JP2018056854A - Self-excited class-d amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a self-excited class-D amplifier capable of increasing a negative feedback amount from an output side to an input side to improve distortion characteristics.SOLUTION: A self-excited class-D amplifier comprises: a pre-processing circuit that amplifies a signal obtained by feeding a feedback signal from a second feedback circuit back to an input signal; an integration circuit that integrates the signal of the pre-processing circuit; a comparator that compares a voltage of the integrated signal with a predetermined reference voltage, and outputs a pulse width modulation signal; a switching circuit that amplifies a power of the pulse width modulation signal; a low-pass filter that generates an amplified output signal from the pulse width modulation signal with an amplified power; a first feedback circuit that feeds an output of the switching circuit back to an input of the integration circuit; and a second feedback circuit that feeds an output of the low-pass filter back to an input of the pre-processing circuit.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a self-excited class D amplifier.

  2. Description of the Related Art Conventionally, a self-excited class D amplifier is known as one of amplifiers that amplify power of audio signals and the like.

  In FIG. 1 of the following Patent Document 1, an integrator that integrates an input audio signal, a comparator that compares an output of the integrator with a predetermined reference value, and outputs a PWM (pulse width modulation) signal, A drive circuit and a switch circuit that amplify and output the PWM signal from the comparator, and a low-pass filter that demodulates the power signal after removing the carrier component from the PWM signal output from the switch circuit ( LPF), a first feedback circuit for feeding back the output of the switch circuit to the inverting input of the integrator, and a second feedback circuit for differentiating the LPF output by the differentiation circuit and feeding back to the inverting input of the integrator. A self-excited class D amplifier is disclosed. The loop circuit that reaches the switch circuit from the integrator and feeds back to the inverting input of the integrator by the first feedback circuit oscillates at a frequency at which the phase delay of the loop circuit is 180 degrees. In this self-excited class D amplifier, the LPF output is differentiated by a differentiating circuit and negative feedback is applied to the inverting input of the integrator, thereby improving the distortion characteristics in the audio band of the amplifier and increasing the gain of the amplifier. Determined.

JP 2005-123949 A

  In the circuit of Patent Document 1 described above, negative feedback is applied to the input side of the integrator via a differentiating circuit. However, since the differential circuit is provided, there is a problem that the amount of negative feedback is reduced particularly in a low frequency band. there were. There is a demand for an amplifier for an audio signal to further increase the amount of negative feedback in a low frequency band (audio band) in order to further improve the distortion characteristics.

  An object of the present invention is to improve the distortion characteristics by increasing the amount of negative feedback from the output side to the input side in a self-excited class D amplifier.

  To achieve the above object, a self-excited class D amplifier according to claim 1 is amplified by a preprocessing circuit that amplifies a signal obtained by feeding back a feedback signal from a second feedback circuit to an input signal, and the preprocessing circuit. An integration circuit that integrates the obtained signal and outputs the result as an integration signal, a comparator that compares the voltage of the integration signal with a predetermined reference voltage and outputs a pulse width modulation signal, and outputs from the comparator A switching circuit that amplifies the pulse width modulation signal to be amplified, a low-pass filter that generates an amplified output signal from the power amplified pulse width modulation signal, and an output of the switching circuit is fed back to the input of the integration circuit A first feedback circuit and a second feedback circuit that feeds back an output of the low-pass filter to an input of the pre-processing circuit.

  The self-excited class D amplifier according to claim 2 further integrates a preprocessing circuit that integrates a signal obtained by feeding back the feedback signal from the second feedback circuit to the input signal, and a signal integrated by the preprocessing circuit, An integration circuit that outputs the result as an integration signal, a comparator that compares the voltage of the integration signal with a predetermined reference voltage and outputs a pulse width modulation signal, and a pulse width modulation signal output from the comparator A switching circuit that amplifies the power, a low-pass filter that generates an amplified output signal from the power-amplified pulse width modulation signal, a first feedback circuit that feeds back the output of the switching circuit to the input of the integrating circuit, And a second feedback circuit that feeds back the output of the low-pass filter to the input of the preprocessing circuit.

  The invention according to claim 3 is the self-excited class D amplifier according to claim 1 or 2, wherein the second feedback circuit is a differentiating circuit.

