U.S. patent application number 12/393524 was filed with the patent office on 2009-10-08 for switching amplifier.
This patent application is currently assigned to STMicroelectronics (Grenoble) SAS. Invention is credited to Remy Cellier, Gael Pillonnet.
Application Number | 20090251212 12/393524 |
Document ID | / |
Family ID | 39673473 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090251212 |
Kind Code |
A1 |
Pillonnet; Gael ; et
al. |
October 8, 2009 |
SWITCHING AMPLIFIER
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 V.sub.ref-.alpha.V.sub.DD and the
high threshold voltage is equal to V.sub.ref+.alpha.V.sub.DD, where
V.sub.ref is a common mode voltage level, .alpha. is a non-zero
constant, and V.sub.DD is a power supply voltage.
Inventors: |
Pillonnet; Gael; (Lyon,
FR) ; Cellier; Remy; (Grenoble, FR) |
Correspondence
Address: |
STMicroelectronics Inc.;c/o WOLF, GREENFIELD & SACKS, P.C.
600 Atlantic Avenue
BOSTON
MA
02210-2206
US
|
Assignee: |
STMicroelectronics (Grenoble)
SAS
Grenoble
FR
|
Family ID: |
39673473 |
Appl. No.: |
12/393524 |
Filed: |
February 26, 2009 |
Current U.S.
Class: |
330/251 |
Current CPC
Class: |
H03F 3/2171
20130101 |
Class at
Publication: |
330/251 |
International
Class: |
H03F 3/217 20060101
H03F003/217 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2008 |
EP |
EP 08300125.5 |
Claims
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 V.sub.ref-.alpha.V.sub.DD, and said high threshold voltage is
equal to V.sub.ref+.alpha.V.sub.DD, where V.sub.ref is a common
mode voltage level, .alpha. is a non-zero constant, and V.sub.DD 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
V.sub.ref-.alpha.V.sub.DD and said high threshold voltage is equal
to V.sub.ref+.alpha.V.sub.DD, where V.sub.ref is a common mode
voltage level, .alpha. is a non-zero constant, and V.sub.DD is a
power supply voltage.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] 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.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an amplifier and in
particular to an amplifier comprising one or more switches
controlled by a hysteresis block.
[0004] 2. Discussion of the Related Art
[0005] 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.
[0006] It has been proposed to provide a hysteresis stage in the
feedback loop for controlling the switches.
[0007] 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.
SUMMARY OF THE INVENTION
[0008] It is one aim of embodiments of the present invention to at
least partially address one or more needs in the prior art.
[0009] 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.
[0010] According to an embodiment of the present invention, the low
threshold voltage is equal to V.sub.ref-.alpha.V.sub.DD and the
high threshold voltage is equal to V.sub.ref+.alpha.V.sub.DD, where
V.sub.ref is the reference voltage level, .alpha. is a non-zero
constant, and V.sub.DD is a power supply voltage.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] According to a further aspect of the present invention,
there is provided an electronic device, or a mobile communications
device comprising the above amplifier.
[0017] 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 V.sub.ref-.alpha.V.sub.DD
and said high threshold voltage is equal to
V.sub.ref+.alpha.V.sub.DD, where V.sub.ref is a common mode voltage
level, .alpha. is a constant, and V.sub.DD is a power supply
voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] 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.
[0019] FIG. 1 illustrates a switching amplifier using sliding mode
control;
[0020] FIG. 2 illustrates the hysteresis and power stages of the
switching amplifier of FIG. 1 in more detail;
[0021] FIG. 3 illustrates an amplifier according to an embodiment
of the present invention;
[0022] FIG. 4 illustrates circuitry for generating hysteresis
threshold values according to an embodiment of the present
invention;
[0023] FIG. 5 illustrates a switching amplifier according to a
further embodiment of the present invention;
[0024] FIGS. 6A and 6B are timing diagrams illustrating the output
of integrator and hysteresis blocks according to an embodiment of
the present invention; and
[0025] FIG. 7 illustrates a electronic device comprising a
switching amplifier according to embodiments of the present
invention.
