US20180091102A1 - Amplifier with configurable final output stage - Google Patents
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
- H03F3/21—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers with semiconductor devices only
- H03F3/217—Class D power amplifiers; Switching amplifiers
- H03F3/2171—Class D power amplifiers; Switching amplifiers with field-effect devices
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
- H03F3/21—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers with semiconductor devices only
- H03F3/217—Class D power amplifiers; Switching amplifiers
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/02—Modifications of amplifiers to raise the efficiency, e.g. gliding Class A stages, use of an auxiliary oscillation
- H03F1/0205—Modifications of amplifiers to raise the efficiency, e.g. gliding Class A stages, use of an auxiliary oscillation in transistor amplifiers
- H03F1/0277—Selecting one or more amplifiers from a plurality of amplifiers
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/32—Modifications of amplifiers to reduce non-linear distortion
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F1/00—Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
- H03F1/34—Negative-feedback-circuit arrangements with or without positive feedback
- H03F1/342—Negative-feedback-circuit arrangements with or without positive feedback in field-effect transistor amplifiers
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/181—Low frequency amplifiers, e.g. audio preamplifiers
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/181—Low frequency amplifiers, e.g. audio preamplifiers
- H03F3/183—Low frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
- H03F3/187—Low frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only in integrated circuits
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/72—Gated amplifiers, i.e. amplifiers which are rendered operative or inoperative by means of a control signal
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F2200/00—Indexing scheme relating to amplifiers
- H03F2200/345—Pulse density modulation being used in an amplifying circuit
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F2200/00—Indexing scheme relating to amplifiers
- H03F2200/351—Pulse width modulation being used in an amplifying circuit
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/20—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers
- H03F3/21—Power amplifiers, e.g. Class B amplifiers, Class C amplifiers with semiconductor devices only
- H03F3/217—Class D power amplifiers; Switching amplifiers
- H03F3/2173—Class D power amplifiers; Switching amplifiers of the bridge type
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03F—AMPLIFIERS
- H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
- H03F3/38—Dc amplifiers with modulator at input and demodulator at output; Modulators or demodulators specially adapted for use in such amplifiers
Definitions
- the present disclosure relates in general to circuits for audio devices, including without limitation personal audio devices, such as wireless telephones and media players, and more specifically, to systems and methods relating to an amplifier with a configurable final output stage.
- Personal audio devices including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use.
- Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers.
- Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers.
- a power amplifier amplifies an audio signal by taking energy from a power supply and controlling an audio output signal to match an input signal shape but with a larger amplitude.
- a class-D amplifier (also known as a “switching amplifier”) may comprise an electronic amplifier in which the amplifying devices (e.g., transistors, typically metal-oxide-semiconductor field effect transistors) operate as electronic switches, and not as linear gain devices as in other amplifiers (e.g., class-A, class-B, and class-AB amplifiers).
- the amplifying devices e.g., transistors, typically metal-oxide-semiconductor field effect transistors
- the amplifying devices e.g., transistors, typically metal-oxide-semiconductor field effect transistors
- an analog signal to be amplified may be converted to a series of pulses by pulse-width modulation, pulse-density modulation, or other method of modulation, such that the analog signal is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) are a function of the magnitude of the analog signal.
- the output pulse train may be converted back to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in the class-D amplifier or a load driven by the class-D amplifier.
- Class-D amplifiers are often used due to the fact that they may be more power efficient than linear analog amplifiers, in that class-D amplifiers may dissipate less power as heat in active devices as compared to linear analog amplifiers. However, class-D amplifiers may have high quiescent power when amplifying low-magnitude signals and may require a large amount of area in order to meet stringent dynamic range requirements in audio devices.
- one or more disadvantages and problems associated with existing approaches to signal amplification in an audio system may be reduced or eliminated.
- an amplifier may include a plurality of stages comprising at least a first stage configured to receive an input signal at an amplifier input and generate an intermediate signal which is a function of the input signal, and a final output stage configured to generate an output signal at an amplifier output, wherein the output signal is a function of the intermediate signal, and a signal feedback network coupled between the amplifier output and the amplifier input.
- the final output stage may be switchable among a plurality of modes including at least a first mode in which the final output stage generates the output signal as a modulated output signal which is a function of the intermediate signal, and a second mode in which the final output stage generates the output signal as an unmodulated output signal which is a function of the intermediate signal.
- Structure of the feedback network and the first stage may remain static when switching between the first mode and the second mode and when switching between the second mode and the first mode.
- a method for operating an amplifier having a plurality of stages comprising at least a first stage configured to receive an input signal at an amplifier input and generate an intermediate signal which is a function of the input signal, and a final output stage configured to generate an output signal at an amplifier output, wherein the output signal is a function of the intermediate signal, and a signal feedback network coupled between the amplifier output and the amplifier input, may be provided.
- the method may include switching the final output stage among a plurality of modes including at least a first mode in which the final output stage generates the output signal as a modulated output signal which is a function of the intermediate signal, and a second mode in which the final output stage generates the output signal as an unmodulated output signal which is a function of the intermediate signal, wherein the structure of the feedback network and the first stage remains static when switching between the first mode and the second mode and when switching between the second mode and the first mode.
- FIG. 1 is an illustration of an example personal audio device, in accordance with embodiments of the present disclosure
- FIG. 2 is a block diagram of selected components of an example audio integrated circuit of a personal audio device, in accordance with embodiments of the present disclosure
- FIG. 3 is a block diagram of selected components of an example amplifier, in accordance with embodiments of the present disclosure.
- FIG. 4 is a flow chart of an example method for switching between a first mode of a final output stage of an amplifier and a second mode of the final output stage of the amplifier, in accordance with embodiments of the present disclosure.
