CN111698628B - Spread spectrum modulation method and circuit, audio amplifier - Google Patents
Spread spectrum modulation method and circuit, audio amplifier Download PDFInfo
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
- H03F3/183—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
- H03F3/185—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only with field-effect devices
- H03F3/1855—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only with field-effect devices with junction-FET devices
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K4/00—Generating pulses having essentially a finite slope or stepped portions
- H03K4/06—Generating pulses having essentially a finite slope or stepped portions having triangular shape
- H03K4/08—Generating pulses having essentially a finite slope or stepped portions having triangular shape having sawtooth shape
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Abstract
The invention discloses a spread spectrum modulation method, a spread spectrum modulation circuit, a pulse width modulation method of an audio signal and an audio amplifier, wherein the spread spectrum modulation method comprises the following steps: generating a spread spectrum modulation signal and generating a sawtooth wave signal based on the spread spectrum modulation signal; generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal; and changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal. The multi-frequency sawtooth wave signal can be obtained by the spread spectrum modulation method, and the EMI radiation interference of the audio amplifier can be reduced by the multi-frequency sawtooth wave signal.
Description
Technical Field
The invention relates to the technical field of audio amplification, in particular to a spread spectrum modulation method, a spread spectrum modulation circuit, a pulse width modulation method of an audio signal and an audio amplifier.
Background
An audio amplifier, such as a class D audio amplifier, performs PWM (Pulse Width Modulation) processing on an audio signal through a sawtooth wave signal such as a triangular wave signal, converts an analog signal into a square wave signal for output, and obtains a clock signal based on the square wave signal output by the Pulse Width Modulation, and applies the clock signal to a power output stage circuit of the audio amplifier.
The inventor finds that, when a sawtooth wave signal with a single frequency is adopted, the square wave signal output by the pulse width modulation of the audio amplifier causes the frequency energy of the clock signal to be relatively large, and the problem of relatively large EMI (Electromagnetic Interference) exists.
Disclosure of Invention
Accordingly, the present invention provides a spread spectrum modulation method, a spread spectrum modulation circuit, a pulse width modulation method of an audio signal, and an audio amplifier, which can reduce EMI.
In a first aspect, a spread spectrum modulation method is provided, including:
generating a spread spectrum modulation signal and generating a sawtooth wave signal based on the spread spectrum modulation signal;
generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal;
and changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal.
In one embodiment, the step of generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal comprises: and generating a pseudo random code according to the output frequency of the spread spectrum modulation signal, and generating the spread spectrum random code signal according to the pseudo random code.
In one embodiment, the step of generating the pseudo random code according to the output frequency of the spread spectrum modulation signal comprises correspondingly generating a pseudo random code in each period of the spread spectrum modulation signal.
In one embodiment, the spread spectrum modulation signal is a square wave signal.
In one embodiment, the step of generating the spread spectrum modulation signal comprises: generating a reference square wave signal, and carrying out frequency division on the reference square wave signal to obtain at least one frequency division square wave signal, wherein the frequency of each frequency division square wave signal is different;
the step of generating a sawtooth wave signal based on the spread spectrum modulation signal comprises: generating each path of sawtooth wave signals with the same frequency as each square wave signal based on the reference square wave signal and each frequency division square wave signal;
the step of generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal comprises: generating a spread spectrum random code signal by using the output frequency of the frequency division square wave signal with the lowest frequency in each square wave signal;
the step of changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal comprises: and changing the frequencies of the reference square wave signal and each path of frequency division square wave signal according to the spread spectrum random code signal, thereby correspondingly generating each path of multi-frequency sawtooth wave signal.
In a second aspect, a method for pulse width modulating an audio signal is provided, including:
generating a multi-frequency sawtooth wave signal according to the spread spectrum modulation method in any one of the embodiments;
and performing pulse width modulation on the audio signal by using the sawtooth wave signal to obtain a pulse width modulation signal with broadened frequency.
In a third aspect, a spread spectrum modulation circuit is provided, including:
the spread spectrum logic control circuit is used for generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal;
and the spread spectrum modulation signal generating circuit is used for generating the spread spectrum modulation signal, generating a sawtooth wave signal based on the spread spectrum modulation signal and changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal.
In one embodiment, the spread spectrum modulation signal generating circuit is used for generating a square wave signal as the spread spectrum modulation signal.
In one embodiment, the spread spectrum modulation signal generation circuit is configured to generate a reference square wave signal and divide the frequency of the reference square wave signal to obtain at least one frequency-divided square wave signal, where the frequency of each frequency-divided square wave signal is different; the square wave signal comprises the reference square wave signal and a frequency division square wave signal;
the modulation signal circuit is used for generating each path of sawtooth wave signals with the same frequency as each square wave signal based on the reference square wave signal and each frequency division square wave signal;
the pseudo-random code generating circuit is used for generating the spread spectrum random code signal by using the frequency division square wave signal with the lowest frequency in each square wave signal; the spread spectrum modulation signal generating circuit is used for changing the frequency of the reference square wave signal and each path of frequency division square wave signal according to the spread spectrum random code signal, thereby correspondingly generating each path of multi-frequency sawtooth wave signal.
In one embodiment, the spread spectrum modulation signal generation circuit comprises a comparator, at least one charge and discharge circuit and at least one group of switches, wherein the output end of the comparator is connected with the charge and discharge circuit, and the charge and discharge circuit is connected with the switches; the first input end of the comparator is used for selectively accessing a first reference voltage and a second reference voltage, and the first reference voltage is higher than the second reference voltage; the second input end of the comparator is connected with the charge-discharge circuit to access charge-discharge voltage;
the comparator is used for outputting a square wave signal by utilizing the charging and discharging function of the charging and discharging circuit, outputting a high level when a first reference voltage is connected, charging the charging and discharging circuit, outputting a low level when the charging voltage reaches the first reference voltage, switching to be connected to a second reference voltage, discharging the charging and discharging circuit, outputting a high level when the discharging voltage reaches the second reference voltage, and switching to be connected to the first reference voltage;
the charging and discharging circuit is used for outputting a sawtooth wave signal according to the square wave signal output by the comparator;
the switch is also connected with the spread spectrum logic control circuit, and the spread spectrum logic control circuit is used for changing the on-off of the switch by using a spread spectrum random code signal to change the charging and discharging frequency so as to change the frequency of the square wave signal.
