U.S. patent application number 10/622051 was filed with the patent office on 2005-01-20 for efficient class-g amplifier with wide output voltage swing.
Invention is credited to Somerville, Thomas A., Wortel, Klaas.
Application Number | 20050012554 10/622051 |
Document ID | / |
Family ID | 33541430 |
Filed Date | 2005-01-20 |
United States Patent
Application |
20050012554 |
Kind Code |
A1 |
Somerville, Thomas A. ; et
al. |
January 20, 2005 |
EFFICIENT CLASS-G AMPLIFIER WITH WIDE OUTPUT VOLTAGE SWING
Abstract
Various embodiments of methods and apparatus for an amplifier
with wide output voltage swing are disclosed. The amplifier may
include multiple output stages, each associated with a distinct
supply voltage. Each output stage may contribute current to the
output of the amplifier over a range of amplifier output voltages
and these ranges may overlap. Each output stage may contribute
current until the amplifier output voltage reaches the supply
voltage associated with that output stage. The amplifier output may
be as great as the largest supply voltage minus a drop equal to
Rdson for an output transistor multiplied by the output current. In
a CMOS implementation, this voltage drop may be approximately
0.15V. When the amplifier output voltage is close to the supply
voltage associated with an output stage, both that output stage and
the output stage associated with the next highest supply voltage
may contribute to the amplifier output.
Inventors: |
Somerville, Thomas A.;
(Tempe, AZ) ; Wortel, Klaas; (Phoenix,
AZ) |
Correspondence
Address: |
Jeffrey C. Hood
Meyertons, Hood, Kivlin, Kowert & Goetzel PC
P.O. Box 398
Austin
TX
78767
US
|
Family ID: |
33541430 |
Appl. No.: |
10/622051 |
Filed: |
July 17, 2003 |
Current U.S.
Class: |
330/297 |
Current CPC
Class: |
H03F 3/72 20130101; H03F
1/025 20130101; H03F 3/211 20130101 |
Class at
Publication: |
330/297 |
International
Class: |
H03F 003/04 |
Claims
What is claimed is:
1. An amplifier, comprising: an input stage configured to receive
an amplifier input signal; a plurality of output stages configured
to combinatorially produce an amplifier output; and an output stage
controller coupled to the input stage and the output stages;
wherein each of the plurality of output stages is configured to
receive a supply voltage that is different from any other output
stage; wherein each of the plurality of output stages comprises an
output transistor, and wherein in response to the amplifier input
signal, the output stage controller is configured to generate
control signals that cause each of the plurality of output stages
to contribute current to the output of the amplifier when the
output of the amplifier is less than the supply voltage received by
the output stage.
2. The amplifier of claim 1 configured as a class G amplifier.
3. The amplifier of claim 1, wherein the maximum output voltage of
the amplifier is equal to a highest supply voltage minus the
product of the amplifier output current and Rdson for the output
transistor comprised in the one of the plurality of output stages
associated with the highest supply voltage.
4. The amplifier of claim 1, wherein the Rdson for the output
transistors is inversely proportional to the channel width of the
transistors.
5. The amplifier of claim 1, wherein the product of the amplifier
output current and Rdson for the output transistor comprised in the
one of the plurality of output stages associated with the highest
supply voltage is approximately 0.15V.
6. The amplifier as recited in claim 1, wherein for each of the
plurality of output stages except for the one of the plurality of
output stages configured to receive a highest supply voltage, the
output stage controller is configured to generate a control signal,
which changes state in response to the amplifier output voltage
reaching the voltage level of the supply voltage received by the
output stage.
7. The amplifier as recited in claim 6, wherein each of the control
signals controls an analog switch coupled between an input and
output of the corresponding output stage, wherein if the amplifier
output is increasing, the control signal is configured to close the
analog switch, wherein if the amplifier output is decreasing, the
control signal is configured to open the analog switch.
8. The amplifier as recited in claim 7, wherein when the analog
switch is closed the output transistor comprised in the output
stage is configured to inhibit current from flowing from the
amplifier output into a power supply associated with the output
stage.
9. The amplifier as recited in claim 7, wherein when the analog
switch is open the output transistor comprised in the output stage
is configured to allow current to flow from a power supply
associated with the output stage to the amplifier output.
