Patent Publication Number: US-7911267-B2

Title: Sigma-delta class-D amplifier and control method for a sigma-delta class-D amplifier

Description:
FIELD OF THE INVENTION 
     The present invention is related generally to a power amplifier and, more particularly, to a sigma-delta class-D amplifier and a control method for a sigma-delta class-D amplifier. 
     BACKGROUND OF THE INVENTION 
     Class-D amplifiers may be categorized into pulse width modulation (PWM) type and sigma-delta (ΣΔ) modulation type, and the ΣΔ-based class-D amplifiers may be further categorized into 1-bit ΣΔ class-D amplifier and 1.5-bit ΣΔ class-D amplifier, where the terms “1-bit” and “1.5-bit” represent the degrees of quantization, or otherwise called “resolution”. More specifically, “1-bit” means that the input of the ΣΔ class-D amplifier is quantized into two logical levels, such as ‘0’ and ‘1’, and “1.5-bit” means that the input of the ΣΔ class-D amplifier is quantized into three logical levels, such as ‘−1’, ‘0’ and ‘1’.  FIG. 1  is a block diagram of a conventional 1-bit ΣΔ class-D amplifier  100 , which includes a filter  102  to filter an input consisting of signals VINP and VINN opposite in phase to each other to produce signals SOP and SON, a 1-bit quantizer  104  to quantize the signals SOP and SON to produce a digital signal consisting of signals QuP and QuN, and a switching logic  106  to drive a power stage  108  to produce an output Vout for a load according to the signals QuP and QuN produced by the quantizer  104 . The output Vout of the ΣΔ class-D amplifier  100  is the difference between two voltages OUTP and OUTN produced by the output stage  108 . For better signal-to-noise ratio (SNR), the sigma-delta modulation scheme further includes a feedback mechanism to feed back the voltages OUTP and OUTN in the output stage  108  to the filter  102 . The class-D amplifier with 1-bit sigma-delta modulation (ΣΔ-based class-D amplifier) has good SNR compared to that with pulse width modulation, but suffers from instability and high switching loss. 
     Specifically, a conventional 1-bit ΣΔ modulator is only stable to 50% modulation, which greatly limits the maximum output power to 0.25× of theoretical full-scale. This is because the output Vout from the 1-bit ΣΔ class-D amplifier  100  has only two voltage levels, say +5V and −5V, so that a minimum voltage variation fed back to its input is 10V. This will cause significant increase of the input of the filter  102 , leading to abnormal operation of the filter  102  due to saturation thereof. Consequently, the SNR is lowered. 
     In addition, at least 64× oversampling is need for a conventional 1-bit ΣΔ class-D amplifier to achieve sufficient audio-band SNR, so the typical output data rate is higher than 2 MHz, which increases the switching loss and degrades the power efficiency. For more details on ΣΔ class-D amplifiers, readers are referred to U.S. Pat. Nos. 7,170,340, 6,724,830 and 6,472,933. 
     Therefore, it is desired a more stable and less switching loss ΣΔ class-D amplifier. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a ΣΔ-based class-D amplifier by using an “at least five logical levels bit quantization scheme” to improve the stability and reduce the output data rate thereof. 
     According to the present invention, a sigma-delta class-D amplifier comprises a quantizer with a smaller step size for quantizing an input into a digital signal of at least five logical levels, and a power stage for providing an output for a load according to the digital signal. The voltage level of the output varies with the logical level of the digital signal. In an embodiment, the power stage includes at least two independent power supplies and a plurality of power switches, and each of the power supplies and four of the power switches are configured together with the load to be an H-bridge circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a conventional 1-bit ΣΔ class-D amplifier; 
         FIG. 2  is a block diagram of a 2.5-bit ΣΔ class-D amplifier according to the present invention; 
         FIG. 3  is an embodiment for the power stage in the amplifier of  FIG. 2 ; 
         FIG. 4  is a diagram to illustrate the operations of the power stage shown in  FIG. 3 ; 
         FIG. 5  is a simulated output spectrum of third-order 1-bit, 1.5-bit and 2.5-bit ΣΔ class-D amplifiers at 50% modulation; and 
         FIG. 6  is a simulated output spectrum of third-order 1-bit, 1.5-bit and 2.5-bit ΣΔ class-D amplifiers at 90% modulation. 
     
