation will be described below.

  In FIG. 1, a PWM reference signal input 1 and an audio out-of-band signal 2 are input to the frequency modulation circuit 3a of the PWM signal generating unit 3, and the PWM reference signal frequency is frequency-modulated by the audio out-of-band signal. A rectangular wave signal is output. This PWM reference signal frequency corresponds to the PWM carrier frequency. FIG. 2 shows a case where frequency modulation by an out-of-band signal is not applied to the PWM reference signal frequency for the sake of simplicity, but is based on the output of the frequency modulation circuit 3a as shown in FIG. The triangular wave generation circuit 3b generates a triangular wave signal as shown in FIG. 2B, and the triangular wave output is input to the comparison circuit 3c.

  On the other hand, the analog audio signal input 4 is subjected to distortion correction by the negative feedback signal from the power switch 6 in the correction circuit 5 and input to the comparison circuit 3 c constituting the PWM signal generation unit 3. In the comparison circuit 3c, as shown in FIG. 2C, the triangular wave signal output from the triangular wave generation circuit 3b is compared with the analog audio signal output from the correction circuit 5, and the PWM signal shown in FIG. 2D is generated and output. . This PWM signal is amplified by the power switch 6, smoothed by the filter 7, and output as a PWM demodulated output 8 to a speaker (not shown). The correction circuit 5 is generally used for the purpose of reducing distortion components included in the output of the power switch 6, such as nonlinear distortion of a triangular wave signal, pulse width distortion in the power switch 6, and distortion due to power supply voltage fluctuation. Is.

  The triangular wave signal is a triangular wave signal that is frequency-modulated by the out-of-band signal because the rectangular wave signal output from the frequency modulation circuit 3a is frequency-modulated by the out-of-band signal with respect to the PWM reference signal frequency. Yes. Therefore, the PWM signal output from the comparison circuit 3c is also subjected to frequency modulation by the audio out-of-band signal. Hereinafter, the case where frequency modulation is performed by an out-of-band audio signal will be described in detail.

  FIG. 3 is a diagram illustrating an example of the configuration of the PWM signal generation unit 3, and FIG. 4 is a diagram illustrating the operation of the triangular wave generation circuit 3 b in the PWM signal generation unit 3. Hereinafter, the operation of the triangular wave generating circuit 3b will be described.

  In FIG. 3, the triangular wave generation circuit 3 b includes an integration circuit 3 b 1 that integrates the output of the frequency modulation circuit 3 a and an amplitude correction circuit 3 b 2 that corrects the amplitude of the output of the integration circuit 3 b 1 based on the audio out-of-band signal 2. When the rectangular wave output of the frequency modulation circuit 3a is not modulated by the audio out-of-band signal 2 as shown in FIG. 4A, the output of the integration circuit 3b1 has an amplitude as shown in FIG. 4B. And a triangular wave signal with the same period. On the other hand, when modulation by the out-of-band signal 2 is applied as shown in FIG. 4C, the amplitude of the triangular wave signal varies as shown in FIG. FIG. 4E shows the audio out-of-band signal 2. Since the amplitude change corresponds to the frequency change of the frequency modulation circuit 3a, if amplitude correction is performed in the amplitude correction circuit 3b2 based on this, A triangular wave signal having the same amplitude as shown in FIG. 4F can be obtained.

  In this way, by controlling the amplitude of the triangular wave signal input to the comparison circuit 3c to a constant amplitude regardless of the frequency, the analog audio signal 4 is not affected by the frequency modulation of the PWM signal made into the triangular wave signal. Correctly generated based on

  In this embodiment, the case where the time constant of the integration circuit 3b1 is fixed is described. However, if the time constant is changed in accordance with the input, it is possible to realize the configuration without using the amplitude correction circuit 3b2. .

  As an example of the frequency modulation circuit 3a, there is a method of superimposing a sine wave out-of-band signal 2 on the control voltage using a voltage controlled oscillator.