10. The amplifier as recited in claim 1 implemented as an
integrated circuit using CMOS technology.
11. The amplifier as recited in claim 1 configured to drive a
fan.
12. The amplifier as recited in claim 1, wherein each of the
plurality of output stages is configured to provide a voltage gain
greater than unity.
13. A class G amplifier, wherein when the amplifier output voltage
is in a range between the supply voltage received by an output
stage and a voltage differential, delta V, below said supply
voltage, both that output stage and another output stage receiving
a next higher supply voltage contribute current to the amplifier
output.
14. The class G amplifier as recited in claim 13, wherein delta V
is determined by the W/L ratio of transistors comprised in the
output stage controller.
15. The class G amplifier as recited in claim 13, wherein delta V
is in the range of 0V to 0.8V.
16. The class G amplifier as recited in claim 13, wherein delta V
is approximately 0.3V.
17. A class G amplifier, wherein each of the plurality of output
stages, except for an output stage configured to receive a lowest
supply voltage, is configured to contribute current to the
amplifier output when the amplifier output voltage is in a range
from the supply voltage received by that output stage to a next
lower supply voltage less a differential voltage, delta V.
18. The class G amplifier as recited in claim 17, wherein the
output stage associated with the lowest supply voltage is
configured to contribute current to the amplifier output when the
amplifier output voltage is in a range from 0V, to the lowest
supply voltage.
19. The class G amplifier as recited in claim 17, wherein delta V
is in a range of 0V to approximately 0.8V.
20. The class G amplifier as recited in claim 17, wherein delta V
is approximately 0.3V
21. A method comprising: outputting current from an output stage
associated with a lowest supply voltage when an amplifier output
voltage is less than the lowest supply voltage; outputting current
from an output stage associated with a highest supply voltage when
the amplifier output voltage is greater than the next lower supply
voltage minus delta V; and outputting current from an output stage
associated with a supply voltage other than the lowest or highest
supply voltage when the amplifier output voltage is between a next
lower supply voltage minus delta V, and the supply voltage
associated with that output stage.
22. The method of claim 21, wherein delta V is determined by the
W/L ratio of transistors comprised in an output stage
controller.
23. The method of claim 21, wherein delta V is in the range of 0V
to 0.8V.
24. The method of claim 21, wherein delta V is approximately
0.3V.
25. The method of claim 21, wherein the number of output stages and
corresponding supply voltages is greater than 3.
26. The method of claim 21, wherein the maximum output voltage of
the amplifier is equal to a highest supply voltage minus the
product of the amplifier output current and Rdson for the output
transistor comprised in the one of the plurality of output stages
associated with the highest supply voltage.
27. The method of claim 21, wherein the product of the amplifier
output current and Rdson for the output transistor comprised in the
one of the plurality of output stages associated with the highest
supply voltage is approximately 0.15V.
28. The method of claim 21, further comprising isolating the output
of each output stage from the amplifier output voltage when the
amplifier output voltage is greater than the supply voltage
associated with that output stage.
29. A method comprising: generating a first current that is
inversely proportional to an output voltage of an amplifier;
generating a second current that is directly proportional to a sum
of currents output by the output stages of the amplifier; combining
the first and second currents such that under normal amplifier
operation the resulting current is constant and less than a
limiting value, Ilim, for a range of output voltages for the
amplifier.
30. The method of claim 29, wherein the combined first and second
currents control a resistance between an input voltage of the
amplifier and ground.
31. The method of claim 30, further comprising reducing the
resistance between the input voltage of the amplifier and ground in
response to the value of the combined first and second currents
exceeding Ilim.
32. The method of claim 31, wherein the reduction in resistance
between the input voltage of the amplifier and ground is
proportional to the amount by which the combined first and second
currents exceed Ilim.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to integrated circuits, and more
particularly, to amplifiers implemented as integrated circuits.
[0003] 2. Description of the Related Art
[0004] The basic function of an amplifier is to produce and output
signal whose power is a multiple of the power of an input signal.
In many applications it is desirable that the output waveform
faithfully reproduce the shape of the input signal while magnifying
its voltage and/or current in a linear fashion. Traditionally,
amplifiers designed for these types of applications have been
configured for class A operation.
