Class-d audio amplifier and associated sound bar

Abstract

The present invention relates to a class-D audio amplifier comprising: - a main input (10) for a digital signal to be amplified; - a digital amplification module (16); - a feedback loop (19) comprising a summing unit (32), an amplification input of which is connected to the output of the digital amplification module and another input of which is connected to the main input through a digital-to-analog converter suitable for supplying an input current, the output of the summing unit being connected through an analog-to-digital converter to the input of a subtractor, the other input of which is connected to the main input and the output of which is connected to the input of the digital amplification module, characterized in that the summing unit comprises means (38, 40, 42) for converting the output voltage of the digital amplification module into a return current and is suitable for summing the input current and the return current.

Description

[0001] DESCRIPTION

[0002] Class D audio amplifier and associated soundbar

[0003] The present invention relates to a class D audio amplifier.

[0004] Given the technologies available today, a person skilled in the art is capable of manufacturing high-performance audio amplifiers to produce speakers and soundbars with very good sound quality or with easily accessible technology.

[0005] US10256777 describes a digital Class D audio amplifier comprising a digital input signal, a digital amplification module, and a feedback loop. The digital input signal is first compared to a digital feedback signal from the feedback loop. The output signal from the feedback loop subtractor is then modulated by a modulation block, amplified by a push-pull configuration, and filtered using passive components. The input signal is also used within the feedback loop to improve the accuracy of the Class D audio amplifier.

[0006] However, the feedback loop is often very complex. Indeed, the feedback loop includes an analog summing junction, and this junction is designed to add two voltages: the output signal voltage and the input signal voltage. Since the input signal is a digital signal, a digital-to-analog converter (DAC) is necessary to add the two voltages. Existing DACs on the market convert the digital signal into an analog output signal, but this output signal is, by default, a current. Therefore, a current-to-voltage converter is also required.

[0007] In addition, to ensure that the summed signal is not disturbed, an analog filter is connected at the output of the summing stump and then an analog-to-digital converter is used to obtain the digital return signal.

[0008] It is then observed that the class D audio amplifier contains several complex and not very robust components which make it less functional.

[0009] The aim of the invention is to provide a simpler and more efficient Class D audio amplifier. To this end, the invention relates to a Class D audio amplifier comprising:

[0010] - a main input for a digital signal to be amplified,

[0011] - a digital amplification module,

[0012] - a feedback loop comprising a summing junction, one amplification input of which is connected to the output of the digital amplification module and another input of which is connected to the main input through a digital-to-analog converter suitable for providing an input current, the output of the summing junction being connected through an analog-to-digital converter to the input of a subtractor, the other input of which is connected to the main input and the output of which is connected to the input of the digital amplification module, the summing junction comprising means for converting the output voltage of the digital amplification module into a return current and is suitable for ensuring the summation of the input current and the return current.

[0013] The class D audio amplifier proposed by the invention makes it possible to create a summing amplifier capable of adding two currents without the need for a current-voltage converter.

[0014] According to other advantageous aspects of the invention, the class D audio amplifier comprises one or more of the following features, taken individually or in any technically possible combination:

[0015] - the summing amplifier includes a linear amplifier chosen from the group consisting of an operational amplifier and a transconductance amplifier;

[0016] - the summing amplifier includes an input resistor connected to the output of the digital amplification module, and a feedback resistor connected between an inverting input and the output of the linear amplifier;

[0017] - the amplification module includes a push-pull circuit and an analog filter connected to the output of the push-pull circuit and the amplification input of the summing ...

[0018] - the amplification module includes a push-pull assembly connected to the output of a digital modulator connected to the output of the subtractor, said digital modulator including a low-pass digital filter;

[0019] - the digital modulator includes a programmable digital controller, the controller being suitable for implementing a type of controller chosen from the group consisting of: a phase-lead controller, an integral controller, a proportional controller, a differentiator controller, a proportional-differentiator-integral controller and a phase-lag controller; - the proportional-integral controller is suitable for correcting a linear or non-linear approximation of the analog filter;

[0020] - The Class D audio amplifier further includes a digital filter upstream of the subtractor, with the main input connected to the subtractor through the digital filter; and

[0021] - The digital filter is one of:

[0022] - a pure gain,

[0023] - two cascaded advance-lag compensators,

[0024] - three identical lead-lag compensators in cascade, or

[0025] - a second-order filter.

