US20090251212A1 - Switching amplifier - Google Patents
Switching amplifier Download PDFInfo
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- US20090251212A1 US20090251212A1 US12/393,524 US39352409A US2009251212A1 US 20090251212 A1 US20090251212 A1 US 20090251212A1 US 39352409 A US39352409 A US 39352409A US 2009251212 A1 US2009251212 A1 US 2009251212A1
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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
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Abstract
An amplifier having at least one switch controlled by an output voltage of a hysteresis block, wherein the hysteresis block is adapted to receive an input voltage signal based on an integration of an error signal, a low threshold voltage and a high threshold voltage, and is arranged to change the output voltage from a first value to a second value when the input voltage signal is higher than the high threshold voltage and to change the output voltage from the second value to the first value when the input voltage signal is lower than the low threshold voltage, and wherein the low threshold voltage is equal to Vref−αVDD and the high threshold voltage is equal to Vref+αVDD, where Vref is a common mode voltage level, α is a non-zero constant, and VDD is a power supply voltage.
Description
- This application claims the priority benefit of European patent application number 08300125.5, filed on Feb. 29, 2008, entitled “SWITCHING AMPLIFIER,” which is hereby incorporated by reference to the maximum extent allowable by law.
- 1. Field of the Invention
- The present invention relates to an amplifier and in particular to an amplifier comprising one or more switches controlled by a hysteresis block.
- 2. Discussion of the Related Art
- Switching amplifiers, such as class D audio amplifiers, are amplifiers that generate an output voltage by switching an output node between a supply voltage and a ground voltage. The duration of the pulses during which the output node is coupled to the supply voltage determines the output voltage level. This is controlled by a feedback loop that drives the switches.
- It has been proposed to provide a hysteresis stage in the feedback loop for controlling the switches.
- It is often an aim when designing such amplifiers to minimize noise and distortion at the output node. However, known designs that use hysteresis stages tend to have poor power supply rejection ratio (PSRR) meaning the effect of ripples in the power supply is relatively high on the output voltage. There is thus a need for a switching amplifier with a good PSRR.
- It is one aim of embodiments of the present invention to at least partially address one or more needs in the prior art.
- According to one aspect of the present invention, there is provided an amplifier comprising at least one switch controlled by an output voltage of a hysteresis block, wherein the hysteresis block is adapted to receive a low threshold voltage, a high threshold voltage and an input voltage signal based on an integration of an error signal, the hysteresis block being arranged to change the output voltage from a first value to a second value when the input voltage signal is higher than the high threshold voltage and to change the output voltage from the second value to the first value when the input voltage signal is lower than the low threshold voltage, and wherein the low threshold voltage is equal to a reference voltage level minus a supply voltage level, and the high threshold voltage is equal to the reference voltage level plus the supply voltage level.
- According to an embodiment of the present invention, the low threshold voltage is equal to Vref−αVDD and the high threshold voltage is equal to Vref+αVDD, where Vref is the reference voltage level, α is a non-zero constant, and VDD is a power supply voltage.
- According to another embodiment of the present invention, the amplifier further comprises an integrator coupled to the hysteresis block to provide the input voltage signal; a power stage comprising the switches and coupled to receive the output voltage; and adder circuitry arranged to receive an input signal of the amplifier and an output voltage of the power stage, and to provide the error signal to an input of the integrator, the error signal based on the input signal and the output voltage.
- According to another embodiment of the present invention, the hysteresis block comprises a first comparator for comparing said input voltage signal to the high threshold voltage and a second comparator for comparing the input voltage signal to the low threshold voltage.
- According to another embodiment of the present invention, the hysteresis block further comprises an SR flip-flop adapted to be set by the output of one of the first and second comparators and reset by the output of the other of the first and second comparators.
- According to another embodiment of the present invention, the amplifier further comprises a class D amplifier coupled to receive the output voltage of the hysteresis block, the class D amplifier comprising the switches.
