Output Driver and Its Pre-Charging Method

Abstract

The present invention provides an output driver and a pre-charging method thereof for an audio system, which includes a pre-charge circuit. The pre-charge circuit includes a charging amplifier and a feedback bias circuit. The charging amplifier includes an output node for coupling to a capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charging amplifier, and a bias node for receiving a bias current. The output current of the charging amplifier changes with the bias current. The feedback bias circuit is coupled to the output node to sense the output voltage of the charging amplifier, and is configured to provide a bias current that changes with the output voltage of the charging amplifier.

Description

Technical Field

[0001] The present invention relates to the field of audio amplifiers. In particular, embodiments of the present invention are dedicated to reducing or eliminating pop noise and click noise in an audio playback system. However, the circuits and methods described herein can be used in other applications to improve the charging of load devices. Background Art

[0002] Pop noise and click noise may occur when a playback system is powered on or off, and when a source driver causes rapid charging or discharging due to improper equalization, and can cause damage to the headphone speaker.

[0003] Existing methods for solving pop noise and click noise often suffer from many drawbacks. These drawbacks may include complex circuits, large silicon area, noise generation, and insufficient accuracy. These drawbacks and their improvement methods and circuit embodiments will be described in more detail below. Summary of the Invention

[0004] Embodiments of the present invention provide methods and embodiments for pre-charging the output of an audio amplifier, which control the bias current of the charging amplifier in a smooth and efficient manner to minimize pop noise and click noise. This method senses the output voltage through a feedback amplifier to monitor and automatically track the output voltage and adjust the bias current according to the output voltage. This method can use a programmable resistor to control the charging time to set the bias current of the charging amplifier according to the capacitive load and the required rise time. This method can also deliberately create an input offset on the differential amplifier used in a closed-loop system to bias the gate voltage of an n-type metal oxide semiconductor channel transistor so that the n-type metal oxide semiconductor channel transistor is biased in the sub-threshold region.

[0005] According to some embodiments of the present invention, an output driver for an audio system includes a driving amplifier circuit for driving a capacitive load and a pre-charge circuit for pre-charging the capacitive load to a common-mode voltage. The pre-charge circuit includes a charging amplifier, which includes an output node for coupling to the capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charging amplifier, and a bias node for receiving a bias current. The output current of the charging amplifier changes with the bias current. The pre-charge circuit also includes a feedback bias circuit, which includes a first current source, a second current source, and a summing circuit. The first current source includes a first transistor and a feedback amplifier. The feedback amplifier includes an output node coupled to the gate of the first transistor, a first input node for coupling to the output node of the charging amplifier, a second input node for coupling to the source of the first transistor, and a resistor coupled to the source of the first transistor. The first current source is configured to provide a first current such that the first current changes with the output voltage of the charging amplifier, and the second current source is used to provide a second current. The summing circuit includes a first current mirror and a second current mirror to provide a bias current, which represents the sum of the first current and the second current. The summing circuit is coupled to the bias node of the charging amplifier to provide the bias current to the charging amplifier.

[0006] In some embodiments of the above output driver, the feedback amplifier has a pre-set input offset configured to bias the first transistor to be weakly-on in the sub-threshold region.

[0007] In some embodiments of the above output driver, the output voltage of the charging amplifier changes proportionally with the bias current.

[0008] In some embodiments of the above output driver, the resistor is a programmable resistor configured to set the bias current of the charging amplifier according to the capacitive load and the desired rise time.

[0009] In some embodiments of the above output driver, the output driver of the present invention includes a switching circuit configured to couple the output node of the charging amplifier to the capacitive load or a ground node.

[0010] In some embodiments of the above output driver, the charging amplifier and the feedback amplifier are operational amplifiers.

[0011] In some embodiments of the above output driver, the second current is a starting current, and the starting current is characterized in that its current amount can be selected as the initial step of the charging current.

[0012] According to some embodiments of the present invention, an output driver for an audio system includes a pre-charge circuit. The pre-charge circuit includes a charge amplifier and a feedback bias circuit. The charge amplifier includes an output node for coupling to a capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charge amplifier, and a bias node for receiving a bias current. The output current of the charge amplifier changes with the bias current. The feedback bias circuit is coupled to the output node to sense the output voltage of the charge amplifier and is configured to provide a bias current that changes with the output voltage of the charge amplifier.