[0026] The invention also relates to a soundbar comprising a class D audio amplifier as defined above.

[0027] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which: Figure 1 is a schematic of a Class D audio amplifier; and Figure 2 is a Bode plot of a filter gain of the Class D audio amplifier, a corrector gain of the Class D audio amplifier and a Class D audio amplifier gain.

[0028] The Class D audio amplifier 8 shown in Figure 1 is, for example, integrated into a soundbar. The amplifier 8 includes a main input 10 for a digital signal to be amplified. The signal to be amplified is, for example, a voltage.

[0029] The main input 10 is connected to a digital subtractor 14. The output of the subtractor 14 is connected to the input of a digital amplifier module 16. The subtractor 14 is capable of performing a subtraction between the main input 10 and another input 18 from a feedback loop 19.

[0030] Preferably, the digital amplification module 16 includes a digital modulator 20 connected to the output of the subtractor 14.

[0031] Preferably, the digital amplifier module 16 also includes a push-pull circuit 22 connected to the output of the digital modulator 20. The push-pull circuit 22, as is known, comprises two transistors 22A and 22B connected in series between two supply voltages. The output of the push-pull circuit 22 is taken from a midpoint 22M between the two transistors. The gates of the two transistors 22A and 22B are connected to a control circuit 23, which is itself connected to the output of the digital modulator 20.

[0032] For example, one modulation method used by the digital modulator 20 is pulse-width modulation (PWM). This type of modulation is used to discretize a signal at the output of the subtractor 14 using a digital circuit (not shown). The modulation method used by the digital modulator 20 generates a pseudo-analog signal to control the transistors 22A and 22B of the push-pull circuit 22 (in their open or closed states).

[0033] The digital amplification module 16 includes at the output of the push-pull circuit 22 an analog filter 24. As an example, in figure 1 the analog filter 24 comprises a coil 25 connected to the midpoint 22M between the two transistors 22A, 22B on one side and is connected, on the other side, to a capacitor 26. The analog filter 24 is, for example, a low-pass filter itself connected to ground M.

[0034] A loudspeaker 27 is connected to the output of the filter between the coil 25 and the capacitor 26.

[0035] Alternatively, the digital amplification module 16 does not include an analog filter 24 and the midpoint between the two transistors 22A, 22B is connected directly to the loudspeaker 27. In this case, the digital modulator 20 includes a digital low-pass filter 30 in order to reproduce the integrating effect of the analog filter 24.

[0036] Preferably, the digital modulator 20 includes a programmable digital controller 28. Depending on its programming, the controller 28 is capable of implementing many types of correction, including a phase-lead corrector, an integral corrector, a proportional corrector, a differentiator corrector, a proportional-differentiator, an integral corrector, and a phase-lag corrector. By way of example, the controller 28 implements a phase-lead corrector and compensates for a bandwidth 29 limited by the analog filter 24, represented by the Bode plot in Figure 2. Figure 2 shows a curve C1 corresponding to the gain of the Class D audio amplifier 8 for several frequencies. A bandwidth 29 of the amplifier 8 is also shown in Figure 2. A curve C2 shows the gain of the low-pass filter 24, and a curve C3 shows the gain of the phase-lead corrector.

[0037] Curves C2 and C3 have the same slope but opposite signs between 40 kHz and 400 kHz. Since the gain of amplifier 8 depends on the gain of the low-pass filter 24 and the phase-lead corrector of curves C2 and C3, it follows that the gain curve C1 of amplifier 8 is a straight line 32 with a zero slope between 40 kHz and 400 kHz. As an example, the controller 28 has 2, 3, or 4 poles allowing the bandwidth of the Class D audio amplifier 8 to be modified. Indeed, if the number of poles increases, the shape and therefore the slope of curve C3 is modified across all frequencies. Consequently, the shape of curve C1 and the bandwidth 29 are also modified across all frequencies.