DETAILED DESCRIPTION
[0026] FIG. 1 illustrates a switching amplifier 100 according to
one embodiment comprising an input line 102 for receiving an input
signal V.sub.e. Line 102 is coupled to an adder block 104, which
subtracts a value received from a feedback line 105, and passes the
output to an integrator 106. Integrator 106 performs an integration
1/.tau.p of the input signal with a constant of integration .tau.
and wherein p is the laplace variable.
[0027] The integrated signal V.sub.i at the output of integrator
106 is provided to a hysteresis block 108, which outputs a high or
low voltage based on high and low threshold levels. In particular,
when the output V.sub.C of the hysteresis block is at the low
voltage level, it changes to the high voltage level when the input
signal V.sub.i goes higher than the high threshold, and when the
output V.sub.C is at the high voltage level, it goes to the low
voltage level when the input signal V.sub.i goes lower than the low
threshold.
[0028] The signal V.sub.C is provided to a power stage 110 of the
amplifier, which comprises switches controlled by signal V.sub.C,
known as a class D amplifier. The switches in the power stage 110
control when an output node 112 is coupled to a supply voltage
level or a ground voltage level. The output signal V.sub.S
therefore comprises pulses having durations depending on the
control signal V.sub.C, and its duty cycle can thus be controlled.
Node 112 is coupled via feedback line 105 to adder 104.
[0029] The switching frequency f.sub.S of the switches that
generates the pulsed output voltage V.sub.S 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.
[0030] FIG. 2 illustrates circuitry 200 that has been proposed for
providing the hysteresis block 108 and the power stage 110 of the
switching amplifier of FIG. 1.
[0031] The output V.sub.i of the integrator is provided on line 202
to the negative input of a differential amplifier 204, which has
supply voltage inputs coupled to a supply voltage level
V.sub.DD,.sub.- for example at 3.6 V, and a ground voltage level,
which is, for example, 0 V. The output V.sub.C of the operational
amplifier 204 is coupled to a node 206, which is in turn coupled to
a common mode voltage level V.sub.cm via resistors 208 and 210
coupled in series. Common mode voltage V.sub.cm is for example a
clean voltage level, for example halfway between the supply voltage
level and the ground voltage level. A node 211 between resistors
208 and 210 is coupled to the positive input of the operational
amplifier 204.
[0032] Node 206 is coupled to a gate driver block 212 of the power
stage 110, which generates signals for controlling a pair of
switches 214, 216 based on signal V.sub.C. Switches 214, 216 are
coupled via their main current terminals between the supply voltage
level V.sub.DD and a ground voltage level. A node 217 between
switches 214, 216 provides an output signal V.sub.S, which in this
example is filtered by an inductor 218 and a capacitor 220,
inductor 218 being coupled in series between node 217 and an output
load 222, and capacitor 220 being coupled in parallel with the
output load 222 to ground. In the example of FIG. 2, the output
load is an audio speaker.
[0033] In operation, the threshold levels of the hysteresis block
are determined by the voltage at node 211, which depends on the
output voltage V.sub.C of the operational amplifier 204. In
particular, when the output voltage V.sub.C of the amplifier 204 is
at the supply voltage level V.sub.DD, the voltage at node 211, and
thus the high threshold level U.sub.+ is equal to:
U.sub.+=K.sub.1V.sub.DD+K.sub.2V.sub.cm
wherein K.sub.1 is equal to R.sub.1/(R.sub.1+R.sub.2) and K.sub.2
is equal to R.sub.2/(R.sub.1+R.sub.2), R1 being the resistance of
resistor 210 and R2 being the resistance of resistor 208. When the
output voltage of the amplifier is at the ground reference voltage
level, for example at 0 V, the voltage at node 211, and thus the
low threshold U.sub.- is:
U.sub.-=K.sub.2V.sub.cm
[0034] Thus the average voltage U between the high and low
thresholds U.sub.+ and U.sub.- is equal to:
U=K.sub.2V.sub.cm+K.sub.1V.sub.DD/2
[0035] The term containing V.sub.DD necessitates a high dependence
between the switching of the output voltage V.sub.C of the
hysteresis block 108 and the supply voltage V.sub.DD, leading to a
poor PSRR of the amplifier. In particular, the PSRR due to the
hysteresis block 108 can be determined as:
PSRR=20
log(.pi.V.sub.e(V.sub.DD-V.sub.e)f.sub.p/V.sub.DD.sup.2f.sub.S
[0036] where V.sub.e is the input voltage of the integrator of the
switching amplifier, f.sub.S is the switching frequency, and
f.sub.p is the frequency of a voltage ripple on the supply voltage
V.sub.DD. Assuming V.sub.DD=3.6 V, V.sub.e=V.sub.DD/2, f.sub.S=1
MHz and f.sub.p=217 Hz, the PSRR is approximately 75 dB, which is
insufficient in many applications.