- FIG. 5 is a flow chart of an example method for switching between a second mode of a final output stage of an amplifier and a first mode of the final output stage of the amplifier, in accordance with embodiments of the present disclosure.
- FIG. 1 is an illustration of an example personal audio device 1 , in accordance with embodiments of the present disclosure.
- FIG. 1 depicts personal audio device 1 coupled to a headset 3 in the form of a pair of earbud speakers 8 A and 8 B.
- Headset 3 depicted in FIG. 1 is merely an example, and it is understood that personal audio device 1 may be used in connection with a variety of audio transducers, including without limitation, headphones, earbuds, in-ear earphones, and external speakers.
- a plug 4 may provide for connection of headset 3 to an electrical terminal of personal audio device 1 .
- Personal audio device 1 may provide a display to a user and receive user input using a touch screen 2 , or alternatively, a standard liquid crystal display (LCD) may be combined with various buttons, sliders, and/or dials disposed on the face and/or sides of personal audio device 1 . As also shown in FIG. 1 , personal audio device 1 may include an audio integrated circuit (IC) 9 for generating an analog audio signal for transmission to headset 3 and/or another audio transducer.
- IC audio integrated circuit
- FIG. 2 is a block diagram of selected components of an example audio IC 9 of a personal audio device, in accordance with embodiments of the present disclosure.
- example audio IC 9 may be used to implement audio IC 9 of FIG. 1 .
- a microcontroller core 18 may supply a digital audio input signal DIG_IN to a digital-to-analog converter (DAC) 14 , which may convert the digital audio input signal to an analog input signal V IN .
- DAC 14 may supply analog signal V IN to an amplifier 16 which may amplify or attenuate analog input signal V IN to provide an audio output signal V OUT , which may operate a speaker, headphone transducer, a line level signal output, and/or other suitable output.
- DAC digital-to-analog converter
- FIG. 3 is a block diagram of selected components of an example amplifier 16 , in accordance with embodiments of the present disclosure.
- amplifier 16 may include a first stage 22 (e.g., an analog front end) configured to receive analog input signal V IN at an amplifier input of amplifier 16 and generate an intermediate signal V INT which is a function of analog input signal V IN , a final output stage 24 configured to generate audio output signal V OUT at an amplifier output of amplifier 16 as a function of intermediate signal V NT , a signal feedback network 26 coupled between the amplifier output and the amplifier input, and a control circuit 28 for controlling the operation of certain components of amplifier 16 , as described in greater detail below.
- first stage 22 e.g., an analog front end
- V INT which is a function of analog input signal V IN
- final output stage 24 configured to generate audio output signal V OUT at an amplifier output of amplifier 16 as a function of intermediate signal V NT
- signal feedback network 26 coupled between the amplifier output and the amplifier input
- a control circuit 28 for controlling the operation of certain components
- First stage 22 may include any suitable analog front end circuit for conditioning analog input signal V IN for use by final output stage 24 .
- first stage 22 may include one or more analog integrators 32 cascaded in series, as shown in FIG. 3 .
- Final output stage 24 may include any suitable driving circuit for driving audio output signal V OUT as a function of intermediate signal V INT (thus, also making audio output signal V OUT a function of analog input signal V IN ) wherein final output stage 24 is switchable among a plurality of modes including at least a first mode in which final output stage 24 generates audio output signal V OUT as a modulated output signal which is a function of intermediate signal V INT and a second mode in which final output stage 24 generates audio output signal V OUT as an unmodulated output signal which is a function of intermediate signal V INT .
- final output stage 24 may include a class-D audio output stage 42 which may be enabled in the first mode (and disabled in the second mode) to generate audio output signal V OUT as a modulated output signal which is a function of intermediate signal V INT and a class-AB audio output stage 44 which may be enabled in the second mode (and disabled in the first mode) to generate audio output signal V OUT as an unmodulated output signal which is a function of intermediate signal V INT .
- Class-D audio output stage 42 may comprise any suitable system, device, or apparatus configured to amplify intermediate signal V INT and convert intermediate signal V INT into a series of pulses by pulse-width modulation, pulse-density modulation, or another method of modulation, such that intermediate signal V INT is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) is a function of the magnitude of intermediate signal V INT .
- a characteristic of the pulses of the modulated signal e.g., pulse widths, pulse density, etc.
- class-D audio output stage 42 After amplification by class-D audio output stage 42 , its output pulse train may be converted back to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in output circuitry of class-D audio output stage 42 or a load driven by final output stage 24 . As shown in FIG.
- class-D audio output stage 42 may include a control input for receiving a control input from control circuit 28 in order to selectively enable class-D audio output stage 42 during the first mode and disable class-D audio output stage 42 during the second mode (e.g., prevent class-D audio output stage 42 from driving the amplifier output of amplifier 16 by disabling or decoupling a supply voltage from class-D audio output stage 42 or by disabling or decoupling driving devices of the amplifier output of amplifier 16 ).
- Class-AB audio output stage 44 may comprise any suitable system, device, or apparatus configured to amplify intermediate signal V INT with a linear gain and convert intermediate signal V INT into an unmodulated audio output signal V OUT .
- unmodulated audio output signal V OUT may include a continuous-time baseband signal (e.g., an audio baseband signal). As shown in FIG.
- class-AB audio output stage 44 may include a control input for receiving a control input from control circuit 28 in order to selectively enable class-AB audio output stage 44 during the second mode and disable class-AB audio output stage 44 during the first mode (e.g., prevent class-AB audio output stage 44 from driving the amplifier output of amplifier 16 by disabling or decoupling a supply voltage from class-AB audio output stage 44 or by disabling or decoupling driving devices of the amplifier output of amplifier 16 ).
