Patent Description:
A class-D amplifier is also known as a switching amplifier or a pulse width modulation (PWM) amplifier. By PWM control, the metal-oxide-silicon (MOS) transistors of the power driver of the class-D amplifier are either fully on or fully off, significantly reducing power loss. High-efficiency amplification is achieved.

There are several ways to convert analog signals to PWM signals to drive the MOSs of the power driver. To eliminate an output filter, a BD modulation switching technique has been developed. According to the BD modulation switching technique, the duty cycle of the difference between two output signals VOP and VON of the class-D amplifier is modulated to make the average content of the amplified result correspond to the analog input signal. The class-D amplifier is a closed-loop circuit. The larger the analog input signal, the more significant the difference of PWM width is between the two output signals.

However, the BD modulation switching technique may result in common-mode voltage fluctuation. <FIG> shows waveforms of the output signals VOP and VON and their common-mode voltage Out_CM. An ideal value of the common-mode voltage Out_CM is VDD/<NUM>. When both the output signals VOP and VON are low ('L', e.g., 0V), the common-mode voltage Out_CM is 0V. When both the output signals VOP and VON are high ('H', e.g., VDD), the common-mode voltage Out_CM is VDD. The common-mode voltage Out_CM fluctuates significantly (0V or VDD) rather than being kept at VDD/<NUM>. When fed back into a loop filter of a class-D amplifier, the common-mode voltage fluctuation degrades the gain of a differential operational amplifier within the loop filter.

A class-D amplifier without the phenomenon of common-mode voltage fluctuation is called for. <CIT> (<NUM>-<NUM>-<NUM>) and<NPL> disclose a class-D power amplifier with a similar common-mode voltage compensation circuit as disclosed by the present application.

An amplification circuit (e.g., a class-D amplifier using a PWM technique) that successfully eliminates the common-mode voltage fluctuation is proposed.

A amplification circuit in accordance with the present invention as defined by claim <NUM> includes a loop filter, a pulse-width modulation signal generator, a gate driver, a power driver, a signal feedback circuit, and a common-mode voltage compensation circuit. The loop filter, the pulse-width modulation signal generator, the gate driver, the power driver, and the signal feedback circuit are configured to form a loop for signal amplification. The common-mode voltage compensation circuit provides a control node that is coupled to a first input terminal and a second input terminal of the loop filter. The common-mode voltage compensation circuit couples the control node to a power ground voltage when both a first output signal and a second output signal of the amplification circuit are high. The common-mode voltage compensation circuit couples the control node to a power supply voltage when both the first output signal and the second output signal of the amplification circuit are low.

According to the present invention as defined by claim <NUM>, the loop filter has a differential operational amplifier. The first input terminal and the second input terminal of the loop filter are coupled to a pair of input terminals of the differential operational amplifier. The signal feedback circuit includes a first feedback resistor coupling the first output signal of the amplification circuit to the second input terminal of the loop filter, and a second feedback resistor coupling the second output signal of the amplification circuit to the first input terminal of the loop filter. The common-mode voltage compensation circuit includes a first compensation resistor coupled between the second input terminal of the loop filter and the control node, and a second compensation resistor coupled between the first input terminal of the loop filter and the control node.

The common-mode voltage compensation circuit may be a switching-type digital-to-analog converter, comprising a first switch controlled to couple the control node to the power supply voltage, and a second switch, controlled to couple the control node to the power ground voltage.

Unlike the use of an active component, the switching-type digital-to-analog converter saves power.

Furthermore, the signal feedback is not affected by common-mode voltage compensation.

The following description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense.

<FIG> depicts an amplification circuit in a differential-in and differential-out architecture in accordance with an exemplary embodiment of the present invention. The amplification circuit includes a loop filter <NUM>, a pulse-width modulation (PWM) signal generator <NUM>, a gate driver <NUM>, a power driver <NUM>, a signal feedback circuit <NUM>, and a common-mode voltage compensation circuit <NUM>. The loop filter <NUM>, PWM signal generator <NUM>, gate driver <NUM>, power driver <NUM>, and signal feedback circuit <NUM> are configured to form a loop for signal amplification. The common-mode voltage compensation circuit <NUM> is designed to compensate for common-mode voltage fluctuation.

As shown, a differential input pair (Vi <NUM>, Vi2) received by the amplification circuit is coupled to the loop filter <NUM>. The loop filter <NUM> is coupled to the PWM signal generator <NUM>. The PWM signal generator <NUM> generates PWM signals Vpwmp and Vpwmn by comparing the signals received from the loop filter <NUM> with a saw signal Vsaw. The gate driver <NUM> transforms the PWM signals Vpwmp and Vpwmn into control signals CSAp, CSAn, CSBp, and CSBn. The power driver <NUM> includes an H-bridge circuit controlled by the control signals CSAp, CSAn, CSBp, and CSBn to generate output signals VOP and VON to drive a load <NUM>. The output signals VOP and VON are fed back to the loop filter <NUM> through the signal feedback circuit <NUM>. The common-mode voltage compensation circuit <NUM> is provided to compensate for the fluctuation of a common-mode voltage Out_CM of the output signals VOP and VON.