[0013] In some embodiments of the above output driver, the feedback bias circuit includes a first current source. The first current source includes a first transistor and a feedback amplifier. The feedback amplifier includes an output node coupled to the gate of the first transistor, a first input node for coupling to the output node of the charge amplifier, and a second input node for coupling to the source of the first transistor. The first current source also includes a resistor for coupling to the source of the first transistor. The first current source is configured to provide a first current that changes with the output voltage of the charge amplifier.

[0014] In some embodiments of the above output driver, the feedback bias circuit includes a second current source for providing a second current, and a summing circuit including a first current mirror and a second current mirror to provide a sum of bias currents, where the bias current represents the sum of the first current and the second current. The summing circuit is coupled to the bias node of the charge amplifier to provide a bias current to the charge amplifier. The pre-charge circuit is configured to provide a capacitive load output current at the output node of the charge amplifier. The output current changes with the output voltage of the output node.

[0015] In some embodiments of the above output driver, the second current is a starting current, and the starting current is characterized in that its current value can be selected as the starting stage of the charging current.

[0016] In some embodiments of the above output driver, the feedback amplifier has a preset input offset, and the preset input offset is configured to bias the first transistor to be weakly turned on in the sub-threshold region.

[0017] In some embodiments of the above output driver, the resistor is a programmable resistor, and the programmable resistor is configured to set the bias current of the charge amplifier according to the capacitive load and the required rise time.

[0018] In some embodiments of the above output driver, the output current of the charge amplifier changes with the bias current.

[0019] In some embodiments of the above output driver, the output driver of the present invention further includes a switching circuit configured to couple the output node of the charging amplifier to a capacitive load or a ground node.

[0020] According to some embodiments of the present invention, a pre-charging method for an output driver in an audio system is provided. The method includes providing a charging amplifier including an output node for coupling to a capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charging amplifier, and a bias node for receiving a bias current. The method also includes sensing the output voltage of the charging amplifier, providing a first current according to the output voltage of the charging amplifier, coupling the first current to the bias node of the charging amplifier to provide a bias current such that the bias current changes with the output voltage, and charging the capacitive load using the output voltage of the charging amplifier.

[0021] In some embodiments of the above output driver, the step of sensing the output voltage of the charging amplifier includes providing a feedback amplifier, coupling a first input node of the feedback amplifier to the output node of the charging amplifier, coupling a second input node of the feedback amplifier to the source of a first transistor, and coupling the output node of the feedback amplifier to the gate of the first transistor.

[0022] In some embodiments of the above output driver, the step of providing a first current includes coupling the source of the first transistor to a resistor.

[0023] In some embodiments of the above output driver, the method of the present invention further includes providing a second current as a programmable current.

[0024] In some embodiments of the above output driver, the method of the present invention further includes coupling the second current to the bias node of the charging amplifier as an initial charging voltage step.

[0025] The following describes information regarding the advantages and properties claimed by the present invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 FIG. is a simplified schematic diagram of a pipeline output driver in an existing audio system;

[0027] Figure 2 FIG. is a simplified block diagram of an audio driving circuit in an audio system according to some embodiments of the present invention;

[0028] Figure 3A and Figure 3B FIG. is a simplified schematic diagram of a pre-charge circuit for an output driver in an audio system according to some embodiments of the present invention;

[0029] Figure 4 Simplified schematic diagram of a precharge circuit for an output driver in an audio system according to some embodiments of the present invention;

[0030] Figure 5 Schematic diagram of an operational amplifier that can use the precharge circuit in Figure 4 according to some embodiments of the present invention;

[0031] Figure 6A and Figure 6B Voltage and current diagrams of the precharge circuit in Figure 4 according to some embodiments of the present invention;

[0032] Figure 7 Performance data table shown according to the simulation results of the precharge circuit in Figure 4 ; and

[0033] Figure 8 Flowchart of a method for precharging an output driver in an audio system according to some embodiments of the present invention.

