Class-D amplifier with pulse-width modulation common-mode control and associated method for performing class-D amplification

[0001] This application claims the benefit of U.S. provisional application No. 62/217,178 filed on Sep. 11, 2015 and U.S. provisional application No. 62/218,744 filed on Sep. 15, 2015. The entire contents of the related U.S. provisional applications are incorporated herein by reference.

[0002] The present invention relates to an amplifier design, and more particularly, to a class-D amplifier with pulse-width modulation (PWM) common-mode control and an associated method for performing class-D amplification.

[0003] A class-D amplifier is basically a switching amplifier. It has a power efficiency approaching 100%. That is, most of the power supplied to the class-D amplifier is delivered to the load. Conventionally, a low-dropout (LDO) regulator with high power supply rejection ratio (PSRR) is adopted to supply a clean power to the class-D amplifier. This LDO is used to mitigate the noise coupling through or generated from the power management circuit that may include buck/boost converters. However, this LDO causes additional power loss and decreases the energy efficiency of the whole system. Therefore, in order to avoid this undesired power loss, the supply voltage of a class-D amplifier is preferred to be powered by a power source (e.g., a battery) without using a LDO, which lays critical emphasis on the PSRR of the Class-D audio amplifier. Consequently, for high performance of audio applications, there is a need for an innovative class-D amplifier (CDA) design with high power-supply rejection ratio (PSRR) and high linearity (e.g., low Total Harmonic Distortion with Noise (THD+N)).

[0004] One of the objectives of the claimed invention is to provide a class-D amplifier with pulse-width modulation (PWM) common-mode control and an associated method for performing class-D amplification.

[0005] According to a first aspect of the present invention, an exemplary class-D amplifier is disclosed. The exemplary class-D amplifier includes a loop filter, a pulse-width modulation (PWM) circuit, an output circuit, and a common-mode control circuit. The loop filter is arranged to receive an input signal of the class-D amplifier to generate a filtered signal. The PWM circuit is arranged to convert a non-PWM signal into a PWM signal, wherein the non-PWM signal is derived from at least the filtered signal. The output circuit is arranged to generate an output signal of the class-D amplifier according to the PWM signal. The common-mode control circuit is arranged to monitor a common-mode level of the output signal to generate a common-mode control signal for PWM common-mode control.

[0006] According to a second aspect of the present invention, an exemplary class-D amplifier is disclosed. The exemplary class-D amplifier includes a loop filter, a combining circuit, an input feed-forward circuit, a pulse-width modulation (PWM) circuit, and an output circuit. The loop filter is arranged to receive an input signal of the class-D amplifier to generate a filtered signal. The combining circuit is arranged to combine at least the filtered signal and the input signal to generate a non-PWM signal, wherein the combining circuit is composed of passive components only. The input feed-forward circuit is arranged to feed the input signal to the combining circuit. The PWM circuit is arranged to convert the non-PWM signal into a PWM signal. The output circuit is arranged to generate an output signal of the class-D amplifier according to the PWM signal.

[0007] According to a third aspect of the present invention, an exemplary method for performing class-D amplification is disclosed. The exemplary method includes: performing a loop filtering operation upon an input signal of the class-D amplification to generate a filtered signal; performing a pulse-width modulation (PWM) operation to convert a non-PWM signal into a PWM signal, wherein the non-PWM signal is derived from at least the filtered signal; generating an output signal of the class-D amplification according to the PWM signal; and monitoring a common-mode level of the output signal to generate a common-mode control signal for PWM common-mode control.

[0008] According to a fourth aspect of the present invention, an exemplary method for performing class-D amplification is disclosed. The exemplary method includes: performing a loop filtering operation upon an input signal of the class-D amplification to generate a filtered signal; utilizing a combining circuit to combine at least the filtered signal and the input signal to generate a non-PWM signal, wherein the combining circuit is composed of passive components only; performing a pulse-width modulation (PWM) operation to convert the non-PWM signal into a PWM signal; and generating an output signal of the class-D amplifier according to the PWM signal.

