Adjustable duration for driving pulse width modulation (PWM) output to reduce thermal noise

Abstract

Noise introduced in the output signal of a pulse width modulator (PWM) can be reduced by varying the duration of the switch drive output node. Because the power supply coupled to the switch is a source of noise in the output signal of the PWM, the duration of the power supply drive output can be reduced to result in a subsequent reduction of noise in the output signal. For example, when a small signal is desired to be output by the PWM, the switch can be operated for a short duration. Thus, the switch couples the noise source to ground for the duration of a cycle to reduce the contribution of noise to the output. But when a larger signal is desired to be output by the PWM, the switch can be operated for a longer duration or the conventional duration described above.

Description

[0001] This application is a divisional application of the original application filed in the China Patent Office on June 15, 2018 (international filing date of June 19, 2016) and designated as Patent Application No. 201680073860.5, entitled "Adjustable Duration for Driving Pulse Width Modulation (PWM) Output to Reduce Thermal Noise." TECHNICAL FIELD

[0002] The present disclosure relates to signal modulation. More specifically, portions of the present disclosure relate to pulse width modulation (PWM) with reduced noise. BACKGROUND

[0003] Information can be represented as a signal in many ways. One such way is modulation, in which information is encoded by changing a characteristic of a carrier signal. One example of modulation is amplitude modulation (AM) employed in AM radios. In an AM radio signal, the amplitude of a signal at a fixed frequency changes over time according to the information to be transmitted. Another example of modulation is pulse width modulation (PWM). Pulse width modulation (PWM) is frequently used in electronics and communications (e.g., mobile phones) to transmit information from one point to another, whether across a circuit board or across radio waves kilometers away. Pulse width modulation (PWM) encodes data as a series of pulses in a pulsating signal. The pulses are generated by a circuit such as the one shown in Figure 1 .

[0004] Figure 1 is a circuit schematic diagram illustrating a conventional pulse width modulator. The modulator 110 can include switches 116 and 118 coupled to a positive power supply and a negative power supply, respectively. The switches 116 and 118 can be configured to drive an output node 102 when coupled to a node 112 and not to drive the output node 102 when coupled to a node 114. Control signals CTRL1 and CTRL2 supplied to the switches 116 and 118 can couple the switches 116 and 118 to the node 112 to produce a pulse at the output node 102. For example, the control signals shown in Figure 2A may be applied to the switches 116 and 118 of Figure 1 . The first control signal CTRL1 202 and the second control signal CTRL2 204 can be high at time 212 to couple the switch 116 to the node 112 and the switch 118 to the node 114. At time 214, the switches 116 and 118 are reversed so that the switch 116 is coupled to the node 114 and the switch 118 is coupled to the node 112. As another example, the control signals shown in Figure 2B may be applied to the switches 116 and 118 of Figure 1Switches 116 and 118 are used to generate an average positive output. A first control signal CTRL1 206 may go high at time 212, followed by a second control signal CTRL2 208 at time 216. Now, the output at output node 102 is the average of the drive strengths from switches 116 and 118, but weighted by the amount of time each switch is coupled to node 112. When the drives from switches 116 and 118 are equal, the output at output node 102 is a positive pulse between time 212 and time 214 because the first control signal CTRL1 couples switch 116 to node 112 during the longer duration of control signal CTRL2 coupling switch 118 to node 112.

[0005] The above about Figure 1 In the pulse width modulation (PWM) technique described above, the output signal at output node 102 is not perfect. Noise is generated by the power supplies of switches 116 and 118, and this noise becomes part of the output at output node 102. One type of noise introduced by switches 116 and 118 is thermal noise. Thermal noise is generated as a result of thermal fluctuations within the power supplies of switches 116 and 118 due to variations in the drive of node 112. These thermal fluctuations may be caused by environmental changes or heat generated by operating the power supply. In the conventional pulse width modulation (PWM) technique described above, thermal noise is always present in the output signal because switches 112 and 118 are never both coupled to node 114. Instead, one of switches 116 and 118 always drives output node 102.

[0006] As electronic devices continue to advance, the total power consumed by the devices decreases, leading to significant noise issues. This reduction in power consumption is often due to a decrease in the amplitude of the supply voltage to the electronic devices, which is necessary for the miniaturization of transistors (standard components in electronic devices). However, the sources of noise (such as thermal noise in the power supply) do not necessarily decrease along with power consumption. Therefore, as power consumption decreases, the relative contribution of noise to the output signal increases. Because many applications require a sufficient signal-to-noise ratio (SNR), noise introduced by the power supply becomes a problem when signal levels are low.