[0005] In an amplifier designed for class A operation, both output
devices conduct continuously for the entire cycle of signal swing,
or the bias current flows in the output devices at all times. The
key ingredient of class A operation is that both devices are always
on. There is no condition where one or the other is turned off.
Because of this, class A amplifiers in reality are not
complementary designs. They are single-ended designs with only one
type polarity output devices. They may have "bottom side"
transistors but these are operated as fixed current sources, not
amplifying devices.
[0006] Since a class A amplifier operates from only one power
supply, the voltage level of the supply must be somewhat greater
than the level of the peak output specified. Therefore, during
times in which the input signal is very small, the difference
between the amplitude of the output signal and the voltage of the
power supply will be large. The amount of non-usable power to be
dissipated in the output devices is the aforementioned voltage
difference multiplied by the output current. Even in those
instances where the output is at its maximum level, there will
still be a non-negligible voltage drop across the output devices
and corresponding level of non-usable power dissipated in the
devices.
[0007] Consequently class A is the most inefficient of all power
amplifier designs, averaging only around 20% (meaning it consumes
about 5 times as much power from the source as it delivers to the
load!) Thus class A amplifiers are large, heavy and run very hot.
All this is due to the amplifier constantly operating at full
power. The positive effect of all this is that class A designs are
inherently the most linear, with the least amount of
distortion.
[0008] In order to increase the efficiency of an amplifier while
maintaining a high degree of linearity, a class G design may be
employed. Class G operation involves changing the power supply
voltage from a lower level to a higher level when larger output
swings are required. There have been several ways to do this. The
simplest involves a single class AB output stage that is connected
to two power supply rails by a diode, or a transistor switch. The
design is such that under most circumstances, the output stage is
connected to the lower supply voltage, and automatically switches
to the higher rails for large signal peaks. Another approach uses
two class AB output stages, each connected to a different power
supply voltage, with the magnitude of the input signal determining
the signal path. Using two power supplies improves efficiency
enough to allow significantly more power for a given size and
weight.
[0009] Typically, class G amplifier implementations employ current
blocking diodes to prevent current from being driven into a lower
voltage supply when the amplifier output exceeds the lower supply
voltage. This effectively protects the lower voltage power
supplies, but also limits the efficiency of their contribution to
the amplifier output. The power dissipated in the diode will be the
voltage drop across the diode times the output current. This power
loss will occur any time a lower voltage supply is contributing to
the output of the amplifier. In addition, each output stage
normally includes a power device to control the flow of current to
the load. This device dissipates power equal to the difference
between the supply voltage and the amplifier output multiplied by
the load current. Again, this power will be wasted any time the
supply is contributing to the amplifier output.
[0010] When a power supply is contributing maximum current to the
amplifier output, the output device will typically be saturated and
drop a few tenths of a volt. When added to the diode drop for a
lower voltage supply, the total difference between the supply
voltage and the amplifier output may be around one volt. While such
a voltage drop and corresponding inefficiency may be acceptable in
a relatively high voltage amplifier design where the output is
several tens of volts, integrated circuit amplifiers for low-power
applications are typically designed to operate with minimum supply
voltages below two volts and such a drop in output stage voltage
would limit the amplifier's maximum efficiency to less than fifty
percent. Therefore, a more efficient design for a class G amplifier
may be desirable.
SUMMARY
[0011] Various embodiments of methods and apparatus for an
amplifier with wide output voltage swing are disclosed. The
amplifier may include multiple output stages, each associated with
a distinct supply voltage. Each output stage may contribute current
to the output of the amplifier over a range of amplifier output
voltages and these ranges may overlap. Each output stage may
contribute current until the amplifier output voltage reaches the
supply voltage associated with that output stage. The amplifier
output may be as great as the largest supply voltage minus a drop
equal to Rdson for an output transistor multiplied by the output
current. In a CMOS implementation, this voltage drop may be
approximately 0.15V. When the amplifier output voltage is close to
the supply voltage associated with an output stage, both that
output stage and the output stage associated with the next highest
supply voltage may contribute to the amplifier output.