The loop filter <NUM> includes a differential operational amplifier op. The loop filter <NUM> has two input terminals IN1 and IN2, which are coupled to a pair of input terminals of the differential operational amplifier op within the loop filter <NUM>. A stable common-mode voltage is required at the two input terminal IN1 and IN2.

In the exemplary embodiment shown in <FIG>, the signal feedback circuit <NUM> includes a feedback resistor Rfbp coupling the output signal VOP to the input terminal IN2 of the loop filter <NUM>, and a feedback resistor Rfbn coupling the output signal VON to the input terminal IN1 of the loop filter <NUM>. The common-mode voltage compensation circuit <NUM> has a compensation resistor RCMp coupled between the input terminal IN2 of the loop filter <NUM> and a control node ncs, and a compensation resistor RCMn coupled between the input terminal IN1 of the loop filter <NUM> and the control node ncs. The common-mode voltage compensation circuit <NUM> is a switching-type DAC (digital-to-analog converter). In addition to the resistors RCMp and RCMn, the common-mode voltage compensation circuit <NUM> includes a switching design and a logic circuit controlling the switching design. When both the output signals VOP and VON are high, the switching design is controlled to couple the control node ncs to a power ground voltage (GND). When both the output signals VOP and VON are low, the switching design is controlled to couple the control node ncs to a power ground voltage (VDD). In these situations, the feedback resistor Rfbp is combined with the compensation resistor RCMp and the feedback resistor Rfbn is combined with the compensation resistor RCMn; two voltage dividers are established. Through the two voltage dividers, the two input terminals IN1 and IN2 coupled to the pair of input terminals of the differential operational amplifier op are biased at the same stable direct current voltage level. The common-mode voltage fluctuation is suppressed.

The structure of the power driver <NUM> is described in this paragraph. The power driver <NUM> has p-channel metal-oxide-silicon (PMOS) transistors Mp1 and Mp2, and n-channel metal-oxide-silicon (NMOS) transistors Mn1 and Mn2. The PMOS transistor Mp1 is controlled by the control signal CSAp to pull up the output signal VOP to the power supply voltage VDD. The NMOS transistor Mn1 is controlled by the control signal CSAn to pull down the output signal VOP to the power ground voltage GND. The PMOS transistor Mp2 is controlled by the control signal CSBp to pull up the output signal VON to the power supply voltage VDD. The NMOS transistor Mn2 is controlled by the control signal CSBn to pull down the output signal VON to the power ground voltage GND. The control signals CSAp, CSAn, CSBp, and CSBn are further used to switch the switching design of the common-mode voltage compensation circuit <NUM>.

The common-mode voltage compensation circuit <NUM> uses a PMOS transistor Mp3 and an NMOS transistor Mn3 in the switching design. When the output signals VOP and VON are both high, the switch implemented by the PMOS transistor Mp3 is open, and the switch implemented by the NMOS transistor Mn3 is close. The control node ncs is coupled to the power ground voltage VGG. The high voltage level of the output signal VOP is divided by the resistors Rfbp and RCMp. A stable DC voltage level RCMp/(Rfbp+ RCMp) is supplied to the input terminal IN2 of the loop filter <NUM>. Similarly, the high voltage level of the output signal VON is divided by the resistors Rfbn and RCMn. A stable DC voltage level RCMn/(Rfbn+ RCMn) is supplied to the input terminal IN1 of the loop filter <NUM>. In an exemplary embodiment, the switches Mp3 and Mn3 are opened or closed in accordance with the control signals CSAp, CSAn, CSBp, and CSBn generated by the gate driver <NUM>.