[0034] Symbol description:

[0035] 100: Output driver

[0036] 200: Audio drive circuit

[0037] 210: Drive amplifier circuit

[0038] 220, 440: Capacitive load

[0039] 230, 300, 350, 400: Precharge circuit

[0040] 241, 242, S1, S2: Switch

[0041] 310, 410, 500: Charge amplifier

[0042] 320, 420: Feedback bias circuit

[0043] 412, 424, 530: Output node

[0044] 414, 426, 510: First input node

[0045] 416, 427, 520: Second input node

[0046] 418, 540: Bias node

[0047] 421: First current source

[0048] 422: First transistor

[0049] 423: Feedback amplifier

[0050] 425: Gate

[0051] 428: Source

[0052] 429: Resistor

[0053] 431: Second current source

[0054] 432: Summing circuit

[0055] 800: Method

[0056] 810~850: Steps

[0057] clk: Clock signal

[0058] Co: Capacitor

[0059] C L : Load capacitance

[0060] I1: First current

[0061] I2: Second current

[0062] Iout: Output current

[0063] Ibias: Bias current

[0064] Ib<1:0>: 2-bit code

[0065] M1~M4: Transistor

[0066] OUT: Output node

[0067] OUTN, OUTP: Connection options

[0068] Ro: Resistor

[0069] R: Resistance value

[0070] Rampgen: Ramp signal generator

[0071] rst: Reset signal

[0072] THD: Total harmonic distortion

[0073] Vout, Vo: Output voltage

[0074] V1: Voltage

[0075] VCM: Common-mode voltage Detailed implementation manners

[0076] Figure 1 To illustrate a simplified schematic diagram of a pipeline output driver in an existing audio system. Figure 1Illustrate existing methods and devices for suppressing pop noise and click noise. In this example, the output driver 100 is a discrete sampling system and requires a clock signal (clk). Initially, the reset signal (rst) is high, which forces the potential V1 to be pulled down to ground potential. When the reset signal is turned off, the circuit starts to let the amplifier AMP drive the voltage V1 of the node up to VCM (common mode voltage). At the same time, the ramp signal generator Rampgen is turned on. I bias The input is the bias current of the ramp signal generator Rampgen. The output of the ramp signal generator then drives the gate of the n-type metal oxide semiconductor (NMOS) transistor M1 and slowly raises the gate voltage step by step at each pulse time. The rise time of the output voltage depends on the external DC blocking capacitor Co and the load resistor Ro.

[0077] Figure 1 Is a discrete system that requires the signal of the ramp signal generator to control the gate, and pre-charges the output capacitor to VCM (common mode voltage) in this control manner. Figure 1 One of the disadvantages of the shown system is that the clock signal requires a ramp signal generator, which can cause clock coupling interference and increase power consumption. In addition, it should be carefully noted that it is necessary to minimize the layout parasitic capacitance to prevent the interference caused by the clock coupling to the node voltage V1, because it may cause, for example, pop noise or click noise.

[0078] Figure 2 Illustrate a simplified block diagram of an audio drive circuit in an audio system according to some embodiments of the present invention. Figure 2 Draw the audio drive circuit 200 in the audio system. The audio drive circuit 200 includes a drive amplifier circuit 210 for driving the capacitive load 220 and a pre-charge circuit 230 for pre-charging the capacitive load 220 to a reference voltage such as the common mode voltage (VCM). The capacitive load 220 may include a capacitor Co and a resistor Ro at the output node OUT. The capacitor Co may be a coupling capacitor, and the resistor Ro may represent a speaker.

[0079] In Figure 2 the audio drive circuit 200 also includes switches 241 and 242 to couple the drive amplifier circuit 210 and the pre-charge circuit 230. When switch 241 is off and switch 242 is on, the pre-charge circuit 230 is activated and the drive amplifier circuit 210 is not connected to the output. The pre-charge circuit 230 takes over and charges the voltage of the output (the end point commonly connected by switches 241 and 242) to the common mode voltage VCM. When the voltage of the output is charged to the common mode voltage VCM, the pre-charge circuit 230 disconnects from the output, and the drive amplifier circuit 210 takes over to start driving the audio signal to the output.

[0080] Figure 3A andFigure 3B FIG. 1 is a simplified schematic diagram of a pre - charge circuit for an output driver in an audio system according to some embodiments of the present invention. Figure 3A FIG. 2 shows a pre - charge circuit 300 including a charge amplifier 310 and a feedback biasing circuit 320. The pre - charge circuit 300 is one example of various pre - charge circuits and can be used as Figure 2 the pre - charge circuit 230 of the audio driver circuit 200 shown in FIG. 3. The pre - charge circuit 300 includes a continuous - time feedback system and does not require a clock signal. This method can avoid clock coupling generated in existing systems as Figure 1 shown. The pre - charge circuit 300 can be configured to pre - charge the output voltage to a reference voltage such as a common - mode voltage (VCM) in a smooth and efficient manner and avoid charging glitches, thereby minimizing pop noise and click noise when the playback system is powered on.