- According to another embodiment of the present invention, the amplifier further comprises circuitry adapted to generate the high and low threshold voltages, the circuitry comprising at least one variable resistance for allowing the supply voltage level to be adjusted.
- According to a further aspect of the present invention, there is provided an electronic device, or a mobile communications device comprising the above amplifier.
- According to yet a further aspect of the present invention, there is provided a method of controlling a power stage of an amplifier comprising: performing integration on an error signal to generate an integrated signal, the error signal being based on an input signal to be amplified and the output voltage of the power stage; comparing by a hysteresis block the integrated signal with a high threshold voltage and a low threshold voltage, and generating an output signal that changes from a first value to a second value if the integrated signal is higher than the high threshold voltage and changes from the second value to the first value if the integrated signal is lower than the low threshold voltage, wherein said low threshold voltage is equal to Vref−αVDD and said high threshold voltage is equal to Vref+αVDD, where Vref is a common mode voltage level, α is a constant, and VDD is a power supply voltage.
- The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings.
-
FIG. 1 illustrates a switching amplifier using sliding mode control; -
FIG. 2 illustrates the hysteresis and power stages of the switching amplifier ofFIG. 1 in more detail; -
FIG. 3 illustrates an amplifier according to an embodiment of the present invention; -
FIG. 4 illustrates circuitry for generating hysteresis threshold values according to an embodiment of the present invention; -
FIG. 5 illustrates a switching amplifier according to a further embodiment of the present invention; -
FIGS. 6A and 6B are timing diagrams illustrating the output of integrator and hysteresis blocks according to an embodiment of the present invention; and -
FIG. 7 illustrates a electronic device comprising a switching amplifier according to embodiments of the present invention. -
FIG. 1 illustrates aswitching amplifier 100 according to one embodiment comprising aninput line 102 for receiving an input signal Ve.Line 102 is coupled to anadder block 104, which subtracts a value received from afeedback line 105, and passes the output to anintegrator 106.Integrator 106 performs anintegration 1/τp of the input signal with a constant of integration τ and wherein p is the laplace variable. - The integrated signal Vi at the output of
integrator 106 is provided to ahysteresis block 108, which outputs a high or low voltage based on high and low threshold levels. In particular, when the output VC of the hysteresis block is at the low voltage level, it changes to the high voltage level when the input signal Vi goes higher than the high threshold, and when the output VC is at the high voltage level, it goes to the low voltage level when the input signal Vi goes lower than the low threshold. - The signal VC is provided to a
power stage 110 of the amplifier, which comprises switches controlled by signal VC, known as a class D amplifier. The switches in thepower stage 110 control when anoutput node 112 is coupled to a supply voltage level or a ground voltage level. The output signal VS therefore comprises pulses having durations depending on the control signal VC, and its duty cycle can thus be controlled. Node 112 is coupled viafeedback line 105 to adder 104. - The switching frequency fS of the switches that generates the pulsed output voltage VS is generally chosen to be much higher than the frequency band of any information contained in the signal, such that this can be filtered out resulting in a clean signal containing the information.