[0007] The disadvantages mentioned herein are merely representative and are included only to emphasize the need for improved electrical components, particularly for pulse width modulation used in consumer-grade devices such as smartphones. The embodiments described herein address certain disadvantages, but are not necessarily every disadvantage described herein or known in the art. Summary of the Invention

[0008] Noise introduced in an output signal of a pulse width modulator (PWM) can be reduced by varying the duration of the switch drive output node. Because the power supply coupled to the switch is a source of noise in the output signal of the PWM, the duration of the power supply drive output can be reduced to result in a subsequent reduction of noise in the output signal. For example, when a small signal is desired to be output by the PWM, the switch can be operated during a shorter duration. Thus, the switch couples a noise source to ground during the duration of a cycle to reduce the contribution of noise to the output. This switching technique can be implemented in one embodiment as less than 50% duty cycle of the switch. But when a larger signal is desired to be output by the PWM, the switch can be operated during a longer duration or a regular duration as described above.

[0009] According to one embodiment, an apparatus can include an output node; a first source to drive the output node in a positive direction; a first switch coupled to the first source and the output node, wherein the first switch is configured such that when connected as a first phase, the output node is driven in the positive direction and contributes noise, and when connected as a second phase, does not contribute to the output noise or drives the output node in any direction; a second source to drive the output node in a negative direction; a second switch coupled to the second source and the output node, wherein the second switch is configured such that when connected as a third phase, the output node is driven in the negative direction and contributes noise, and when connected as a fourth phase, does not contribute to the output noise or drives the output node in any direction; and / or a controller coupled to the first switch and coupled to the second switch. The controller can be configured to perform steps including receiving a reference signal; operating the first switch and the second switch to produce a pulse width modulated (PWM) representation of the reference signal at the output node; and / or adjusting a duration that the first switch and the second switch are coupled to the output node based at least in part on an envelope level of a desired output at the output node.

[0010] In certain embodiments, the controller can be configured to perform steps including adjusting the duration that the first switch and the second switch are coupled to the output node in proportion to an envelope level; in a first mode, coupling the first switch and the second switch to the output node for a first duration; in a second mode, coupling the first switch and the second switch to the output node for a second duration that is shorter than the first duration; switching between the first mode and the second mode based at least in part on the envelope level; switching from the first mode to the second mode when the envelope level is at least 10 decibels below a total scale; adjusting the duration based at least in part on an amplitude of the reference signal; adjusting the duration based at least in part on a desired volume level; operating the first switch to produce a pulse for a duration that is longer than the second switch; and / or coupling the first switch and the second switch to the output node at a duty cycle that is less than 50%.

[0011] In certain embodiments, the first source can be stronger than the second source, and the controller can be configured to operate the first switch for a longer duration than the second switch to produce a positive pulse at the output node.

[0012] According to another embodiment, a method can include receiving, by a controller, a reference signal; operating, by the controller, a first switch and a second switch of a pulse width modulator to produce a pulse width modulated (PWM) representation of the reference signal at an output node, wherein the first switch operates in a first phase to drive the output node in a positive direction and contribute to noise and operates in a second phase to not contribute to the output noise or drive the output node in any direction, and wherein the second switch operates in a third phase to drive the output node in a negative direction and contribute to noise and operates in a fourth phase to not contribute to the output noise or drive the output node in any direction; and adjusting, by the controller, a duration for which the first switch and the second switch are coupled to the output node based at least in part on an envelope level of a desired output signal.

[0013] In certain embodiments, the step of adjusting the duration can include adjusting the duration proportionally to the envelope level; the step of adjusting the duration can include, for a first range of envelope levels, coupling the first switch and the second switch to the output node for a first duration; the step of adjusting the duration can include, for a second range of envelope levels, coupling the first switch and the second switch to the output node for a second duration that is shorter than the first duration; the step of adjusting the duration can include switching from the first duration to the second duration when the envelope level is at least 10 decibels below a total scale; the step of adjusting the duration can be based at least in part on an amplitude of the reference signal; and / or the step of adjusting the duration can be based at least in part on a desired gain level.

