CN102857176A - Class D power amplifier modulator for digital audio frequencies - Google Patents

Abstract

The present invention relates to modulators for class D power amplifiers for digital audio. The invention belongs to the field of integrated circuit design and the field of digital audio amplification. The modulator is mainly composed of a noise shaping module, two comparators, a triangular wave generating module, an inversion unit, a division by 2 unit, and an adder; wherein, the output end of the noise shaping module is directly connected to the first comparator. input end, the other way is connected to the input end of the second comparator after passing through an inverting unit; the other input ends of the two comparators are connected to the triangular wave generating module; the output ends of the two comparators are connected to an adder The input terminal and the output terminal of the adder are connected to an input port of the noise shaping module through a divide-by-2 unit to form a feedback loop, and the other input port of the noise shaping module is the input port of the modulator. The modulator can directly generate BD mode PWM signals. And it has the characteristics of insensitivity to non-ideal factors and high signal-to-noise ratio.

Description

Modulator for Class D Power Amplifiers for Digital Audio

technical field

The invention belongs to the field of integrated circuit design and the field of digital audio amplification. In particular, it relates to a signal for driving a class-D power amplifier operating in BD mode for generating a high signal-to-noise ratio.

Background technique

A class D power amplifier is an amplifier that works in switch mode, and uses the on and off of the switch to realize the function of power amplification. Figure 1 is a structural diagram of a typical digital full-bridge Class D power amplifier. The amplifier is mainly composed of an interpolation filter, a modulator and a class D power tube in series; its input signal is interpolated by the interpolation filter and then input to the modulator, and the modulator generates two 1-bit signals H+ and H- to drive the D Class power tube. A typical class D power tube is shown in Figure 2. The power tube consists of four MOS tubes M1, M2, M3, and M4. Its input signals H+ and H- are the output signals of the modulator.

The class D power tube of the full bridge class D power amplifier can work in AD mode and BD mode. Figure 3 is the schematic diagram of AD mode, and Figure 4 is the schematic diagram of BD mode. For the AD mode signal, H+ is the inverse of H-. After being amplified by the power tube, the difference between H+ and H- is obtained on the speaker Diff, which is equal to twice the size of H+. If the rising or falling edges of H+ and H- are not aligned, it is easy to introduce additional noise in the circuit. The rising and falling edges of the signals H+ and H- in BD mode do not need to be aligned, and the difference signal Diff of H+ and H- obtained on the speaker does not have additional noise due to lack of alignment, and the high-frequency noise of the circuit is reduced.

The structure of the existing modulator and the way to generate the BD mode signal are shown in Figure 5. The modulator is mainly composed of a Delta-Sigma modulator and a pulse width modulation (PWM) signal generator connected in series. The way the modulator generates the BD mode signal is as follows: the original audio signal has a bit width of N and a sampling rate of fs, and after passing through an interpolation filter, it becomes a signal with a bit width of N and a sampling rate of k times fs, which is interpolated The subsequent high-speed signal is used as the input of the modulator. After passing through the Delta-Sigma modulator in the modulator, a signal with a sampling rate k times fs and a bit width of n bits is output, where n is less than N. The signal can achieve the same accuracy as the original input signal with a sampling rate of fs and a bit width of N within a half fs bandwidth. The output signal of the Delta-Sigma modulator is used as the input signal of the pulse width modulation (PWM) signal generator. The PWM signal generation module divides the input signal into two paths. All the way is directly compared with the internal triangle wave to generate H+ signal. In the other way, the input signal is reversed, and then compared with the internal triangle wave to generate an H-signal. The H+ signal and the H- signal together constitute a signal for driving the BD mode of the class D power transistor.

The PWM signal generation of the existing modulator structure is located outside the Delta-Sigma modulator, the noise and harmonics introduced by this part cannot be suppressed, and the signal-to-noise harmonic ratio (SNDR) cannot meet the requirements of high-end devices. Among the published papers, "Class D Audio Power Amplifiers for Mobile Applications, Berkhout, M Dooper, L, Circuits and SystemsI: Regular Papers, IEEE Transactions on" achieved the best SNDR, and only 80dB .