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FIG. 2 illustratescircuitry 200 that has been proposed for providing thehysteresis block 108 and thepower stage 110 of the switching amplifier ofFIG. 1 . - The output Vi of the integrator is provided on
line 202 to the negative input of adifferential amplifier 204, which has supply voltage inputs coupled to a supply voltage level VDD,− for example at 3.6 V, and a ground voltage level, which is, for example, 0 V. The output VC of theoperational amplifier 204 is coupled to anode 206, which is in turn coupled to a common mode voltage level Vcm viaresistors node 211 betweenresistors operational amplifier 204. - Node 206 is coupled to a
gate driver block 212 of thepower stage 110, which generates signals for controlling a pair ofswitches node 217 betweenswitches inductor 218 and acapacitor 220,inductor 218 being coupled in series betweennode 217 and anoutput load 222, andcapacitor 220 being coupled in parallel with theoutput load 222 to ground. In the example ofFIG. 2 , the output load is an audio speaker. - In operation, the threshold levels of the hysteresis block are determined by the voltage at
node 211, which depends on the output voltage VC of theoperational amplifier 204. In particular, when the output voltage VC of theamplifier 204 is at the supply voltage level VDD, the voltage atnode 211, and thus the high threshold level U+ is equal to: -
U + =K 1 V DD +K 2 V cm - wherein K1 is equal to R1/(R1+R2) and K2 is equal to R2/(R1+R2), R1 being the resistance of
resistor 210 and R2 being the resistance ofresistor 208. When the output voltage of the amplifier is at the ground reference voltage level, for example at 0 V, the voltage atnode 211, and thus the low threshold U− is: -
U−=K2Vcm - Thus the average voltage U between the high and low thresholds U+ and U− is equal to:
-
U=K 2 V cm +K 1 V DD/2 - The term containing VDD necessitates a high dependence between the switching of the output voltage VC of the
hysteresis block 108 and the supply voltage VDD, leading to a poor PSRR of the amplifier. In particular, the PSRR due to thehysteresis block 108 can be determined as: -
PSRR=20 log(πV e(V DD −V e)f p /V DD 2 f S - where Ve is the input voltage of the integrator of the switching amplifier, fS is the switching frequency, and fp is the frequency of a voltage ripple on the supply voltage VDD. Assuming VDD=3.6 V, Ve=VDD/2, fS=1 MHz and fp=217 Hz, the PSRR is approximately 75 dB, which is insufficient in many applications.
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FIG. 3 illustrates anamplifier 300 comprising aninput line 302 receiving an input voltage signal Ve, coupled to anadder 304, which subtracts the signal received on afeedback line 305 from signal Ve. The output ofadder 304 is coupled to anintegrator 306, which performs anintegration 1/τp having a constant of integration τ and wherein p is the laplace variable. The output signal Vi of the integrator is provided to ahysteresis block 308, which in this embodiment also receives a high threshold voltage online 312 equal to Vref+αVDD, and a low threshold voltage online 314 equal to Vref−αVDD, where Vref is a reference voltage based on a common mode voltage level, which is a clean voltage reference, VDD is a supply voltage and α is a constant, but may be adjusted in some embodiments as described in more detail below. A difference between the supply voltage VDD and the reference voltage Vref is that while a current can generally be supplied by VDD to supply a load, Vref is a reference voltage generally only used as a reference from which no current or a very little current is drawn. By way of example, assuming Vref is equal to 1.8 V, VDD is equal to 3.6 V, and α is equal to 0.2, the low threshold voltage is 1.08 V and the high threshold 2.52 V. α is preferably in a range 0.05 to 0.2. - The
hysteresis block 308 generates an output signal VC, which, as withhysteresis block 108, has either a high or low value. As withhysteresis block 108, when the output is low and the input voltage Vi is greater that the high threshold voltage, the output goes high, while when the output is high, and the input voltage Vi is lower than the low threshold voltage, the output goes low. - The output signal VC of the
hysteresis block 308 is provided to apower stage 310 comprising switches, and provides the output voltage VS of the amplifier online 312. Theoutput line 312 is also coupled to adder 304 viafeedback line 305. - By providing the threshold values U+ and U− equal to a reference voltage plus or minus respectively a multiple of the supply voltage, the mean threshold U is the reference voltage Vref, while the difference between the thresholds ΔU=(U+−U−)/2 is αVDD. This means that ripples in the supply voltage affect the high and low thresholds by the same amount in opposite directions, cancelling the effect of the ripple, while as the average value of the thresholds does not change, the overall duty cycle of the output pulses will not be affected by the ripple.