    
    
     DETAIL DESCRIPTION OF THE INVENTION 
       FIG. 2  is a block diagram of a third-order 2.5-bit ΣΔ class-D amplifier  200  according to the present invention, in which a filter  202  includes three serially coupled integrators  20202  to convert an input consisting of opposite-phase signals VINP and VINN into signals SOP and SON, the integrators  20202  can be of continuous-time or discrete-time types, and a 2.5-bit quantizer  204  quantizes the signals SOP and SON into a digital signal consisting of signals QuP and QuN each having one of five logical levels, for example +2, +1, 0, −1 and −2. Since the signals VINP and VINN have opposite phases to each other, the signals QuP and QuN also have opposite phases to each other. According to the output QuP and QuN of the 2.5-bit quantizer  204 , a switching logic  206  drives a power stage  208  to produce an output Vout for a load  210 . The output Vout is the difference between two voltages OUTP and OUTN produced by the output stage  208  and is fed back to the filter  202 . The voltage level of the output Vout varies with the logical level of the signals QuP and QuN. For instance, the output Vout is +5V when the signal QuP is +2; the output Vout is +2.5V when the signal QuP is +1; the output Vout is 0V when the signal QuP is 0; the output Vout is −2.5V when the signal QuP is −1; and the output Vout is −5V when the signal QuP is −2. In this embodiment, the ΣΔ class-D amplifier  200  is a third-order one, so that the filter  202  includes three integrators  20202 . In other embodiments, for example a fourth-order ΣΔ class-D amplifier, the filter  202  will have four integrators  20202 . 
     It is known that the in-band noise power for an L th -order sigma-delta modulator is approximately
 
q 2   rms ≈(π 2L Δ 2 )/[12×(2L+1)×(OSR) 2L+1 ],  [Eq-1]
 
where Δ is the step size of quantization and OSR is the oversampling rate. Obviously, three parameters, Δ, L and OSR, can be used to reduce the in-band noise power. The SNR increases as the in-band noise power decreases. For conventional 1-bit or 1.5-bit quantization, the step size Δ is large and the modulator thus requires high-order, high-OSR architecture to achieve desired SNR. However, the higher the order L of the modulator is, the easier the modulator becomes instable, which limits the maximum output power. The use of 2.5-bit quantization (five logical levels) can improve the stable range to 70%-90% of modulation (i.e. the input is 70%-90% of the output), which enhances the maximum output power. Moreover, since the step size Δ of 2.5-bit quantization is smaller than that of 1-bit or 1.5-bit quantization, the in-band noise power of L th -order ΣΔ modulator with 2.5-bit quantization can be reduced. Thus, the desired SNR of the modulator can be achieved with lower OSR. This, in turn, reduces the output data rate and switching loss.
 
