Patent Publication Number: US-2022224293-A1

Title: Class-d amplifier, a method of controlling a gain of an input audio signal in a class-d amplifier

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Application No. PCT/JP2020/037284, filed Sep. 30, 2020, which claims a priority to Japanese Patent Application No. 2019-180508, filed Sep. 30, 2019. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present application relates to a class-D amplifier and a method of controlling a gain of an input audio signal in a class-D amplifier for driving a load with a pulse that is pulse-width modulated based on an input signal. 
     BACKGROUND INFORMATION 
     A class-D amplifier is known that generates a first pulse whose pulse width increases in response to a change in an input signal in a positive direction and a second pulse whose pulse width increases in response to a change in the input signal in a negative direction, and drives a load such as a speaker with the first and second pulses. 
     In a filterless class-D amplifier among class-D amplifiers of this type, in a small output region in which the level of an input signal is close to zero and an output power with respect to the load is small, the range of the input signal from which the first pulse is output and the range of the input signal from which the second pulse is output overlap each other. 
     In the small output region, the input signal is falls between the lower limit of the input signal for generating the first pulse and the upper limit of the input signal for generating the second pulse. For this reason, in the small output region, both the pulse width of the first pulse and the pulse width of the second pulse are short. Therefore, the power consumption in the small output region is reduced. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2018-137548A 
       
    
     SUMMARY 
     In the above-described conventional class-D amplifier, in the small output region, a first pulse whose pulse width increases and a second pulse whose pulse width decreases in accordance with, for example, a change in the positive direction of the input signal are output. For this reason, inclination of input/output characteristic of the class-D amplifier is different between the small output region and the region other than the small output region, and there is the problem of the total harmonic distortion factor increasing. 
     A technique for preventing distortion of an output signal of a class-D amplifier is disclosed in, for example, Patent Document 1. In the technique disclosed in Patent Document 1, an offset voltage that generates distortion that cancels distortion generated by an output stage in a class-D amplifier is applied to an input signal of a pulse width modulator of the class-D amplifier. 
     However, even if such offset adjustment is performed, the voltage difference between the input signal (input signal to which the offset voltage is added) supplied to the pulse width modulator of the class-D amplifier and the carrier wave changes, and the pulse width of each pulse output from the pulse width modulator is only uniformly corrected. In the above-described conventional class-D amplifier, inclination of the input/output characteristic in the small output region is different from inclination of the input/output characteristic in the other region, and thus distortion occurs. For this reason, even if the offset is adjusted, the occurrence of distortion in the class-D amplifier is not suppressed. 
     The present disclosure has been made in view of the above-described circumstances, and provides a class-D amplifier and a method of controlling a gain of an input audio signal in a class-D amplifier that reduces power consumption in a small output region, and in which the total harmonic distortion factor does not increase. 
     The present disclosure provides a class-D amplifier including: a gain control unit configured to amplify an input audio signal in accordance with a compensation gain to generate an input signal; and a pulse width modulator configured to generate a first pulse whose pulse width changes according to the generated input signal within a first input range where a value of the generated input signal is higher than a first boundary, and a second pulse whose pulse width changes according to generated the input signal within a second input range. The second input range is the range where a value of the generated input signal is lower than a second boundary and partially overlapping the first input range. The gain control unit controls the compensation gain so that first inclination of an input/output characteristic of the class-D amplifier in a first section where the pulse width modulator outputs both the first pulse and the second pulse and second inclination of the input/output characteristic in a second section other than the first section are similar to each other. 
     The present disclosure provides a method of controlling a gain of an input audio signal in a class-D amplifier, the method comprising: amplifying the input audio signal in accordance with a compensation gain to generate an input signal; and generating a first pulse whose pulse width changes according to the generated input signal within a first input range where a value of the generated input signal is higher than a first boundary; generating a second pulse whose pulse width changes according to the generated input signal within a second input range where the value of the generated input signal is lower than a second boundary while partially overlapping the first input range; and wherein the amplifying controls the compensation gain so that inclination of an input/output characteristic of the class-D amplifier in a first section where the pulse width modulator outputs both the first pulse and the second pulse and inclination of the input/output characteristic in a second section other than the first section are similar to each other. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of an example of a class-D amplifier according to an embodiment. 
         FIG. 2  shows waveform examples of an input signal of a pulse width modulator of the class-D amplifier and first and second pulses output from an output stage of the class-D amplifier. 