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FIG. 4 illustrates an example ofcircuitry 400 for generating the high and low thresholds provided onlines Circuitry 400 comprises aninput line 402 for receiving the supply voltage VDD, which could be a battery voltage, provided by one or more batteries or a different supply voltage.Line 402 is coupled to the negative input terminal of anoperational amplifier 404 via avariable resistor 406. An input line 408 is also provided for receiving a common mode voltage Vcm, which is a clean voltage reference that is, for example, generated by a band gap cell. Input line 408 is coupled to the positive input node of theoperational amplifier 404 via a resistor 410. The positive input terminal of operational amplifier is further coupled to a ground reference voltage via avariable resistor 412. The voltage at the positive input node ofoperational amplifier 404 is the reference voltage Vref, which, in this example, is between Vcm and the ground reference voltage. - The negative input terminal of
operational amplifier 404 is coupled to theoutput terminal 414 of the operational amplifier via aresistor 416. Theoutput node 414 provides the low threshold U−=Vref−αVDD.Node 414 is coupled to the negative input terminal of anoperational amplifier 418 via aresistor 420. The positive input terminal ofoperational amplifier 418 is coupled to aline 422 for receiving the reference voltage Vref. The negative input terminal of the operational amplifier is further coupled to theoutput terminal 424 of theoperational amplifier 418 via aresistor 426.Resistors operational amplifier 418 is −1.Output node 424 provides the high threshold U+=Vref+αVDD. - The value of α can be adjusted by
variable resistor 406. In particular, assumingresistor 406 has a resistance R1 and resistor 416 a resistance R2, α can be determined as being equal to R2/R1. The value of α can be used to control the switching frequency of the switching amplifier as will be described in more detail below. - Furthermore, the voltage Vref can be adjusted by varying
resistor 412. Assuming resistor 410 has a resistance R1, whileresistor 412 has a resistance R2, it follows that Vref=VcmR2/(R1+R2). -
FIG. 5 illustrates a switchingamplifier 500 according to one example. - As illustrated, an
input line 502 is provided for receiving an input voltage signal Ve to be amplified.Line 502 is coupled to the negative input terminal of anoperational amplifier 504 via aresistor 506. Furthermore, theoutput node 508 of theoperational amplifier 504 is coupled to the negative input terminal via acapacitor 510, and the positive input terminal of theoperational amplifier 504 is coupled to a ground reference voltage. Theoperational amplifier 504,resistor 506 andcapacitor 510 provide the function of the integrator, wherein the value ofcapacitor 510 andresistor 506 provide the integration constant τ. For example,capacitor 510 has a capacitance of 1 pF, whileresistor 506 has a resistance of 1 M Ohms, although these values will depend on the requirements of the amplifier, such as the switching frequency. - The negative input terminal of
operational amplifier 504 is also coupled to afeedback line 512 comprising afeedback resistor 514. - The
output node 508 ofoperational amplifier 504 is coupled to the negative input terminal ofoperational amplifier 516 and to the positive input terminal ofoperational amplifier 518.Operational amplifier 516 has its positive input terminal coupled to aline 520 for receiving the low threshold voltage U−=Vref−αVDD, whileoperational amplifier 518 has its negative input terminal coupled to a line 522 for receiving the high threshold voltage U+=Vref+αVDD, the high and low threshold voltages, for example, being generated by the circuitry ofFIG. 4 . Theoutput nodes operational amplifiers flop 528, which has its Q output coupled to aline 530. The combination ofoperational amplifiers flop 528 provide the hysteresis block of the switching amplifier. -
Line 530 is coupled to the gate node of atransistor 532 via aninverter 534, and to the gate node of atransistor 536,transistors Transistors Transistors inverter 534 provide the power stage of the switching amplifier, and anode 538 between the transistors provides the output voltage signal VS of the switching amplifier.Node 538 is coupled to the negative input terminal ofoperational amplifier 504 via thefeedback resistor 514.Node 538 is, for example, coupled to a load (not shown inFIG. 5 ), which in the case of audio applications could be a speaker, such as speaker 22 ofFIG. 2 , coupled via filtering circuitry such asinductor 218 andcapacitor 220 connected as shown inFIG. 2 . - In operation, the voltage signal at