In one embodiment, the set of switches includes at least two switches;
the charging and discharging circuit is an RC charging and discharging circuit and comprises at least two charging and discharging resistors, the charging and discharging resistors are connected in series, the switches are connected in parallel, and two adjacent switches are connected with one charging and discharging resistor; or the charging and discharging circuit is an RC charging and discharging circuit and comprises at least two charging and discharging capacitors, the charging and discharging capacitors are connected in parallel, and each switch is connected with one charging and discharging capacitor in series; or the charge and discharge circuit is an IC charge and discharge circuit and comprises at least two charge and discharge capacitors, the charge and discharge capacitors are connected in parallel, and each switch is connected with one charge and discharge capacitor in series.
In one embodiment, the spread spectrum modulation signal generating circuit comprises at least two paths of charging and discharging circuits and at least two groups of switches; each charge and discharge circuit is correspondingly connected with a group of switches;
the spread spectrum modulation signal generation circuit further comprises a frequency divider, the output end of the comparator is connected with the charge and discharge circuit through the frequency divider so as to obtain each frequency division square wave signal through the frequency divider, and the output end of the comparator is directly connected with the charge and discharge circuit so as to obtain the reference square wave signal.
In one embodiment, the spread spectrum modulation circuit further comprises a reference voltage generation circuit and a buffer circuit, the buffer circuit is respectively connected with the reference voltage generation circuit and the comparator, the reference voltage generation circuit is used for providing a first reference voltage and a second reference voltage, and the buffer circuit is used for selectively outputting the first reference voltage and the second reference voltage and isolating the interference of the inversion of the output of the comparator.
In one embodiment, the buffer circuit comprises a first operational amplifier, a first RC circuit, a P-type MOS transistor M1, a second operational amplifier, a second RC circuit and an N-type MOS transistor; the first RC circuit comprises a capacitor C3 and a resistor R5, and the second RC circuit comprises a capacitor C4 and a resistor R6;
the positive input end of the first operational amplifier is connected with a reference voltage circuit, the negative input end of the first operational amplifier is connected with the output end of the first operational amplifier, the output end of the first operational amplifier is also respectively connected with the first end of a capacitor C3 and the first end of a resistor R5, the second end of a capacitor C3 is grounded, the second end of a resistor R5 is connected with the source end of a P-type MOS tube, the drain end of the P-type MOS tube is connected with the first input end of a comparator, and the P-type MOS tube provides a first reference voltage when being conducted;
the positive input end of the second operational amplifier is connected with a reference voltage circuit, the negative input end of the second operational amplifier is connected with the output end of the second operational amplifier, the output end of the second operational amplifier is further connected with the first end of a capacitor C4 and the first end of a resistor R6 respectively, the second end of a capacitor C4 is used for decoding, the second end of a resistor R6 is connected with the source end of an N-type MOS tube, the drain end of the N-type MOS tube is connected with the first input end of a comparator, and the N-type MOS tube provides a second reference voltage when being conducted.
In one embodiment, the spread spectrum logic control circuit comprises a pseudo random code generation circuit and a spread spectrum random code signal generation circuit; the pseudo-random code generating circuit is used for generating pseudo-random codes according to the output frequency of the spread spectrum modulation signal, and the spread spectrum random code signal generating circuit is used for generating spread spectrum random code signals according to the pseudo-random codes.
In a fourth aspect, an audio amplifier is provided, which includes the spread spectrum modulation circuit as described in any of the above embodiments, and is configured to access an audio signal, perform pulse width modulation on the audio signal according to a sawtooth wave signal generated by the spread spectrum modulation circuit, and send the audio signal after pulse width modulation to a speaker.
In one embodiment, the audio amplifier further comprises a first stage amplifier, a second stage amplifying integrator, a pulse width modulation comparator, a power output stage circuit and a loudspeaker; the input end of the first-stage amplifier is used for accessing an audio signal, the output end of the first-stage amplifier is connected with the input end of the second-stage amplification integrator, the output end of the second-stage amplification integrator is connected with the first input end of the pulse width modulation comparator, the second input end of the pulse width modulation comparator is connected with the spread spectrum modulation circuit so as to access a sawtooth wave signal, the output end of the pulse width modulation comparator is connected with the input end of the power output stage circuit, the pulse width modulation comparator is used for carrying out pulse width modulation on the audio signal amplified by two stages by utilizing the sawtooth wave signal and outputting the pulse width modulation signal to the power output stage circuit, and the output end of the power output stage circuit is connected with the loudspeaker and used for carrying out power amplification on the pulse width modulation signal and then transmitting the amplified signal to the loudspeaker for loudspeaker to carry out loudspeaker.
The spread spectrum modulation method, the spread spectrum modulation circuit, the pulse width modulation method of the audio signal and the audio amplifier can obtain multi-frequency sawtooth wave signals, the audio amplifier can broaden the frequency of square wave signals output by pulse width modulation by using the multi-frequency sawtooth wave signals, and the frequency of clock signals obtained based on the square wave signals output by the pulse width modulation can be dispersed, so that the energy of the clock signals is dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier can be reduced.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
Fig. 1 is a flow chart of a spread spectrum modulation method according to an embodiment of the present invention;
fig. 2 is a flow chart of a spread spectrum modulation method according to another embodiment of the present invention;
FIG. 3 is a schematic diagram of a spread spectrum modulation circuit according to an embodiment of the present invention;
FIG. 4 is a schematic diagram of a spread spectrum modulation signal generating circuit according to an embodiment of the present invention;
FIG. 5a is a schematic diagram of a circuit for generating a reference square wave signal according to an embodiment of the present invention;
FIG. 5b is a schematic diagram of a circuit configuration for generating a frequency-divided square wave signal according to another embodiment of the present invention;
FIG. 6 is a waveform diagram of a signal generated by a spread spectrum modulation signal generating circuit in an embodiment;
FIG. 7 is a schematic diagram of a reference voltage generating circuit and a buffer circuit of a spread spectrum modulation circuit according to another embodiment of the present invention;
FIG. 8 is a waveform diagram of a portion of a clock signal generated by a spread spectrum modulation signal generating circuit according to an embodiment of the present invention;
FIG. 9 is a schematic diagram of a spread spectrum logic control circuit according to an embodiment of the present invention;
FIG. 10 is a diagram illustrating a variation of a pseudorandom code with a spread spectrum modulated signal in an embodiment of the present invention;
FIG. 11 is a graph illustrating the effect of energy comparison before and after clock signal frequency dispersion according to an embodiment of the present invention;
fig. 12 is a schematic structural diagram of an audio amplifier according to an embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the accompanying drawings, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. The following embodiments and their technical features may be combined with each other without conflict.