- final output stage 24 may include a signal feedback network 50 for feeding back a signal indicative of audio output signal V OUT to the input of final output stage 24 , thus forming a feedback loop around Class-AB audio output stage 44 .
- signal feedback network 50 may include resistors and/or other suitable circuit elements.
- a signal gain (e.g., V OUT /V INT ) of final output stage 24 in the first mode may be approximately equal to the signal gain of final output stage 24 in the second mode.
- an offset (e.g., direct current offset) of final output stage 24 in the first mode may be approximately equal to the offset of final output stage 24 in the second mode.
- final output stage 24 may also include a clamp 46 , which may be embodied as a switch, coupled between the output terminals of the amplifier output of amplifier 16 , with clamp 46 having a control input received from control circuit 28 for selectively enabling clamp 46 (to short the output terminals together) and disabling clamp 46 , as described in greater detail below.
- a clamp 46 which may be embodied as a switch, coupled between the output terminals of the amplifier output of amplifier 16 , with clamp 46 having a control input received from control circuit 28 for selectively enabling clamp 46 (to short the output terminals together) and disabling clamp 46 , as described in greater detail below.
- Signal feedback network 26 may include any suitable feedback network for feeding back a signal indicative of audio output signal V OUT to the amplifier input of amplifier 16 ).
- signal feedback network 26 may include variable feedback resistors 48 , wherein resistances of variable feedback resistors 48 are controlled by control signals received from control circuit 28 , as described in greater detail below.
- final output stage 24 may operate as an open-loop switched-mode driver in the first mode and may operate as a continuous-time closed-loop amplifier in the second mode.
- amplifier 16 may comprise a first feedback loop including signal feedback network 26 and a second feedback loop coupled between the amplifier output and the intermediate output implemented by signal feedback network 50 .
- Control circuit 28 may include any suitable system, device, or apparatus configured to receive information indicative of audio output voltage V OUT , intermediate signal V INT , and/or other operational characteristic of amplifier 16 , and based at least thereon, control operation of one or more components of amplifier 16 .
- control circuit 28 may be configured to, based on a characteristic of analog input signal V IN (e.g., which may be determined from receiving and analyzing intermediate signal V INT and/or audio output signal V OUT ), switch between the first mode and the second mode of final output stage 24 .
- control circuit 28 may be configured to switch final output stage 24 from the first mode to the second mode when an amplitude of analog input signal V IN decreases below a threshold amplitude, and may be configured to switch final output stage 24 from the second mode to the first mode when an amplitude of analog input signal V IN increases above the same threshold amplitude or another threshold amplitude.
- control circuit 28 may also be configured to switch between modes only when the amplitude of audio output signal V OUT is approximately zero (e.g., when a modulated signal generated by class-D audio output stage 42 is at its minimum voltage in its generated pulse train).
- control circuit 28 may further be configured to, in order to reduce audio artifacts induced by switching between the two modes, cause final output stage 24 to switch between the first mode and the second mode at an approximate completion of a modulation period of the modulated output signal output by Class-D audio output stage 42 , and cause final output stage 24 to switch between the second mode and the first mode at an approximate beginning of another modulation period of the modulated output signal output by Class-D audio output stage 42 .
- control circuit 28 may also be configured to perform calibration of final output stage 24 .
- control circuit 28 may receive and analyze intermediate signal V INT and audio output signal V OUT to determine a gain of class-D audio output stage 42 (e.g., the signal gain of final output stage 24 in the first mode) and a gain of class-AB audio output stage 44 (e.g., the signal gain of final output stage 24 in the second mode), and based thereon, modify the gain of class-D audio output stage 42 and/or the gain of class-AB audio output stage 44 in order to calibrate the signal gain of final output stage 24 in the second mode to match the signal gain of final output stage 24 in the first mode.
- a gain of class-D audio output stage 42 e.g., the signal gain of final output stage 24 in the first mode
- a gain of class-AB audio output stage 44 e.g., the signal gain of final output stage 24 in the second mode
- control circuit 28 may receive and analyze intermediate signal V INT and/or audio output signal V OUT to determine an offset (e.g., direct current offset) of class-D audio output stage 42 (e.g., the offset of final output stage 24 in the first mode) and an offset of class-AB audio output stage 44 (e.g., the offset of final output stage 24 in the second mode), and based thereon, modify the offset of class-D audio output stage 42 and/or the offset of class-AB audio output stage 44 in order to calibrate the offset of final output stage 24 in the second mode to match the offset of final output stage 24 in the first mode.
- an offset e.g., direct current offset
- class-D audio output stage 42 e.g., the offset of final output stage 24 in the first mode
- an offset of class-AB audio output stage 44 e.g., the offset of final output stage 24 in the second mode
- control circuit 28 may also be configured to control characteristics of first stage 22 (e.g., integrator 32 ) and/or signal feedback network 26 .
- Control circuit 28 may maintain such characteristics and structure of first stage 22 and signal feedback network 26 as static when switching between the first mode and the second mode of final output stage 24 and when switching between the second mode and the first mode. Maintaining the characteristics and structure of first stage 22 and signal feedback network 26 as static when switching between modes allows the modes to share the same analog front end and feedback network, thus reducing or minimizing the likelihood of mismatched signal gain and offset between the modes, and thus reducing or minimizing audio artifacts caused by switching between modes.
- control circuit 28 may modify characteristics of first stage 22 and/or signal feedback network 26 in order to decrease a noise floor of amplifier 16 .