         FIG. 3  shows an example of an input/output characteristic of a section from the pulse width modulator to the output stage in the class-D amplifier. 
         FIG. 4  shows an example of an input/output characteristic of a section from a gain control unit to the output stage in the class-D amplifier. 
         FIG. 5  is a block diagram of an example of a class-D amplifier according to another embodiment. 
         FIG. 6  is a block diagram of an example of a class-D amplifier according to still another embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. 
       FIG. 1  is a block diagram of a class-D amplifier  1  according to an embodiment. In  FIG. 1 , LC filters  161  and  162  and a speaker SP serving as a load are shown together with the class-D amplifier  1  to facilitate understanding of the configuration of the class-D amplifier  1 . The LC filters  161  and  162  are respectively connected to terminals  151  and  152  of the class-D amplifier  1 . The speaker SP is connected between the LC filters  161  and  162 . The LC filters  161  and  162  serve to remove the high-frequency component of the pulses output from the terminals  151  and  152 . 
     In  FIG. 1 , a subtractor  111  subtracts a feedback signal Vf output from a feedback circuit  170  from an input audio signal Ain supplied via an input terminal  101 , and outputs a signal indicating the subtraction result. An analog signal Ain varies within a voltage range from a maximum value A to a minimum value −A. An integrator  112  integrates and outputs an output signal of the subtractor  111 . The output signal from the integrator  112  is supplied as an input signal Vin to a pulse width modulator  131  via a gain control unit  120 . 
     A carrier wave generator  132  is a circuit that generates a periodic carrier wave C. The carrier wave C in the present embodiment is a triangular wave in which a rising section with a constant gradient and a falling section with a constant gradient are alternately repeated. 
     The pulse width modulator  131  outputs pulses Vp and Vn that are pulse-width modulated according to the input signal Vin based on the input signal Vin and the carrier wave C. More specifically, the pulse width modulator  131  compares a signal C+Vofs obtained by adding a positive offset voltage +Vofs to the carrier wave C with the input signal Vin, and compares a signal C−Vofs obtained by adding a negative offset voltage −Vofs to the carrier wave C with the input signal Vin. Then, the pulse width modulator  131  outputs the first pulse Vp and the second pulse Vn. The first pulse Vp is ON (H level or “1”) in a period in which the value (voltage) of the signal Vin is higher than the value (voltage) of the carrier wave C+Vofs, and Vos is otherwise OFF (L level or “0”). The second pulse Vn is ON (H level or “1”) in a period in which the value of the signal Vin is lower than the value of the carrier wave C−Vofs, and is otherwise OFF (L level or “0”). That is to say, the pulse width modulator  131  generates the first pulse Vp whose pulse width changes according to the input signal Vin within the first input range in which the value of the input signal Vin is larger than a first boundary Vb 1  (=−A+Vofs). Also, the pulse width modulator  131  generates the second pulse Vn whose pulse width changes according to the input signal Vin within a second input range in which the value of the input signal Vin is smaller than a second boundary Vb 2  (=+A−Vofs) and partially overlaps the first input range. Therefore, the pulse width modulator  131  has three states in total: a section (a positive second section) in which only the first pulse Vp changes according to the input signal Vin; a section (a first section) in which both the first pulse Vp and the second pulse Vn change; and a section (a negative second section) in which only the second pulse Vn changes according to the input signal Vin. 
     An output stage  140  amplifies the first pulse Vp and the second pulse Vn, and outputs the amplified pulses as a first pulse P and a second pulse N from the terminals  151  and  152  to the LC filters  161  and  162 . The first pulse Vp and the first pulse P have the same shape. The second pulse Vn and the second pulse N have the same shape. The positive-side input of the speaker SP is supplied with a positive-side voltage of an audio signal obtained as a result of the LC filter  161  removing a high-frequency component from the first pulse P. The negative-side input of the speaker SP is supplied with a negative-side voltage of the audio signal obtained as a result of the LC filter  162  removing the high-frequency component from the second pulse N by. The feedback circuit  170  generates the above-described feedback signal Vf as a result of removing the high-frequency component of the first pulse P and the second pulse N, and supplies the feedback signal Vf to the subtractor  111 . As a result of negatively feeding back the feedback signal Vf, the audio signal supplied to the speaker SP has a voltage waveform having substantially the same shape as the input audio signal Ain. 