As described in the background, the inventor has found that when a sawtooth wave signal with a single frequency is used, the square wave signal output by the pulse width modulation of the audio amplifier causes the frequency energy of the clock signal to be relatively large, which causes a problem of relatively large EMI (Electromagnetic Interference) and affects the performance of the audio amplifier.
Based on this, the invention provides a spread spectrum modulation method, which can obtain a multi-frequency sawtooth wave signal, and the EMI of the audio amplifier can be reduced through the multi-frequency sawtooth wave signal.
Fig. 1 is a flowchart illustrating a spread spectrum modulation method according to an embodiment of the present invention, as shown in fig. 1, the spread spectrum modulation method in this embodiment includes steps 102 to 106:
The spread spectrum modulated signal may be a square wave signal. The sawtooth wave signal can be obtained by combining an RC charging and discharging circuit based on the square wave signal.
And 104, generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal.
In one embodiment, a pseudo random code may be generated based on the output frequency of the spread spectrum modulated signal, the spread spectrum random code signal being generated in accordance with the pseudo random code. The spread spectrum random code signals corresponding to different pseudo random codes are different, and the output frequencies of the spread spectrum modulation signals changed by using different spread spectrum modulation signals are different, so that multi-frequency sawtooth wave signals can be obtained. The spread spectrum random code signal can be a level signal, and the output frequency of the spread spectrum modulation signal changed by different level signals is different.
Specifically, a pseudo random code can be correspondingly generated in each signal period of the spread spectrum modulation signal, and compared with a pseudo random code generated in more than two signal periods, the pseudo random code generated in each signal period can enable the frequency of the subsequent spread spectrum modulation signal to be changed to be more, which is more beneficial to obtaining a multi-frequency sawtooth wave signal.
And 106, changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal, thereby generating a multi-frequency sawtooth wave signal.
In this step, changing the output frequency of the spread spectrum modulation signal means changing the output frequency of the spread spectrum modulation signal along with the time, and the multi-frequency sawtooth wave signal means that one sawtooth wave signal has different frequencies in different time periods, including at least two different frequencies.
In this step, the output frequency of the spread spectrum modulation signal is changed, so that the spread spectrum modulation signal obviously comprises more than two different output frequencies in the time dimension, and the sawtooth wave signal is generated based on the spread spectrum modulation signal, so that the generated sawtooth wave signal is multi-frequency.
After the audio amplifier is connected with the audio signal, the audio amplifier can output low-frequency, intermediate-frequency, high-frequency and other audio signals according to the requirement, and correspondingly, sawtooth wave signals of corresponding frequency bands are required for modulation. The sawtooth wave signals can be generated based on square wave signals of corresponding frequency bands, and the generated sawtooth wave signals can be multi-frequency so as to reduce EMI interference. Fig. 2 is a diagram illustrating a spread spectrum modulation method according to another embodiment of the present invention, which can generate multiple multi-frequency sawtooth signals.
As shown in fig. 2, in the spread spectrum modulation method in this embodiment, the step of generating the spread spectrum modulation signal includes the step 202: generating a reference square wave signal, and carrying out frequency division on the reference square wave signal to obtain at least one frequency-divided square wave signal, wherein the frequency of each frequency-divided square wave signal is different. The step of generating a sawtooth wave signal based on the spread spectrum modulation signal comprises the step 204: and generating sawtooth wave signals with the same frequency as each square wave signal based on the reference square wave signal and each frequency division square wave signal. The step of generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal comprises the step 206: and generating the spread spectrum random code signal by using the output frequency of the frequency division square wave signal with the lowest frequency. The step of generating a multi-frequency sawtooth signal by changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal, wherein the spread spectrum modulation signal includes a reference square wave signal and each frequency-divided square wave signal, includes step 208: and changing the frequencies of the reference square wave signal and each frequency division square wave signal according to the spread spectrum random code signal, thereby correspondingly generating each path of multi-frequency sawtooth wave signal.
In this embodiment, the spread spectrum random code signal obtained by using the frequency-divided square wave signal with the lowest frequency changes the frequency of the reference square wave signal and the frequencies of the other frequency-divided square wave signals when the frequency of the frequency-divided square wave signal with the lowest frequency, that is, the frequency of the frequency-divided square wave signal with the largest period, changes once.
In other embodiments, any one of the frequency-divided square wave signals may be selected as the spread spectrum modulation signal.
In summary, the spread spectrum modulation method in the embodiment of the present invention may obtain a multi-frequency sawtooth wave signal, and the audio amplifier may broaden the frequency of the square wave signal output by the pulse width modulation by using the multi-frequency sawtooth wave signal, so that the frequency of the clock signal obtained based on the square wave signal output by the pulse width modulation may also be dispersed, and thus the energy of the clock signal is also dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier may be reduced.