- control circuit 28 may modify characteristics of integrator 32 (e.g., resistances and/or capacitances of filters internal to integrator 32 ) and/or other components of first stage 22 in order to decrease a noise floor of amplifier 16 when final output stage 24 operates in the second mode.
- control circuit 28 may modify characteristics of signal feedback network 26 (e.g., resistances of variable feedback resistors 48 ) in order to decrease a noise floor of amplifier 16 when final output stage 24 operates in the second mode.
- control circuit 28 may, before switching final output stage from the second mode to the first mode, return such characteristics to their unmodified states.
- FIG. 4 is a flow chart of an example method 50 for switching between a first mode of a final output stage 24 of amplifier 16 and a second mode of final output stage 24 of amplifier 16 , in accordance with embodiments of the present disclosure.
- method 50 begins at step 52 .
- teachings of the present disclosure are implemented in a variety of configurations of personal audio device 1 . As such, the preferred initialization point for method 50 and the order of the steps comprising method 50 may depend on the implementation chosen.
- control circuit 28 may monitor intermediate signal V INT , audio output signal V OUT , or another signal indicative of analog input signal V IN , to determine if analog input signal V IN has decreased from above to below a threshold amplitude. If analog input signal V IN has decreased from above to below the threshold amplitude, method 50 may proceed to step 54 . Otherwise, method 50 may remain at step 52 until such threshold amplitude crossing occurs.
- control circuit 28 may monitor audio output signal V OUT to determine when the amplitude of audio output signal V OUT is approximately zero (e.g., when a modulated signal generated by class-D audio output stage 42 is at its minimum voltage in its generated pulse train). If audio output signal V OUT has reached approximately zero, method 50 may proceed to step 55 . Otherwise, method 50 may remain at step 54 until audio output signal V OUT reaches approximately zero.
- control circuit 28 may monitor audio output signal V OUT to determine when the modulated output signal output by Class-D audio output stage 42 is at an approximate completion of a modulation period. If the modulated output signal output by Class-D audio output stage 42 is at an approximate completion of a modulation period, method 50 may proceed to step 56 . Otherwise, method 50 may remain at step 55 until the modulated output signal output by Class-D audio output stage 42 is at an approximate completion of a modulation period.
- control circuit 28 may enable clamp 46 , thus shorting the output terminals at the amplifier output of amplifier 16 together, forcing audio output signal V OUT to zero.
- class-AB audio output stage 44 (or another auxiliary amplifier, not shown in FIG. 3 ) may ramp a common mode voltage of audio output signal V OUT to a predetermined value (e.g., one-half of a supply voltage for class-AB audio output stage 44 ).
- control circuit 28 may fully enable class-AB audio output stage 44 such that audio output signal V OUT is an unmodulated signal which is a function of intermediate signal V INT .
- control circuit 28 may disable clamp 46 , thus allowing audio output signal V OUT to take on a non-zero value driven by class-AB audio output stage 44 .
- method 50 may end.
- FIG. 4 discloses a particular number of steps to be taken with respect to method 50
- method 50 may be executed with greater or fewer steps than those depicted in FIG. 4 .
- FIG. 4 discloses a certain order of steps to be taken with respect to method 50
- the steps comprising method 50 may be completed in any suitable order.
- Method 50 may be implemented using personal audio device 1 or any other system operable to implement method 50 .
- method 50 may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller.
- FIG. 5 is a flow chart of an example method 70 for switching between a second mode of final output stage 24 of amplifier 16 and a first mode of final output stage 24 of amplifier 16 , in accordance with embodiments of the present disclosure.
- method 70 begins at step 72 .
- teachings of the present disclosure are implemented in a variety of configurations of personal audio device 1 . As such, the preferred initialization point for method 70 and the order of the steps comprising method 70 may depend on the implementation chosen.
- control circuit 28 may monitor intermediate signal V INT , audio output signal V OUT , or another signal indicative of analog input signal V IN , to determine if analog input signal V IN has increased from below to above a threshold amplitude (which may be the same threshold as that of step 52 , or a different threshold). If analog input signal V IN has increased from below to above the threshold amplitude, method 70 may proceed to step 74 . Otherwise, method 70 may remain at step 72 until such threshold amplitude crossing occurs.
- a threshold amplitude which may be the same threshold as that of step 52 , or a different threshold.
- control circuit 28 may monitor audio output signal V OUT to determine when the amplitude of audio output signal V OUT is approximately zero (e.g., when audio output signal V OUT experiences a zero crossing). If audio output signal V OUT is approximately zero, method 70 may proceed to step 75 . Otherwise, method 70 may remain at step 74 until audio output signal V OUT is approximately zero.
- control circuit 28 may monitor audio output signal V OUT to determine when the modulated output signal output by Class-D audio output stage 42 is at an approximate beginning of a modulation period. If the modulated output signal output by Class-D audio output stage 42 is at an approximate beginning of a modulation period, method 70 may proceed to step 76 . Otherwise, method 70 may remain at step 75 until the modulated output signal output by Class-D audio output stage 42 is at an approximate beginning of a modulation period.
- control circuit 28 may enable clamp 46 , thus shorting the output terminals at the amplifier output of amplifier 16 together, forcing audio output signal V OUT to zero.
- class-D audio output stage 42 (or another auxiliary amplifier, not shown in FIG. 3 ) may ramp a common mode voltage of audio output signal V OUT to zero.
- control circuit 28 may fully enable class-D audio output stage 42 such that audio output signal V OUT is a modulated signal which is a function of intermediate signal V INT .
- control circuit 28 may disable clamp 46 , thus allowing audio output signal V OUT to take on a non-zero value driven by class-D audio output stage 42 . After completion of step 82 , method 70 may end.