[0014] According to another embodiment, an apparatus can include a pulse width modulation controller configured to control a pulse width modulator to produce a pulse width modulated (PWM) representation of a reference signal at an output node at least by switching a first switch to and from the output node and by switching a second switch to and from the output node, wherein the first switch operates in a first phase to drive the output node in a positive direction and contribute to noise and operates in a second phase to not contribute to the output noise or drive the output node in any direction, wherein the second switch operates in a third phase to drive the output node in a negative direction and contribute to noise and operates in a fourth phase to not contribute to the output noise or drive the output node in any direction, and wherein the pulse width modulation controller is configured to adjust a duration within a period for which both the first switch and the second switch are coupled to the output node based at least in part on an envelope level of a desired output signal at the output node.

[0015] In certain embodiments, the controller can be configured to adjust the duration proportional to the envelope level; the controller configured to adjust the duration by coupling the first switch and the second switch to the output node for a first duration for a first range of the envelope level; the controller can be configured to adjust the duration by coupling the first switch and the second switch to the output node for a second duration shorter than the first duration for a second range of the envelope level; the controller can be configured to switch from the first duration to the second duration when the envelope level is at least 10 decibels below the total scale.

[0016] The foregoing has outlined rather broadly certain features and technical advantages of embodiments of the present application in order that the detailed description that follows can be better understood. Additional features and advantages will be described in the description which follows, and portions of the description which will follow can be readily understood by those skilled in the art in view of the accompanying drawings, the detailed description, and its drawings. It is to be expected that BRIEF DESCRIPTION OF DRAWINGS

[0017] For a more complete understanding of the disclosed system and method, reference is now made to the following description taken in connection with the accompanying drawings in which:

[0018] Figure 1 is a circuit schematic diagram illustrating a conventional pulse width modulator.

[0019] Figure 2A is a plot illustrating control signals for operating the circuit to produce a zero output. Figure 1

[0020] Figure 2B is a plot illustrating control signals for operating the circuit to produce a positive output. Figure 1

[0021] Figure 3A is a plot illustrating control signals for operating a pulse width modulator with reduced noise to produce a zero output according to one embodiment of the disclosure.

[0022] Figure 3B is a plot illustrating control signals for operating a pulse width modulator with reduced noise to produce a positive output according to one embodiment of the disclosure.

[0023] Figure 4 ​​is a flowchart illustrating operation of a pulse width modulator for adjusting duration (switch is active) according to one embodiment of the disclosure.

[0024] Figure 5 is a circuit schematic diagram illustrating operation of a pulse width modulator by a controller to adjust duration (switch is active) according to one embodiment of the disclosure.

[0025] Figure 6 is a flowchart illustrating a method of operating a switch of a pulse width modulator by a controller to reduce noise according to one embodiment of the disclosure.

[0026] Figure 7 is a flowchart illustrating a method of operating a switch of a pulse width modulator by a controller to reduce noise according to one embodiment of the disclosure. DETAILED DESCRIPTION

[0027] Noise in an output signal of a pulse width modulator (PWM) can be reduced by reducing the amount of time a noise source is coupled to an output node of the pulse width modulator. For example, the time a switch is turned on to couple a power supply to the output node can be reduced compared to conventional techniques, resulting in a corresponding reduction in thermal noise contributed to the output node of the PWM. Operation of the switch can be controlled by Figure 3A a control signal as shown.

[0028] Figure 3A is a plot illustrating a control signal for operating a pulse width modulator with reduced noise to produce a zero output according to one embodiment of the disclosure. The first control signal CTRL1 302 can go high at time 312 to turn on a switch coupling a power supply to an output node along with the second control signal CTRL2 304. The control signals CTRL1 and CTRL2 can go low at time 314 to turn off the power supply coupled to the output node. The duration 322 of the power supply coupled to the output node of the pulse width modulator (PWM) is shorter in time than the duration 324 between the corresponding times 212 and 214 of the prior art.

[0029] Also, Figure 3B is a plot illustrating a control signal for operating a pulse width modulator with reduced noise to produce a positive output according to one embodiment of the disclosure. The first control signal CTRL1 306 can go high at time 312 to turn on a switch coupling a power supply to an output node. The second control signal CTRL2 308 can go high at time 316 after time 312. Then, the control signals CTRL1 and CTRL2 can go low at time 314. As with Figure 3ASimilar to the control signal, the durations 322 and 326 when the switch couples the power supply to the output node are reduced from the duration 324 in the prior art. Therefore, noise reduction at the output of the pulse width modulator (PWM) can be achieved.