The embodiment of the invention also provides a pulse width modulation method of the audio signal, which generates a sawtooth wave signal according to the spread spectrum modulation method in any embodiment; and performing pulse width modulation on the audio signal by using the multi-frequency sawtooth wave signal to obtain a pulse width modulation signal with the frequency widened. The frequency broadening, i.e. the range of the frequency band of the pulse width modulation signal, is expanded, for example, the frequency band before broadening is fixed at 100HZ, and the frequency band after broadening is expanded to 50 HZ-500 HZ. The pulse width modulated signal is a square wave signal. According to the pulse width modulation method of the audio signal, the frequency of the square wave signal output by the pulse width modulation can be broadened through the multi-frequency sawtooth wave signal, and then the frequency of the clock signal obtained based on the square wave signal output by the pulse width modulation can be dispersed, so that the energy of the clock signal is dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier can be reduced.
The invention also provides a spread spectrum modulation circuit, which can obtain multi-frequency sawtooth wave signals, and the EMI of the audio amplifier can be reduced through the multi-frequency sawtooth wave signals.
Fig. 3 is a schematic structural diagram of a spread spectrum modulation circuit 300 according to an embodiment of the present invention, and as shown in fig. 3, the spread spectrum modulation circuit 300 in this embodiment includes:
a spread spectrum logic control circuit 310, configured to generate a spread spectrum random code signal according to an output frequency of the spread spectrum modulation signal; the spread spectrum modulation signal generating circuit 320 is configured to generate the spread spectrum modulation signal and generate a sawtooth wave signal based on the spread spectrum modulation signal, and is configured to change a frequency of the spread spectrum modulation signal according to a spread spectrum random code signal, thereby outputting a sawtooth wave signal of multiple frequencies.
In the spread spectrum modulation circuit 300, the changed spread spectrum modulation signal obviously has more than two output frequencies in the time dimension, and the sawtooth wave signal is generated based on the spread spectrum modulation signal, so that the generated sawtooth wave signal is multi-frequency. The frequency of the square wave signal output by the pulse width modulation can be broadened by the audio amplifier through the multi-frequency sawtooth wave signal, and then the frequency of the clock signal obtained based on the square wave signal output by the pulse width modulation can be dispersed, so that the energy of the clock signal is dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier can be reduced.
The sawtooth signal may be generated based on a square wave signal, the frequency of which is changed by changing the frequency of the square wave signal. Therefore, in one embodiment, the spread spectrum modulation signal generating circuit 320 is used to generate a square wave signal as the spread spectrum modulation signal.
In one embodiment, as shown in fig. 4, the spread spectrum modulation signal generating circuit 320 includes a comparator COMP1, at least one charging and discharging circuit 410, and at least one set of switches 420, wherein an output terminal of the comparator is connected to the charging and discharging circuit, and the charging and discharging circuit 410 is connected to the switches 420; a first input end (non-inverting input end) of the comparator COMP1 is used for selectively connecting a first reference voltage and a second reference voltage, wherein the first reference voltage is higher than the second reference voltage; a second input end (an inverting input end) of the comparator COMP1 is connected to the charge and discharge circuit 410 to receive a charge and discharge voltage.
The comparator COMP1 is configured to output a square wave signal by using a charging and discharging function of the charging and discharging circuit 410, wherein when a first reference voltage is connected, a high level is output, the charging and discharging circuit 410 is charged, when a charging voltage reaches the first reference voltage, the comparator COMP1 outputs a low level, the comparator COMP1 is switched to be connected to a second reference voltage, the charging and discharging circuit 410 is discharged, and when a discharging voltage reaches the second reference voltage, the comparator COMP1 outputs a high level and is switched to be connected to the first reference voltage.
The charging and discharging circuit 410 is used for outputting a sawtooth wave signal according to the square wave signal output by the comparator COMP 1; the switch is further connected to the spread spectrum logic control circuit 310, and the spread spectrum logic control circuit 310 is configured to change the charging and discharging frequency by changing the on/off state of the switch using a spread spectrum random code signal to change the frequency of the square wave signal, so as to change the frequency of the sawtooth wave signal.
For example, the first reference voltage is 0.8VDD, and the second reference voltage is 0.2 VDD. When the comparator COMP1 is connected to 0.8VDD, the output of the comparator COMP1 is at a high level, and a charging and discharging circuit is charged; when the charging voltage reaches 0.8VDD, the output of the comparator COMP1 changes to low level, meanwhile, the reference of the comparator COMP1 switches to 0.2VDD, the charging and discharging circuit discharges, when the discharging voltage reaches 0.2VDD, the output of the comparator COMP1 changes to high level, and meanwhile, the reference of the comparator COMP1 switches to 0.8 VDD.
In one embodiment, the set of switches includes at least two switches; the charge and discharge circuit 410 is an RC charge and discharge circuit, and includes at least two charge and discharge resistors, each charge and discharge resistor is connected in series, each switch is connected in parallel, two adjacent switches are connected with a charge and discharge resistor, and the frequency of the square wave signal can be changed based on the principle that the frequency (or charge and discharge time) of the RC charge and discharge circuit is inversely proportional to the RC value. The spread spectrum logic control circuit 310 changes the on/off of the switch by using the spread spectrum random code signal, so as to change the magnitude of the charging and discharging resistor connected to the charging and discharging circuit to change the RC value to change the frequency of the square wave signal. Or, the RC charging and discharging circuit includes at least two charging and discharging capacitors, each charging and discharging capacitor is connected in parallel, each switch is connected in series with one charging and discharging capacitor, and the spread spectrum logic control circuit 310 changes the on and off of the switch by using the spread spectrum random code signal, so as to change the capacitance value connected to the charging and discharging circuit to change the frequency of the square wave signal.
In another embodiment, the charging and discharging circuit 410 may also be an IC charging and discharging circuit, and includes at least two charging and discharging capacitors, each charging and discharging capacitor is connected in parallel, and each switch is connected in series with a charging and discharging capacitor. The capacitance value connected to the charge and discharge circuit is changed by changing the on-off state of the switch 420, so that the charge and discharge frequency (or the charge and discharge time) is changed to change the frequency of the square wave signal, and the frequency of the sawtooth wave signal is changed.
