CN101931372B - Class D Amplifier with Second-Order Noise Filtering - Google Patents

Abstract

The present invention relates to a class D amplifier with a second-order noise filtering circuit, which allows two differential output terminals to form a second-order feedback loop with a first differential amplifier and a second differential amplifier, respectively, thereby providing a higher high-frequency attenuation value, effectively allowing nonlinear elements such as noise of the first and second differential amplifiers to decay more quickly, thereby improving the overall noise-distortion-to-noise ratio; furthermore, the present invention uses the first and second differential amplifiers to synthesize and then perform secondary differential amplification on the distorted components (Distortion) contained in the output signals of the differential output terminals, respectively, to effectively improve the overall loop gain and form a high-gain system, and the differential output signal of the class D amplifier of the present invention can be more finely corrected after being processed by a second-level gain.

Description

Class D Amplifier with Second-Order Noise Filtering

technical field

The present invention relates to a class D amplifier, in particular to a class D amplifier with a second-order noise filtering circuit.

Background technique

Generally speaking, the amplifier circuit of the output stage includes amplifiers such as class A, class B, class AB, and class D. In the early days, class AB amplifiers were more common, but with the maturity of semiconductor manufacturing processes, D amplifiers with lower power consumption Class amplifiers are becoming more and more common.

The biggest difference between a class D amplifier and a class AB amplifier is that the output pulse width modulation signal drives the inductive load instead of a linear signal; the pulse width modulation signal includes sound signals, pulse width modulation switching signals and harmonics Signal. Since the Class D amplifier outputs a pulse width modulation signal, each switch of the output stage switching circuit switches from high impedance to extremely low impedance, and the conduction time is short, so that the conduction time of the conduction current flowing through the conduction resistance is relatively shortened, and compared with AB Class amplifiers are more efficient and consume less power.

Please refer to Fig. 5, which is an existing open-loop class D amplifier 70, which includes a gain amplifier 71, a PWM modulator 72, an internal oscillator 73 and an H-type bridge switch Circuit 74; wherein the input terminal of the gain amplifier 71 receives an external analog sound signal (Vi+, Vi-), and after the external analog sound signal is amplified, the PWM modulator 72 outputs the oscillation signal according to the internal oscillator 73 pulse width modulation signal, and output the pulse width modulation signal to the H-bridge switch circuit 74 to control the conduction time and conduction loop of each switch of the H-bridge switch circuit 74 .

Since the above-mentioned class D amplifier 70 belongs to an open-loop differential circuit, and the gain amplifier 71 includes a differential amplifier 701, and the differential amplifier 701 itself includes a noise floor (NoiseFloor), wherein the noise floor belongs to a kind of dynamic noise, Therefore, when the audio signal is input to the gain amplifier 71 , the dynamic noise will be incorporated into the amplified audio signal, causing the audio signal restored by the inductive load to be distorted, which has a relatively poor signal-to-distortion-to-noise ratio (SDNR).

Therefore, please refer to FIG. 7, which is a circuit block diagram of a class D amplifier 70a with a closed feedback loop, which includes:

A gain adjustment circuit 711, which includes a group of analog input terminals (Vi+, Vi-) for connecting sound signals;

A first differential amplifier 712, the input terminals of which are respectively connected to the analog input terminals (Vi+, Vi-) of the gain adjustment circuit 711, and the gain value of the first differential amplifier 712 is adjusted by the gain adjustment circuit 711;

The first-order integration circuit 75 includes a second differential amplifier 751 and two groups of RC circuits, wherein the two groups of RC circuits are respectively connected to the two differential output terminals (D0+, D0-) of the class D amplifier and the non-inverting terminals of the second differential amplifier 751. Between the input terminals (+, -), the differential output signal output by the class D amplifier 70a and the amplified sound signal output by the first differential amplifier 712 are combined and then output;

