JP2007005957A - Digital amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a "digital amplifier" capable of sufficiently eliminating a carrier component caused in its PWM module without the need for decreasing the cut-off frequency of an LPF provided on a feedback loop. <P>SOLUTION: In the digital amplifier provided with a switching circuit 3 driven by a PWM signal, an LC filter 4 for receiving the PWM signal amplified by the switching circuit 3; and the LPF 5 on the feedback loop, an output signal of the switching circuit 3 is added to an output signal of the LC filter 4, and its summation result is given to the LPF 5 so as to allow the carrier components included in both the signals to be cancelled out as a result of the summation of the output signal of the switching circuit 3 and the output signal of the LC filter 4 wherein the phases of both the signals are almost inverse to each other, resulting in to eliminate the need for lowering the cut-off frequency of the LPF 5 for eliminating the carrier component. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a digital amplifier, and is particularly suitable for use in a digital amplifier having a low-pass filter on a feedback loop.

  A conventional class A / AB amplifier is referred to as an analog amplifier, whereas a class D amplifier is also referred to as a digital amplifier because it has a feature of driving a load such as a speaker by switching the power MOSFET. Digital amplifiers are more power efficient than conventional analog amplifiers. For this reason, audio devices that employ digital amplifiers are increasing against the background of recent demands for smaller and lower power consumption of audio devices.

  One of digital amplifier systems is a PWM (Pulse Width Modulation) system. The PWM method is a method in which an analog audio signal and the amplitude of a triangular wave are compared to generate a pulse width modulated PWM signal, and the power MOSFET is switched by the PWM signal. Most of the PWM methods are composed of analog circuits.

  FIG. 5 is a block diagram schematically showing a configuration of a conventional digital amplifier employing the PWM method. In FIG. 5, an audio signal input from an external signal source 20 is input to the PWM module 2 via the feedback module 1. The PWM module 2 performs a conversion process based on PWM modulation on the audio signal input from the feedback module 1 to obtain a PWM signal. Then, the obtained PWM signal is output as a control signal for driving the switching circuit 3.

  When no audio signal is input to the PWM module 2, the PWM module 2 outputs a pulse signal having a duty ratio of 50%. On the other hand, when an audio signal is input to the PWM module 2, a PWM signal in which the above pulse signal is pulse-width modulated in accordance with the input signal is output. The PWM signal is amplified by alternately turning on / off the power MOSFETs Q1 and Q2 of the switching circuit 3 by this PWM signal, and a load such as the speaker 30 is driven through the LC filter 4 by the amplified PWM signal. The

Negative feedback is applied to the feedback module 1 from the output side of the LC filter 4 for the purpose of realizing a low distortion rate and low output impedance of the sound output from the speaker 30. Conventionally, a technique aimed at stably performing this negative feedback operation has been proposed (see, for example, Patent Document 1).
JP 2003-115730 A

  In addition, when generating a PWM signal by the PWM module 2, a carrier component is generated in the PWM signal. When this is fed back from the LC filter 4 to the feedback module 1, a nonlinear phenomenon occurs in the feedback module 1, and residual noise and distortion are generated. Leads to worsening. In order to prevent such a non-linear phenomenon from occurring, a low-pass filter (LPF) 5 for removing a carrier component is provided in the feedback loop.

  However, in order to sufficiently remove the carrier component by the LPF 5, it is necessary to considerably reduce the cut-off frequency of the LPF 5. That is, as shown in FIG. 6, in order to double the attenuation factor of the LPF 5 so that the level of the carrier component can be sufficiently lowered, the cut-off frequency needs to be halved. However, if the cut-off frequency is reduced, there is a problem that high-frequency feedback is not applied and the meaning of applying negative feedback is lost.

  The present invention has been made to solve such problems, and can sufficiently remove the carrier component generated in the PWM module without reducing the cutoff frequency of the LPF provided on the feedback loop. The purpose is to.

In order to solve the above-described problem, the digital amplifier of the present invention adds the output signal of the LC filter and the output signal of the switching circuit, and inputs the addition result to the low-pass filter.
Preferably, the phase adjustment is performed so that the phase of the output signal of the LC filter and the phase of the output signal of the switching circuit are inverted from each other. In addition, the amplitude is adjusted so that the amplitude of the output signal of the LC filter and the amplitude of the output signal of the switching circuit are both the same.