     A detector  180  is a circuit that detects the state (at least one of the three states described above) of the pulse width modulator  131  according to the value of the input signal Vin. The detector  180  is, for example, a circuit that monitors the first pulse Vp and the second pulse Vn output from the pulse width modulator  131 , and detects the first section in which both the first pulse Vp and the second pulse Vn are generated. The detector  180  sets a detection signal DET to the H level (or “1”) in the first section in which both the first pulse Vp and the second pulse Vn are generated, and sets the detection signal DET to the L level (or “0”) in a second section other than the first section. 
     In the first section, for example, when the input signal Vin changes in the positive direction, the pulse width modulator  131  outputs the first pulse Vp whose pulse width increases in accordance with the change in the positive direction and the second pulse Vn whose pulse width decreases in accordance with the change in the positive direction. First inclination of the input/output characteristic of the class-D amplifier  1  in the first section is twice as large as second inclination of the input/output characteristic of the class-D amplifier  1  in the positive second section in which only the first pulse Vp is output, or in the negative second section in which only the second pulse Vn is output. For this reason, unless some sort of measure is taken, second inclination of the input/output characteristic of the class-D amplifier  1  in the second section becomes half of first inclination of the input/output characteristic in the first section, and the input/output characteristic of the class-D amplifier  1  becomes nonlinear. As a result, the total harmonic distortion factor is higher than that in the case where inclination of the input/output characteristic is linear in all sections. 
     Accordingly, in the present embodiment, the pulse width modulator  131  and the gain control unit  120  are connected in series in the section between the input terminal  101  and the terminals  151  and  152 . Specifically, the gain control unit  120  is provided in a preceding stage of the pulse width modulator  131 . 
     The gain control unit  120  is configured to compensate for a nonlinear input/output characteristic of the class-D amplifier  1  to make it linear. On the basis of the detection signal DET, for example, the gain control unit  120  is a circuit that performs control such that first of the input/output characteristic of the class-D amplifier  1  in the first section and second inclination of the input/output characteristic of the class-D amplifier  1  in the second section other than the first section are similar to each other or become the same. Specifically, the gain control unit  120  sets the gain (inclination of the input/output characteristic) of the gain control unit  120  when the detection signal DET is at the L level to be twice as large as the gain of the gain control unit  120  when the detection signal DET is at the H level. 
     The configuration of the class-D amplifier  1  has been described above. 
     Next, operations of the present embodiment will be described. In the present embodiment, the duty ratio of the first pulse P and the duty ratio of the second pulse N when there is no signal depend on the offset voltages ±Vofs. As the absolute values of the offset voltages ±Vofs increase, the two duty ratios decrease. In the present embodiment, by appropriately setting the offset voltages ±Vofs, the two duty ratios are each set to an appropriate duty ratio of less than 50%, for example, about 10%. 
     In the present embodiment, when an input audio signal Ain varying within a voltage range from the maximum value A to the minimum value −A is supplied to the input terminal  101 , and then when an input signal Vin is supplied to the pulse width modulator  131 , the first pulse P and the second pulse N that have been pulse-width modulated according to the input signal Vin are output from the terminals  151  and  152 . 
       FIG. 2  is a waveform diagram illustrating the input signal Vin of the pulse width modulator  131  and the first pulse P and the second pulse N output from the output stage  140 . In  FIG. 2 , the horizontal axis represents time and the vertical axis represents voltage. In the example shown in  FIG. 2 , the input signal Vin is sinusoidal, and has a level of zero volts in the middle between its positive and negative peaks. 
     In  FIG. 2 , to facilitate understanding of the relationship between the input signal Vin and the pulse width of each of the first pulse P and the second pulse N, the first pulse P is superimposed on the positive half region of the waveform of the input signal Vin, and the second pulse N is superimposed on the negative half region of the input signal Vin. 
     In the present embodiment, the first pulse P whose pulse width changes according to the input signal Vin is output within the first input range in which the value of the input signal Vin is higher than the first boundary Vb 1  (=−A+Vofs). Also, the second pulse N whose pulse width changes according to the input signal Vin is output within the second input range in which the value of the input signal Vin is lower than the second boundary Vb 2  (=+A−Vofs) and partially overlaps the first input range. 
     In  FIG. 2 , the first pulse P is output in a section TP in which the input signal Vin is higher than the first boundary Vb 1 , and the second pulse N is output in a section TN in which the input signal Vin is lower than the second boundary. In the present embodiment, because the first input range and the second input range overlap each other, there is a section in which the section TP and the section TN overlap each other, that is to say, there is a first section T 1  in which both the first pulse P and the second pulse N are output and a second section T 2  other than the first section T 1 . 