[0037] FIG. 3 illustrates an amplifier 300 comprising an input line
302 receiving an input voltage signal V.sub.e, coupled to an adder
304, which subtracts the signal received on a feedback line 305
from signal V.sub.e. The output of adder 304 is coupled to an
integrator 306, which performs an integration 1/.tau.p having a
constant of integration .tau. and wherein p is the laplace
variable. The output signal V.sub.i of the integrator is provided
to a hysteresis block 308, which in this embodiment also receives a
high threshold voltage on line 312 equal to
V.sub.ref+.alpha.V.sub.DD, and a low threshold voltage on line 314
equal to V.sub.ref-.alpha.V.sub.DD, where V.sub.ref is a reference
voltage based on a common mode voltage level, which is a clean
voltage reference, V.sub.DD is a supply voltage and .alpha. is a
constant, but may be adjusted in some embodiments as described in
more detail below. A difference between the supply voltage V.sub.DD
and the reference voltage V.sub.ref is that while a current can
generally be supplied by V.sub.DD to supply a load, V.sub.ref 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 V.sub.ref is equal to 1.8 V, V.sub.DD is equal to 3.6 V,
and .alpha. is equal to 0.2, the low threshold voltage is 1.08 V
and the high threshold 2.52 V. .alpha. is preferably in a range
0.05 to 0.2.
[0038] The hysteresis block 308 generates an output signal V.sub.C,
which, as with hysteresis block 108, has either a high or low
value. As with hysteresis block 108, when the output is low and the
input voltage V.sub.i is greater that the high threshold voltage,
the output goes high, while when the output is high, and the input
voltage V.sub.i is lower than the low threshold voltage, the output
goes low.
[0039] The output signal V.sub.C of the hysteresis block 308 is
provided to a power stage 310 comprising switches, and provides the
output voltage V.sub.S of the amplifier on line 312. The output
line 312 is also coupled to adder 304 via feedback line 305.
[0040] By providing the threshold values U.sub.+ and U.sub.- equal
to a reference voltage plus or minus respectively a multiple of the
supply voltage, the mean threshold U is the reference voltage
V.sub.ref, while the difference between the thresholds
.DELTA..sub.U=(U.sub.+-U.sub.-)/2 is .alpha.V.sub.DD. 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.
[0041] FIG. 4 illustrates an example of circuitry 400 for
generating the high and low thresholds provided on lines 312 and
314 to the hysteresis block. Circuitry 400 comprises an input line
402 for receiving the supply voltage V.sub.DD, 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 an operational amplifier 404 via a variable resistor 406. An
input line 408 is also provided for receiving a common mode voltage
V.sub.cm, 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 the operational amplifier 404 via a resistor
410. The positive input terminal of operational amplifier is
further coupled to a ground reference voltage via a variable
resistor 412. The voltage at the positive input node of operational
amplifier 404 is the reference voltage V.sub.ref, which, in this
example, is between V.sub.cm and the ground reference voltage.