  According to a fourth aspect of the present invention, in the self-excited class D amplifier according to any one of the first to third aspects, the second feedback circuit is a snubber circuit provided at a subsequent stage of the low-pass filter. Features.

  According to the present invention, in a self-excited class D amplifier, the amount of negative feedback from the output side to the input side can be increased to improve the distortion characteristics.

Circuit diagram of self-excited class D amplifier according to an embodiment of the present invention Figure showing a modification

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a circuit diagram of a self-excited class D amplifier 10 according to an embodiment of the present invention. 1 is an input of the amplifier, and 2 is an output of the amplifier 10. The amplifier 10 includes a preprocessing circuit 12, an integration circuit 14, a comparator 16, a switching circuit 17, an LPF 18, a first feedback circuit 21, and a second feedback circuit 22.

  The preprocessing circuit 12 includes an operational amplifier 121 and a β circuit 122. The β circuit 122 includes a resistor 123 connected between the inverting input terminal and the output terminal of the operational amplifier 121 and a resistor 124 connected between the inverting input terminal of the operational amplifier 121 and the ground. Therefore, the preprocessing circuit 12 as a whole constitutes a non-inverting amplifier circuit. The preprocessing circuit 12, which is a non-inverting amplifier circuit, amplifies the audio signal obtained by applying feedback by the second feedback circuit 22 to the input 1 audio signal, and outputs the amplified audio signal to the integration circuit 14. The integration circuit 14 includes a resistor 141, an operational amplifier 142, and a capacitor 143. The integrating circuit 14 integrates the signal from the preprocessing circuit 12 and outputs the resulting integrated signal to the comparator 16. The integration circuit 14 is an inverting integration circuit, and outputs an integration signal having a phase opposite to that of the input signal.

  The comparator 16 compares the integrated signal with a reference voltage (the ground voltage of the comparator 16), thereby generating and outputting a PWM signal obtained by PWM-modulating the audio signal of the input 1. The switching circuit 17 includes a switch drive circuit 171 and a switch circuit 172, and amplifies and outputs the PWM signal from the comparator 16. The LPF 18 includes an inductor 181 and a capacitor 182, removes a carrier component (frequency component of self-excited oscillation) from the power amplified PWM signal, demodulates the power amplified audio signal, and outputs from the output 2 of the amplifier 10. Output.

  The first feedback circuit 21 feeds back the output signal of the switching circuit 17 to the inverting input of the operational amplifier 142 of the integrating circuit 14. By this feedback, the amplifier 10 self-oscillates as a self-excited class D amplifier. Since the integration circuit 14 is provided in the loop circuit for self-oscillation (the loop circuit from the integration circuit 14 to the switching circuit 17 and fed back to the integration circuit 14 by the first feedback circuit 21), the integration circuit 14 has a certain audio band. The effect of increasing the amount of feedback can be expected.

  The second feedback circuit 22 is a differentiation circuit including a capacitor 221 and a resistor 222. The second feedback circuit 22 feeds back the output 2 audio signal to the audio signal input to the preprocessing circuit 12. This feedback improves the distortion characteristics of the amplifier 10 in the audio band. In the present embodiment, since the output of the LPF 18 is fed back by the second feedback circuit 22 and further amplified by the preprocessing circuit 12, the amount of feedback is further increased and the distortion characteristics are improved as compared with the case without the preprocessing circuit 12. Can do. In particular, the preprocessing circuit 12 which is a non-inverting amplifier circuit has a frequency characteristic showing a high gain in the audio band (low frequency band). Therefore, the presence of the preprocessing circuit 12 can increase the amount of negative feedback in the audio band and further improve the distortion characteristics as compared with the conventional circuit.

  FIG. 2A shows a modification of the preprocessing circuit 12 in FIG. 1 and shows a modification in which a non-inverting integration circuit is applied as the preprocessing circuit 12. The part of the preprocessing circuit 12 in FIG. 1 is replaced with FIG. 2A, and the other configurations in FIG. 1 are the same. The pre-processing circuit 12 in FIG. 2A includes an operational amplifier 121, a β circuit 122 including a capacitor 125 and a resistor 126, a resistor 127, and a capacitor 128, and an audio signal subjected to feedback by the second feedback circuit 22 is provided. Integration is performed, and the resultant integration signal is output to the integration circuit 14. The subsequent signal processing is the same as in FIG.