       FIG. 3  shows the input/output characteristic of the section from the pulse width modulator  131  to the terminals  151  and  152 . In  FIG. 3 , the horizontal axis represents input, that is to say, the value (voltage) of the input signal Vin, and the vertical axis represents output, that is to say, the pulse width of the first pulse P or the second pulse N, or the pulse width (time or duty ratio) of a pulse P-N obtained by combining the first pulse P and the second pulse N. In the present embodiment, as shown in  FIG. 3 , the duty ratio of the first pulse P and the duty ratio of the second pulse N when there is no signal (input signal Vin=0) are each smaller than 50%. 
     As shown in  FIG. 3 , in the first input range A 1  in which the input signal Vin is higher than the first boundary Vb 1  (=−A+Vofs), the pulse width of the first pulse P increases with a constant gain GP (inclination of the input/output characteristic) in accordance with the change in the input signal Vin in the positive direction. Also, in the second input range A 2  in which the input signal Vin is lower than the second boundary Vb 2  (=+A−Vofs), the pulse width of the second pulse N increases with a constant gain GN (inclination of the input/output characteristic) in accordance with the change in the input signal Vin in the negative direction. Here, because the first pulse P and the second pulse N are generated based on the common carrier wave C and have the same amplitude, the gain GP and the gain GN have the same value (Gpwm). 
     In  FIG. 3 , the first input range A 1  and the second input range A 2  overlap in an input range A 3  having a lower limit −A+Vofs and an upper limit +A−Vofs. The section in which the input signal Vin is within the input range A 3  is the first section T 1  in which both the first pulse P and the second pulse N are output. In the pulse width modulator  131 , the value of the gain G (inclination of the input/output characteristic) for the pulse P-N obtained by combining the first pulse P and the second pulse N is twice as large as the gains GP and GN of the first pulse P and the second pulse N alone (Gpwm×2). 
     Accordingly, unless some sort of measure is taken, second inclination of the input/output characteristic of the class-D amplifier  1  in the second section T 2  (the gain in the second section T 2 ) becomes half of the gain G in the first section T 1 , and the input/output characteristic becomes nonlinear, and thus the total harmonic distortion factor increases. 
     In the present embodiment, the gain control unit  120  provided in the preceding stage of the pulse width modulator  131  performs control to set the gain G 2  of the gain control unit  120  in the second section T 2  to be twice as large as the compensation gain G 1  of the first section T 1  (G 1 ×2). Specifically, when the signal Ain is within the first section, the gain control unit  120  amplifies the signal Ain with the compensation gain G 1 , and outputs a signal Vin having a value of Ain×G 1 . When the signal Ain is within the positive second section, the gain control unit  120  amplifies the signal Ain with the compensation gain G 2  that is twice as large as the compensation gain G 1  (=G 1 ×2), and outputs a signal Vin having a value of (Ain×2−Vb 2 )×G 1 . When the signal Ain is within the negative second section, the gain control unit  120  amplifies the signal Ain with the compensation gain G 2  that is twice as large as the compensation gain G 1 , and outputs a signal Vin having a value of (Ain×2−Vb 1 )×G 1 . The analog circuit can be shared between the positive and negative second sections because only the boundaries that are used are different. 
     It is difficult to generate accurate analog values of the boundaries Vb 1  and Vb 2 , and the boundaries Vb 1  and Vb 2  may vary depending on temperature, moisture, and the like. Accordingly, the signal Ain at the time of the signal Ain entering the second section from the first section may be sampled and held, and the held value Vh may be used as the boundaries Vb 1  and Vb 2 . Specifically, when the signal Vin enters the positive second section from the first section, +A−Vofs is held as the value Vh, and when the signal Vin enters the negative second section from the first section, −A+Vofs is held as the value Vh. Then, when the signal Ain is within the second section, the gain control unit  120  outputs a signal Vin having a value of Vh×G 1 +(Ain−Vh)×2×G 1 =(Ain×2−Vh)×G 1 . The boundary to be used is automatically switched, and it is not necessary to distinguish whether the second section is the positive second section or the negative second section. The boundaries Vb 1  and Vb 2  obtained are highly accurate. 