[0042] The negative input terminal of operational amplifier 404 is
coupled to the output terminal 414 of the operational amplifier via
a resistor 416. The output node 414 provides the low threshold
U.sub.-=V.sub.ref-.alpha.V.sub.DD. Node 414 is coupled to the
negative input terminal of an operational amplifier 418 via a
resistor 420. The positive input terminal of operational amplifier
418 is coupled to a line 422 for receiving the reference voltage
V.sub.ref. The negative input terminal of the operational amplifier
is further coupled to the output terminal 424 of the operational
amplifier 418 via a resistor 426. Resistors 420 and 426 for example
have the same resistance as each other, such that the gain provided
by operational amplifier 418 is -1. Output node 424 provides the
high threshold U.sub.+=V.sub.ref+.alpha.V.sub.DD.
[0043] The value of .alpha. can be adjusted by variable resistor
406. In particular, assuming resistor 406 has a resistance R.sub.1
and resistor 416 a resistance R.sub.2, .alpha. can be determined as
being equal to R2/R1. The value of .alpha. can be used to control
the switching frequency of the switching amplifier as will be
described in more detail below.
[0044] Furthermore, the voltage V.sub.ref can be adjusted by
varying resistor 412. Assuming resistor 410 has a resistance R1,
while resistor 412 has a resistance R2, it follows that
V.sub.ref=V.sub.cmR2/(R1+R2).
[0045] FIG. 5 illustrates a switching amplifier 500 according to
one example.
[0046] As illustrated, an input line 502 is provided for receiving
an input voltage signal V.sub.e to be amplified. Line 502 is
coupled to the negative input terminal of an operational amplifier
504 via a resistor 506. Furthermore, the output node 508 of the
operational amplifier 504 is coupled to the negative input terminal
via a capacitor 510, and the positive input terminal of the
operational amplifier 504 is coupled to a ground reference voltage.
The operational amplifier 504, resistor 506 and capacitor 510
provide the function of the integrator, wherein the value of
capacitor 510 and resistor 506 provide the integration constant
.tau.. For example, capacitor 510 has a capacitance of 1 pF, while
resistor 506 has a resistance of 1 M Ohms, although these values
will depend on the requirements of the amplifier, such as the
switching frequency.
[0047] The negative input terminal of operational amplifier 504 is
also coupled to a feedback line 512 comprising a feedback resistor
514.
[0048] The output node 508 of operational amplifier 504 is coupled
to the negative input terminal of operational amplifier 516 and to
the positive input terminal of operational amplifier 518.
Operational amplifier 516 has its positive input terminal coupled
to a line 520 for receiving the low threshold voltage
U.sub.-=V.sub.ref-.alpha.V.sub.DD, while operational amplifier 518
has its negative input terminal coupled to a line 522 for receiving
the high threshold voltage U.sub.+=V.sub.ref+.alpha.V.sub.DD, the
high and low threshold voltages, for example, being generated by
the circuitry of FIG. 4. The output nodes 524 and 526 of
operational amplifiers 516 and 518 respectively are coupled to the
R and S inputs respectively of an RS flip-flop 528, which has its Q
output coupled to a line 530. The combination of operational
amplifiers 516, 518 and SR flip-flop 528 provide the hysteresis
block of the switching amplifier.
[0049] Line 530 is coupled to the gate node of a transistor 532 via
an inverter 534, and to the gate node of a transistor 536,
transistors 532 and 536 for example being power MOSFETs (metal
oxide semiconductor field effect transistors). Transistors 532 and
536 are coupled in series via their main current terminals between
a supply voltage terminal and a ground reference terminal.
Transistors 532, 536 and inverter 534 provide the power stage of
the switching amplifier, and a node 538 between the transistors
provides the output voltage signal V.sub.S of the switching
amplifier. Node 538 is coupled to the negative input terminal of
operational amplifier 504 via the feedback resistor 514. Node 538
is, for example, coupled to a load (not shown in FIG. 5), which in
the case of audio applications could be a speaker, such as speaker
22 of FIG. 2, coupled via filtering circuitry such as inductor 218
and capacitor 220 connected as shown in FIG. 2.