[0038] Advantageously, the controller 28 is suitable for correcting a linear or non-linear approximation of the analog filter 24.

[0039] Depending on the type of corrector chosen for adapter 28, the shape of the C3 curve is different.

[0040] Figure 2 shows an example where the compensator has two pairs of zero poles. Depending on the number of poles, the slope of curves C1 and C2 is modified.

[0041] The feedback loop 19 includes a summing 32, one amplification input 34 of which is connected to the output of the digital amplification module 16 and the other input 35 of which is connected to the main input 10 through a digital-to-analog converter 36 suitable for providing an input current 11.

[0042] The 36 digital-to-analog converter, for example, is suitable for achieving a fairly high signal-to-noise ratio (SNR). A signal-to-noise ratio is considered fairly high when it exceeds a value between 100 dB and 110 dB.

[0043] The summing 32 is suitable for converting the output voltage denoted V2 at the output of the digital amplification module 16 received on the input 34 into a return current I2 and is suitable for ensuring the summation of the input current 11 received on the input 35 and the return current I2.

[0044] The summing unit includes, after the input 34, a resistance 37 for converting the voltage V1 into the current 11.

[0045] It also includes a linear amplifier 38 whose inverting input is connected to the resistor 37 and to the input 35 to receive the sum of the currents 11 and I2.

[0046] The linear amplifier 38 is selected from the group consisting of an operational amplifier and a transconductance amplifier. The non-inverting input of the linear amplifier 38 is connected to a reference voltage V re f.

[0047] A feedback resistor 42 is connected between the inverting input 41 and the output of the linear amplifier 38 to receive the input current 11 and the return current I2 representative of the voltage V1.

[0048] The output of the summing 32 is connected via an analog-to-digital converter 44 to the input 18 of the subtractor 14.

[0049] The input of the summing 32 is connected to the output of the analog filter 24. Alternatively, the feedback loop 19, and consequently an input resistor 40, is connected to the output of the push-pull circuit 22 between the midpoint between the two transistors and the inverting input 41 of the linear amplifier 38.

[0050] The 44-bit analog-to-digital converter has a low resolution of between 4 and 10 bits maximum. This implies that the 44-bit analog-to-digital converter is fast and has a latency between 0.1 and 1 microsecond.

[0051] A variant of the Class D 108 audio amplifier is shown in Figure 3.

[0052] In Figure 3, the elements of the Class D 108 audio amplifier that are identical to those of the Class D 8 audio amplifier in Figure 1 will not be listed and bear the same numerical references.

[0053] In other words, the Class D 108 audio amplifier is identical to the Class D 8 audio amplifier except that it also includes a second digital filter 150 upstream of the subtractor 14.

[0054] Ideally, the 150 filter would have a Fidéai transfer function equal to: where K u is the gain of the digital-to-analog converter 36, K yis the gain between the output and the analog summing 32 which is equal to 1 / R where R is the value of the resistor 37, G is the transfer function of the block including the push-pull assembly 22, the analog filter 24, and the loudspeaker 27, and C is the transfer function of the controller 28.

[0055] According to a first example implementation, the 150 filter is a pure gain Fo equal to:

[0056] F Ku

[0057] ° K y G(f0)C(f0) where G( / o) is the gain of the transfer function G at a frequency f0, C(f0) is the gain of the transfer function C at the frequency f0, and f0 is a predefined frequency that minimizes the input of the analog-to-digital converter 44.

[0058] According to a second example, the 150 filter is implemented as two cascaded lead-lag compensators. The 150 filter then has a transfer function Fi equal to: where b 01 , b llt has 01 , b 02 , b 12 , n 12 are coefficients determined empirically using standard techniques for determining these parameters.

[0059] According to a third embodiment, filter 150 is implemented as three identical cascaded lead-lag compensators. Filter 150 then has a transfer function F2 equal to: where b 01 , b ll t has 01 are also coefficients determined empirically.

[0060] According to a fourth embodiment, filter 150 is implemented as a second-order filter. Filter 150 then has a transfer function F3 equal to: where b0, b, b2, ai, a 2 are also empirically determined coefficients.