  Since the output signal of the switching circuit and the output signal of the LC filter are in an almost inverted phase, when these two signals are added, the carrier components contained in both signals cancel each other and are removed. Is done. As a result, the carrier component can be sufficiently removed without reducing the cut-off frequency of the low-pass filter. Further, by adjusting the phase and amplitude of the output signal of the LC filter and the output signal of the switching circuit, the degree of cancellation of the carrier component can be increased and the carrier component can be more effectively removed.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of a digital amplifier according to the present embodiment. In FIG. 1, components having the same functions as those shown in FIG.

  In FIG. 1, the feedback module 1 is composed of, for example, a linear amplifier. An audio signal supplied from an external signal source 20 is input to the positive terminal, and an audio signal fed back through the LPF 5 is input to the negative terminal. It has come to be.

  The PWM module 2 (corresponding to the pulse width modulation circuit of the present invention) performs a conversion process based on the pulse width modulation on the audio signal input from the feedback module 1 to thereby generate a PWM signal (the pulse width modulation signal of the present invention). Equivalent). Then, the obtained PWM signal is output as a control signal for driving the switching circuit 3.

  When no audio signal is input to the PWM module 2, the PWM module 2 outputs a pulse signal having a duty ratio of 50%. On the other hand, when an audio signal is input to the PWM module 2, a PWM signal in which the above pulse signal is pulse-width modulated in accordance with the input signal is output.

  The switching circuit 3 is driven by the PWM signal output from the PWM module 2 and amplifies and outputs the PWM signal. That is, the PWM signal is amplified by alternately turning on / off the power MOSFETs Q1 and Q2 of the switching circuit 3 by the PWM signal.

  The PWM signal output from the switching circuit 3 is applied to a load such as a speaker 30 via the LC filter 4. That is, the PWM signal amplified by the switching circuit 3 becomes an analog audio signal through the LC filter 4 and is output from the speaker 30. The LC filter 4 includes a coil L1 and a capacitor C1.

  The audio signal that has passed through the LC filter 4 is fed back to the feedback module 1 provided at the input stage of the PWM module 2. An LPF 5 for removing carrier components is provided on a feedback loop from the output stage of the LC filter 4 to the feedback module 1.

  In this embodiment, in addition to the output signal of the LC filter 4, the output signal of the switching circuit 3 (input signal to the LC filter 4) is also fed back to the feedback module 1 via the LPF 5. At that time, the output signal of the switching circuit 3 is added to the output signal of the LC filter 4, and the addition result is input to the LPF 5.

  When the output signal of the LC filter 4 and the output signal of the switching circuit 3 are added, the phase is adjusted so that the phases of both output signals are inverted (180 ° phase shifted). Is preferred. For this purpose, the phase adjustment circuit 6 is provided in the present embodiment. In addition, when adding the output signal of the LC filter 4 and the output signal of the switching circuit 3, it is preferable to adjust the amplitude so that the amplitudes of both output signals are in the same state. For this purpose, an amplitude adjustment circuit 7 is provided in this embodiment.

  The phase adjustment circuit 6 is provided between the output stage of the LC filter 4 and the LPF 5, and includes a resistor R3 and a capacitor C2. This phase adjustment circuit 6 shifts the phase of the output signal of the LC filter 4 and adjusts the phase so that the phase of the output signal of the LC filter 4 and the phase of the output signal of the switching circuit 3 are inverted from each other. Do.

  The amplitude adjustment circuit 7 is provided between the output stage of the switching circuit 3 (the input stage of the LC filter 4) and the LPF 5, and is composed of resistors R1 and R2 for voltage division. The amplitude adjustment circuit 7 reduces the amplitude of the output signal of the switching circuit 3 and adjusts the amplitude so that the amplitude of the output signal of the LC filter 4 and the amplitude of the output signal of the switching circuit 3 are both in the same state. .

  The signal whose phase is adjusted by the phase adjustment circuit 6 is input to the LPF 5 through the resistor R4. The signal whose amplitude is adjusted by the amplitude adjusting circuit 7 is input to the LPF 5 via the resistor R5. At this time, the signal that has passed through the resistor R4 and the signal that has passed through the resistor R5 are added together and input to the LPF 5.

  FIG. 2 is a diagram illustrating voltage waveforms at the output stage of the LC filter 4 (point A), the output stage of the phase adjustment circuit 6 (point B), and the output stage of the amplitude adjustment circuit 7 (point C). FIG. 3 is a diagram showing frequency-phase characteristics at the output stage (point A) of the LC filter 4 and the output stage (point B) of the phase adjustment circuit 6.