[0081] Figure 3A Also shown are two switches S1 and S2. The switch S1 is used to couple an audio load such as a speaker, which is represented by a capacitive load C L Two connection options are labeled OUTN or OUTP. The switch S2 is used to couple to a ground node. Figure 3A The output Vo of the pre - charge circuit 300 shown in FIG. 4 is coupled to the ground node and the output voltage is 0V. Figure 3B FIG. 5 shows a pre - charge circuit 350, which has the same circuit as the Figure 3A pre - charge circuit 300 shown in FIG. 2. However, the pre - charge circuit 350 is coupled to the output terminal through the operation of switches S1 and S2 as Figure 3B shown in FIG. 6. The output voltage Vo is charged to the common - mode voltage VCM as Figure 3B shown in FIG. 7. In some embodiments, the switches S1 and S2 can be driven by a control circuit in the audio system.

[0082] Initially, as Figure 3A shown in FIG. 8, the charge amplifier 310 is not operating (power off) and the output voltage Vo is 0V to save power. Figure 3A FIG. 9 shows the circuit in an inoperative mode when the switch S1 is configured to be open and the switch S2 is closed. In Figure 3B FIG. 10, at time t = 0, the switch S1 is closed and the switch S2 is open. This causes the charge amplifier 310 to start slowly driving the voltage at the output terminal towards the common - mode voltage VCM. The biasing circuit 320 continuously tracks the output voltage and adjusts the biasing current I of the charge amplifier 310 bias, so as to prompt the output voltage Vo to reach the common-mode voltage VCM completely smoothly. In some audio systems, the common-mode voltage VCM can be set to the middle value of the output supply voltage. For example, if the output supply voltage is 3.3V, the common-mode voltage VCM is 1.65V. In some embodiments, the charge amplifier 310 can be a unity-gain amplifier.

[0083] Figure 4 FIG. is a simplified schematic diagram of a precharge circuit for an output driver in an audio system according to some embodiments of the present invention. The precharge circuit 400 is an example of a precharge circuit, which can be used as Figure 2 the precharge circuit 230 of the audio drive circuit 200 shown in Figure 3A the precharge circuit 300 shown in Figure 3B or the precharge circuit 350 shown in Figure 4 FIG. shows that the precharge circuit 400 includes a charge amplifier 410 and a feedback bias circuit 420. Figure 4 Also shown are two switches S1 and S2. The switch S1 is used to couple a load such as a speaker, which is represented by a capacitive load 440 having a capacitor C L . The switch S2 is used to set the initial state of the output voltage Vo to ground. Two connection options are labeled OUTN or OUTP.

[0084] As Figure 4 shown, the charge amplifier 410 includes an output node 412 for coupling to the capacitive load 440, a first input node 414 for receiving a reference voltage such as the common-mode voltage VCM, and a second input node 416 for coupling to the output node 412 of the charge amplifier 410. The charge amplifier 410 also has a bias node 418 for receiving a bias current I bias , where the output voltage Vo of the charge amplifier 410 changes with the bias current I bias .

[0085] In Figure 4In it, the feedback bias circuit 420 includes a first current source 421 and a second current source 431. The first current source 421 includes a first transistor 422, a feedback amplifier 423, and a resistor 429. The feedback amplifier 423 includes an output node 424 coupled to the gate 425 of the first transistor 422, a first input node 426 for coupling to the output node 412 of the charge amplifier 410, and a second input node 427 for coupling to the source 428 of the first transistor 422. The resistor 429 is coupled to the source 428 of the first transistor 422. The first current source 421 is configured to provide that the first current I1 changes with the output voltage Vo of the charge amplifier 410. The second current source 431 provides a reference current I2 as an initial current. The feedback bias circuit 420 also includes a summing circuit 432, which includes a first current mirror (mirroring the first current I1) composed of transistors M1 and M2 and a second current mirror (mirroring the first current I2) composed of transistors M3 and M4 to provide a bias current I bias . The bias current I bias is expressed as the sum of the first current I1 and the second current I2. The summing circuit 432 is coupled to the bias node 418 of the charge amplifier 410 to provide the bias current I bias to the charge amplifier 410.