[0030] The duration 322 of the control pulse used to operate the switch can vary based on a reference input signal converted into a pulse width modulation representation. That is, the duration can be based on the desired output level of the pulse width modulator indicated by the reference input signal. If a larger input signal is detected and a larger output is thus desired, the duration can be increased; if a smaller input signal is detected and a smaller output is thus desired, the duration can be decreased. "Smaller" and "larger" can refer to the envelope level of the input or output signal. The desired envelope level can be indicated by factors such as desired volume or the amplitude of the reference input signal. It can be determined according to... Figure 4 The method shown is used to perform control of the pulse width modulator (PWM).

[0031] Figure 4 This is a flowchart illustrating the operation of a pulse width modulator (PWM) for adjusting duration (the switches are active) according to an embodiment of the present disclosure. Method 400 begins at block 402 with receiving a reference input signal for conversion into a PWM output signal representing a reference input signal. Then at block 404, a first switch and a second switch of the PWM can be operated based on the reference input signal. At block 406, the duration of the coupling of the first and second switches to the output node can be adjusted based on the envelope level of the desired output signal at the output of the PWM. The steps of block 406 can occur in real-time along with the operation of the switches in block 404, such that the duration changes dynamically in response to continuous input received as the reference input signal.

[0032] Figure 4 The method can be implemented in a pulse width modulator, and in one embodiment, it is implemented in a controller configured to operate a switch of the pulse width modulator (e.g., Figure 5 (as shown in the controller). Figure 5is a circuit schematic diagram showing a pulse width modulator operated by a controller to adjust durations (switches are active) according to one embodiment of the disclosure. A portion of the pulse width modulator (PWM) 500 can include switches 516 and 518 configured to couple power sources 512 and 514 to nodes 532 and 534, respectively. Switch 516 can switch between coupling power source 512 to node 532 to drive an output at output node 502 in a positive direction and coupling power source 512 to ground. Switch 518 can switch between coupling power source 514 to node 534 to drive output node 502 in a negative direction and coupling power source 514 to ground.

[0033] Controller 522 can be configured to operate switches 516 and 518 by outputting control signals CTRL1 and CTRL2, which switch switches 516 and 518. Controller 522 can be coupled to input node 504 for receiving a reference input signal at output node 502 for conversion to a pulse width representation. Although only one set of switches 516 and 518 and corresponding power sources 512 and 514 are shown in Figure 5 additional sets of switches and power sources can be included. For example, a finite impulse response (FIR) filter can include many sets of switches 516 and 518, for example between 8 and 128 sets. Power sources 512 and 514 can be, for example, current sources that drive current to the nodes selected by switches 516 and 518.

[0034] Controller 522 can generate control signals CTRL1 and CTRL2 with various durations selected according to a desired envelope level of an output signal generated at output node 502. Controller 522 can implement shorter durations by generating control signals CTRL1 and CTRL2 similar to the signals shown in Figure 3A and Figure 3B Controller 522 can implement longer durations by generating control signals CTRL1 and CTRL2 similar to the signals shown in Figure 2A and Figure 2B The shorter durations and the longer durations can be switched according to an algorithm executed by controller 522. One such algorithm is shown in Figure 6

[0035] Figure 6 ​This is a flowchart illustrating a method of operating a switch of a pulse width modulator by a controller to reduce noise according to an embodiment of the present disclosure. Method 600 begins at block 602 with the controller receiving a reference input signal for conversion into a pulse width modulated representation. At block 604, the controller determines whether the reference input signal is a small signal or a non-small signal. In one embodiment, a small signal may be a signal having a signal-to-noise ratio (SNR) of 20 dB or less. In other embodiments, a small signal may be a signal having a SNR of 10 dB or less, or it may be a signal having an SNR of 60 dB or less. Optionally or additionally, block 604 may include determining whether the envelope level of the desired output signal is a small signal. When a small signal is detected at block 604, method 600 continues to block 606 to operate the switch of the pulse width modulator for a short duration. For example, block 606 may be implemented by controller 522, which utilizes... Figure 3A and Figure 3B The control signals CTRL1 and CTRL2 shown are used to operate. Figure 5 Switches 516 and 518. (Refer back for further details.) Figure 6 When no small signal is detected at block 604, method 600 proceeds to block 608 to operate the switch of the pulse width modulator for a long duration. For example, block 608 can be implemented by controller 522, which utilizes... Figure 2A and Figure 2B The control signals CTRL1 and CTRL2 shown are used to operate. Figure 5 Switches 516 and 518. Although mentioned... Figure 2A and Figure 2B The signal can be implemented in box 608 for any duration longer than the short duration in box 606.