[0009] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

[0023] Certain terms are used throughout the following description and claims, which refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not in function. In the following description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . . ”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections.

[0024] FIG. 1 is a diagram illustrating a class-D amplifier without proposed pulse-width modulation (PWM) common-mode control according to an embodiment of the present invention. The class-D amplifier 100 may be a fully-differential closed-loop class-D amplifier, including a loop filter (denoted by “LF”) 102, a PWM circuit 104, an output circuit 106, and a feedback circuit 108. The loop filter 102 is arranged to receive an input signal V_IN of the class-D amplifier 100 to generate a filtered signal V_LF, wherein the input signal V_IN is a fully-differential signal composed of a positive signal V_IP and a negative signal V_IN, and the filtered signal V_LF is a fully-differential signal composed of a positive signal V_LFP and a negative signal V_LFN. The PWM circuit 104 is arranged to convert a non-PWM signal V_SUM into a PWM signal V_PWM. The non-PWM signal V_SUM may be derived from the filtered signal V_LF, directly or indirectly. For example, the filtered signal V_LF may be directly fed into the PWM circuit 104 to serve as the non-PWM signal V_SUM. For another example, the filtered signal V_LF may be processed (e.g., combined with an input feed-forward signal) before being fed into the PWM circuit 104. The non-PWM signal V_sum is composed of a positive signal V_SUMP and a negative signal V_SUMN, and the PWM signal V_PWM is composed of a positive signal V_PWMP and a negative signal V_PWMN. The output circuit 106 is arranged to generate an output signal V_OUT of the class-D amplifier 100 according to the PWM signal V_PWM, where the output signal V_OUT is composed of a positive signal V_OP and a negative signal V_ON. For example, the output circuit 106 may include a non-overlapping clock generator and an H-bridge output driver. The feedback circuit 108 is coupled between an output of the output circuit 106 and an input of the loop filter 102, and includes a plurality of feedback paths (e.g., feedback resistors) 109_1 and 109_2. In an actual implementation, the feedback paths 109_1 and 109_2 have a gain mismatch δ, thus resulting in undesired terms (e.g., distortions and noise) in the differential output (V_OP-V_ON).

[0025] FIG. 2 is a diagram illustrating a linear model of the class-D amplifier 100 in FIG. 1 according to an embodiment of the present invention. The class-D amplifier 100 can be linearly modeled because the PWM switching frequency is much higher than the signal bandwidth. The supply noise and/or the even harmonic distortions may be represented by e_C. The differential noise and/or the odd harmonic distortions may be represented by e_D. The loop gain of the class-D amplifier 100 may be represented by H_D(s). The differential output (V_OP-V_ON) may be expressed using the following formula.

[0026] $\begin{array}{cc}\left(V_{\mathrm{OP}}-V_{\mathrm{ON}}\right)\approx \left(V_{\mathrm{IP}}-V_{\mathrm{IN}}\right)-\delta \times e_C+\frac{2e_D}{H_D},\mathrm{where}{H}_D\text{>>}1& \left(1\right)\end{array}$
With the gain mismatch δ of the differential feedback path, the supply noise and/or the even harmonic distortions e_C contribute δ×e_C to the differential output (V_op-V_ON). The differential noise and/or the odd harmonic distortions e_D can be suppressed by the loop gain H_D (s). PSRR can only be improved by reducing the gain mismatch δ. For example, a stringent matching requirement of δ<0.001% is required in order to achieve PSRR>100 dB. However, it is very hard to meet the stringent matching requirement of δ<0.001% in the real world. The present invention therefore proposes applying PWM common-mode (CM) control to a class-D amplifier to suppress the supply noise and/or the even harmonic distortions at the class-D amplifier output δ×e_C.