Two comparators 76, wherein one input end of each comparator 76 is connected to the differential output end of the second differential amplifier 751 corresponding to the first-order integration circuit 75, and each other input end of the two comparators 76 is then commonly connected to a Triangular wave generator 77, so each comparator 76 compares the output signal of the first-order integral circuit 75 with the triangular wave signal, as shown in Figure 8, because the output of the first-order integral circuit 75 is a sine wave signal (Vi+, Vi-), After comparing with the triangular wave signal S2, the pulse width modulation signal is output; and

A logic circuit 78, whose input terminal is connected to the output terminals of the two comparators 76, determines two groups of driving signals according to the two groups of pulse width modulation signals; and

An H-type bridge switch circuit 74 is composed of two half-bridge switch circuits 741. The two series connection nodes of the two half-bridge switch circuits 741 are connected to an inductive load 60, and the control terminals of the two half-bridge switch circuits 741 are connected to To the output terminal of the logic circuit 78, it is turned on and off by the two sets of driving signals output by the logic circuit 78.

In the structure of the above-mentioned class D amplifier 70a, because the second differential amplifier 751 and the two sets of RC circuits form a first-order feedback loop, the signal input to the comparator 76 is combined with the error signal generated by the second differential amplifier 751 The first differential amplifier 712 amplifies the input sound signal and the real output signal (high-voltage square wave), and the input sound signal amplified by the first differential amplifier 712 contains some nonlinear terms (non-linear terms), such as the differential amplifier frequency limits, amplifier noise, reference voltage noise, gain/span product limits, and feedback output signals containing nonlinear values of triangle wave generators, etc. These non-linear components are eliminated by a first-order feedback loop.

However, the above-mentioned first-order feedback loop only uses a single second differential amplifier to synthesize the sound signals for differential amplification, so the overall open-loop gain is quite limited.

Contents of the invention

In view of the above problems, the main purpose of the present invention is to provide a class D amplifier with a second-order noise filtering circuit, which can improve the overall open-loop gain and provide a better noise-to-distortion signal ratio.

The main technical means used to achieve the above-mentioned purpose is to make the Class D amplifier include a gain adjustment circuit, a second-order integrator, two comparators, a logic circuit and an H-type bridge switch circuit; wherein the second-order integrator includes :

A first differential amplifier includes a positive input terminal, a reverse input terminal, a reverse differential output terminal and a positive differential output terminal, wherein the positive input terminal is connected to the gain adjustment circuit to adjust the first the gain of a differential amplifier;

Two first RC circuits, respectively connected between the differential output terminals of the class D amplifier and the forward and reverse input terminals of the first differential amplifier, to form two sets of first-order integration circuits;

A second differential amplifier comprising a positive input terminal and an inverting input terminal, wherein the positive input terminal is connected to the positive differential output terminal of the first differential amplifier, and the inverting input terminal is connected to the first differential amplifier an inverting differential output of a differential amplifier; and

Two second RC circuits, respectively connected between the differential output terminals of the class D amplifier and the forward and reverse input terminals of the second differential amplifier, to form two sets of second-order integration circuits;

As can be seen from the above description, the beneficial effect of the class D amplifier of the present invention is that the two differential output terminals respectively form a second-order feedback loop with the first and second differential amplifiers, thus providing a higher high-frequency attenuation value, effectively making the first and second differential amplifiers Non-linear components such as the noise of the two differential amplifiers themselves can attenuate more quickly, and improve the overall noise-distortion-to-noise ratio; moreover, the present invention uses the first and second distortion components (Distortion) in the output signal of the differential output The differential amplifiers are pre-synthesized and then differentially amplified twice to effectively increase the overall loop gain to form a high-gain system. The differential output signal of the class D amplifier of the present invention can be more finely corrected after two-stage gain processing.