- FIG. 5 discloses a particular number of steps to be taken with respect to method 70
- method 70 may be executed with greater or fewer steps than those depicted in FIG. 5 .
- FIG. 5 discloses a certain order of steps to be taken with respect to method 70
- the steps comprising method 70 may be completed in any suitable order.
- Method 70 may be implemented using personal audio device 1 or any other system operable to implement method 70 .
- method 70 may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller.
- references in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
Abstract
Description
- The present disclosure relates in general to circuits for audio devices, including without limitation personal audio devices, such as wireless telephones and media players, and more specifically, to systems and methods relating to an amplifier with a configurable final output stage.
- Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers. Generally speaking, a power amplifier amplifies an audio signal by taking energy from a power supply and controlling an audio output signal to match an input signal shape but with a larger amplitude.
- One example of an audio amplifier is a class-D amplifier. A class-D amplifier (also known as a “switching amplifier”) may comprise an electronic amplifier in which the amplifying devices (e.g., transistors, typically metal-oxide-semiconductor field effect transistors) operate as electronic switches, and not as linear gain devices as in other amplifiers (e.g., class-A, class-B, and class-AB amplifiers). In a class-D amplifier, an analog signal to be amplified may be converted to a series of pulses by pulse-width modulation, pulse-density modulation, or other method of modulation, such that the analog signal is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) are a function of the magnitude of the analog signal. After amplification with a class-D amplifier, the output pulse train may be converted back to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in the class-D amplifier or a load driven by the class-D amplifier. Class-D amplifiers are often used due to the fact that they may be more power efficient than linear analog amplifiers, in that class-D amplifiers may dissipate less power as heat in active devices as compared to linear analog amplifiers. However, class-D amplifiers may have high quiescent power when amplifying low-magnitude signals and may require a large amount of area in order to meet stringent dynamic range requirements in audio devices.
- In accordance with the teachings of the present disclosure, one or more disadvantages and problems associated with existing approaches to signal amplification in an audio system may be reduced or eliminated.
- In accordance with embodiments of the present disclosure, an amplifier may include a plurality of stages comprising at least a first stage configured to receive an input signal at an amplifier input and generate an intermediate signal which is a function of the input signal, and a final output stage configured to generate an output signal at an amplifier output, wherein the output signal is a function of the intermediate signal, and a signal feedback network coupled between the amplifier output and the amplifier input. The final output stage may be switchable among a plurality of modes including at least a first mode in which the final output stage generates the output signal as a modulated output signal which is a function of the intermediate signal, and a second mode in which the final output stage generates the output signal as an unmodulated output signal which is a function of the intermediate signal. Structure of the feedback network and the first stage may remain static when switching between the first mode and the second mode and when switching between the second mode and the first mode.
- In accordance with these and other embodiments of the present disclosure, a method for operating an amplifier having a plurality of stages comprising at least a first stage configured to receive an input signal at an amplifier input and generate an intermediate signal which is a function of the input signal, and a final output stage configured to generate an output signal at an amplifier output, wherein the output signal is a function of the intermediate signal, and a signal feedback network coupled between the amplifier output and the amplifier input, may be provided. The method may include switching the final output stage among a plurality of modes including at least a first mode in which the final output stage generates the output signal as a modulated output signal which is a function of the intermediate signal, and a second mode in which the final output stage generates the output signal as an unmodulated output signal which is a function of the intermediate signal, wherein the structure of the feedback network and the first stage remains static when switching between the first mode and the second mode and when switching between the second mode and the first mode.
- Technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims.
- It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are not restrictive of the claims set forth in this disclosure.
- A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:
-
FIG. 1 is an illustration of an example personal audio device, in accordance with embodiments of the present disclosure; -
FIG. 2 is a block diagram of selected components of an example audio integrated circuit of a personal audio device, in accordance with embodiments of the present disclosure; -
FIG. 3 is a block diagram of selected components of an example amplifier, in accordance with embodiments of the present disclosure; -
FIG. 4 is a flow chart of an example method for switching between a first mode of a final output stage of an amplifier and a second mode of the final output stage of the amplifier, in accordance with embodiments of the present disclosure; and -
FIG. 5 is a flow chart of an example method for switching between a second mode of a final output stage of an amplifier and a first mode of the final output stage of the amplifier, in accordance with embodiments of the present disclosure. -
FIG. 1 is an illustration of an examplepersonal audio device 1, in accordance with embodiments of the present disclosure.FIG. 1 depictspersonal audio device 1 coupled to aheadset 3 in the form of a pair ofearbud speakers Headset 3 depicted inFIG. 1 is merely an example, and it is understood thatpersonal audio device 1 may be used in connection with a variety of audio transducers, including without limitation, headphones, earbuds, in-ear earphones, and external speakers. A plug 4 may provide for connection ofheadset 3 to an electrical terminal ofpersonal audio device 1.Personal audio device 1 may provide a display to a user and receive user input using a touch screen 2, or alternatively, a standard liquid crystal display (LCD) may be combined with various buttons, sliders, and/or dials disposed on the face and/or sides ofpersonal audio device 1. As also shown inFIG. 1 ,personal audio device 1 may include an audio integrated circuit (IC) 9 for generating an analog audio signal for transmission toheadset 3 and/or another audio transducer. -