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and propose a modulator for a class D power amplifier for digital audio, the modulator can directly generate a BD mode PWM signal. And it has the characteristics of insensitivity to non-ideal factors and high signal-to-noise ratio.

A kind of modulator used for the class D power amplifier of digital audio frequency that the present invention proposes, is characterized in that, this modulator mainly is made up of noise shaping module, two comparators, triangular wave generating module, a negation unit, a dividing unit by 2 and an adder; wherein, one of the output ends of the noise shaping module is directly connected to the input end of the first comparator, and the other is connected to the input end of the second comparator after passing through an inverting unit; the other end of the two comparators One input terminal is connected to the triangular wave generation module; the output terminals of the two comparators are connected to the input terminal of an adder, and the output terminal of the adder is connected to an input port of the noise shaping module after passing through a divide-by-2 unit to form a feedback loop, the other input port of the noise shaping module is the input end of the modulator.

Features and beneficial effects of the present invention:

The present invention is characterized in that a modulator is used to directly generate a signal for driving a class D power tube to work in a BD mode. Compared with the structure of the pulse width modulation (PWM) signal converted from the pulse density modulation (PDM) signal to the BD mode, all the noise-introducing parts of the present invention are all within the filter loop, which can obtain higher SNR.

The invention can directly generate the PWM signal of the BD mode, the SNDR of the generated PWM signal can reach 120dB, and the SNDR of the signal output by the class D power tube can reach 98dB.

Description of drawings

Fig. 1 is the structure and functional block diagram of existing full-bridge class D power amplifier;

Figure 2 is a circuit diagram of a typical class D power tube;

Fig. 3 is a schematic diagram of the generation and working principle of the AD mode signal of the class D power tube of the full bridge class D power amplifier;

Fig. 4 is a schematic diagram of the generation and working principle of the BD mode signal of the class D power tube of the full bridge class D power amplifier;

Fig. 5 is the structure and functional block diagram of existing modulator;

Fig. 6 is the modulator structure and functional block diagram of the class D power amplifier for digital audio of the present invention;

Fig. 7 is a structural block diagram of an implementation solution of the present invention.

Detailed ways

The modulator of the class D power amplifier for digital audio proposed by the present invention is described in detail as follows in conjunction with accompanying drawings and embodiments:

The structure and principle of the modulator of the present invention are shown in FIG. 6 . The present invention proposes a completely new modulator structure. The modulator is mainly composed of a noise shaping module, two comparators, a triangular wave generating module, an inversion unit, a division by 2 unit, and an adder; wherein, the output end of the noise shaping module is directly connected to the first comparator. The other input terminal is connected to the input terminal of the second comparator after passing through an inverting unit (the triangle marked -1 in Figure 4); the other input terminals of the two comparators are connected to the triangle wave generation module; the two comparators The output terminal of the adder is connected to the input terminal of an adder, and the output terminal of the adder is connected to an input port of the noise shaping module after passing through a divide-by-2 unit (the triangle marked as 0.5 in Figure 4) to form a feedback loop, Another input port of the noise shaping module is the input end of the modulator.

The working principle of the present invention is as follows: the noise shaping module outputs a high-speed code stream with n bits and a rate of Fs. One end of the comparator has an input rate of Fs and a triangle wave whose peak-to-peak value is 2 ⁿ . Split the output of the noise shaping block into two paths. All the way is directly sent to the first comparator, in the first comparator, the signal is subtracted from the triangular wave of the triangular wave generating module, if it is greater than 0, it will output 1, if it is less than 0, it will output 0, and the output signal is H+. The other path is sent to the second comparator after being inverted by the inverting unit, and subtracted from the triangle wave in the second comparator. Similarly, if it is greater than 0, it will output 1, and if it is less than 0, it will output 0, and the output signal is H-. The 1-bit signals H+ and H- from the two comparators are subtracted by the adder to obtain a signal V, and the value of the signal V is equal to the value of the signal obtained at both ends of the speaker. V is fed back to the noise shaping module after being multiplied by 0.5 by the divide-by-2 unit.