The spread spectrum modulation circuit 300 in the embodiment of the present invention may generate one square wave signal or more than two square wave signals. After the audio amplifier is connected to the audio signal, the audio amplifier may output low-frequency, intermediate-frequency, high-frequency and other audio signals according to the requirement, and accordingly, a sawtooth wave signal of a corresponding frequency band is required for modulation. These sawtooth signals, which may also be all multi-frequency, may be generated based on square wave signals of the respective frequency bands. In the embodiment where the spread spectrum modulation signal generating circuit 320 generates more than two square wave signals, the spread spectrum modulation signal generating circuit 320 is configured to generate a reference square wave signal and frequency-divide the reference square wave signal to obtain at least one frequency-divided square wave signal; the square wave signals comprise reference square wave signals and frequency division square wave signals, and the frequency of each frequency division square wave signal is different. The spread spectrum modulation signal generating circuit 320 is configured to generate a sawtooth wave signal having the same frequency as each square wave signal according to each square wave signal; the spread spectrum logic control circuit 310 is configured to generate the spread spectrum random code signal by using the square wave signal with the lowest frequency in each square wave signal; the spread spectrum modulation signal generating circuit 320 is configured to change the frequency of each square wave signal according to the spread spectrum random code signal, so as to obtain corresponding sawtooth wave signals of each multiple frequency.
In this embodiment, the spread spectrum random code signal obtained by using the frequency-divided square wave signal with the lowest frequency changes the frequency of the reference square wave signal and the frequencies of the other frequency-divided square wave signals when the frequency of the frequency-divided square wave signal with the lowest frequency, that is, the frequency of the frequency-divided square wave signal with the largest period, changes once.
Specifically, the spread spectrum modulation signal generating circuit 320 includes a frequency divider, at least two charge and discharge circuits, and at least two sets of switches, where each charge and discharge circuit is correspondingly connected to one set of switches; the output end of the comparator is connected with the charging and discharging circuit through the frequency divider so as to obtain each frequency division square wave signal through the frequency divider, and the output end of the comparator is also directly connected with the charging and discharging circuit so as to obtain a reference square wave signal. The direct connection means that the output end of the comparator is directly connected with the charging and discharging circuit without a frequency divider.
For example, as shown in fig. 5a and 5b, the comparator COMP1 of the spread spectrum modulation signal generating circuit 320 is connected to the RC charging and discharging circuit in fig. 5a without frequency division, outputs a reference square wave signal with a frequency of 12.8MHz, and obtains a first high-frequency sawtooth wave signal VOSC 0. The comparator COMP1 further performs frequency division by 16 through the frequency divider in fig. 5b to obtain a square wave signal with frequency division by 16, and obtains a sawtooth wave signal VOSC with a low frequency of 0.8MHz through the RC charging and discharging circuit in fig. 5b based on the square wave signal with frequency division by 16.
The 16-frequency-divided square wave signal CLK _ OSC in fig. 5b generates a spread spectrum random code signal to change the on/off of the switches K1-K7 in fig. 5a to change the magnitude of the resistance connected to the charge and discharge circuit, thereby changing the frequency of the sawtooth wave signal VOSC0, and also to change the on/off of the switches K8-K14 in fig. 5b to change the magnitude of the resistance connected to the charge and discharge circuit, thereby changing the frequency of the sawtooth wave signal VOSC.
Fig. 6 is a schematic waveform diagram of the square wave signal COMP _ OUT, the frequency-divided square wave signal CLK _ OSC, the first sawtooth wave signal VOSC0, and the second sawtooth wave signal VOSC output by the comparator COMP1 according to an embodiment. The frequency variation of the second sawtooth wave signal VOSC is consistent with the frequency variation of the frequency-divided square wave signal CLK _ OSC which is a spread spectrum random code signal, and the frequency variation period of the first sawtooth wave signal VOSC0 is 16 times of the frequency variation period of the frequency-divided square wave signal CLK _ OSC. When the frequency of the second sawtooth wave signal VOSC changes once through a cycle, the frequency of the first sawtooth wave signal VOSC0 is also changed.
In the specific implementation process, all the charging and discharging circuits are matched, so that the amplitudes of all the output sawtooth wave signals are consistent. The matching at least comprises the consistency of types and numbers of components in the charging and discharging circuit, the consistency of layout environments and the like.
In another embodiment of the present invention, as shown in fig. 7, the spread spectrum modulation circuit 300 further includes a reference voltage generating circuit 710 and a buffer circuit 720, where the buffer circuit 720 is respectively connected to the reference voltage generating circuit 710 and the comparator COMP1, the reference voltage generating circuit 710 is configured to provide a first reference voltage and a second reference voltage, and the buffer circuit 720 is configured to selectively provide the first reference voltage or the second reference voltage. The buffer circuit 720 can also be used to isolate the interference that the comparator COMP1 outputs flip.
Specifically, as shown in fig. 7, the buffer circuit 720 includes a first operational amplifier OP1, a first RC circuit, a P-type MOS transistor M1, a second operational amplifier OP2, a second RC circuit, and an N-type MOS transistor M2; the first RC circuit comprises a capacitor C3 and a resistor R5, and the second RC circuit comprises a capacitor C4 and a resistor R6; the positive input end of the first operational amplifier OP1 is connected to a reference voltage circuit, the negative input end of the first operational amplifier OP1 is connected to the output end of the first operational amplifier OP1, the output end of the first operational amplifier OP1 is further connected to the first end of a capacitor C3 and the first end of a resistor R5, the second end of a capacitor C3 is used for decoding, the second end of the resistor R5 is connected to the source end of a P-type MOS transistor M1, the drain end of the P-type MOS transistor M1 is connected to the first input end of a comparator COMP1, and the P-type MOS transistor M1 provides a first reference voltage when turned on; the positive input end of the second operational amplifier OP2 is connected to a reference voltage circuit, the negative input end of the second operational amplifier OP2 is connected to the output end of the second operational amplifier OP2, the output end of the second operational amplifier OP2 is further connected to the first end of the capacitor C4 and the first end of the resistor R6, the second end of the capacitor C4 is used for decoding, the second end of the resistor R6 is connected to the source end of the N-type MOS transistor M2, the drain end of the N-type MOS transistor M2 is connected to the first input end of the comparator COMP1, and the N-type MOS transistor M2 provides a second reference voltage when turned on. Specifically, as shown in fig. 7, the reference voltage circuit includes a resistor R1, a resistor R2, a resistor R3, and a resistor R4 connected in series in sequence, a first end of the resistor R1 is connected to the supply voltage, a second end of the resistor R4 is connected to ground, a second end of the resistor R1 is connected to the positive input end of the first operational amplifier OP1, and a second end of the resistor R3 is connected to the positive input end of the second operational amplifier OP 2.