[0012] As the amplifier input voltage increases from zero, the
output stage associated with the lowest supply voltage may supply
all of the amplifier output current until the amplifier output
voltage reaches a level that is delta V below the lowest supply
voltage, where delta V may be a few tenths of a volt and may be set
by ratio of channel geometries of transistors in the output stage
controller. At this point the output stage associated with the next
highest supply voltage may begin to contribute current to the
output of the amplifier. Both output stages may continue to
contribute current to the amplifier output until the amplifier
output voltage reaches the level of the lowest supply voltage. At
this point the output stage controller may assert a signal that
causes the output of the output stage associated with the lowest
supply voltage to be coupled to the input for that stage through an
analog switch, thus isolating the lowest supply voltage from the
amplifier output when the amplifier output voltage is above the
lowest supply voltage.
[0013] When the amplifier output voltage is above the lowest supply
voltage the output stage associated with the next highest supply
voltage may contribute all the current to the amplifier output.
When the amplifier output rises to within delta V of the supply
voltage for this output stage the output stage associated with the
next highest supply voltage may begin to contribute current to the
amplifier output. The transition regions in which two output stages
whose supply voltages are adjacent share the current load of the
amplifier may begin at delta V below each supply voltage. At this
voltage, the output stage associated with the supply voltage may be
supplying the preponderance of the amplifier output current, while
the output stage associated with the next highest supply voltage
may supply only negligible current. As the output voltage of the
amplifier increases through delta V, the brunt of the amplifier
output current may switch sources, so that by the time the
amplifier output voltage reaches the supply voltage, the output
stage associated with the next highest supply voltage may bear the
complete burden. This transition mechanism may serve to reduce
noise associated with switching the current supply between output
stages.
[0014] The transitioning of amplifier output load between output
stages may be repeated for each pair of output stages included in
the amplifier. A larger number of output stages may correspond to a
greater amplifier efficiency. In one embodiment, an amplifier may
be implemented as an IC using CMOS technology and delta V may be
designed to be approximately 0.3 V. This IC may be used to drive a
cooling fan in a personal computer environment and may use supply
voltages of 2.5 V, 3.3 V, and 5 V that are common in this type of
system. Each output stage of the amplifier may provide a voltage
gain greater than unity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] Other aspects of the invention will become apparent upon
reading the following detailed description and upon reference to
the accompanying drawings in which:
[0016] FIG. 1 shows a diagram of a class G amplifier, according to
one embodiment.
[0017] FIG. 1A shows a schematic diagram of one embodiment of a
output stage controller included in a class G amplifier.
[0018] FIG. 2 is a flowchart of a method for operating a class G
amplifier with three output stages, according to one
embodiment.
[0019] FIG. 3 illustrates a mechanism for protecting the power
supply of an output stage of a class G amplifier from reverse
current flow, according to one embodiment.
[0020] FIG. 4 is a flowchart of a method for operating an output
stage of a class G amplifier, according to one embodiment.
[0021] FIG. 5 illustrates circuitry for limiting the total output
current of an amplifier as a function of the amplifier output
voltage, according to one embodiment.
[0022] FIG. 6 is a flowchart of a method for providing output
current limiting in a class G amplifier, according to one
embodiment.
[0023] While the invention is susceptible to various modifications
and alternative forms, specific embodiments thereof are shown by
way of example in the drawings and will herein be described in
detail. It should be understood, however, that the drawings and
description thereto are not intended to limit the invention to the
particular form disclosed, but, on the contrary, the invention is
to cover all modifications, equivalents, and alternatives falling
with the spirit and scope of the present invention as defined by
the appended claims.
DETAILED DESCRIPTION OF EMBODIMENTS
[0024] FIG. 1 shows a diagram of a class G amplifier, according to
one embodiment. Voltage-controlled current sink 100 may draw
current from the control sections of the output stages 190 in
proportion to the amplifier input voltage 180. For example, output
stage 190A may be associated with the lowest power supply voltage
120 of any output stage. When current sink 100 begins to draw
current in response to input voltage 180, switches 105 and 110
associated with the control sections of output stages 190B and 190C
respectively may be open. In this instance, all current drawn by
current sink 100 may come from the control section of output stage
190A through resistor 115.