Furthermore, the error signal (error signal) generated by the second differential amplifier is a combination of the output signal of the first stage amplifier and the feedback signal (high voltage square wave), and this loop contains some non-linear terms (non-linear terms), including Amplifier frequency limitation, amplification noise, reference voltage noise, gain-frequency product limitation, and nonlinear components containing triangular wave generators, these nonlinear components can be eliminated through negative feedback; similarly, the present invention also converts the first-stage differential amplifier to Incorporated into the feedback system to form a second-order feedback loop, that is, the entire Class D amplifier is fully linearized, and relatively better total harmonic distortion plus noise ratio (THD+N) performance is obtained.

The second object of the present invention is to provide a class D amplifier that can greatly reduce electromagnetic interference, which means that the above-mentioned H-type bridge switch circuit includes two half-bridge circuits, and each half-bridge circuit is composed of a plurality of high-side switch groups and a plurality of low-side switches. The switch groups are connected in series, and each half-bridge circuit divides the high-side switch group and the low-side switch group into two sub-half-bridge switch circuits, wherein the number of high/low-side switches in the first sub-half-bridge switch group is more than that of the second The number of high/low side switches of the sub-half-bridge switch group makes the above-mentioned logic circuit output four groups of pulse width modulation signals to the H-type bridge switch circuit of the present invention, wherein the logic operation formula of the logic circuit is X-=(Y -)-(Y+); X+=(Y+)-(Y-), the X- and X+ are the first sub-half-bridge switch circuits that control two sets of half-bridge circuits, and Y- and Y+ are control two sets of half-bridge switches The second sub-half-bridge switch circuit of the circuit, from this logical operation formula, it can be seen that when the differential output terminal of the class D amplifier has no output, the pulse width modulation ratio (Duty Cycle- are 50% respectively, and X- and X+ are the difference between Y- and Y+ respectively, so the pulse width modulation ratios output to X- and X+ (Duty Cycle- are 0% respectively, so the two-half-bridge circuit The first sub-half-bridge switching circuit does not need to switch at all when the class D amplifier has no signal output; and when the class D amplifier has signal output, only one of the two first sub-half bridges has an action; therefore, it can effectively reduce The switching loss and output switch slew rate of the half-bridge circuit can reduce the EMI value of the overall class D amplifier.

Description of drawings

Fig. 1 is the circuit block diagram of the first preferred embodiment of the present invention;

Fig. 2 is the circuit block diagram of the second preferred embodiment of the present invention;

FIG. 3A is a waveform diagram of FIG. 2 when the class D amplifier has no signal output;

FIG. 3B is a waveform diagram of FIG. 2 when the class D amplifier has a signal output;

Fig. 4 is the frequency-domain diagram of the amplified sound signal under the second-order and single-order feedback loops;

Fig. 5 is a circuit block diagram of an open-loop class D amplifier;

Fig. 6 is the frequency domain diagram of Fig. 5 outputting the amplified sound signal;

Fig. 7 is a circuit block diagram of a closed-loop class D amplifier;

FIG. 8 is a waveform diagram of FIG. 7 .

Explanation of reference signs:

10, 10a-class D amplifier; 11-gain adjustment circuit; 20-first differential amplifier; 21-first RC circuit; 22-second differential amplifier; 23-second RC circuit; 30-comparator; 31-comparison 40, 40a-logic circuit; 50, 50a-H-type bridge switch circuit; 51-half-bridge circuit; 511-first sub-half-bridge switch circuit; 512-second sub-half-bridge switch circuit; 52-half-bridge Circuit; 521-first sub-half-bridge switching circuit; 522-second sub-half-bridge switching circuit; 60-inductive load; 70, 70a-class D amplifier; 701-differential amplifier; 71-gain amplifier; 711-gain adjustment Circuit; 712-first differential amplifier; 72-PWM modulator; 73-internal oscillator; 74-H-type bridge switch circuit; 741-half-bridge switch circuit; 75-first-order integrator; 751-second differential Amplifier; 76-comparator; 77-triangular wave signal; 78-logic circuit; 80-low-pass filter.