  5 and 6 are diagrams for explaining the effect of EMI reduction according to the present invention. In both figures, when there is no modulation by the analog audio signal 4 (when the duty ratio of the PWM signal is 50%), the electromagnetic wave emitted from the class D amplifier by the presence or absence of frequency modulation with respect to the PWM carrier frequency by the audio out-of-band signal 2 This is an example of actually measuring the frequency distribution of intensity. In both figures, (a) PWM carrier frequency is 400 kHz, and when there is no modulation by the out-of-band signal 2, (b) is 30% out-of-band signal 2 and frequency modulation of 0.5% modulation is applied. An example of the case is shown. In addition, a horizontal axis is a frequency, (a) is 0-100 MHz, (b) is 39.9-40.9 MHz.

  The peak level of the radiated electromagnetic wave power in FIG. 5A is due to the odd-order harmonics of the PWM carrier signal, and substantially coincides with the coefficient change of the Fourier series expansion formula of the PWM carrier signal rectangular wave. FIG. 6A is an enlarged view of the vicinity of the 101st harmonic (40.4 MHz). In addition, since it is not an ideal rectangular wave, even-order harmonics are also seen.

  From the comparison between FIGS. 5 and 6A and 6B, by applying frequency modulation to the PWM carrier frequency by the out-of-band audio signal 2, the energy of the harmonic center frequency of the PWM carrier signal is dispersed, and class D It can be seen that the peak level of the electromagnetic wave power radiated from the amplifier is reduced over the entire frequency.

  As described above, in the class D amplifier according to the present invention, the PWM signal generation unit receives the audio out-of-band signal and the analog audio signal as input, and the PWM signal based on the analog audio signal is frequency-modulated by the out-of-audio signal. Since it is configured to generate, harmonic noise derived from the switching operation of the power switch is also frequency-modulated, and the frequency components of the noise are dispersed. Therefore, even when the analog audio signal is unmodulated, energy is not concentrated on the harmonic center frequency of the PWM signal, so that it is possible to provide a class D amplifier with reduced peak level of harmonic noise and less electromagnetic interference.

Embodiment 2. FIG.
FIG. 7 is a block diagram showing the configuration of the class D amplifier according to the second embodiment of the present invention, and FIG. 8 is a diagram for explaining the operation of the triangular wave generation circuit 10b in FIG. 7 is different from FIG. 1 only in the configuration of the triangular wave generating circuit 10b and the triangular wave generating operation. The operation will be described below, but the description of the parts common to FIG. 1 will be omitted.

  In FIG. 7, the PWM reference signal input 1 and the audio out-of-band signal 2 are input to the frequency modulation circuit 10a of the PWM signal generation unit 10, and the PWM reference signal frequency as shown in FIG. 8A or 8E is obtained. Thus, a rectangular wave signal having a form subjected to frequency modulation by an out-of-band audio signal is output.

  For example, if FIG. 8A is a basic rectangular wave signal when frequency modulation is not applied, FIG. 8E is a rectangular wave signal frequency-modulated in a higher frequency direction. (B) and (f) are triangular wave pattern signals in which the triangular wave pattern read from the memory circuit 10b1 by the rectangular wave signal is converted into an analog signal by the digital-analog converter (DAC) 10b2, and (c) And (g) are triangular wave signals obtained by smoothing the triangular wave pattern signal by the smoothing circuit 10b2.

  That is, by changing the readout clock signal of the memory circuit 10b1 to a signal frequency-modulated by the audio out-of-band signal 2, the readout speed of the triangular wave pattern is changed, and a triangular-wave signal frequency-modulated by the out-of-band signal 2 is generated. . A PWM signal based on the analog audio signal 4 is generated in the comparison circuit 10c using this triangular wave signal. Since the subsequent steps are the same as described above, the description thereof is omitted.