FIG. 2 is a block diagram of selected components of anexample audio IC 9 of a personal audio device, in accordance with embodiments of the present disclosure. In some embodiments, example audio IC 9 may be used to implementaudio IC 9 ofFIG. 1 . As shown inFIG. 2 , amicrocontroller core 18 may supply a digital audio input signal DIG_IN to a digital-to-analog converter (DAC) 14, which may convert the digital audio input signal to an analog input signal VIN. DAC 14 may supply analog signal VIN to anamplifier 16 which may amplify or attenuate analog input signal VIN to provide an audio output signal VOUT, which may operate a speaker, headphone transducer, a line level signal output, and/or other suitable output. -
FIG. 3 is a block diagram of selected components of anexample amplifier 16, in accordance with embodiments of the present disclosure. As shown inFIG. 3 ,amplifier 16 may include a first stage 22 (e.g., an analog front end) configured to receive analog input signal VIN at an amplifier input ofamplifier 16 and generate an intermediate signal VINT which is a function of analog input signal VIN, afinal output stage 24 configured to generate audio output signal VOUT at an amplifier output ofamplifier 16 as a function of intermediate signal VNT, asignal feedback network 26 coupled between the amplifier output and the amplifier input, and acontrol circuit 28 for controlling the operation of certain components ofamplifier 16, as described in greater detail below. -
First stage 22 may include any suitable analog front end circuit for conditioning analog input signal VIN for use byfinal output stage 24. For example,first stage 22 may include one or moreanalog integrators 32 cascaded in series, as shown inFIG. 3 . -
Final output stage 24 may include any suitable driving circuit for driving audio output signal VOUT as a function of intermediate signal VINT (thus, also making audio output signal VOUT a function of analog input signal VIN) whereinfinal output stage 24 is switchable among a plurality of modes including at least a first mode in whichfinal output stage 24 generates audio output signal VOUT as a modulated output signal which is a function of intermediate signal VINT and a second mode in whichfinal output stage 24 generates audio output signal VOUT as an unmodulated output signal which is a function of intermediate signal VINT. To carry out this functionality,final output stage 24 may include a class-Daudio output stage 42 which may be enabled in the first mode (and disabled in the second mode) to generate audio output signal VOUT as a modulated output signal which is a function of intermediate signal VINT and a class-ABaudio output stage 44 which may be enabled in the second mode (and disabled in the first mode) to generate audio output signal VOUT as an unmodulated output signal which is a function of intermediate signal VINT. - Class-D
audio output stage 42 may comprise any suitable system, device, or apparatus configured to amplify intermediate signal VINT and convert intermediate signal VINT into a series of pulses by pulse-width modulation, pulse-density modulation, or another method of modulation, such that intermediate signal VINT is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) is a function of the magnitude of intermediate signal VINT. After amplification by class-Daudio output stage 42, its output pulse train may be converted back to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in output circuitry of class-Daudio output stage 42 or a load driven byfinal output stage 24. As shown inFIG. 3 , class-Daudio output stage 42 may include a control input for receiving a control input fromcontrol circuit 28 in order to selectively enable class-Daudio output stage 42 during the first mode and disable class-Daudio output stage 42 during the second mode (e.g., prevent class-Daudio output stage 42 from driving the amplifier output ofamplifier 16 by disabling or decoupling a supply voltage from class-Daudio output stage 42 or by disabling or decoupling driving devices of the amplifier output of amplifier 16). - Class-AB
audio output stage 44 may comprise any suitable system, device, or apparatus configured to amplify intermediate signal VINT with a linear gain and convert intermediate signal VINT into an unmodulated audio output signal VOUT. For example, in some embodiments, unmodulated audio output signal VOUT may include a continuous-time baseband signal (e.g., an audio baseband signal). As shown inFIG. 3 , class-ABaudio output stage 44 may include a control input for receiving a control input fromcontrol circuit 28 in order to selectively enable class-ABaudio output stage 44 during the second mode and disable class-ABaudio output stage 44 during the first mode (e.g., prevent class-ABaudio output stage 44 from driving the amplifier output ofamplifier 16 by disabling or decoupling a supply voltage from class-ABaudio output stage 44 or by disabling or decoupling driving devices of the amplifier output of amplifier 16). - As shown in
FIG. 3 ,final output stage 24 may include asignal feedback network 50 for feeding back a signal indicative of audio output signal VOUT to the input offinal output stage 24, thus forming a feedback loop around Class-ABaudio output stage 44. For example, as shown inFIG. 3 ,signal feedback network 50 may include resistors and/or other suitable circuit elements. - In some embodiments, a signal gain (e.g., VOUT/VINT) of
final output stage 24 in the first mode may be approximately equal to the signal gain offinal output stage 24 in the second mode. In these and other embodiments, an offset (e.g., direct current offset) offinal output stage 24 in the first mode may be approximately equal to the offset offinal output stage 24 in the second mode. - As shown in
FIG. 3 ,final output stage 24 may also include aclamp 46, which may be embodied as a switch, coupled between the output terminals of the amplifier output ofamplifier 16, withclamp 46 having a control input received fromcontrol circuit 28 for selectively enabling clamp 46 (to short the output terminals together) and disablingclamp 46, as described in greater detail below. -
Signal feedback network 26 may include any suitable feedback network for feeding back a signal indicative of audio output signal VOUT to the amplifier input of amplifier 16). For example, as shown inFIG. 3 ,signal feedback network 26 may includevariable feedback resistors 48, wherein resistances ofvariable feedback resistors 48 are controlled by control signals received fromcontrol circuit 28, as described in greater detail below. - Thus,