Detailed ways

Please refer to Fig. 1, which is a circuit block diagram of a preferred embodiment of a Class D amplifier 10 with a second-order noise filtering circuit according to the present invention, which includes a gain adjustment circuit 11, a second-order integrator, and two comparators 30. 31. A logic circuit 40 and an H-type bridge switch circuit 50, wherein the H-type bridge switch circuit 50 includes two half-bridge circuits 51, 52, and the control terminals of each half-bridge circuit 51, 52 are respectively connected to the logic The output terminal of the circuit, and the serial connection node of each half-bridge circuit 51, 52 is the differential output terminal (D0+, D0-) of the class D amplifier 10. Wherein the second-order integrator includes:

A first differential amplifier 20 includes a forward input terminal, a reverse input terminal, a reverse differential output terminal and a forward differential output terminal, wherein the positive input terminal is connected to the gain adjustment circuit 11 to adjust the gain of the first differential amplifier 20;

Two first RC circuits 21 are respectively connected between the differential output terminals (D0+, D0-) of the class D amplifier and the forward and reverse input terminals of the first differential amplifier 20 to form two sets of first-order integration circuits;

A second differential amplifier 22 includes a positive input terminal and an inverting input terminal, wherein the positive input terminal is connected to the positive differential output terminal of the first differential amplifier 20, and the inverting input terminal is connected to the inverting differential output of the first differential amplifier 20; and

Two second RC circuits 23 are respectively connected between the differential output terminals (D0+, D0-) of the class D amplifier and the forward and reverse input terminals of the second differential amplifier 22 to form two sets of second-order integration circuits.

As can be seen from the above description, the two differential output terminals (D0+, D0-) of the class D amplifier 10 of the present invention form a second-order feedback loop with the first and second differential amplifiers 20, 22 respectively, thus providing a higher high-frequency attenuation value ( -40dB), effectively make the nonlinear elements such as the noise of the first and second differential amplifiers 20,22 themselves can attenuate more quickly, and improve the overall noise distortion noise ratio; moreover, the present invention includes in the output signal of the differential output terminal The distorted component (Distortion), with the first and second differential amplifiers 20,22 respectively to give the secondary differential amplification after synthesis, effectively improve the overall loop gain, constitute a high-gain system, and the class D amplifier 10 differential output signal of the present invention Finer corrections can be obtained after secondary gain processing. Furthermore, the error signal (error signal) generated by the second differential amplifier 22 is a combination of the output signal and the feedback signal (high voltage square wave) of the first stage amplifier 20, and this loop contains some non-linear terms (non-linear terms), Including amplifier frequency limitation, amplification noise, reference voltage noise, gain-frequency product limitation, and nonlinear components including triangular wave generator, because the second differential amplifier 22 and the second RC circuit 23 are connected to the two differential output terminals (D0+, D0-) By forming a negative feedback loop, these nonlinear elements can be eliminated; similarly, the present invention also incorporates the first-stage differential amplifier into the feedback system to form a second-order feedback loop, that is, the entire Class D amplifier is fully linearized, and relatively more Good total harmonic distortion plus noise ratio (THD+N) performance.

See also shown in Fig. 2, it is the second preferred embodiment of class D amplifier 10a of the present invention, and its circuit structure is roughly the same as that of the first preferred embodiment, but because each half-bridge circuit 51,52 is made up of a plurality of high-side switch groups and a plurality of low-side switch groups connected in series, so this embodiment further makes the high-side switch group and the low-side switch group of each half-bridge circuit be divided into two groups of sub-half-bridge circuits, that is, a first sub-half-bridge switch circuit is included 511,521 and the second sub-half-bridge switch circuit 512,522, wherein the number of high/low-side switches of the first sub-half-bridge switch circuit 511,521 is more than the number of high/low-side switches of the second sub-half-bridge switch circuit 512,522 (about 3-5 times).