[0027] FIG. 3 is a diagram illustrating a class-D amplifier with proposed PWM common-mode control according to an embodiment of the present invention. The class-D amplifier 300 may be a fully-differential closed-loop class-D amplifier, including a common-mode control circuit 302 and the aforementioned loop filter 102, PWM circuit 104, output circuit 106 and feedback circuit 108. That is, the class-D amplifier 300 shown in FIG. 3 may be created by adding the common-mode control circuit 302 to the class-D amplifier 100 shown in FIG. 1. In this embodiment, the common-mode control circuit 302 is arranged to monitor a common-mode level of the output signal V_OUT (which is composed of the positive signal V_OP and the negative signal V_ON) to generate a common-mode control signal V_{CM_CTRL} for PWM common-mode (CM) control. For example, the common-mode level of the output signal V_OUT is compared with a reference common-mode voltage V_CM to set the common-mode control signal V_{CM_CTRL}.

[0028] FIG. 4 is a diagram illustrating a linear model of the class-D amplifier 300 in FIG. 3 according to an embodiment of the present invention. The class-D amplifier 300 can be linearly modeled because the PWM switching frequency is much higher than the signal bandwidth. The supply noise and/or the even harmonic distortions may be represented by e_C. The differential noise and/or the odd harmonic distortions may be represented by e_D). The loop gain of the class-D amplifier 300 may be represented by H_D (s). The PWM common-mode control loop provided by the common-mode control circuit 302 is capable of further suppressing the supply noise and/or the even harmonic distortions at the class-D amplifier output δ×e_C. The loop gain of the PWM common-mode control loop may be represented by H_C(s). The differential output (V_OP-V_ON) may be expressed using the following formula.

[0029] $\begin{array}{cc}\left(V_{\mathrm{OP}}-V_{\mathrm{ON}}\right)\approx \left(V_{\mathrm{IP}}-V_{\mathrm{IN}}\right)-\delta \times \frac{e_C}{H_C}+\frac{2e_D}{H_D}-\delta \times V_{\mathrm{CM}},\mathrm{where}{H}_D\text{>>}1\mathrm{and}{H}_c\text{>>}1& \left(2\right)\end{array}$
The above derivation shows that the supply noise and/or the even harmonic distortions e_C are further suppressed by the loop gain H_C(s) of the PWM common-mode control loop. Hence, the supply noise and/or the even harmonic distortions at the class-D amplifier output can be improved from

[0030] $\delta \times e_C\mathrm{to}\delta \times \frac{e_C}{H_C}.$
The matching requirement of the differential feedback path can be relaxed by the introduced PWM common-mode control loop. To put it simply, the proposed PWM common-mode control loop is of great help to improve the PSRR and reduce the THD+N of the fully-differential closed-loop class-D amplifier. With the gain mismatch δ of the differential feedback path, the reference common-mode voltage V_CM contributes δ×V_CM to the differential output (V_OP-V_ON). In one exemplary design, a high PSRR bandgap reference voltage generator may be used to provide a bandgap reference voltage for setting the reference common-mode voltage V_CM, thereby mitigating or avoiding the noise caused by the reference common-mode voltage V_CM at the class-D amplifier output.