final output stage 24 may operate as an open-loop switched-mode driver in the first mode and may operate as a continuous-time closed-loop amplifier in the second mode. In addition, when the final output stage is operating in the second mode,amplifier 16 may comprise a first feedback loop includingsignal feedback network 26 and a second feedback loop coupled between the amplifier output and the intermediate output implemented bysignal feedback network 50. -
Control circuit 28 may include any suitable system, device, or apparatus configured to receive information indicative of audio output voltage VOUT, intermediate signal VINT, and/or other operational characteristic ofamplifier 16, and based at least thereon, control operation of one or more components ofamplifier 16. For example,control circuit 28 may be configured to, based on a characteristic of analog input signal VIN (e.g., which may be determined from receiving and analyzing intermediate signal VINT and/or audio output signal VOUT), switch between the first mode and the second mode offinal output stage 24. Such characteristic may include one or more of a frequency of analog input signal VIN, an amplitude of analog input signal VIN, a signal-to-noise ratio of analog input signal VIN, a noise floor of analog input signal VIN, or another noise characteristic of analog input signal VIN. For example, in some embodiments,control circuit 28 may be configured to switchfinal output stage 24 from the first mode to the second mode when an amplitude of analog input signal VIN decreases below a threshold amplitude, and may be configured to switchfinal output stage 24 from the second mode to the first mode when an amplitude of analog input signal VIN increases above the same threshold amplitude or another threshold amplitude. In some embodiments, to reduce audio artifacts associated with switching between modes,control circuit 28 may also be configured to switch between modes only when the amplitude of audio output signal VOUT is approximately zero (e.g., when a modulated signal generated by class-Daudio output stage 42 is at its minimum voltage in its generated pulse train). - In these and other embodiments,
control circuit 28 may further be configured to, in order to reduce audio artifacts induced by switching between the two modes, causefinal output stage 24 to switch between the first mode and the second mode at an approximate completion of a modulation period of the modulated output signal output by Class-Daudio output stage 42, and causefinal output stage 24 to switch between the second mode and the first mode at an approximate beginning of another modulation period of the modulated output signal output by Class-Daudio output stage 42. - In addition,
control circuit 28 may also be configured to perform calibration offinal output stage 24. For example,control circuit 28 may receive and analyze intermediate signal VINT and audio output signal VOUT to determine a gain of class-D audio output stage 42 (e.g., the signal gain offinal output stage 24 in the first mode) and a gain of class-AB audio output stage 44 (e.g., the signal gain offinal output stage 24 in the second mode), and based thereon, modify the gain of class-Daudio output stage 42 and/or the gain of class-ABaudio output stage 44 in order to calibrate the signal gain offinal output stage 24 in the second mode to match the signal gain offinal output stage 24 in the first mode. As another example,control circuit 28 may receive and analyze intermediate signal VINT and/or audio output signal VOUT to determine an offset (e.g., direct current offset) of class-D audio output stage 42 (e.g., the offset offinal output stage 24 in the first mode) and an offset of class-AB audio output stage 44 (e.g., the offset offinal output stage 24 in the second mode), and based thereon, modify the offset of class-Daudio output stage 42 and/or the offset of class-ABaudio output stage 44 in order to calibrate the offset offinal output stage 24 in the second mode to match the offset offinal output stage 24 in the first mode. - In these and other embodiments,
control circuit 28 may also be configured to control characteristics of first stage 22 (e.g., integrator 32) and/or signalfeedback network 26.Control circuit 28 may maintain such characteristics and structure offirst stage 22 andsignal feedback network 26 as static when switching between the first mode and the second mode offinal output stage 24 and when switching between the second mode and the first mode. Maintaining the characteristics and structure offirst stage 22 andsignal feedback network 26 as static when switching between modes allows the modes to share the same analog front end and feedback network, thus reducing or minimizing the likelihood of mismatched signal gain and offset between the modes, and thus reducing or minimizing audio artifacts caused by switching between modes. However, aftercontrol circuit 28 has switchedfinal output stage 24 to the second mode (e.g., amplifier output driven by class-AB audio output stage 44),control circuit 28 may modify characteristics offirst stage 22 and/or signalfeedback network 26 in order to decrease a noise floor ofamplifier 16. For example, in some embodiments,control circuit 28 may modify characteristics of integrator 32 (e.g., resistances and/or capacitances of filters internal to integrator 32) and/or other components offirst stage 22 in order to decrease a noise floor ofamplifier 16 whenfinal output stage 24 operates in the second mode. As another example, in these and other embodiments,control circuit 28 may modify characteristics of signal feedback network 26 (e.g., resistances of variable feedback resistors 48) in order to decrease a noise floor ofamplifier 16 whenfinal output stage 24 operates in the second mode. When making such modification,control circuit 28 may, before switching final output stage from the second mode to the first mode, return such characteristics to their unmodified states. -
FIG. 4 is a flow chart of anexample method 50 for switching between a first mode of afinal output stage 24 ofamplifier 16 and a second mode offinal output stage 24 ofamplifier 16, in accordance with embodiments of the present disclosure. According to some embodiments,method 50 begins atstep 52. As noted above, teachings of the present disclosure are implemented in a variety of configurations ofpersonal audio device 1. As such, the preferred initialization point formethod 50 and the order of thesteps comprising method 50 may depend on the implementation chosen. - At
step 52,control circuit 28 may monitor intermediate signal VINT, audio output signal VOUT, or another signal indicative of analog input signal VIN, to determine if analog input signal VIN has decreased from above to below a threshold amplitude. If analog input signal VIN has decreased from above to below the threshold amplitude,method 50 may proceed to step 54. Otherwise,method 50 may remain atstep 52 until such threshold amplitude crossing occurs. - At