Further, the spread spectrum modulation signal generating circuit 320 further includes a first inverter N1, an output terminal of the comparator COMP1 is connected to an input terminal of the first inverter N1, a gate terminal of the P-type MOS transistor M1 is connected to an output terminal of the first inverter N1 at an output terminal of the comparator COMP1, and when the comparator COMP1 outputs a high level, the first inverter N1 outputs a low level, so that the P-type MOS transistor M1 is maintained to be turned on. The gate terminal of the N-type MOS transistor M2 is connected to the output terminal of the first inverter N1 at the output terminal of the comparator, and when the comparator COMP1 outputs a low level, the first inverter N1 outputs a high level, so that the N-type MOS transistor M2 can be maintained to be turned on.
Still further, on the premise of including the first inverter N1, in order to make the change of the sawtooth wave signal output by the RC charging and discharging circuit in fig. 5a consistent with the square wave signal output by the comparator COMP1, for example, the spread spectrum modulation signal generating circuit 320 further includes a second inverter N2, the output of the first inverter N1 is connected to the input of the second inverter N2, and the output of the second inverter N2 is connected to the RC charging and discharging circuit, that is, the output of the comparator COMP1 passes through two inverters, and then passes through the RC charging and discharging circuit to output the sawtooth wave signal. The introduction of an inverter helps to improve the stability of the square wave signal.
In an embodiment where the square wave signal output by the comparator COMP1 needs to be divided by the frequency divider, as shown in fig. 5b, the frequency divider may divide the frequency of the square wave signal output by the first inverter N1, and the divided signal may be inverted by an inverter and output as the divided square wave signal CLK _ OSC.
In other embodiments of the present invention, the frequency-divided square wave signal output by the frequency divider may also be used as a clock signal for other chips of the audio amplifying circuit. For example, as shown in fig. 5b, the frequency divider outputs some divided clock signals, and assuming that the frequency of the square wave signal is 12.8MHz, the clock signal CLK _ PA is a 16-divided square wave signal with a frequency of 0.8MHz, which can be used as the clock signal of the class D audio power amplifier. The clock signal CLK _ AGC is an 8-frequency-division square wave signal with a frequency of 1.6MHz, and is used for timing of an AGC (Automatic Gain Control) chip. The clock signal CLK _ CP is a frequency-divided-by-2 square wave signal having a frequency of 6.4MHz, and can be used as a clock signal for a charge pump in an audio power amplifier. Fig. 8 is a schematic diagram of waveforms of the clock signal CLK _ AGC and the clock signal CLK _ CP.
With respect to spread logic control circuit 310, in one embodiment, spread logic control circuit 310 includes a pseudo-random code generation circuit 910 and a spread random code signal generation circuit 920; the pseudo random code generating circuit 910 is configured to generate a pseudo random code according to an output frequency of the spread spectrum modulation signal, and the spread spectrum random code signal generating circuit 920 is configured to generate a spread spectrum random code signal according to the pseudo random code. The spread spectrum random code signals corresponding to different pseudo random codes are different, and the output frequencies of the spread spectrum modulation signals changed by using different spread spectrum modulation signals are also different.
Specifically, a pseudo-random code may be generated for each period of the spread spectrum modulated signal. Compared with the method that more than two signal periods generate one pseudo random code, the method that one pseudo random code is correspondingly generated in each signal period can change the output frequency of the spread spectrum modulation signal for a plurality of times, and is more favorable for obtaining a multi-frequency sawtooth wave signal.
The degree of each increase or decrease in the output frequency of the spread spectrum modulated signal may be, but is not limited to, 3%, 6%, 12%, etc. of the last frequency.
The generated pseudo-random code may vary cyclically from small to large, then from large to small, and then from small to large over time. For example, as shown in FIG. 10, each cycle of the divided square wave signal CLK _ OSC generates a pseudo random code. The pseudo-random code may be a 3-bit binary code, SW0 is low, SW1 is medium, SW2 is high, and one cycle of the pseudo-random code is 000, 001, 010, 011, 100, 101, 110, 101, 100, 011, 010, 001, 000.
In a specific embodiment where the spread spectrum modulation signal is changed by changing the on/off of the switch, the pseudo random code may be a binary code, and the electrical signal is a high level signal or a low level signal, when the binary code is 0, the electrical signal is a high level signal, the switch is turned on, and when the binary code is 1, the switch is turned off when the electrical signal is a low level signal.
For example, as shown in fig. 9, the spread spectrum random code signal generating circuit 920 may include a three-eight decoder for decoding a 3-bit pseudo random code into an 8-bit binary code, and the spread spectrum random code signal generating circuit 920 may correspondingly output 8 electrical signals, and the one-bit binary code controls one electrical signal. For example, each of fig. 5a and 5b has 7 switches, and the 7 switches can be respectively controlled to be turned on and off by using 7 electrical signals.
In summary, the spread spectrum modulation circuit 300 in the embodiment of the invention can obtain a multi-frequency sawtooth signal. The frequency of the square wave signal output by the pulse width modulation can be broadened by the audio amplifier through the multi-frequency sawtooth wave signal, and then the frequency of the clock signal obtained based on the square wave signal output by the pulse width modulation can be dispersed, so that the energy of the clock signal is dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier can be reduced. Fig. 11 is a graph showing a comparison of the energy before and after the frequency dispersion of the clock signal, and it can be seen from fig. 11 that the peak value of the energy of the clock signal is significantly reduced.