[0031] FIG. 5 is a diagram illustrating a common-mode control circuit according to an embodiment of the present invention. For example, the common-mode control circuit 302 shown in FIG. 3 may be implemented using the common-mode control circuit 500 shown in FIG. 5. The common-mode control circuit 500 includes an operational amplifier circuit 502 and a reference common-mode voltage generator 504. In this embodiment, the operational amplifier circuit 502 is realized by a 1^st-order active-RC integrator that includes an operational amplifier OP, a capacitor C1, and a plurality of resistors R1. The operational amplifier circuit 502 senses an average of the class-D amplifier output (i.e., a common-mode voltage V_OCM of the positive signal V_OP and the negative signal V_ON), and compares the sensed common-mode voltage V_OCM with the reference common-mode voltage V_CM to output the common-mode control signal V_{CM_CTRL} for PWM common-mode control. In one exemplary embodiment, the PWM circuit 104 and the output circuit 106 both obtain the supply voltage V_BAT from a battery (not shown) directly. Hence, the reference common-mode voltage V_CM may be set by ½V_BAT to enlarge the output swing. As shown in FIG. 5, the reference common-mode voltage generator 504 includes a plurality of circuits, such as a voltage divider, a comparator, and a digital-to-analog converter. The voltage divider is implemented by resistors R2 for dividing the supply voltage V_BAT that is directly provided by the battery. The comparator CMP is arranged to compare a voltage output ½V_BAT of the voltage divider and a voltage output V_CM of the digital-to-analog converter DAC, and generate a comparison result to a control circuit CTRL of the digital-to-analog converter DAC. The digital-to-analog converter DAC may be a low-rate digital-to-analog converter that operates under a low-frequency clock signal f_CLK (e.g., f_CLK<<217 Hz) and has a high PSRR bandgap reference voltage generator BG arranged to provide a bandgap reference voltage. The digital-to-analog converter DAC provides a voltage output (i.e., reference common-mode voltage V_CM) according to the bandgap reference voltage. That is, the reference common-mode voltage V_CM is set by the bandgap reference voltage under the control of the comparison result generated from the comparator CMP. Specifically, the control circuit CTRL refers to the comparison result generated from the comparator CMP to dynamically adjust the reference common-mode voltage V_CM. In this way, V_CM=½V_BAT.

[0032] The proposed PWM common-mode control is implemented to control the common-mode level of the PWM signal V_PWM (which is composed of the positive signal V_PWMP and the negative signal V_PWMN) generated from the PWM circuit 104 to the output circuit 106. The PWM circuit 104 is used to convert the non-PWM signal V_SUM (which is composed of the positive signal V_SUMP and the negative signal V_SUMN) into the PWM signal V_PWM (which is composed of the positive signal V_PWMP and the negative signal V_PWMN). FIG. 6 is a diagram illustrating a PWM circuit according to an embodiment of the present invention. The PWM circuit 104 shown in FIG. 3 may be implemented using the PWM circuit 600 shown in FIG. 6. The PWM circuit 600 includes a comparing circuit (e.g., two comparators 602 and 604) and a sawtooth signal generator (or triangular wave generator) 606. The sawtooth signal generator 606 is a circuit designed for generating a sawtooth signal V_SAW to each of the comparators 602 and 604. The comparator 602 is arranged to compare the positive signal V_SUMP of the non-PWM signal V_SUM with the sawtooth signal V_SAW to generate the positive signal V_PWMP of the PWM signal V_PWM. The comparator 604 is arranged to compare the negative signal V_SUMN of the non-PWM signal V_SUM with the sawtooth signal V_SAW to generate the negative signal V_PWMN of the PWM signal V_PWM. FIG. 7 is a diagram illustrating a PWM operation performed by the PWM circuit 600 in FIG. 6 according to an embodiment of the present invention. Take the generation of the negative signal V_PWMN for example. Assume V_CM,SAW is the common-mode level of the sawtooth signal V_SAW (i.e., the mid-point of the peak to peak voltage of the sawtooth signal V_SAW). In a case where the negative signal V_PWMN is set by the fixed voltage level V_SUMN1, a negative signal V_PWMN1 is generated by comparing the sawtooth signal V_SAW with the fixed voltage level V_SUMN1. In another case where the negative signal V_PWMN is set by the fixed voltage level V_SUMN2, a negative signal V_PWMN2 is generated by comparing the sawtooth signal V_SAW with the fixed voltage level V_SUMN2. Since the fixed voltage level V_SUMN2 is lower than the fixed voltage level V_SUMN1, the duty cycle of the negative signal V_PWMN1 is larger than the duty cycle of the negative signal V_PWMN2. Hence, the DC level of the negative signal V_PWMN1 is higher than the DC level of the negative signal V_PWMN2. As can be seen from FIG. 7, a common-mode level (DC level) of a PWM signal can be adjusted by adjusting magnitude of a non-PWM signal to be processed by pulse-width modulation. Hence, the common-mode level (DC level) of the PWM signal V_PWM is affected by the common-mode level of the non-PWM signal V_SUM.