  As shown in FIG. 3, the phase of the voltage waveform at the output stage (point A) of the LC filter 4 is slightly shifted from −180 ° at the carrier frequency. On the other hand, by providing the phase adjustment circuit 6 and setting the values of the resistor R3 and the capacitor C2 to appropriate values, the phase of the voltage waveform at the output stage (point B) of the phase adjustment circuit 6 becomes the carrier frequency. In the meantime, it is reversed 180 °. As a result, as shown in FIG. 2, the phase of the output signal of the LC filter 4 and the phase of the output signal of the switching circuit 3 are inverted by 180 °.

  On the other hand, the amplitude adjustment circuit 7 is provided at the output stage of the switching circuit 3, and the voltage at the output stage (point C) of the switching circuit 3 is lowered by setting the voltage dividing resistors R1 and R2 to appropriate values. As a result, the peak value of the output signal of the LC filter 4 and the peak value of the output signal of the switching circuit 3 are equal to each other as shown in FIG.

  In this way, by adding the signals whose phases are inverted by 180 ° and having the same voltage level, the carrier components contained in both the output signal of the LC filter 4 and the output signal of the switching circuit 3 are mutually changed. The carrier components at the output stage of the LC filter 4 are removed by canceling each other.

  As shown in FIG. 3, the phase shift between the output signal of the LC filter 4 and the output signal of the switching circuit 3 (shift from 180 ° inverted state) is slight. Even if the output signal of the LC filter 4 is directly added, an effect of removing the carrier component can be expected. Even if the output signal of the switching circuit 3 is added as it is without providing the amplitude adjustment circuit 7, the level of the carrier component can be attenuated. However, by adjusting the phase and amplitude of the output signal of the LC filter 4 and the output signal of the switching circuit 3, the degree to which the carrier component is canceled can be increased, and the carrier component can be more effectively removed. .

  As described above in detail, according to the present embodiment, the output signal of the LC filter 4 and the output signal of the switching circuit 3 are added, and the addition result is input to the LPF 5. The carrier component included in the signal can be canceled by the carrier component included in the switching circuit 3 and removed. Thereby, it is possible to eliminate the need to reduce the cutoff frequency of the LPF 5 in order to remove the carrier component. When an n-th order filter is used for the LPF 5, the cutoff frequency of the LPF 5 can be set to about 2n times compared to the conventional case in order to remove carrier components to the same extent as in the conventional case.

  By the way, since the output signal of the switching circuit 3 is a square wave, harmonics other than the carrier frequency are included. However, for example, since the frequency of the second harmonic is twice the frequency of the fundamental wave (carrier), the attenuation of the LPF 5 at the frequency of the second harmonic is 6 [dB] compared to the attenuation at the carrier frequency. / Oct]. Further, the attenuation amount of the LPF 5 at the third-order or higher harmonic frequency is further increased. Therefore, even if the output signal of the switching circuit 3 is added to the output signal of the LC filter 4, the harmonics are sufficiently attenuated by passing through the LPF 5.

  In the above embodiment, the example in which the phase is adjusted for the output signal of the LC filter 4 and the amplitude is adjusted for the output signal of the switching circuit 3 has been described, but the present invention is not limited to this. The method of adjusting the phase and amplitude of the feedback signal may be different from that of the above embodiment. For example, the phase and amplitude of the output signal of the LC filter 4 may be adjusted.

  FIG. 4 is a diagram illustrating another configuration example of the digital amplifier according to the present embodiment. In FIG. 4, those given the same reference numerals as those shown in FIG. 1 have the same functions, and therefore redundant description is omitted here. In the digital amplifier shown in FIG. 4, the amplitude adjustment circuit 7 shown in FIG. 1 is not provided, and a phase / amplitude adjustment circuit 8 is provided instead of the phase adjustment circuit 6.

  The phase / amplitude adjustment circuit 8 is provided between the output stage of the LC filter 4 and the LPF 5 and includes an operational amplifier OP, resistors R6 to R8, and a capacitor C3. The operational amplifier OP functions to amplify the amplitude of the output signal of the LC filter 4. Further, the capacitor C3 functions to shift the phase of the output signal of the LC filter 4.