[0036] Figure 6 The duration adjustment shows two durations, however, a large number of durations can be controlled by controller 522 (e.g., during execution). Figure 7 The method shown is implemented when... Figure 7is a flowchart illustrating a method of operating a switch of a pulse width modulator using predetermined thresholds by a controller to reduce noise according to one embodiment of the present disclosure. The method 700 begins at block 702 by receiving a reference input signal for conversion to a pulse width representation. At block 704, the controller determines whether the input signal is less than a first threshold. If so, the method 700 continues to block 706 to operate the switch using a first duration. If not, the method 700 continues to block 708 where the controller determines whether the input signal is less than a second, larger threshold. If so, the method 700 continues to block 710 to operate the switch using a second, larger duration. If not, the method 700 continues to block 712 to operate the switch using a third, even larger duration that is longer than both the first and second durations. Alternatively or in addition, blocks 704 and 708 can include determining whether a desired envelope level of the input signal is less than the first or second threshold.

[0037] Although the thresholds are shown and used by the controller to adjust the duration of operating the switch in Figure 6 and Figure 7 , the duration can also be set in a continuous or near-continuous manner. For example, the duration can be set in proportion to the amplitude or envelope level of the reference input signal. In this embodiment, the duration can be set based on a formula that uses the amplitude or envelope level of the reference input signal as a variable.

[0038] Figure 4 、 Figure 6 and Figure 7 The illustrative flowcharts of methods 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3100, 3200, 3300, 3400, 3500, 3600, 3700, 3800, 3900, 4000, 4100, 4200, 4300, 4400, 4500, 4600, 4700, 4800, 4900, 5000, 5100, 5200, 5300, 5400, 5500, 5600, 5700, 5800, 5900, 6000, 6100, 6200, 6300, 6400, 6500, 6600, 6700, 6800, 6900, 7000, 7100, 7200, 7300, 7400, 7500, 7600, 7700, 7800, 7900, 8000, 8100, 8200, 8300, 8400, 8500, 8600, 8700, 8800, 8900, 9000, 9100, 9200, 9300, 9400, 9500, 9600, 9700, 9800, 9900, and 10000 are generally set forth as logical flowcharts. As such, the depicted order and numbered steps indicate one embodiment of the disclosed methods. Other steps and methods can be con ceived that are equivalent in function, logic, or effect to one or more steps or portions thereof, with the

[0039] If implemented in firmware and / or software, the functions can be stored as one or more instructions or code on a computer-readable medium. Examples include non-transitory computer- readable media encoded with data structures containing instructions that are executable by a computer to perform the methods described above and other examples of modules or other machine-accessible media. Computer-readable media can include physical computer storage media. A storage medium can be any available medium or means of storing data that is accessible by a computer. By way of example, and not limitation, such computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Disk and disc, as used herein, includes compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks and blu-ray discs. Generally, disks reproduce data magnetically, while discs reproduce data optically. Combinations of the above should also be included within the scope of computer-readable media.

[0040] In addition to storage on computer-readable medium, instructions and / or data can be provided as signals on transmission media included in a communication apparatus. For example, a communication apparatus can include a transceiver having signals indicative of instructions and data. The instructions and data are configured to cause one or more processors to implement the functions outlined in the claims.

[0041] While the present disclosure and certain representative advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, other devices, methods, compositions of matter, processes, machines, article of manufacture, means, methods or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein can be utilized. Accordingly, the appended claims are intended to include within their scope such devices, methods, compositions of matter, processes, machines, article of manufacture, means, methods or steps.

Claims

1. An apparatus to operate a pulse width modulator, comprising: a controller coupled to a plurality of switches to produce a pulse width modulated signal at an output node, and wherein the controller is configured to: receive an input signal for conversion to a pulse width modulated representation; determine whether the input signal is a small signal by determining whether the input signal is below a threshold level; when the input signal is a small signal, operate the plurality of switches in a short duration having a first duration; and when the input signal is not a small signal, operate the plurality of switches in a long duration having a second duration that is longer than the first duration.