step 54,control circuit 28 may monitor audio output signal VOUT to determine when the amplitude of audio output signal VOUT is approximately zero (e.g., when a modulated signal generated by class-Daudio output stage 42 is at its minimum voltage in its generated pulse train). If audio output signal VOUT has reached approximately zero,method 50 may proceed to step 55. Otherwise,method 50 may remain atstep 54 until audio output signal VOUT reaches approximately zero. - At
step 55,control circuit 28 may monitor audio output signal VOUT to determine when the modulated output signal output by Class-Daudio output stage 42 is at an approximate completion of a modulation period. If the modulated output signal output by Class-Daudio output stage 42 is at an approximate completion of a modulation period,method 50 may proceed to step 56. Otherwise,method 50 may remain atstep 55 until the modulated output signal output by Class-Daudio output stage 42 is at an approximate completion of a modulation period. - At
step 56,control circuit 28 may enableclamp 46, thus shorting the output terminals at the amplifier output ofamplifier 16 together, forcing audio output signal VOUT to zero. Atstep 58, class-AB audio output stage 44 (or another auxiliary amplifier, not shown inFIG. 3 ) may ramp a common mode voltage of audio output signal VOUT to a predetermined value (e.g., one-half of a supply voltage for class-AB audio output stage 44). Atstep 60,control circuit 28 may fully enable class-ABaudio output stage 44 such that audio output signal VOUT is an unmodulated signal which is a function of intermediate signal VINT. Atstep 62,control circuit 28 may disableclamp 46, thus allowing audio output signal VOUT to take on a non-zero value driven by class-ABaudio output stage 44. After completion ofstep 62,method 50 may end. - Although
FIG. 4 discloses a particular number of steps to be taken with respect tomethod 50,method 50 may be executed with greater or fewer steps than those depicted inFIG. 4 . In addition, althoughFIG. 4 discloses a certain order of steps to be taken with respect tomethod 50, thesteps comprising method 50 may be completed in any suitable order. -
Method 50 may be implemented usingpersonal audio device 1 or any other system operable to implementmethod 50. In certain embodiments,method 50 may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller. -
FIG. 5 is a flow chart of anexample method 70 for switching between a second mode offinal output stage 24 ofamplifier 16 and a first mode offinal output stage 24 ofamplifier 16, in accordance with embodiments of the present disclosure. According to some embodiments,method 70 begins atstep 72. As noted above, teachings of the present disclosure are implemented in a variety of configurations ofpersonal audio device 1. As such, the preferred initialization point formethod 70 and the order of thesteps comprising method 70 may depend on the implementation chosen. - At
step 72,control circuit 28 may monitor intermediate signal VINT, audio output signal VOUT, or another signal indicative of analog input signal VIN, to determine if analog input signal VIN has increased from below to above a threshold amplitude (which may be the same threshold as that ofstep 52, or a different threshold). If analog input signal VIN has increased from below to above the threshold amplitude,method 70 may proceed to step 74. Otherwise,method 70 may remain atstep 72 until such threshold amplitude crossing occurs. - At
step 74,control circuit 28 may monitor audio output signal VOUT to determine when the amplitude of audio output signal VOUT is approximately zero (e.g., when audio output signal VOUT experiences a zero crossing). If audio output signal VOUT is approximately zero,method 70 may proceed to step 75. Otherwise,method 70 may remain atstep 74 until audio output signal VOUT is approximately zero. - At
step 75,control circuit 28 may monitor audio output signal VOUT to determine when the modulated output signal output by Class-Daudio output stage 42 is at an approximate beginning of a modulation period. If the modulated output signal output by Class-Daudio output stage 42 is at an approximate beginning of a modulation period,method 70 may proceed to step 76. Otherwise,method 70 may remain atstep 75 until the modulated output signal output by Class-Daudio output stage 42 is at an approximate beginning of a modulation period. - At
step 76,control circuit 28 may enableclamp 46, thus shorting the output terminals at the amplifier output ofamplifier 16 together, forcing audio output signal VOUT to zero. Atstep 78, class-D audio output stage 42 (or another auxiliary amplifier, not shown inFIG. 3 ) may ramp a common mode voltage of audio output signal VOUT to zero. Atstep 80,control circuit 28 may fully enable class-Daudio output stage 42 such that audio output signal VOUT is a modulated signal which is a function of intermediate signal VINT. Atstep 82,control circuit 28 may disableclamp 46, thus allowing audio output signal VOUT to take on a non-zero value driven by class-Daudio output stage 42. After completion ofstep 82,method 70 may end. - Although
FIG. 5 discloses a particular number of steps to be taken with respect tomethod 70,method 70 may be executed with greater or fewer steps than those depicted inFIG. 5 . In addition, althoughFIG. 5 discloses a certain order of steps to be taken with respect tomethod 70, thesteps comprising method 70 may be completed in any suitable order. -
Method 70 may be implemented usingpersonal audio device 1 or any other system operable to implementmethod 70. In certain embodiments,method 70 may be implemented partially or fully in software and/or firmware embodied in computer-readable media and executable by a controller. - As used herein, when two or more elements are referred to as “coupled” to one another, such term indicates that such two or more elements are in electronic communication or mechanical communication, as applicable, whether connected indirectly or directly, with or without intervening elements.
- This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the exemplary embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative.
- All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present inventions have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.
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2016
- 2016-09-27 US US15/277,465 patent/US9929703B1/en active Active
- 2016-12-01 GB GB1620427.3A patent/GB2557244B/en active Active
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CN109792235A (en) | 2019-05-21 |
CN117439552A (en) | 2024-01-23 |
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US10447217B2 (en) | 2019-10-15 |
WO2018063878A1 (en) | 2018-04-05 |
GB201620427D0 (en) | 2017-01-18 |
CN109792235B (en) | 2023-11-21 |
US9929703B1 (en) | 2018-03-27 |
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