[0086] Figure 4 Illustrates an example of an auto-tracking and self-bias circuit for controlling the pre-charge of the output voltage Vo. Initially, the reset switch S2 (reset signal) is turned on, and the output voltage Vo is pulled to the ground voltage. To start the pre-charge, the reset switch S2 is turned off. The feedback amplifier 423 has a deliberate input offset to turn off the first transistor 422 (n-type metal oxide semiconductor transistor) and bias it in the cut-off region or weakly-on in the sub-threshold region. The current flowing through the first transistor 422 is set to Vo / R, where R is the resistance value of the resistor 429, and the resistor 429 can be a programmable resistor. In some embodiments, with a 2-bit programming code, the resistance value R can have 4 values. In another embodiment, the range of the selectable resistance value R is 60KΩ to 75KΩ. The embodiment with resistance adjustment allows fine granularity to set the different rise times of the output voltage by setting the first current I1 flowing through the first transistor 422.

[0087] In one case, initially, at time t = 0+, switch S2 is off and switch S1 is on, causing charge amplifier 410 to start slowly charging the output voltage Vo to the common-mode voltage VCM. Here, switch S2 can act as a reset switch and switch S1 can act as a charge switch. An initial near-zero first current I1 starts to increase through the first transistor 422. The first current I1 is mirrored through transistors M3 and M4 (p-type metal oxide semiconductor transistors) and increases to a second current I2 of transistor M2 (p-type metal oxide semiconductor transistor). Thus, as the output voltage Vo rises, the first current I1 will also continue to increase. The second current I2 determines the initial step during charging. In some embodiments, for example, the target voltage for pre-charging and the common-mode voltage VCM can also be programmable to compensate for power supply variations.

[0088] Please refer to Figure 4 , the bias current flowing into the bias node 418 of charge amplifier 410 can be expressed as follows.

[0089] I bias = I1 + I2 = (Vo / R) + I2

[0090] In some embodiments, charge amplifier 410 with an input bias current I bias can also be expressed as Figure 5 shown.

[0091] As Figure 4 an operating example of the pre-charge circuit 400, the initial value of the second current I2 is preset to 750 nA. When the first current source 421 senses and tracks the increase in the output voltage Vo of charge amplifier 410, the first current I1 (Vo / R) continues to increase. Thus, the output voltage Vo increases in a smooth control manner, so that any pop noise and click noise can be minimized during charging.

[0092] Figure 5 FIG. is a schematic diagram of an operational transconductance amplifier that can be used as the pre-charge circuit in Figure 4 according to some embodiments of the present invention. As Figure 5 shown, amplifier 500 can be an example of various amplifiers, which can be used as Figure 2 the charge amplifier 410 shown. Amplifier 500 includes an output node 530 for coupling to a capacitive load, a first input node 510 for receiving a reference voltage (common-mode voltage VCM), a second input node 520 for coupling to the output node 530 of charge amplifier 500. Amplifier 500 also includes a bias node 540 for receiving the bias current I bias . It can be understood that the output current Iout of amplifier 500 varies with the bias current I biasand change. The amplifier 500 also includes a plurality of transistors forming a current mirror. The functions of the plurality of transistors are not described in detail herein.

[0093] Figure 6A and Figure 6B is shown for a partial embodiment according to the present invention Figure 4 the voltage and current diagrams of the precharge circuit in Figure 6A is shown Figure 4 in which the charge amplifier 410 simulates the output voltage Vout as a function of time. The output voltage Vout is charged to the common-mode voltage VCM, which is approximately 1.65V when the 3.3V power supply is taken as an example. Figure 6B is shown Figure 4 in which the bias current I is simulated as a function of time in bias . It can be understood that the output voltage Vout of the output node changes with the bias current I bias . Furthermore, when the charging operation starts (t = 0), the initial value of the bias current I bias depends on Figure 4 the initial value of the second current I2 in Figure 6A As shown, the bias current I bias can select a lower initial value to obtain a low voltage step in the output voltage Vout.

[0094] Figure 7 is for Figure 4 the simulation results of the precharge circuit to show the performance data table. Figure 7 shows the simulation results of the precharge circuit under normal operating conditions (27°C, 3.3V). This simulation is for the programmable resistor of the feedback bias circuit to select its resistance value R from 60KΩ to 75KΩ using the 2-bit code Ib<1:0>. The capacitive load C L at the outputs OUTN and OUTP is 1μF. When the output voltage reaches 90% of the final common-mode voltage VCM with an average rise time of 225ms, the measured value of the total harmonic distortion (THD) is approximately -91dB. Generally speaking, the value of the total harmonic distortion required for pop noise / click noise suppression is -80dB or more. It can be understood that the -91dB of the total harmonic distortion of the aforementioned precharge circuit is substantially better than the general standard of -80dB.