In <FIG>, a logic circuit including a NAND gate <NUM> and a NOR gate <NUM> is provided to control the switches Mp3 and Mn3, respectively. The NAND gate <NUM> has two input terminals receiving the control signals CSAp and CSBp, and an output terminal coupled to a gate terminal of the PMOS Mp3. The NOR gate <NUM> has two input terminals receiving the control signals CSAn and CSBn, and an output terminal coupled to a gate terminal of the NMOS Mn3. When the output signals VOP and VON are both high (VDD), it means that the control signals CSAp and CSBp are low to turn on the PMOS transistors Mp1 and Mp2, and the control signals CSAn and CSBn are low to turn off the NMOS transistors Mn1 and Mn2. In response to the control signals CSAp and CSBp which are low, the NAND gate <NUM> outputs a high signal and the switch Mp3 is open. In response to the control signals CSAn and CSAn which are low, the NOR gate <NUM> output a high signal and the switch Mn3 is close. The control node ncs is coupled to the power ground voltage through the switch Mn3. The resistors Rfbp and RCMp form a voltage divider between VOP (H) and the grounded control node ncs. The input terminal IN2 of the loop filter <NUM> is biased at a stable voltage level RCMp/(RCMp+Rfbp). When RCMp is equal to Rfbp, the input terminal IN2 of the loop filter <NUM> is biased at VDD//<NUM>. The resistors Rfbn and RCMn form another voltage divider between VON (H) and the grounded control node ncs. The input terminal IN1 of the loop filter <NUM> is biased at a stable voltage level RCMn/(RCMn+Rfbn). When RCMn is equal to Rfbn, the input terminal IN1 of the loop filter <NUM> is biased at VDD//<NUM>. In such a design, the common mode voltage is kept at the ideal value VDD/<NUM>. The common-mode voltage fluctuation is successfully suppressed.

In comparison with active components, the switching-type DAC implementing the common-mode voltage compensation circuit <NUM> saves power.

Furthermore, the common-mode voltage compensation circuit <NUM> does not affect the feedback of the output signals VOP and VON. No matter the common-mode voltage compensation circuit <NUM> is turned on or not, the output signals VOP and VON is feedback the loop filter <NUM> as usual.

In an exemplary embodiment, the resistance ratio between the feedback resistor Rfbp and the compensation resistor RCMp is equal to the resistance ratio between the feedback resistor Rfbn and the compensation resistor RCMn.

In an exemplary embodiment, the resistors Rfbp, Rfbn, RCMp, and RCMn have the same resistance.

In an exemplary embodiment, the input terminal IN1 of the loop filter <NUM> is coupled to the positive input terminal "+" of the differential operational amplifier op, and the input terminal IN2 of the loop filter <NUM> is coupled to the negative input terminal "-" of the differential operational amplifier op.

In an exemplary embodiment, the input terminal IN1 of the loop filter <NUM> is coupled to the negative input terminal "-" of the differential operational amplifier op, and the input terminal IN2 of the loop filter <NUM> is coupled to the positive input terminal "+" of the differential operational amplifier op.

<FIG> depicts an amplification circuit in accordance with the present invention as defined by claim <NUM>. In comparison with the common-mode voltage compensation circuit <NUM> of <FIG>, the common-mode voltage compensation circuit <NUM> shown in <FIG> further includes two capacitors Cp and Cn. The capacitor Cp is coupled between the input terminal IN2 of the loop filter <NUM> and the control node ncs in series with the compensation resistor RCMp. The capacitor Cn is coupled between the input terminal IN1 of the loop filter <NUM> and the control node ncs in series with the compensation resistor RCMn. The in-band noise of the common-mode voltage compensation circuit <NUM> can be filtered by the capacitors Cp and Cn.

Claim 1:
An amplification circuit, comprising:
a loop filter (<NUM>), a pulse-width modulation signal generator (<NUM>), a gate driver (<NUM>), a power driver (<NUM>), and a signal feedback circuit (<NUM>), configured to form a loop for signal amplification; and
a common-mode voltage compensation circuit (<NUM>), providing a control node (ncs) that is coupled to a first input terminal (IN1) and a second input terminal (IN2) of the loop filter,
wherein:
the common-mode voltage compensation circuit couples the control node (ncs) to a power ground voltage when both a first output signal and a second output signal of the amplification circuit are high;
the common-mode voltage compensation circuit couples the control node to a power supply voltage when both the first output signal and the second output signal of the amplification circuit are low;
the loop filter has a differential operational amplifier (op), and the first input terminal and the second input terminal of the loop filter are coupled to a pair of input terminals of the differential operational amplifier;
the signal feedback circuit includes a first feedback resistor (Rfbp) coupling the first output signal of the amplification circuit to the second input terminal of the loop filter, and a second feedback resistor (Rfbn) coupling the second output signal of the amplification circuit to the first input terminal of the loop filter;
the common-mode voltage compensation circuit includes a first compensation resistor (RCMn) coupled between the second input terminal of the loop filter and the control node, and a second compensation resistor (RCMp) coupled between the first input terminal of the loop filter and the control node; the amplification circuit being characterized in that:
the common-mode voltage compensation circuit further includes a first capacitor (Cn), coupled between the second input terminal of the loop filter and the control node in series with the first compensation resistor; and
the common-mode voltage compensation circuit further includes a second capacitor (Cp), coupled between the first input terminal of the loop filter and the control node in series with the second compensation resistor, wherein the first and the second capacitors are configured to filter in-band noise of the common-mode voltage compensation circuit.