  According to the modification of FIG. 2A, an audio signal obtained by applying negative feedback by the second feedback circuit 22 to the audio signal of the input 1 is integrated, and the resultant integrated signal is further integrated by the integrating circuit 14. . As a result, the amount of feedback can be increased to improve the distortion characteristics. In particular, the preprocessing circuit 12 which is a non-inverting integration circuit has a frequency characteristic showing a high gain in the audio band (low frequency band). Therefore, the presence of the preprocessing circuit 12 can increase the amount of negative feedback in the audio band and further improve the distortion characteristics as compared with the conventional circuit.

  FIG. 2B shows a modification of the second feedback circuit 22. In this modification, a circuit called a snubber circuit (or a damper circuit) including a capacitor 19 and a resistor 20 is provided after the LPF 18 in FIG. 1, and the other configuration is the same as that in FIG. The snubber circuit is for suppressing an output peak generated at the resonance frequency of the LPF 18 when the amplifier 10 is unloaded or lightly loaded. When such a circuit is provided, a feedback signal may be taken out from the connection position of the capacitor 19 and the resistor 20 and fed back to the input 1 side as shown in FIG. . In this case, since the capacitor 19 and the resistor 20 serve as the second feedback circuit 22 (differential circuit) in FIG. 1, the second feedback circuit 22 may be omitted.

  According to the above embodiment, a large amount of negative feedback can be applied from the front stage of the LPF 18, and as a result, low distortion can be realized. The primary type is easy to design, but if the stagger ratio is ensured, the integration type of the second or higher order is also possible. In addition, noise resistance is excellent because no point at which sensitivity is increased in principle is generated. If a high-speed, wide-band type is used, low distortion can be realized even at a high frequency (from 5 kHz).

  Note that the specific circuit configuration in the above embodiment is not limited to that described, and can be implemented with appropriate modifications. For example, the configurations of the integration circuit 22, the first feedback circuit 27, the second feedback circuit 28, and the like are not limited to those shown in the drawing. The first feedback circuit 27 and the second feedback circuit 28 may be differentiation circuits.

  DESCRIPTION OF SYMBOLS 1 ... Input, 2 ... Output, 12 ... Pre-processing circuit, 14 ... Integration circuit, 16 ... Comparator, 17 ... Switching circuit, 18 ... LPF, 21 ... First feedback circuit, 22 ... Second feedback circuit.

Claims (4)

A preprocessing circuit for amplifying a signal obtained by feeding back the feedback signal from the second feedback circuit to the input signal;
Integrating the signal amplified by the preprocessing circuit, and outputting the result as an integration signal;
A comparator that compares the voltage of the integrated signal with a predetermined reference voltage and outputs a pulse width modulation signal;
A switching circuit that amplifies the power of the pulse width modulation signal output from the comparator;
A low-pass filter that generates an amplified output signal from the power-amplified pulse width modulation signal;
A first feedback circuit that feeds back an output of the switching circuit to an input of the integrating circuit;
A self-excited class D amplifier comprising: a second feedback circuit that feeds back an output of the low-pass filter to an input of the preprocessing circuit. A preprocessing circuit for integrating a signal obtained by feeding back the feedback signal from the second feedback circuit to the input signal;
An integration circuit for further integrating the signal integrated in the preprocessing circuit and outputting the result as an integration signal;
A comparator that compares the voltage of the integrated signal with a predetermined reference voltage and outputs a pulse width modulation signal;
A switching circuit that amplifies the power of the pulse width modulation signal output from the comparator;
A low-pass filter that generates an amplified output signal from the power-amplified pulse width modulation signal;
A first feedback circuit that feeds back an output of the switching circuit to an input of the integrating circuit;
A self-excited class D amplifier comprising: a second feedback circuit that feeds back an output of the low-pass filter to an input of the preprocessing circuit. The self-excited class D amplifier according to claim 1 or 2,
The self-excited class D amplifier, wherein the second feedback circuit is a differentiation circuit. The self-excited class D amplifier according to any one of claims 1 to 3,
The self-excited class D amplifier, wherein the second feedback circuit is a snubber circuit provided at a subsequent stage of the low-pass filter.

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