  As described above, since the triangular wave pattern stored in the memory circuit is read by the read clock signal frequency-modulated by the audio out-of-band signal 2, the amplitude of the triangular wave signal input to the comparison circuit 3c is related to the frequency. Therefore, the PWM signal can be correctly generated based on the analog audio signal 4 without being affected by the frequency modulation performed on the triangular wave signal.

Embodiment 3 FIG.
9 is a block diagram showing the configuration of the class D amplifier according to the second embodiment of the present invention, FIG. 10 is a diagram showing an example of the configuration of the PWM signal generation unit 11 of FIG. 9, and FIG. 11 explains the operation thereof. It is a figure to do. 9 and 10 are different from FIG. 1 only in the configuration of the frequency modulation circuit 11a and the sawtooth wave generation circuit 11b and the sawtooth wave generation operation in the PWM signal generation unit 11. The operation will be described below, but the description of the parts common to FIG. 1 will be omitted.

  In FIG. 10, the PWM reference signal input 1 and the audio out-of-band signal 2 are input to the frequency modulation circuit 11a of the PWM signal generation unit 11, and an asymmetric rectangular wave signal as shown in FIG. Is output. The generation of the asymmetric rectangular wave signal can be realized, for example, by binarizing the sine wave output signal of the voltage controlled oscillator of the frequency modulation unit 11a with a comparator that shifts the binarization threshold from the center of amplitude change. .

  FIG. 11A shows a rectangular wave signal when frequency modulation is not performed. This rectangular wave signal is input to the sawtooth wave generation circuit 11b, and the sawtooth wave signal shown in FIG. 11B is output. The generation of the sawtooth wave signal can be realized, for example, by making the rise and fall time constants of the integrating circuit 11b1 of the sawtooth wave generating circuit 11b asymmetric.

  FIG. 11C shows an output of a rectangular wave signal when the frequency modulation circuit 11a performs frequency modulation with the audio out-of-band signal 2. This rectangular wave signal is input to the sawtooth wave generation circuit 11b. The frequency-modulated sawtooth signal shown in FIG. 11 (d) is output. Since the frequency-modulated sawtooth signal changes not only in frequency but also in amplitude, amplitude correction is performed by an amplitude correction circuit 11b2 using the audio out-of-band signal 2 shown in FIG. 11 (e) as a control input. 11 (f), a sawtooth wave whose frequency is modulated at a constant amplitude is output, and is used for generating a PWM signal in the comparison circuit 11c as in the first embodiment.

  In addition, when a triangular wave signal is input to the comparison circuit 11c, a PWM signal with double edge modulation is generated, and when a sawtooth wave signal is input, a PWM signal with single edge modulation is generated. This is the same as the first embodiment.

Embodiment 4 FIG.
FIG. 12 is a block diagram showing the configuration of the class D amplifier according to the fourth embodiment of the present invention, and FIG. 13 is a diagram for explaining the operation of the sawtooth wave generating circuit 12b in the configuration of FIG. 12 is different from FIG. 7 only in that the memory content of the memory circuit 12b1 of the sawtooth wave generation circuit 12b is a sawtooth wave pattern, and the operation content is similar to that of the second embodiment. Yes. The operation will be described below, but the description of the parts common to FIG. 7 will be omitted.

  FIGS. 13A and 13E are rectangular wave signal outputs of the frequency modulation circuit 12a, and show an example of presence / absence of frequency modulation by the audio out-of-band signal 2. FIG. If FIG. 13A is a basic clock signal not subjected to frequency modulation, FIG. 13E is a clock signal frequency-modulated in the higher direction. FIGS. 13B and 13F show a sawtooth wave pattern signal in which the sawtooth wave pattern read from the memory circuit 12b1 by the rectangular wave signal is converted into an analog signal by the digital / analog converter (DAC) 12b2. FIGS. 13C and 13G show the result of smoothing them by the smoothing circuit 12b3.