[0033] The non-PWM signal V_SUM (which is composed of the positive signal V_SUMP and the negative signal V_SUMN) is derived from the filtered signal V_LF (which is composed of the positive signal V_LFP and the negative signal V_LFN). That is, the non-PWM signal V_SUM may be set by the filtered signal V_LF, directly or indirectly. Hence, the common-mode level of the non-PWM signal V_SUM is affected by the common-mode level of the filtered signal V_LF. Accordingly, the common-mode level (DC level) of the PWM signal V_PWM is affected by the common-mode level of the filtered signal V_LF.

[0034] In a first exemplary PWM common-mode control design, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to the loop filter 102 so as to adjust the common-mode level of the filtered signal V_LF for achieving PWM common-mode control. The loop filter 102 may include one or more integrators for performing the loop filtering operation. For example, the loop filter 102 may be implemented by a 3^rd-order loop filter having a 1^st-order integrator and a 2^nd-order integrator that is cascaded to the 1^st-order integrator. In one exemplary implementation, the 1^st-order integrator may be a 1^st-order active-RC integrator, and the 2^nd-order integrator may be a 2^nd-order filter that adopts the single-amplifier-biquad technique to save one operational amplifier and its associated power consumption. Since the 2^nd-order integrator is the last one of cascaded integrators in the loop filter, the 2^nd-order integrator (particularly, the single operational amplifier of the 2^nd-order integrator) is responsible for generating the filtered signal V_LF (which is composed of the positive signal V_LFP and the negative signal V_LFN). The single operational amplifier of the 2^nd-order integrator may be configured to have a common-mode feedback (CMFB) circuit that is responsive to the common-mode control signal V_{CM_CTRL} for PWM common-mode control.

[0035] FIG. 8 is a diagram illustrating an operational amplifier used for generating a filtered signal of a loop filter according to an embodiment of the present invention. The operational amplifier 800 may be the aforementioned single operational amplifier of the 2^nd-order integrator that adopts the single-amplifier-biquad technique. As shown in FIG. 8, the operational amplifier 800 includes a 1^st-stage circuit, a 2^nd-stage circuit, and a CMFB circuit. The 1^st-stage circuit receives an input signal (V_IP2, V_IN2) from a preceding circuit (e.g., an output signal of the 1^st-order integrator in the loop filter). The 2^nd-stage circuit generates and outputs the positive signal V_LFP and the negative signal V_LFN of the filtered signal V_LF. The common-mode level of the filtered signal V_LF generated from the 2^nd-stage circuit is controlled by the CMFB circuit. In this embodiment, the CMFB circuit receives the common-mode control signal V_{CM_CTRL} from the common-mode control circuit 302, and adaptively adjusts the common-mode level of the filtered signal V_LF according to the common-mode control signal V_{CM_CTRL}.

[0036] As mentioned above, the non-PWM signal V_SUM (which is composed of the positive signal V_SUMP and the negative signal V_SUMN) is derived from the filtered signal V_LF (which is composed of the positive signal V_LFP and the negative signal V_LFN), and the common-mode level of the PWM signal V_PWM is affected by the common-mode level of the non-PWM signal V_SUM. In a second exemplary PWM common-mode control design, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to a combining circuit located between the loop filter 102 and the PWM circuit 104 so as to adjust the common-mode level of the non-PWM signal V_SUM for achieving PWM common-mode control.

[0037] FIG. 9 is a diagram illustrating a combining circuit located between a loop filter and a PWM circuit according to an embodiment of the present invention. For example, the class-D amplifier 300 shown in FIG. 3 may have the combining circuit 900 located between an output of the loop filter 102 and an input of the PWM circuit 104. In this embodiment, the combining circuit 900 receives the common-mode control signal V_{CM_CTRL} from the common-mode control circuit 302, and combines the filtered signal V_LF (which is composed of the positive signal V_LFP and the negative signal V_LFN) and the common-mode control signal V_{CM_CTRL} to generate the non-PWM signal V_SUM (which is composed of the positive signal V_SUMP and the negative signal V_SUMN), such that the common-mode level of the non-PWM signal V_SUM is adjusted/shifted by the injected common-mode control signal V_{CM_CTRL}.