       FIG. 3  provides an embodiment for the power stage  208  in the circuit of  FIG. 2 , which is a dual-supply H bridge circuit and contains six power transistors  20802 ,  20804 ,  20806 ,  20808 ,  20810  and  20812  and two independent power supplies Vdd 1  and Vdd 2 . The power supply Vdd 1  provides a higher voltage and the power supply Vdd 2  provides a lower voltage. The transistor  20802  is coupled between the power supply Vdd 1  and a positive output node OUTP; the transistor  20804  is coupled between the power supply Vdd 2  and the positive output node OUTP; the transistor  20806  is coupled between the power supply Vdd 2  and a negative output node OUTN; the transistor  20808  is coupled between the power supply Vdd 1  and the negative output node OUTN; the transistor  20810  is coupled between the positive output node OUTP and a ground node GND; and the transistor  20812  is coupled between the negative output node OUTN and the ground node GND. As shown in  FIG. 3 , the power supply Vdd 1 , the transistors  20802 ,  20808 ,  20810  and  20812  and the load  210  are configured to be an H-bridge circuit, and the power supply Vdd 2 , the transistors  20804 ,  20806 ,  20810  and  20812  and the load  210  are configured to be another H-bridge circuit. 
       FIG. 4  is a diagram to illustrate the operations of the power stage  208  shown in  FIG. 3 . Assuming that the signals SOP and SON are quantized by the 2.5-bit quantizer  204  into the signals QuP and QuN of five logical levels, +2, +1, 0, −1 and −2, when the signal QuP is +2, the transistors  20802  and  20812  in the power stage  208  are turned on, so that the positive output node OUTP is connected to the power supply Vdd 1  and the negative output node OUTN is grounded. Therefore, the voltage across the load  210  is +Vdd 1 . When the signal QuP is +1, the transistors  20804  and  20812  in the power stage  208  are turned on, so that the positive output node OUTP is connected to the power supply Vdd 2  and the negative output node OUTN is grounded. As a result, the voltage across the load  210  is +Vdd 2 . When the QuP is 0, the transistors  20810  and  20812  in the power stage  208  are turned on, thereby having both the positive and negative output nodes OUTP and OUTN grounded and the voltage across the load  210  to be zero. When the signal QuP is −1, the transistors  20806  and  20810  in the power stage  208  are turned on to ground the positive output node OUTP and connect the negative output node OUTN to the power supply Vdd 2 . Consequently, the voltage across the load  210  is −Vdd 2 . When the signal QuP is −2, the transistors  20808  and  20810  in the power stage  208  are turned on, thereby grounding the positive output node OUTP and connecting the negative output node OUTN to the power supply Vdd 1 . Thus, the voltage across the load  210  is −Vdd 1 . 
       FIG. 5  is a simulated output spectrum of third-order 1-bit, 1.5-bit and 2.5-bit ΣΔ class-D amplifiers at 50% modulation, in which curve  300  represents the output of a third-order 1-bit ΣΔ class-D amplifier, curve  302  represents the output of a third-order 1.5-bit ΣΔ class-D amplifier, and curve  304  represents the output of a third-order 2.5-bit ΣΔ class-D amplifier. As shown in  FIG. 5 , at 50% modulation, the third-order 2.5-bit ΣΔ class-D amplifier has a signal-to-noise plus distortion ratio (SNDR) of 90 dB and a total harmonic distortion+noise (THD+N) of approximately 0.003%. On the other hand, when the modulation rage is 50%, the third-order 1-bit ΣΔ class-D amplifier has an SNDR of about 70 dB and the third-order 1.5-bit ΣΔ class-D amplifier has an SNDR of about 80 dB. 
       FIG. 6  is a simulated output spectrum of third-order 1-bit, 1.5-bit and 2.5-bit ΣΔ class-D amplifiers at 90% modulation, in which curve  400  represents the output of a third-order 1-bit ΣΔ class-D amplifier, curve  402  represents the output of a third-order 1.5-bit ΣΔ class-D amplifier, and curve  404  represents the output of a third-order 2.5-bit ΣΔ class-D amplifier. As shown in  FIG. 6 , the third-order 2.5-bit ΣΔ class-D amplifier has an SNDR of 85 dB, the third-order 1-bit ΣΔ class-D amplifier has an SNDR of about 37 dB, and the third-order 1.5-bit ΣΔ class-D amplifier has an SNDR of about 53 dB. It is thus shown by  FIGS. 5 and 6  that, when the modulation range is increased to 90%, the SNDR values of the 1-bit and 1.5-bit ΣΔ class-D amplifiers drop significantly. This is because a larger input amplitude will saturate the output of the integrator. In contrast, the 2.5-bit ΣΔ class-D amplifier only experiences a slight drop in SNDR, which means the 2.5-bit ΣΔ class-D amplifier remains highly efficient at 90% modulation. 
     According to the simulation results of a third-order 2.5-bit ΣΔ class-D amplifier, when the sampling rate is 1.28 MHz (at a 32-time oversampling rate), the third-order 2.5-bit ΣΔ class-D amplifier has an SNDR as high as 90 dB and a stable modulation rage thereof in the audio band (20 Hz-20 kHz) is increased to 90%. According to the further simulation results of a fourth-order 2.5-bit ΣΔ class-D amplifier, when the sampling rate is 1.28 MHz, the fourth-order 2.5-bit ΣΔ class-D amplifier has an SNDR as high as 110 dB and a stable modulation rage thereof in the audio band (20 Hz-20 kHz) is increased to 90%. Hence, the 2.5-bit ΣΔ class-D amplifier has better efficiency than the conventional 1-bit and 1.5-bit ΣΔ class-D amplifiers. 
     In other embodiment, the bit number of the quantizer  204  can be larger, for example by using a 3.5-bit quantizer. As the quantizer has more bits, the power stage  208  needs to be adjusted accordingly, for example by using more independent power supplies and power switches. 
     While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.