     As a result, the input/output characteristic of the class-D amplifier  1  becomes linear as illustrated in  FIG. 4 . Even when the input signal Vin is within any of the input ranges A 1 , A 2 , and A 3 , the same gain G (=2×G 1 ×Gpwm) is maintained. Therefore, the total harmonic distortion factor does not increase. 
     As described above, according to the present embodiment, the pulse width modulator  131  that generates the first pulse Vp whose pulse width changes according to the input signal Vin in the first input range A 1  in which the value of the input signal Vin is higher than the first boundary Vb 1  (=−A+Vofs), and that generates the second pulse Vn whose pulse width changes according to the input signal Vin in the second input range A 2  in which the value of the input signal Vin is lower than the second boundary Vb 2  (=+A−Vofs) and partially overlaps the first input range A 1 ; and the gain control unit  120  that causes first inclination of the input/output characteristic in the first section T 1  in which the pulse width modulator  131  outputs both the first pulse Vp and the second pulse Vn and second inclination of the input/output characteristic in the second section T 2  other than the first section T 1  to be similar to each other are provided. Therefore, the power consumption in the small output region can be reduced, and the total harmonic distortion factor can be suppressed. 
     Furthermore, according to the present embodiment, because the duty ratio of the first pulse P and the second pulse N when there is no signal is smaller than 50%, the power consumption of the class-D amplifier and the load when there is no signal is reduced. Accordingly, the class-D amplifier according to the present embodiment can achieve a quiet acoustic system in which the operation of an air-cooling fan is reduced. Furthermore, by using the class-D amplifier according to the present embodiment in a battery-driven acoustic system such as a powered speaker, the battery life of the acoustic system can be extended. 
     Other Embodiments 
     While the embodiments have been described above, other embodiments are conceivable. Other embodiments are as follows, for example. 
     (1) The embodiments can be applied to a wide range of class-D amplifiers such as a class-D amplifier with a high output exceeding 100 W and a class-D amplifier with a low output installed in a cellular phone or the like. For a class-D amplifier with a low output, LC filters  161  and  162  may be omitted. 
     (2) In the above embodiment, the pulse width modulator  131  generates the pulse by comparing the input signal with a signal obtained by adding the offset voltage to the carrier wave. However, instead of this, the pulse may be generated by comparing the carrier wave with a signal obtained by adding the offset voltage to the input signal. Alternatively, the pulse may be generated by comparing a signal obtained by adding the offset signal and the carrier wave to the input signal with a threshold value. 
     (3) In the embodiment described above, the gain control unit  120  sets the gain of the gain control unit  120  in the second section T 2  to be twice as large as the gain in the first section T 1 . However, instead of this, the gain of the gain control unit  120  in the first section T 1  may be half as large as the gain in the second section T 2 . Also, instead of changing only the gain of one of the first section T 1  and the second section T 2  in this manner, the gain of both sections may be changed so that first inclination of the input/output characteristic in the first section T 1  and second inclination of the input/output characteristic in the second section T 2  are the same. In short, in the gain control unit  120 , by changing at least one of the gain in the first section T 1  and the gain in the second section T 2  of the gain control unit  120  and setting the ratio of the two gains to 1:2, inclinations of the input/output characteristics of two sections of the class-D amplifier may be made the same. 
     (4) In the above-described embodiment, a triangular wave is used as a carrier wave for pulse-width modulation, but a carrier wave having a waveform other than a triangular wave, such as a sawtooth wave, may be used. 
     (5) In the above embodiment, the gain control unit  120  performs control such that first inclination of the input/output characteristic in the first section T 1  is the same as second inclination of the input/output characteristic in the second section T 2 . However, inclinations of the input/output characteristics in the first section T 1  and in the second section T 2  may not strictly be the same. Even if inclinations of the input/output characteristics in the two sections are not exactly the same, if inclinations are made somewhat similar to each other, the total harmonic distortion factor is reduced by the amount they were made similar to each other. 
     (6)  FIG. 5  is a block diagram illustrating a configuration of a class-D amplifier  1 A according to another embodiment. In  FIG. 5 , the same components shown in  FIG. 1  are denoted by the same reference numerals, and the description thereof will be omitted. 
     In the present embodiment, the detector  180  in the above embodiment is replaced with a detector  180 A. The detector  180  in the above embodiment detects the low output section, based on the output state of the first pulse Vp and the second pulse Vn in the pulse width modulator  131 . In contrast, the detector  180 A detects the low output section by comparing the input audio signal Ain that is supplied from the input terminal  101  with first and second threshold values. 