[0038] As illustrated in FIG. 6, the PWM circuit 600 compares the positive signal V_SUMP and the negative signal V_SUMN of the non-PWM signal V_SUMP with the sawtooth signal V_SAW to generate the positive signal V_PWMP and the negative signal V_PWMM of the PWM signal V_PWM, respectively. In above exemplary designs, the common-mode level of the PWM signal V_PWM is adjusted by adjusting the common-mode level of the non-PWM signal V_SUM, directly or indirectly. Since the sawtooth signal V_SAW is also involved in generating the PWM signal V_PWM, the same objective of adjusting the common-mode level of the PWM signal V_PWM can be achieved by adjusting the common-mode level of the sawtooth signal V_SAW (i.e., the mid-point of the peak-to-peak voltage of the sawtooth signal V_SAW). In a third exemplary PWM common-mode control design, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to a sawtooth signal generator of the PWM circuit 104 so as to adjust the common level of the sawtooth signal (i.e., the mid-point of the peak-to-peak voltage of the sawtooth signal) for achieving PWM common-mode control.

[0039] FIG. 10 is a diagram illustrating another PWM circuit according to an embodiment of the present invention. The PWM circuit 104 shown in FIG. 3 may be implemented using the PWM circuit 1000 shown in FIG. 10. The major difference between the PWM circuits 600 and 1000 is that the PWM circuit 1000 includes a sawtooth signal generator 1006 that is responsive to the common-mode control signal V_{CM_CTRL} generated from the common-mode control circuit 302.

[0040] FIG. 11 is a diagram illustrating a PWM operation performed by the PWM circuit 1000 in FIG. 10 according to an embodiment of the present invention. Take the generation of the negative signal V_PWMN for example. Assume that the negative signal V_PWMN is set by a fixed voltage level V_SUMN2. In a case where the common-mode level of the sawtooth signal V_SAW (i.e., the mid-point of the peak-to-peak voltage of the sawtooth signal V_SAW) is set by V_CM,SAW, a negative signal V_PWMN2 is generated by comparing the sawtooth signal V_SAW with the fixed voltage level V_SUMN2. In another case where the common-mode level of the sawtooth signal V_SAW (i.e., the mid-point of the peak-to-peak voltage of the sawtooth signal V_SAW) is set by V′_CM,SAW, a negative signal V_PWMN1 is generated by comparing the sawtooth signal V_SAW with the fixed voltage level V_SUMN2. Since the common-mode level V′_CM,SAW is lower than the common-mode level V_CM,SAW, the duty cycle of the negative signal V_PWMN1 is larger than the duty cycle of the negative signal V_PWMN2. Hence, the DC level of the negative signal V_PWMN1 is higher than the DC level of the negative signal V_PWMN2. As can be seen from FIG. 11, a common-mode level (DC level) of a PWM signal can be adjusted by adjusting magnitude of a sawtooth signal used for pulse-width modulation. Hence, the common-mode level (DC level) of the PWM signal V_PWM is affected by the common-mode level of the sawtooth signal V_SAW. Based on the above observation, the sawtooth signal generator 1006 adaptively adjusts/shifts the common-mode level of the sawtooth signal V_SAW according to the common-mode control signal V_{CM_CTRL}. In this way, the common-mode level (DC level) of the PWM signal V_PWM generated from the PWM circuit 1000 is adjusted correspondingly.