[0025] Current-controlled current source 125 may source current
from the power supply of output stage 190A in proportion to the
current through resistor 115. Switch 130 may be closed for
amplifier output voltage 175 in the range of zero to the voltage
120 of the power supply associated with output stage 190A. Current
source 125 may supply current through switch 130 to produce
amplifier output voltage 175. For values of amplifier output
voltage 175 below supply voltage 120 minus a small voltage
increment, delta V, output stage 190A may be the only contributor
to the current through amplifier load 195.
[0026] As amplifier input voltage 180 increases, current sink 100
may draw more and more current through resistor 115. Increasing
current through resistor 115 may cause a proportional increase in
current from source 125 through load 195, and a corresponding rise
in amplifier output voltage 175. Output stage 190A may continue to
source current to load 195 until the amplifier output voltage 175
reaches supply voltage 120. At this point the efficiency of
supplying power to the load 195 is quite high. For example, if
supply voltage 120 were 3.3V, the amplifier might be supplying
current to load 195 with an efficiency of greater than 95%.
[0027] In a typical class G amplifier, when the output voltage
increases to a level exceeding the capabilities of a lower voltage
supply, the lower supply is disconnected from the amplifier output
while the next higher voltage supply is switched in to furnish
power to the load. This switching of the load between power
supplies may result in glitches or dropouts in the amplifier output
voltage. If the lower voltage supply switches off before the higher
voltage supply comes on, there may be a momentary drop in the
amplifier's output. Conversely if the lower voltage supply is not
completely out of the circuit before the higher voltage supply
kicks in, a voltage spike may occur at the output. Both of these
conditions may result in the generation of electromagnetic noise,
which could be detrimental to the function of circuitry in the
proximity of the amplifier.
[0028] In order to insure smooth transitions of load current
between power supplies and minimize generated noise, the amplifier
of FIG. 1 establishes a transition zone for each pair of output
stages 190. The transition zone may begin when the amplifier output
voltage 175 increases to a level that is some small voltage, delta
V, below the level of the lower supply voltage. In the case of a
transition between output stages 190A and 190B, switch 105 may
close when amplifier output voltage 175 reaches a level that is
equal to supply voltage 120 minus delta V. When switch 105 closes,
the control section of output stage 190B may start contributing
current from power supply 140 through resistor 135 to current sink
100. Switch 150 may be closed, and the current through resistor 135
may cause current-controlled current source 145 to source a
proportional current to load 195. At this point the current
corresponding to the amplifier input voltage 180 is shared between
resistors 115 and 135, and the load current is shared between
current sources 125 and 145.
[0029] As amplifier output 175 increases from supply voltage 120
minus delta V, the proportion of the current through sink 100
shifts from resistor 115 to resistor 135. Likewise, the load
current source may shift from 125 to 145 such that when the
amplifier output reaches the lower supply voltage 120, output stage
190B is supplying the total output current for load 195. At this
point, a very small amount of current may continue to flow through
resistor 115, but it is insignificant relative to the current
through resistor 135. This transitional sharing of the load current
may insure that the amplifier output will be monotonic and linear
with respect to an increasing input signal.
[0030] Once the amplifier output 175 reaches supply voltage 120,
switch 130 may opened to prevent current from flowing through
current source 125 in the reverse direction and damaging power
supply 120. This protective feature will be described in greater
detail below. As the input voltage increases, current sink 100
draws more current through resistor 135, which causes
proportionally more current to be sourced from 145 through load 195
with a proportional increase in amplifier output 175. A second
transition region occurs between output stages 190B and 190C
beginning when the amplifier output increases to supply voltage 140
minus delta V. Note that the value for the voltage difference,
delta V, for the transition between output stages 190B and 190C may
be the same as or different from the voltage difference for the
transition between output stages 190A and 190B. Delta V may be set
by the width to length parameters of transistors included in the
output stage controller 107. Delta V may be set within a range of
values from 0V to 0.8V or potentially greater depending upon the
specific application. In one embodiment, the value chosen for delta
V may be 0.3V. Smaller values for delta V may increase the overall
efficiency of the amplifier by allowing each output stage to solely
contribute current to the amplifier output until the output is
closer to the supply voltage of the output stage.