2. The apparatus of claim 1, wherein, the controller is configured to operate the plurality of switches in the short duration by operating at least one of the plurality of switches with a duty cycle that is less than fifty percent.

3. The apparatus of claim 1, wherein, the controller is configured to produce the pulse width modulated signal at the output node by operating a first switch and a second switch of the plurality of switches to produce a pulse width modulation (PWM) representation of the input signal at the output node.

4. The apparatus of claim 3, wherein, the controller is further configured to: adjust a duration that the first switch and the second switch are coupled to the output node based at least in part on an envelope level of a desired output signal at the output node, wherein the adjusting comprises: in a first mode, coupling the first switch and the second switch to the output node for the first duration; in a second mode, coupling the first switch and the second switch to the output node for the second duration; and switching between the first mode and the second mode based at least in part on the envelope level.

5. The apparatus of claim 4, wherein, the controller is configured to adjust a duration that the first switch is in a first phase and the second switch is in a third phase such that the duration is shorter at lower desired output signal levels.

6. The apparatus of claim 3, wherein, the first switch is configured to drive the output node in a positive direction and contribute to noise when connected in a first phase, and not contribute to output noise or drive the output node in any direction when connected in a second phase, and wherein the second switch is configured to drive the output node in a negative direction and contribute to noise when connected in a third phase, and not contribute to output noise or drive the output node in any direction when connected in a fourth phase.

7. The apparatus of claim 3, wherein, the controller is configured to adjust the duration based at least in part on a desired volume level.

8. The apparatus of claim 1, wherein, the controller is configured to determine whether the input signal is a small signal includes determining whether an envelope level of the input signal is below the threshold level.

9. The apparatus of claim 1, wherein, the controller is configured to determine whether the input signal is a small signal includes determining whether the input signal is at least 10 decibels (dB) below full scale.

10. The apparatus of claim 1, wherein, the step of determining whether the input signal is a small signal includes determining whether the input signal is below 60 decibels (dB).

11. A method to operate a pulse width modulator, comprising: receiving an input signal for conversion to a pulse width modulated representation; determining whether the input signal is a small signal by determining whether the input signal is below a threshold level; operating a plurality of switches to produce a pulse width modulation representation of the input signal at an output node based on whether the input signal is a small signal, including: operating the plurality of switches with a short duration having a first duration when the input signal is a small signal; and operating the plurality of switches with a long duration having a second duration that is longer than the first duration when the input signal is not a small signal.

12. The method of claim 11, wherein, The step of operating the plurality of switches with a short duration includes operating at least one of the plurality of switches with a duty cycle that is less than fifty percent.

13. The method of claim 11, wherein, The step of producing a pulse width modulation representation of the input signal includes operating a first switch and a second switch of the plurality of switches to produce a pulse width modulation (PWM) representation of the input signal at the output node.

14. The method of claim 13, wherein, The step of adjusting the duration that the first switch and the second switch are coupled to the output node is based at least in part on an envelope level of a desired output signal at the output node, wherein the adjusting includes: in a first mode, coupling the first switch and the second switch to the output node for the first duration; in a second mode, coupling the first switch and the second switch to the output node for the second duration; and switching between the first mode and the second mode based at least in part on the envelope level.

15. The method of claim 13, wherein, The first switch is configured to drive the output node in a positive direction and contribute to noise when connected as a first phase, and not contribute to output noise or drive the output node in any direction when connected as a second phase, and wherein the second switch is configured to drive the output node in a negative direction and contribute to noise when connected as a third phase, and not contribute to output noise or drive the output node in any direction when connected as a fourth phase.

16. The method of claim 15, further comprising: The duration that the first switch is in the first phase and the second switch is in the third phase is adjusted so that the duration is shorter at lower desired output signals.

17. The method of claim 14, further comprising adjusting the duration based at least in part on a desired volume level.

18. The method of claim 11, wherein, The step of determining whether the input signal is a small signal includes determining whether an envelope level of the input signal is below the threshold level.

19. The method of claim 11, wherein, The step of determining whether the input signal is a small signal includes determining whether the input signal is at least 10 decibels (dB) below full scale.

20. The method of claim 11, wherein, The step of determining whether the input signal is a small signal includes determining whether the input signal is below 60 decibels (dB). The step of determining whether the input signal is a small signal includes determining whether the input signal is below 60 decibels (dB).

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