[0095] Figure 8 is a flowchart of a method for precharging an output driver in an audio system according to a partial embodiment of the present invention. Combining the foregoing Figure 2 describes an example of the output driver. Combining the foregoing Figure 3A and Figures 3B to 5 describes Figure 8 the example of the method of executing the precharge circuit. AsFigure 8 As shown, the method 800 of a precharge output driver includes: in step 810, providing a charge amplifier such as Figure 4 the charge amplifier 410 in. The charge amplifier includes an output node for coupling to a capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charge amplifier, and a bias node for receiving a bias current. In some embodiments, as described in conjunction with the foregoing Figure 4 and Figure 5 , the output current of the charge amplifier changes with the bias current.

[0096] In step 820, the method includes sensing the output voltage of the charge amplifier. In some embodiments, sensing the output voltage of the charge amplifier includes sensing the output voltage using a feedback bias circuit such as the foregoing Figure 4 . The method includes providing a feedback amplifier, coupling a first input node of the feedback amplifier to the output node of the charge amplifier, coupling a second input node of the feedback amplifier to the source of a first transistor, and coupling the output node of the feedback amplifier to the gate of the first transistor.

[0097] In step 830, the method includes providing a bias current according to the output voltage of the charge amplifier. In some embodiments, the step of providing a bias current includes providing a bias current including a first current by coupling the source of the first transistor to a resistor. In some embodiments, the step of providing a bias current provides a bias current including a second current by coupling the source of the first transistor to a resistor.

[0098] In step 840, the method includes coupling a bias current to the bias node of the charge amplifier to provide a bias current that changes with the output voltage. The method may further include coupling a second current to the bias node of the charge amplifier as an initial charging voltage step.

[0099] In step 850, the method includes charging a capacitive load using the output voltage of the charge amplifier. Figure 6A , Figure 6B and Figure 7 illustrate the simulation results of smooth precharging of capacitive loads and reduction of pop noise and click noise.

[0100] The methods and processes described herein may be embodied, in part or in whole, as program code and / or data stored in a computer-readable storage medium or device such that, when the computer system reads and executes the program code and / or data, the computer system can perform the relevant methods and processes. The methods and processes may also be embodied, in part or in whole, in hardware modules or devices such that, when the hardware modules or devices are activated, they can perform the relevant methods and processes. A combination of program code, data, and hardware modules or devices may be used to embody the methods and processes disclosed herein.

[0101] Specific embodiments have been described herein. However, various modifications may be made to these embodiments, and the principles presented herein may be applied to other embodiments. Additionally, various components and / or steps / blocks of the methods may be implemented in arrangements other than the specifically disclosed arrangements without departing from the claims. Given these teachings, other embodiments and modifications will be readily apparent to those skilled in the art. Accordingly, the appended claims are intended to cover all such embodiments and modifications when considered in conjunction with the above specification and the accompanying drawings.

Claims

1. An output driver for an audio system, characterized in that, Comprising: A driving and amplifying circuit for driving a capacitive load; A pre-charge circuit for pre-charging the capacitive load to a common-mode voltage, the pre-charge circuit comprising: A charge amplifier, which comprises: An output node for coupling to the capacitive load; A first input node for receiving a reference voltage; A second input node for coupling to the output node of the charge amplifier; and A bias node for receiving a bias current, wherein the output current of the charge amplifier changes with the bias current; and A feedback bias circuit, which comprises: A first current source, which comprises: A first transistor; A feedback amplifier, which comprises: An output node coupled to the gate of the first transistor; A first input node for coupling to the output node of the charge amplifier; and A second input node for coupling to the source of the first transistor; and A resistor for coupling to the source of the first transistor, wherein the first current source is configured to provide a first current, and the first current changes with the output voltage of the charge amplifier; A second current source for providing a second current; and A summing circuit, which comprises a first current mirror and a second current mirror to provide the bias current, and the bias current represents the sum of the first current and the second current, wherein the summing circuit is coupled to the bias node of the charge amplifier to provide the bias current to the charge amplifier.