The embodiment of the present invention further provides an audio amplifier, which includes the spread spectrum modulation circuit 300 as described in any of the above embodiments, and the audio amplifier is configured to access an audio signal, perform pulse width modulation on the audio signal according to a sawtooth wave signal generated by the spread spectrum modulation circuit 300, and send the audio signal after pulse width modulation to a speaker.
In another embodiment, as shown in fig. 12, the audio amplifier further comprises a first stage amplifier 1210, a second stage amplification integrator 1220, a pulse width modulation comparator 1230, a power output stage circuit 1240 and a speaker 1250; the input end of the first stage amplifier 1210 is used for receiving an audio signal, the output end of the first stage amplifier 1210 is connected with the input end of the second stage amplification integrator 1220, the output end of the second stage amplification integrator 1220 is connected to the first input end of the pwm comparator 1230, the second input end of the pwm comparator 1230 is connected to the spread spectrum modulation circuit 300 to receive the sawtooth signal, the output of the pwm comparator 1230 is connected to the input of the power output stage circuit 1240, the pwm comparator 1230 is configured to perform pulse width modulation on the two-stage amplified audio signal by using a sawtooth wave signal, and the pulse width modulation signal is output to the power output stage circuit 1240, and the output end of the power output stage circuit 1240 is connected with the loudspeaker 1250, and is used for performing power amplification on the pulse width modulation signal and then transmitting the pulse width modulation signal to the loudspeaker 1250 for playing. The power output stage circuit 1240 may be specifically a power amplifier.
In some embodiments, if the audio amplifier needs to output multiple audio signals of different frequency bands, the number of the pwm comparators and the number of the sawtooth wave signals need to be multiple, and each pwm comparator outputs a pwm signal according to the corresponding sawtooth wave signal.
For example, as shown in fig. 12, the spread spectrum modulation circuit 300 outputs two sawtooth wave signals with different frequency bands, one of which is transmitted to the pwm comparator 1230a, and the other of which is transmitted to the pwm comparator 1230 b.
In summary, in the audio amplifier in the embodiment of the present invention, the frequency of the square wave signal output by the pulse width modulation can be broadened by using the multi-frequency sawtooth wave signal, and then the frequency of the clock signal obtained based on the square wave signal output by the pulse width modulation can also be dispersed, so that the energy of the clock signal is also dispersed, the energy peak value is reduced, and the influence of EMI on the audio amplifier can be reduced.
The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.
Claims (16)
1. A spread spectrum modulation method, comprising:
generating a spread spectrum modulation signal and generating a sawtooth wave signal based on the spread spectrum modulation signal; the spread spectrum modulation signal is a square wave signal, and the sawtooth wave signal is obtained based on the square wave signal and combined with an RC (resistor-capacitor) charging and discharging circuit;
generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal;
and changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal.
2. The spread spectrum modulation method according to claim 1, wherein the step of generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal comprises: and generating a pseudo random code according to the output frequency of the spread spectrum modulation signal, and generating the spread spectrum random code signal according to the pseudo random code.
3. The spread spectrum modulation method according to claim 2, wherein the step of generating a pseudo random code according to the output frequency of the spread spectrum modulation signal comprises: a pseudo-random code is generated for each period of the spread spectrum modulated signal.
4. The spread spectrum modulation method according to claim 1, wherein the step of generating the spread spectrum modulation signal comprises: generating a reference square wave signal, and carrying out frequency division on the reference square wave signal to obtain at least one frequency division square wave signal, wherein the frequency of each frequency division square wave signal is different; the step of generating a sawtooth wave signal based on the spread spectrum modulation signal comprises: generating each path of sawtooth wave signals with the same frequency as each square wave signal based on the reference square wave signal and each frequency division square wave signal;
the step of generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal comprises: generating the spread spectrum random code signal by using the output frequency of the frequency division square wave signal with the lowest frequency;
the step of changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal comprises: and changing the frequencies of the reference square wave signal and each frequency division square wave signal according to the spread spectrum random code signal, thereby correspondingly generating each path of multi-frequency sawtooth wave signal.
5. A method of pulse width modulation of an audio signal, comprising:
generating a multi-frequency sawtooth signal according to the spread spectrum modulation method of any one of claims 1 to 4;
and performing pulse width modulation on the audio signal by using the multi-frequency sawtooth wave signal to obtain a pulse width modulation signal with the frequency widened.
6. A spread spectrum modulation circuit, comprising:
the spread spectrum logic control circuit is used for generating a spread spectrum random code signal according to the output frequency of the spread spectrum modulation signal; the spread spectrum modulation signal is a square wave signal;
the spread spectrum modulation signal generating circuit is used for generating the spread spectrum modulation signal, generating a sawtooth wave signal based on the spread spectrum modulation signal and changing the output frequency of the spread spectrum modulation signal according to the spread spectrum random code signal so as to generate a multi-frequency sawtooth wave signal; the sawtooth wave signal is obtained by combining an RC (resistor-capacitor) charging and discharging circuit based on the square wave signal.
7. The spread spectrum modulation circuit according to claim 6, wherein the spread spectrum modulation signal generation circuit is configured to generate a square wave signal as the spread spectrum modulation signal.
8. The spread spectrum modulation circuit of claim 7 further comprising a modulation signal circuit and a pseudo random code generation circuit; the spread spectrum modulation signal generating circuit is used for generating a reference square wave signal and dividing the frequency of the reference square wave signal to obtain at least one frequency-divided square wave signal, wherein the frequency of each frequency-divided square wave signal is different; the square wave signal comprises the reference square wave signal and a frequency division square wave signal;
the modulation signal circuit is used for generating each path of sawtooth wave signals with the same frequency as each square wave signal based on the reference square wave signal and each frequency division square wave signal;
the pseudo-random code generating circuit is used for generating the spread spectrum random code signal by utilizing the frequency division square wave signal with the lowest frequency in each square wave signal;
the spread spectrum modulation signal generating circuit is used for changing the frequency of the reference square wave signal and each path of frequency division square wave signal according to the spread spectrum random code signal, thereby correspondingly generating each path of multi-frequency sawtooth wave signal.