[0050] In operation, the voltage signal at node 508 is the signal
V.sub.i, which represents an integration of an error signal based
on the input voltage V.sub.e minus the voltage Vs at the output of
the switch amplifier. The output voltage V.sub.S in this example is
at one of the ground reference and the supply voltage. Assuming
that V.sub.e 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 at node 508 decreases relatively linearly. During this
period, outputs of operational amplifiers 518 and 520 are low as
the voltage at 508 is not lower than V.sub.ref-.alpha.V.sub.DD, nor
higher than V.sub.ref+.alpha.V.sub.DD. However, when the voltage
V.sub.i falls below V.sub.ref-.alpha.V.sub.DD, the output of
operational amplifier 516 will go high, causing the Q output of SR
flip-flop 528 to go low, and thus transistor 532 to turn on, and
transistor 536 to turn off, coupling the output node 538 to the
supply voltage. The integration is then performed on a constant
voltage of -1.8 V, the voltage V.sub.i increases relatively
linearly. When voltage V.sub.i exceeds V.sub.ref+.alpha.V.sub.DD,
the output of operational amplifier 518 will go high, causing the Q
output of SR flip-flop 528 to go high, and transistor 532 to turn
off and transistor 536 to turn on, coupling the output node 538 to
the ground reference voltage again.
[0051] FIG. 6A illustrates the voltage V.sub.i when the high and
low threshold voltages are U.sub.+ and U.sub.- respectively, and
assuming an input voltage V.sub.e at half the supply voltage. As
illustrated by the solid line 602, the voltage V.sub.i increases
linearly until it reaches the high threshold U.sub.+, and then fall
linearly until it reaches the low threshold U.sub.-. The dashed
line in FIG. 6A illustrates the effect of a positive voltage ripple
of the supply voltage. As illustrated, the high threshold increases
to U.sub.+', while the low threshold decreases to U.sub.-', but the
ratio between the time that voltage V.sub.i increases and the time
that it decreases remains the same.
[0052] FIG. 6B illustrates the signal V.sub.S resulting from the
signal V.sub.i of FIG. 6A. The solid line 606 illustrates the case
when the high and low threshold voltagess are U.sub.+ and U.sub.-
respectively, while dashed line 608 illustrates the case that the
high and low threshold voltages become U.sub.+' and U.sub.-'
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.
[0053] FIG. 7 illustrates an electronic device 700 comprising a
power source 702 that provides a supply voltage V.sub.DD. Power
source 702 is, for example, a battery, power supply unit, or other
power source, and V.sub.DD is a DC voltage level. V.sub.DD is
provided to input signal circuitry 704 that provides an input
signal V.sub.e, and to an amplifier 706, which amplifies the input
signal V.sub.e by a gain to generate an output signal V.sub.S.
Amplifier 706 is, for example, the amplifier of FIGS. 3 or 5, which
is, for example, a switching amplifier providing a gain which may
be lower or greater than 1.
[0054] 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 V.sub.e
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 V.sub.e.
[0055] 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.
[0056] 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 V.sub.e minus the output
voltage Vs, is always between U.sub.- and U.sub.+, the circuit is
still stable.
[0057] 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 406, 412, of the
circuitry 400 of FIG. 4, the constant .alpha. 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 F.sub.S can be determined
as:
F.sub.S=(1-M).sup.2/4.alpha..tau.
[0058] wherein M is the index of modulation, .alpha. is the
constant of the high and low threshold voltages as described above,
and .tau. the constant of integration.
[0059] Having thus described illustrative embodiments of the
invention, various alterations, modifications and improvements will
readily occur to those skilled in the art.
[0060] 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.
[0061] While, in the various embodiments, the supply voltage
V.sub.DD 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.
[0062] Furthermore, it will be apparent that while in the circuit
of FIG. 4 variable resistors 406 and 412 have been provided, fixed
resistors could be used, providing a fixed value of .alpha., or a
resistor network comprising a plurality of resistors could be
provided, and selection circuitry provided for selecting one or a
combination of the resistors in the resistor network to provide
these resistances. The latter embodiment would allow an amplifier
to be customized by a customer such that the switching frequency
has a desired frequency range, and thereby reduce possible
interference with surrounding components.
[0063] 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.
[0064] 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.
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