2. The output driver according to claim 1, wherein The feedback amplifier has a preset input offset, and the preset input offset is configured to bias the first transistor to be weakly turned on in the sub-threshold region.

3. The output driver according to claim 1, wherein The output voltage of the charge amplifier changes proportionally with the bias current.

4. The output driver according to claim 1, wherein The resistor is a programmable resistor, and the programmable resistor is configured to set the bias current of the charge amplifier according to the capacitive load and the required rise time.

5. The output driver according to claim 1, wherein It further comprises a switching circuit, and the switching circuit is configured to couple the output node of the charge amplifier to the capacitive load or a ground node.

6. The output driver according to claim 1, wherein The charge amplifier and the feedback amplifier are operational transconductance amplifiers.

7. The output driver according to claim 1, wherein The second current is a starting current, and the starting current is characterized in that its current amount can be selected as the starting stage of the charging current.

8. An output driver for an audio system, characterized in that, Comprising: A pre-charge circuit, wherein the pre-charge circuit comprises: A charge amplifier, which comprises an operational transconductance amplifier and comprises: An output node for coupling to a capacitive load; A first input node for receiving a reference voltage; A second input node for coupling to the output node of the charge amplifier; and A bias node for receiving a bias current, wherein the output current of the charge amplifier changes with the bias current; and A feedback bias circuit, which is coupled to the output node to sense the output voltage of the charge amplifier and is configured to provide the bias current, and the bias current changes with the output voltage of the charge amplifier.

9. The output driver according to claim 8, wherein The feedback bias circuit comprises: A first current source, which comprises: A first transistor; A feedback amplifier, which comprises: An output node coupled to the gate of the first transistor; A first input node for coupling to the output node of the charge amplifier; and A second input node for coupling to the source of the first transistor; and A resistor for coupling to the source of the first transistor, wherein the first current source is configured to provide a first current that changes with the output voltage of the charge amplifier.

10. The output driver according to claim 9, wherein The feedback bias circuit further includes: A second current source for providing a second current; and A summing circuit including a first current mirror and a second current mirror to provide the bias current, the bias current representing the sum of the first current and the second current, wherein the summing circuit is coupled to the bias node of the charge amplifier to provide the bias current to the charge amplifier; wherein, the precharge circuit is configured to provide the capacitive load output current at the output node of the charge amplifier, and the output current changes with the output voltage of the output node.

11. The output driver according to claim 10, wherein The second current is a starting current, and the starting current is characterized in that its current amount can be selected as the starting stage of the charging current.

12. The output driver according to claim 9, wherein The feedback amplifier has a preset input offset, and the preset input offset is configured to bias the first transistor to be weakly turned on in the subthreshold region.

13. The output driver according to claim 9, wherein The resistor is a programmable resistor, and the programmable resistor is configured to set the bias current of the charge amplifier according to the capacitive load and the required rise time.

14. The output driver according to claim 8, wherein The output current of the charge amplifier changes with the bias current.

15. The output driver according to claim 8, characterized in that A switch circuit is further included, and the switch circuit is configured to couple the output node of the charge amplifier to the capacitive load or the ground node.

16. A pre-charging method for an output driver in an audio system, characterized in that, The method includes: Providing a charge amplifier, the charge amplifier including an output node for coupling to a capacitive load, a first input node for receiving a reference voltage, a second input node for coupling to the output node of the charge amplifier, and a bias node for receiving a bias current; Sensing the output voltage of the charge amplifier; Providing a first current according to the output voltage of the charge amplifier; Coupling the first current to the bias node of the charge amplifier to provide the bias current, the bias current changing with the output voltage; and Charging the capacitive load with the output voltage of the charge amplifier; wherein, the step of sensing the output voltage of the charge amplifier includes: Providing a feedback amplifier; Coupling a first input node of the feedback amplifier to the output node of the charge amplifier; Coupling a second input node of the feedback amplifier to the source of a first transistor; and Coupling the output node of the feedback amplifier to the gate of the first transistor.

17. The pre-charging method according to claim 16, wherein The step of providing the first current further includes coupling a source node of the first transistor to a resistor.

18. The pre-charging method according to claim 16, wherein Providing a second current as a programmable current is further included.

19. The pre-charging method according to claim 18, wherein Coupling the second current to the bias node of the charge amplifier as an initial charging voltage stage is further included.

Images