9. The spread spectrum modulation circuit according to claim 8, wherein the spread spectrum modulation signal generating circuit comprises a comparator, at least one charge and discharge circuit and at least one set of switches, wherein the output terminal of the comparator is connected with the charge and discharge circuit, and the charge and discharge circuit is connected with the switches; the first input end of the comparator is used for selectively connecting a first reference voltage and a second reference voltage, and the first reference voltage is higher than the second reference voltage; the second input end of the comparator is connected with the charge-discharge circuit to access charge-discharge voltage; the comparator is used for outputting a square wave signal by utilizing the charging and discharging function of the charging and discharging circuit, outputting a high level when a first reference voltage is connected, charging the charging and discharging circuit, outputting a low level when the charging voltage reaches the first reference voltage, switching to be connected to a second reference voltage, discharging the charging and discharging circuit, outputting a high level when the discharging voltage reaches the second reference voltage, and switching to be connected to the first reference voltage;
the charging and discharging circuit is used for outputting a sawtooth wave signal according to the square wave signal output by the comparator;
the switch is also connected with the spread spectrum logic control circuit, and the spread spectrum logic control circuit is used for changing the on-off of the switch by using a spread spectrum random code signal to change the charging and discharging frequency so as to change the frequency of the square wave signal.
10. The spread spectrum modulation circuit of claim 9, wherein the set of switches comprises at least two switches;
the charging and discharging circuit is an RC charging and discharging circuit and comprises at least two charging and discharging resistors, the charging and discharging resistors are connected in series, the switches are connected in parallel, and two adjacent switches are connected with one charging and discharging resistor; or the charging and discharging circuit is an RC charging and discharging circuit and comprises at least two charging and discharging capacitors, the charging and discharging capacitors are connected in parallel, and each switch is connected with one charging and discharging capacitor in series; or the charge and discharge circuit is an IC charge and discharge circuit and comprises at least two charge and discharge capacitors, the charge and discharge capacitors are connected in parallel, and each switch is connected with one charge and discharge capacitor in series.
11. The spread spectrum modulation circuit according to claim 9, wherein the spread spectrum modulation signal generating circuit comprises at least two charge-discharge circuits and at least two sets of switches; each charge and discharge circuit is correspondingly connected with a group of switches;
the spread spectrum modulation signal generation circuit further comprises a frequency divider, the output end of the comparator is connected with the charge and discharge circuit through the frequency divider so as to obtain each frequency division square wave signal through the frequency divider, and the output end of the comparator is directly connected with the charge and discharge circuit so as to obtain the reference square wave signal.
12. The spread spectrum modulation circuit according to claim 9, further comprising a reference voltage generation circuit and a buffer circuit, the buffer circuit being connected to the reference voltage generation circuit and the comparator, respectively, the reference voltage generation circuit being configured to provide a first reference voltage and a second reference voltage, and the buffer circuit being configured to selectively output the first reference voltage or the second reference voltage.
13. The spread spectrum modulation circuit according to claim 12, wherein the buffer circuit comprises a first operational amplifier, a first RC circuit, a P-type MOS transistor, a second operational amplifier, a second RC circuit, and an N-type MOS transistor; the first RC circuit comprises a capacitor C3 and a resistor R5, and the second RC circuit comprises a capacitor C4 and a resistor R6;
the positive input end of the first operational amplifier is connected with a reference voltage circuit, the negative input end of the first operational amplifier is connected with the output end of the first operational amplifier, the output end of the first operational amplifier is also respectively connected with the first end of a capacitor C3 and the first end of a resistor R5, the second end of a capacitor C3 is used for decoding, the second end of a resistor R5 is connected with the source end of a P-type MOS tube, the drain end of the P-type MOS tube is connected with the first input end of a comparator, and the P-type MOS tube provides a first reference voltage when being conducted;
the positive input end of the second operational amplifier is connected with a reference voltage circuit, the negative input end of the second operational amplifier is connected with the output end of the second operational amplifier, the output end of the second operational amplifier is further connected with the first end of a capacitor C4 and the first end of a resistor R6 respectively, the second end of a capacitor C4 is grounded, the second end of a resistor R6 is connected with the source end of an N-type MOS tube, the drain end of the N-type MOS tube is connected with the first input end of a comparator, and the N-type MOS tube provides a second reference voltage when being conducted.
14. The spread spectrum modulation circuit of claim 6 wherein the spread spectrum logic control circuit comprises a pseudo random code generation circuit and a spread spectrum random code signal generation circuit; the pseudo-random code generating circuit is used for generating pseudo-random codes according to the output frequency of the spread spectrum modulation signal, and the spread spectrum random code signal generating circuit is used for generating spread spectrum random code signals according to the pseudo-random codes.
15. An audio amplifier comprising the spread spectrum modulation circuit as claimed in any one of claims 6 to 14, wherein the audio amplifier is configured to receive an audio signal, pulse-width modulate the audio signal according to a sawtooth signal generated by the spread spectrum modulation circuit, and send the pulse-width modulated audio signal to a speaker.
16. The audio amplifier of claim 15, further comprising a first stage amplifier, a second stage amplifying integrator, a pulse width modulation comparator, a power output stage circuit, and a speaker; the input end of the first-stage amplifier is used for accessing an audio signal, the output end of the first-stage amplifier is connected with the input end of the second-stage amplification integrator, the output end of the second-stage amplification integrator is connected with the first input end of the pulse width modulation comparator, the second input end of the pulse width modulation comparator is connected with the spread spectrum modulation circuit so as to access a sawtooth wave signal, the output end of the pulse width modulation comparator is connected with the input end of the power output stage circuit, the pulse width modulation comparator is used for carrying out pulse width modulation on the audio signal amplified by two stages by utilizing the sawtooth wave signal and outputting the pulse width modulation signal to the power output stage circuit, and the output end of the power output stage circuit is connected with the loudspeaker and used for carrying out power amplification on the pulse width modulation signal and then transmitting the amplified signal to the loudspeaker for loudspeaker to carry out loudspeaker.
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