[0061] According to this variant, the main input 10 is connected to the subtractor 14 through the second digital filter 150. In other words, the main input 10 is directly connected to the second digital filter 150 which is itself directly connected to the subtractor 14.

[0062] According to this variant, the digital-to-analog converter 36 is always connected to the main input 10. Also, the digital-to-analog converter is not connected directly to the input of the subtractor 14 but rather connected upstream of the second digital filter 150.

[0063] Thanks to the second digital filter 150, the amplitude of the signal at the input of the analog-to-digital converter 44 is kept at a low value, for example between 10 mV and 500 mV.

[0064] Also, the second digital filter 150 promotes the fact that the analog-to-digital converter 44 can encode the signal it receives at input on 4 or 10 bits.

[0065] Thanks to the characteristics described above, and more particularly thanks to the summing 32, the amplifier 8, 108 is capable of adding two analog currents directly without needing a current-voltage converter.

[0066] In the case where the digital modulator 20 includes a digital low-pass filter, and therefore a filter much more precise than an analog filter, an analog filter is no longer useful to be placed at the output of the summing 32. The class D audio amplifier 8, 108 then has the same features as a classic class D audio amplifier but in a smaller size.

[0067] In addition, the controller 28 is particularly advantageous since its parameters are not fixed and allow the amplifier 8, 108 to be easily adaptable to any type of circuit.

Claims

8 DEMANDS 1. Class D audio amplifier (8; 108) comprising: - a main input (10) for a digital signal to be amplified, - a digital amplification module (16), - a feedback loop (19) comprising a summing junction (32) having one amplification input (34) connected to the output of the digital amplification module (16) and another input (35) connected to the main input (10) through a digital-to-analog converter (36) suitable for providing an input current (11), the output of the summing junction (32) being connected through an analog-to-digital converter (44) to the input of a subtractor (14) having the other input connected to the main input (10) and having the output connected to the input of the digital amplification module (16), characterized in that the summing junction (32) includes means (38, 40, 42) for converting the output voltage (V1) of the digital amplification module (16) into a feedback current (I2) and is suitable for ensuring the summation of the input current (11) and the feedback current (I2).

2. Class D audio amplifier according to claim 1, wherein the summing junction (32) comprises a linear amplifier (38) selected from the group consisting of an operational amplifier and a transconductance amplifier.

3. Class D audio amplifier according to claim 1 or 2, wherein the summing junction (32) comprises an input resistor (40) connected to the output of the digital amplification module (16), and a feedback resistor (42) connected between an inverting input and the output of the linear amplifier (38).

4. Class D audio amplifier according to any one of the preceding claims, wherein the amplification module (16) comprises a push-pull assembly (22) and an analog filter (24) connected to the output of the push-pull assembly and the amplification input (34) of the summing junction (32) is connected to the output of the analog filter (24).

5. Class D audio amplifier according to any one of the preceding claims, wherein the amplification module (16) comprises a push-pull assembly (22) connected at the output of a digital modulator (20) connected at the output of the subtractor (14), said digital modulator (20) comprising a low-pass digital filter (28). 9 6. Class D audio amplifier according to claim 5, wherein the digital modulator (20) comprises a programmable digital controller (28), the controller (28) being suitable for implementing a type of corrector selected from the group consisting of: a phase-lead corrector, an integral corrector, a proportional corrector, a differentiator corrector, a proportional-differentiator-integral corrector and a phase-lag corrector.

7. Class D audio amplifier according to claims 4 and 6, wherein the integral proportional corrector is suitable for correcting a linear or non-linear approximation of the analog filter (24).

8. Class D audio amplifier (108) according to any one of the preceding claims, further comprising a digital filter (150) upstream of the subtractor (14), the main input (10) being connected to the subtractor (14) through the digital filter (150).

9. Class D audio amplifier according to claim 8, wherein the digital filter (150) is one of: a pure gain (Fo), two cascaded lead-lag compensators (Fi), three identical cascaded lead-lag compensators (F2), or a second-order filter (F3).

10. Soundbar comprising a class D (8;108) audio amplifier according to any one of claims 1 to 9.

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