Let the noise introduced by the two comparators be E ₁ and E ₂ respectively,

H+=U×L ₀ +V×0.5×L ₁ +E ₁

H-=U×L ₀ +V×0.5×L ₁ +E ₂

${\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; STF</span>}}{\mathrm{<span\; class="notranslate">\; NTF</span>}}$

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="L"\; filenames="HDA00001868529800033.png"\; state="\{\{state\}\}">L</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; NTF</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; NTF</span>}}$

V=(H+)-(H-)

Inferred

V=2U×STF+(E ₁ -E ₂ )×NTF

Among them: STF is the signal transfer function, NTF is the noise transfer function, U is the input signal, V is the difference of the output signal of the modulator, L ₀ is the transfer function of the input signal in the noise shaping unit, L ₁ feedback signal in the noise shaping The transfer function within the unit. Through reasonable settings, STF is all-passed to the input signal, and NTF is high-passed to the noise, so that the SNDR of the output signal can be equal to the SNDR of the input signal within the audio bandwidth.

One of the specific implementation schemes of the present invention is shown in Figure 7, the modulator is mainly composed of four integrators, two _g1 amplification modules, a triangular wave generation module, two comparators, an inversion unit, and a division by 2 unit and multiple adders (because the multiplication of coefficients in the implementation process is often implemented by addition, so the number of adders is uncertain); the noise shaping module is composed of four integrators, two g ₁ amplification modules and four adders, four An adder and four integrators are connected in series at intervals, the first g ₁ amplification module is connected to the output of the first adder and the second integrator, and the second g ₁ amplification module is connected to the third adder and the output of the second integrator Output of the fourth integrator.

The four integrators in this embodiment will amplify the input signal to a certain extent, and the amplification factors are c ₁ , c ₂ , c ₃ , and c ₄ respectively. The output of the fourth integrator is divided into two paths, one path is directly connected to the first comparator, and the other path is connected to the second comparator after passing through an inversion unit (the triangle marked -1 in Figure 7). The other inputs of the two comparators are connected to the triangle wave generating block. The outputs of the two comparators are connected to the two inputs of an adder. The output of the adder passes through a divide-by-2 unit (triangle marked 0.5 in Figure 7) as the final feedback.

In this embodiment, the bit width of the output signal of the fourth integrator is n bits, and n can be selected in a range less than N. Considering the required modulation accuracy and the stability of the modulator, this embodiment selects n= 5. The triangle wave generator generates a triangle wave with a peak-to-peak magnitude of 2 ⁿ at a rate Fs equal to the output of the integrator.

Each parameter is as shown in table 1 in the present embodiment:

Table 1

c ₁ c ₂ c ₃ c ₄ g ₁ g ₂ 5/256 5/256 5/64 5/64 1/16 41/131072

For a signal whose input precision is 16 bits in the present invention, the SNDR of the output signal of the modulator can reach 98dB. For a signal with an input precision of 20 bits, the SNDR of the modulator output signal can be obtained at 120dB. Use the circuit simulation tool to simulate, drive the class D power tube with the signal output by the modulator, and you can get a power signal with an SNDR of 97.6dB at the output end of the power tube.

Claims (2)

1. a modulator that is used for the D power-like amplifier of digital audio is characterized in that, this modulator mainly by noise shaping module, two comparators, triangular wave generation module, negate unit, one consist of except Unit 2 and an adder; Wherein, the output one tunnel of noise shaping module is connected directly to the input of the first comparator, and another road is connected to the input of the second comparator after by a negate unit; Another input of two comparators all is connected to the triangular wave generation module; The output of two comparators is connected to the input of an adder, the output of adder consists of a feedback control loop through one except being connected to an input port of noise shaping module behind Unit 2, and another input port of noise shaping module is the input of modulator.

2. modulator as claimed in claim 1 is characterized in that, described noise shaping module is by four integrators, two g ₁Amplification module and four adders consist of; Described four adders and four mutually successively series connection of integrator compartment of terrain, first g ₁Amplification module is connected to the output of first adder and second integral device, second g ₁Amplification module is connected to the output of the 3rd adder and the 4th integrator.

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