[0041] The class-D amplifier 300 with PWM common-mode control as illustrated in FIG. 3 is for illustrative purposes only, and is not meant to be a limitation of the present invention. For example, the class-D amplifier 300 may be modified to include input feed-forward to reduce internal swing and signal-dependent terms to further enhance the linearity. More specifically, when a class-D amplifier employs the input feed-forward, internal nodes are input signal independent, and non-linearity of the loop filter has less impact on the class-D amplifier. FIG. 12 is a diagram illustrating another class-D amplifier with proposed PWM common-mode control according to an embodiment of the present invention. The class-D amplifier 1200 may be a fully-differential closed-loop class-D amplifier, including an input feed-forward circuit (e.g., two input feed-forward paths 1203 and 1204), a combining circuit 1202, and the aforementioned common-mode control circuit 302, loop filter 102, PWM circuit 104, output circuit 106 and feedback circuit 108. That is, the class-D amplifier 1200 shown in FIG. 12 may be created by adding the input feed-forward circuit (e.g., input feed-forward paths 1203 and 1204) and the combining circuit 1202 to the class-D amplifier 300 shown in FIG. 3. In this embodiment, the input feed-forward circuit (e.g., input feed-forward paths 1203 and 1204) is arranged to feed the input signal V_IN to the combining circuit 1202, and the combining circuit 1202 is arranged to combine the input signal V_IN and the filtered signal V_LF to generate the non-PWM signal V_SUM.

[0042] In one exemplary embodiment, the combining circuit 1202 may be composed of passive components only. Hence, input feed-forward with passive resistive summing can be realized by the combining circuit 1202. FIG. 13 is a diagram illustrating a combining circuit according to an embodiment of the present invention. The combining circuit 1202 shown in FIG. 12 may be implemented using the combining circuit 1300 shown in FIG. 13. The combining circuit 1300 only has passive components, including resistors R7, R8 and optional capacitors C7, C8. The resistors R7, R8 are used to realize the input feed-forward passive summation that combines the input signal V_IN (which is composed of the positive signal V_IP and the negative signal V_IN) and the filtered signal V_LF (which is composed of the positive signal V_LFP and the negative signal V_LFN) to generate the non-PWM signal V_PWM (which is composed of the positive signal V_PWMP and the negative signal V_PWMN). Since no active summing amplifier is needed by the combining circuit 1300, the power dissipation and/or the amplifier induced noise can be reduced. The capacitors C7 and C8 may be added to facilitate a low-pass function to filter out high-frequency noise along the signal path. The low-pass filter may be properly designed to reject out-of-band noise without affecting the noise transfer function (NTF) of the class-D amplifier.

[0043] Similarly, one of the aforementioned PWM common-mode control designs may be employed by the class-D amplifier 1200 for providing a common-mode gain H_C to effectively suppress δ×e_C contributed to the differential output (V_OP-V_ON). When the first exemplary PWM common-mode control design is employed, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to the loop filter 102 so as to adjust the common-mode level of the filtered signal V_LF for achieving PWM common-mode control. When the second exemplary PWM common-mode control design is employed, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to a combining circuit located between the loop filter 102 and the PWM circuit 104 so as to adjust the common-mode level of the non-PWM signal V_SUM for achieving PWM common-mode control. For example, the combining circuit 1202 may be configured to combine the input signal V_IN, the common-mode control signal V_{CM_CTRL} and the filtered signal V_LF to generate the non-PWM signal V_SUM. However, this is for illustrative purposes only, and is not meant to be a limitation of the present invention. When the third exemplary PWM common-mode control design is employed, the proposed common-mode control circuit 302 is configured to generate the common-mode control signal V_{CM_CTRL} to a sawtooth signal generator of the PWM circuit 104 so as to adjust the common-mode level of the sawtooth signal for achieving PWM common-mode control.

[0044] In above embodiments, the proposed PWM common-mode control mechanism is applied to a fully-differential closed-loop class-D amplifier. However, these are for illustrative purposes only, and are not meant to be limitations of the present invention. Any class-D amplifier design using the proposed PWM common-mode control mechanism (which may monitor a common-mode level of a differential amplifier output to generate a common-mode control signal to an internal CMFB circuit of a loop filter, an input of a PWM circuit or an internal sawtooth signal generator of the PWM circuit) falls within the scope of the present invention.

[0045] Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.