  That is, by changing the read clock signal of the memory circuit 12b1 to a signal frequency-modulated by the audio out-of-band signal 2, the reading speed of the sawtooth wave pattern is changed, and the saw-tooth wave signal frequency-modulated by the audio out-of-band signal 2 is changed. Is generated. Using this sawtooth signal, a PWM signal based on the analog audio signal 4 is generated in the comparison circuit 12c. Since the subsequent steps are the same as described above, the description thereof is omitted.

  As described above, the sawtooth wave pattern stored in the memory circuit is read out by the read clock signal frequency-modulated by the audio out-of-band signal 2, so that the amplitude of the sawtooth wave signal input to the comparison circuit 12c is changed. Regardless of the frequency, it can be controlled to a constant amplitude, and the PWM signal is correctly generated based on the analog audio signal 4 without being affected by the frequency modulation made to the sawtooth signal.

It is a block diagram which shows the structural example of the class D amplifier of Embodiment 1 of this invention. It is a figure explaining operation | movement of the class D amplifier of Embodiment 1 of this invention. It is a figure which shows the structural example of the PWM signal generation part of Embodiment 1 of this invention. It is a figure explaining the operation | movement of the triangular wave generation circuit of Embodiment 1 of this invention. It is a figure which shows an example of the effect of the PWM signal generation part of Embodiment 1 of this invention. It is a figure which shows an example of the effect of the PWM signal generation part of Embodiment 1 of this invention. It is a block diagram which shows the structural example of the class D amplifier of Embodiment 2 of this invention. It is a figure explaining the operation | movement of the triangular wave generation circuit of Embodiment 2 of this invention. It is a block diagram which shows the structural example of the class D amplifier of Embodiment 3 of this invention. It is a figure which shows the structural example of the PWM signal generation part of Embodiment 3 of this invention. It is a figure explaining operation | movement of the sawtooth wave generation circuit of Embodiment 3 of this invention. It is a block diagram which shows the structural example of the class D amplifier of Embodiment 4 of this invention. It is a figure explaining operation | movement of the sawtooth wave generation circuit of Embodiment 4 of this invention.

Explanation of symbols

1 PWM reference signal input, 2 audio out-of-band signal input, 3 PWM signal generation unit, 3a frequency modulation circuit, 3b triangular wave generation circuit, 3c comparison circuit, 4 analog audio signal input, 5 correction circuit, 6 power switch, 7 filter, 8 PWM demodulation output.

Claims (2)

PWM signal generation means for receiving a voice out-of-band signal and an analog voice signal, and generating a PWM signal that is frequency-modulated by the voice out-of-band signal and based on the analog voice signal;
Power switch means for performing power amplification with the PWM signal generation means output as an input;
Filter means for performing PWM demodulation using the output of the power switch means as an input ,
The PWM signal generating means performs frequency modulation based on the out-of-band signal with respect to a reference frequency of the PWM signal, and triangular wave generating means for generating a triangular wave signal based on the output of the frequency modulating means And a comparing means for generating a PWM signal based on the analog audio signal and the output of the triangular wave generating means,
The triangular wave generating means comprises an integrating means for converting the output of the frequency modulating means into a triangular wave signal, and an amplitude correcting means for correcting the amplitude of the output of the integrating means based on the out-of-sound band signal. Characteristic class D amplifier. PWM signal generation means for receiving a voice out-of-band signal and an analog voice signal, and generating a PWM signal that is frequency-modulated by the voice out-of-band signal and based on the analog voice signal;
Power switch means for performing power amplification with the PWM signal generation means output as an input;
Filter means for performing PWM demodulation using the output of the power switch means as an input,
The PWM signal generation means generates a sawtooth wave generation means for generating a sawtooth wave based on the output of the frequency modulation means, and generates a PWM signal based on the analog audio signal and the output of the sawtooth wave generation means. Comparing means,
The sawtooth wave generating means includes an integrating means for converting the output of the frequency modulating means into a sawtooth wave, and an amplitude correcting means for correcting the amplitude of the output of the integrating means based on the out-of-sound band signal. A class D amplifier characterized by that.

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