node 508 is the signal Vi, which represents an integration of an error signal based on the input voltage Ve minus the voltage Vs at the output of the switch amplifier. The output voltage VS in this example is at one of the ground reference and the supply voltage. Assuming that Ve is equal to 1.8 V and the output is initially at the ground reference, which is, for example, at 0 V, then the integration is performed on a constant voltage of 1.8 V, meaning the voltage atnode 508 decreases relatively linearly. During this period, outputs ofoperational amplifiers operational amplifier 516 will go high, causing the Q output of SR flip-flop 528 to go low, and thustransistor 532 to turn on, andtransistor 536 to turn off, coupling theoutput node 538 to the supply voltage. The integration is then performed on a constant voltage of −1.8 V, the voltage Vi increases relatively linearly. When voltage Vi exceeds Vref+αVDD, the output ofoperational amplifier 518 will go high, causing the Q output of SR flip-flop 528 to go high, andtransistor 532 to turn off andtransistor 536 to turn on, coupling theoutput node 538 to the ground reference voltage again. -
FIG. 6A illustrates the voltage Vi when the high and low threshold voltages are U+ and U− respectively, and assuming an input voltage Ve at half the supply voltage. As illustrated by thesolid line 602, the voltage Vi increases linearly until it reaches the high threshold U+, and then fall linearly until it reaches the low threshold U−. The dashed line inFIG. 6A illustrates the effect of a positive voltage ripple of the supply voltage. As illustrated, the high threshold increases to U+′, while the low threshold decreases to U−′, but the ratio between the time that voltage Vi increases and the time that it decreases remains the same. -
FIG. 6B illustrates the signal VS resulting from the signal Vi ofFIG. 6A . Thesolid line 606 illustrates the case when the high and low threshold voltagess are U+ and U− respectively, while dashedline 608 illustrates the case that the high and low threshold voltages become U+′ and U−′ respectively due to a positive voltage ripple on the supply voltage. As illustrated, the duty cycle of the solid signal and that of the dashed signal during the period illustrated are equal. Thus, the distance between the high and low thresholds is proportional to changes in the supply voltage, while the middle point between the high and low thresholds is unaffected by such a change, leading to a duty cycle relatively independent of changes in the supply voltage. -
FIG. 7 illustrates anelectronic device 700 comprising apower source 702 that provides a supply voltage VDD. Power source 702 is, for example, a battery, power supply unit, or other power source, and VDD is a DC voltage level. VDD is provided to inputsignal circuitry 704 that provides an input signal Ve, and to anamplifier 706, which amplifies the input signal Ve by a gain to generate an output signal VS. Amplifier 706 is, for example, the amplifier ofFIGS. 3 or 5, which is, for example, a switching amplifier providing a gain which may be lower or greater than 1. -
Electronics device 700 is, for example, a mobile telephone, MP3 player, GPS navigator, media player, laptop or desktop PC, set-top box, radio receiver, digital camera, or other electronic device that comprises an amplifier having a power stage based on switches, such as a switching amplifier. The input signal Ve could be an audio signal, and the switching amplifier used to amplify the audio signal to drive internal or external speakers or headphones. Alternatively, the amplifier as described herein could be used as a DC-DC converter, to provide a DC voltage level based on a reference DC voltage Ve. - Advantageously, embodiments of the switching amplifier as described herein provide improved PSRR. In particular, the PSRR is only a function of the switching frequency, and can in theory be infinite for an infinite switching frequency. In practice, it has been found that for a switching frequency of 200 kHz, a PSRR greater than 80 dB can be achieved.
- A further advantage of embodiments of the switching amplifier described herein is that because the error signal, in other words integral of the input voltage Ve minus the output voltage Vs, is always between U− and U+, the circuit is still stable.