Please cooperate with the design of the H-type bridge switch circuit in this embodiment. The logic circuit 40a of the present invention includes four sets of pulse width modulation signal output terminals X+, Y-, Y+, and X- to be connected to the second half-bridge circuit 51 respectively. , 52 of the first sub-half-bridge switching circuit 511,521 and the second sub-half-bridge switching circuit 512,522. Wherein the logic operation formula of this logic circuit 40a is X-=(Y-)-(Y+); A sub-half-bridge switch circuit 511,512, Y- and Y+ control the second sub-half-bridge switch circuit 521,522.

Please refer to FIG. 3A , when the differential output terminals (D0+, D0-) of the class D amplifier 10a have no output, the pulse width modulation ratios (Duty Cycle) of the output of the logic circuit 40a to Y- and Y+ are respectively 50%, according to the above logical operation formula, the X- and X+ are the difference between Y- and Y+ respectively, so the PWM duty cycle (Duty Cycle) output to X- and X+ is 0% respectively, so two and a half The first sub-half-bridge switch circuits 511, 521 in the bridge circuits 51, 52 do not need to switch at all when there is no signal output from the class D amplifier 10a. Please refer to FIG. 3B again. When the class D amplifier 10a has a signal output, only one of the two groups of first sub-half bridges 511, 521 has an action; therefore, the switching loss and output switching slew of the half bridge circuit can be effectively reduced. The output switch slew rate can reduce the EMI value of the overall class D amplifier 10a.

The above description of the present invention is illustrative rather than restrictive. Those skilled in the art understand that many modifications, changes or equivalents can be made to it within the spirit and scope of the claims, but they will all fall into within the protection scope of the present invention.

Claims (3)

1. D class A amplifier A with second-order noise filtering circuit, include a gain adjustment circuit, a second-order integrator, two comparators, a logical circuit and a H type bridge switching circuit, wherein this H type bridge switching circuit includes two half-bridge circuits, the control end of each half-bridge circuit is connected to respectively the output of this logical circuit, and the tandem node of each half-bridge circuit is the difference output end of D class A amplifier A; It is characterized in that, this second-order integrator includes:

One first differential amplifier includes a positive input, a reverse input end, a backward difference output and a forward difference output end, and wherein this positive input is connected to this gain adjustment circuit, to adjust the gain of this first differential amplifier;

2 the one RC circuit are connected between the forward and reverse input end of D class A amplifier A difference output end and this first differential amplifier, consist of two group of first rank integrating circuit;

One second differential amplifier, include a positive input, a reverse input end, wherein this positive input is connected to the forward difference output end of this first differential amplifier, and this reverse input end then is connected to the backward difference output of this first differential amplifier; And

2 the 2nd RC circuit are connected between the forward and reverse input end of D class A amplifier A difference output end and this second differential amplifier, consist of two groups of second-order integrating circuit.

2. the D class A amplifier A with second-order noise filtering circuit as claimed in claim 1, it is characterized in that, described each half-bridge circuit is formed by a plurality of high-side switch groups and a plurality of low side switch group serial connection, described each half-bridge circuit further becomes the first sub-half-bridge switch circuit and the second sub-half-bridge switch circuit with described high-side switch group with described low side switch component, the high/low side number of switches of this first sub-half-bridge switch circuit is more than the high/low side number of switches of the second sub-half-bridge switch circuit;

This logical circuit includes four groups of pulse-width modulation signal output parts, and with the first sub-half-bridge switch circuit and the second sub-half-bridge switch circuit that are connected to respectively this two half-bridge circuit, wherein the logical operation formula of this logical circuit is X-=(Y-)-(Y+); X+=(Y+)-(Y-), this X-and X+ are the second sub-half-bridge switch circuit of described two half-bridge circuits of control for the first sub-half-bridge switch circuit of described two half-bridge circuits of control, Y-and Y+.

3. the D class A amplifier A with second-order noise filtering circuit as claimed in claim 2 is characterized in that, the high/low side number of switches of this first sub-half-bridge switch circuit is 3 to 5 times of high/low side number of switches of the second sub-half-bridge switch circuit.

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