     Specifically, the detector  180 A sets the detection signal DET to the H level when the value of the input audio signal Ain is within the input range A 3  in which both the first pulse Vp and the second pulse Vn are generated, that is to say, when the value of the input audio signal Ain is within the range from the first boundary Vb 1  (=−A+Vofs) to the second boundary Vb 2  (=+A−Vofs), and otherwise sets the detection signal DET to the L level. The operations of other units are the same as those of the above embodiment. Also in the present embodiment, the same effect as that of the above embodiment can be obtained. 
     (7)  FIG. 6  illustrates a configuration of a class-D amplifier  1 B according to still another embodiment. Although pulse-width modulation is performed by the analog circuit in  FIGS. 1 and 5 , pulse-width modulation is performed by a digital circuit in this example. In  FIG. 6 , the same components shown in  FIG. 1  are denoted by the same reference numerals, and the description thereof will be omitted. 
     The class-D amplifier  1 B is provided with an ADC (Analogue Digital Converter)  190 B that performs A/D conversion on the input analog signal Ain. The class-D amplifier  1 B does not include components corresponding to the subtractor  111 , the integrator  112 , the carrier wave generator  132 , and the feedback circuit  170  of the above embodiment ( FIG. 1 ). Furthermore, in the class-D amplifier  1 B, the gain control unit  120 , the pulse width modulator  131 , and the detector  180  of the above embodiment ( FIG. 1 ) are replaced with a gain control unit  120 B, a pulse width modulator  131 B, and a detector  180 B. The output stage  140  is similar to that of the above embodiment. 
     In the class-D amplifier  1 B, the gain control unit  120 B, the pulse width modulator  131 B, the detector  180 B, and the output stage  140  may be digital circuits, or may be functions realized by a processor such as a DSP (Digital Signal Processor) executing a program. 
     In  FIG. 6 , the detector  180 B detects whether or not the value of a digital output signal DAin of the ADC  190 B belongs to the input range A 3  (from Vb 1 =−A+Vofs to Vb 2 =+A−Vofs) of the above embodiment (whether or not the value of DAin is within the first section T 1 ), and supplies the detection signal DET that becomes the H level in the first section to the gain control unit  120 B. As in the above embodiment, the gain control unit  120 B amplifies the digital output signal DAin of the ADC  190 B, so that the compensation gain G 2  in the second section in which the detection signal DET is at the L level is twice (G 1 ×2) as large as the compensation gain G 1  in the first section in which the detection signal DET is at the H level, and supplies the amplified signal to the pulse width modulator  131 B as the input signal Din. Specifically, the gain control unit  120 B amplifies the input signal DAin in the first section with first inclination of the compensation gain G 1  (DAin×G 1 ), and amplifies the input signal DAin in the second section with second inclination of the compensation gain G 2  (=2×G 1 ). More specifically, the value of the amplified signal in the first section is 2×DAin×G 1 . The value of the amplified signal in the second section is (2×DAin−Vb 2 )×G 1 , when the signal DAin is positive. In contrast, the value of the amplified signal in the second section is (2×DAin−Vb 1 )×G 1 , when the signal DAin is negative. Furthermore, the pulse width modulator  131 B generates a first pulse Vp and a second pulse Vn that are pulse-width modulated, based on the input signal Din supplied from the ADC  190 B. 
     Specifically, the pulse width modulator  131 B generates the first pulse Vp that is at the H level when the value of the digital input signal Din is larger than the value obtained by adding the offset value +Vofs to the value corresponding to the carrier wave C (triangular wave) of the above embodiment ( FIG. 1 ), and that otherwise is at the L level. The pulse width modulator  131 B generates the second pulse Vn that is at the H level when the value of the input signal Din 2  is smaller than the value obtained by adding the offset voltage −Vofs to the value corresponding to the carrier wave C, and that otherwise is at the L level. The operations of the gain control unit  120 B and other units are basically the same as those of the above embodiments. Also in the present embodiment, the same effect as that of the above embodiment can be obtained. If the gain control unit  120 B and the detector  180 B shown in  FIG. 6  are replaced with a conversion table for non-linearly converting the value of the signal DAin into the value of the signal Din using the conversion characteristics shown in  FIG. 4 , the same effect as in the above embodiment can be obtained. In the present embodiment, the function of the detector  180 B for determining whether or not the input signal DAin is within the first section is unnecessary.