- A further advantage of embodiments of the switching amplifier described herein is that the variations in the switching frequency can be controlled in order to reduce interference with surrounding electronics. In particular, by controlling, for example, one or both of the
variable resistors circuitry 400 ofFIG. 4 , the constant α can be varied, thus adjusting the high and low thresholds of the hysteresis block and adjusting the switching frequency. For example, in the case of audio data in the audio band of 20 Hz to 20 kHz, the switching frequency can for example be chosen to be around 200 kHz or more. In particular, the switching frequency FS can be determined as: -
F S=(1−M)2/4ατ - wherein M is the index of modulation, α is the constant of the high and low threshold voltages as described above, and τ the constant of integration.
- Having thus described illustrative embodiments of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art.
- For example, while examples of the circuitry of a switching amplifier have been provided in
FIG. 5 , it will be apparent that alternative circuits could be used. For example, the hysteresis function could be implemented in alternative ways. - While, in the various embodiments, the supply voltage VDD has been described as, for example, being a battery voltage, it will be apparent to those skilled in the art that the supply voltage could be from any power source, such a battery, power supply unit, solar cell, etc.
- Furthermore, it will be apparent that while in the circuit of
FIG. 4 variable resistors - While an example of the power stage comprising a class D amplifier comprising a pair of MOS transistors has been described, it will be apparent to those skilled in the art that other types of class D amplifiers or switches could be provided in the power stage.
- Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.
Claims (9)
1. An amplifier comprising at least one switch controlled by an output voltage of a hysteresis block, wherein the hysteresis block is adapted to receive a low threshold voltage, a high threshold voltage and an input voltage signal based on an integration of an error signal, the hysteresis block being arranged to change said output voltage from a first value to a second value when said input voltage signal is higher than said high threshold voltage and to change said output voltage from said second value to said first value when said input voltage signal is lower than said low threshold voltage, and wherein said low threshold voltage is equal to Vref−αVDD, and said high threshold voltage is equal to Vref+αVDD, where Vref is a common mode voltage level, α is a non-zero constant, and VDD is a power supply voltage.
2. The amplifier of claim 1 , further comprising:
an integrator coupled to said hysteresis block to provide said input voltage signal;
a power stage comprising said switches and coupled to receive said output voltage; and
adder circuitry arranged to receive an input signal of said amplifier and an output voltage of said power stage, and to provide said error signal to an input of said integrator, said error signal based on said input signal and said output voltage.
3. The amplifier of claim 1 , wherein said hysteresis block comprises a first comparator for comparing said input voltage signal to said high threshold voltage and a second comparator for comparing said input voltage signal to said low threshold voltage.
4. The amplifier of claim 3 , wherein said hysteresis block further comprises an SR flip-flop adapted to be set by the output of one of the first and second comparators and reset by the output of the other of the first and second comparators.
5. The amplifier of claim 1 , comprising a class D amplifier coupled to receive the output voltage of said hysteresis block, said class D amplifier comprising said switches.
6. The amplifier of claim 1 , further comprising circuitry adapted to generate said high and low threshold voltages, said circuitry comprising at least one variable resistance for allowing said supply voltage level to be adjusted.
7. An electronic device comprising the amplifier of claim 1 .
8. A mobile communications device comprising the amplifier of claim 1 .
9. A method of controlling a power stage of an amplifier comprising:
performing integration on an error signal to generate an integrated signal, said error signal being based on an input signal to be amplified and the output voltage of said power stage; and
comparing by a hysteresis block said integrated signal with a high threshold voltage and a low threshold voltage, and generating an output signal that changes from a first value to a second value if said integrated signal is higher than said high threshold voltage and changes from said second value to said first value if said integrated signal is lower than said low threshold voltage, wherein said low threshold voltage is equal to Vref−αVDD and said high threshold voltage is equal to Vref+αVDD, where Vref is a common mode voltage level, α is a non-zero constant, and VDD is a power supply voltage.
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Also Published As
Publication number | Publication date |
---|---|
EP2096753A1 (en) | 2009-09-02 |
US7961047B2 (en) | 2011-06-14 |
EP2096753B1 (en) | 2011-11-30 |
US20100290646A1 (en) | 2010-11-18 |
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