  That is, the phase / amplitude adjustment circuit 8 shifts the phase of the output signal of the LC filter 4 so that the phase of the output signal of the LC filter 4 and the phase of the output signal of the switching circuit 3 are inverted. In addition, the amplitude of the output signal 4 of the LC filter is increased, and the amplitude is adjusted so that the amplitude of the output signal of the LC filter 4 and the amplitude of the output signal of the switching circuit 3 are both in the same state.

  In the above embodiment, the output signal of the LC filter 4 and the output signal of the switching circuit 3 are added and input to the LPF 5, but the present invention is not limited to this. For example, the output signal of the LC filter 4 is divided into two systems, the phase of one output signal is left as it is, and the phase of the other output signal is inverted by 180 °. Then, these two output signals may be added and input to the LPF 5. In this way, it is not necessary to adjust the amplitude at all.

  In the above embodiment, the PWM method has been described as an example of pulse width modulation, but the present invention is not limited to this. For example, the present invention can be applied to a ΔΣ digital amplifier.

  In addition, each of the above-described embodiments is merely an example of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. In other words, the present invention can be implemented in various forms without departing from the spirit or main features thereof.

  The present invention is useful for a digital amplifier having a low-pass filter on a feedback loop.

It is a figure which shows the structural example of the digital amplifier which concerns on this embodiment. It is a figure which shows the waveform of the voltage in the output stage (A point) of a LC filter, the output stage (B point) of a phase adjustment circuit, and the output stage (C point) of an amplitude adjustment circuit. It is a figure which shows the frequency-phase characteristic in the output stage (A point) of LC filter, and the output stage (B point) of a phase adjustment circuit. It is a figure which shows the other structural example of the digital amplifier which concerns on this embodiment. It is a figure which shows the structure of the conventional digital amplifier. It is a frequency-gain characteristic figure which shows the relationship between the attenuation amount of a low-pass filter, and a cutoff frequency.

Explanation of symbols

1 Feedback Module 2 PWM Module 3 Switching Circuit 4 LC Filter 5 LPF
6 Phase adjustment circuit 7 Amplitude adjustment circuit 8 Phase / amplitude adjustment circuit

Claims (6)

A pulse width modulation circuit that performs conversion processing based on pulse width modulation on an input signal and generates a pulse width modulation signal;
A switching circuit driven by the pulse width modulation signal and amplifying and outputting the pulse width modulation signal;
An LC filter to which the pulse width modulation signal output from the switching circuit is input;
A low pass filter provided on a feedback loop from the output stage of the LC filter to the input stage of the pulse width modulation circuit,
A digital amplifier characterized in that the output signal of the switching circuit is added to the output signal of the LC filter, and the addition result is input to the low-pass filter. 2. The phase adjustment circuit according to claim 1, further comprising a phase adjustment circuit for adjusting a phase so that a phase of an output signal of the LC filter and a phase of an output signal of the switching circuit are inverted by 180 degrees. Digital amplifier. 2. The digital adjustment device according to claim 1, further comprising an amplitude adjustment circuit that adjusts an amplitude so that an amplitude of an output signal of the LC filter and an amplitude of an output signal of the switching circuit are in the same state. Amplifier. A phase adjustment circuit that adjusts the phase so that the phase of the output signal of the LC filter and the phase of the output signal of the switching circuit are inverted by 180 °;
The amplitude adjustment circuit for adjusting the amplitude so that the amplitude of the output signal of the LC filter and the amplitude of the output signal of the switching circuit are both in the same state. Digital amplifier. A phase adjustment circuit that adjusts the phase so that the phase of the output signal of the LC filter and the phase of the output signal of the switching circuit are inverted by 180 ° by shifting the phase of the output signal of the LC filter;
An amplitude adjustment circuit that lowers the amplitude of the output signal of the switching circuit and adjusts the amplitude so that the amplitude of the output signal of the LC filter and the amplitude of the output signal of the switching circuit are both in the same state; The digital amplifier according to claim 1, wherein: The phase of the output signal of the LC filter is shifted to adjust the phase so that the phase of the output signal of the LC filter and the phase of the output signal of the switching circuit are inverted by 180 °, and the LC filter And a phase / amplitude adjustment circuit for adjusting the amplitude so that the amplitude of the output signal of the LC filter and the amplitude of the output signal of the switching circuit are both in the same state. The digital amplifier according to claim 1.

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