[0031] As previously described with regard to the transition
between output stages 190A and 190B, the preponderance of current
through sink 100 switches from resistor 135 to resistor 155 and as
a result, the load current sourced from 145 decreases while that
from 165 increases such that when the amplifier output reaches the
level of supply voltage 140, the entire output current is produced
by output stage 190C. Further increases in input voltage 180 result
in increased current through resistor 155 with a corresponding
increase in current sourced from 165 through load 195. This causes
a proportionate rise in amplifier output 175 until the maximum
level of supply voltage 160 minus the amplifier output current
times Rdson of the output transistor is reached.
[0032] Note that the drain-to-source on resistance for output
transistor implementations using current CMOS technology may be
inversely proportional to the width of the channel. Therefore, the
drain-to-source on resistance of the output transistors used to
implement the amplifier may be reduced and amplifier efficiency
increased by increasing the channel width of the transistors.
Increasing the width of the channel for the output transistors may
require additional IC real estate. Therefore, the desired
efficiency of a CMOS implementation of the disclosed amplifier may
be traded off with the amount of IC area required. Typical values
of a fraction of an OHM are readily achievable for Rdson.
[0033] When the output of the amplifier reaches the level of supply
voltage 160 minus approximately 0.15V, the amplifier may again be
supplying the load at a high efficiency. For example, if the supply
voltage 160 were 7 volts, the amplifier might supply the load
current with around 98% efficiency. Although three output stages
are depicted in FIG. 1, the advantages of this design are readily
extensible to amplifiers with any number of supply voltages and
corresponding output stages greater than or equal to two.
[0034] FIG. 1A presents one embodiment of the amplifier with a more
detailed depiction of the output stage control functionality. Input
voltage 180 may be applied to the positive input of differential
amplifier 181. The output voltage 175 may be divided by resistors
182 and 183, and a portion applied to the negative input of
differential amplifier 181. In this configuration, the output of
the differential amplifier may drive NMOS transistor 184 so as to
draw enough current from current mirrors 191 included in output
stages 190 to satisfy the relationship Vout=Vin(1+R2/R1), under
normal operating conditions.
[0035] If Vin starts at zero and increases monotonically, current
mirror 191A may supply the total amplifier output current 175 until
the amplifier output voltage reaches the level of the lowest supply
voltage 120 minus delta V. Differential amplifier 181 may be
driving transistor 184 to attempt to draw current from transistors
108 and 106. The gate of transistor 108 may be biased from supply
voltage 120 such that it is always on and attempting to draw
current from the input side of current mirror 191A. Current mirrors
191 may be configured to contribute current to Vout 175 until the
point that Vout reaches the level of their input supply, as will be
explained in more detail with regard to FIG. 3. Therefore, in the
stated range of Vout, current mirror 191A of output stage 190A may
be capable of contributing current to the amplifier output 175. On
the other hand, the gate of transistor 106 is biased from Vout and
therefore, no significant current may flow through transistor 106
until Vout reaches a level of voltage at the drain of transistor
184, plus the threshold voltage for transistor 106. In some
embodiments, this voltage may be designed to be equal to the lowest
supply voltage 120 minus delta V.
[0036] At this point transistor 111, biased by supply voltage 140
is able to draw current from current monitor 191B, but Vout is not
great enough to turn on transistor 112, therefore, output stages
190A and 190B may contribute current to amplifier output 175. As
Vout rises toward supply voltage 120, the proportion of output
current contributed by current mirror 191B as compared with that
contributed by current mirror 191A may increase rapidly due to
differences in the channel geometries of transistors 108 and 106,
such that when Vout is equal to supply voltage 120 minus delta V,
current mirror 191A may be contributing almost the entire output
current of the amplifier, but by the time Vout reaches supply
voltage 120, current mirror 191B may be supplying nearly all of the
output current. At this point, protective circuitry may isolate
current mirror 191A from the output node of the amplifier as
described below, causing current mirror 191B to provide all of the
output current for the amplifier.
[0037] Current mirror 191B may supply the entire output current of
the amplifier as Vout rises toward supply voltage 140. When Vout
reaches supply voltage 140 minus delta V, a hand off may occur
between current mirrors 191B and 191C similarly to the one
previously described between current mirrors 191A and 191B. This
hand off of the amplifier output current supply may be initiated
when the gate bias voltage of transistor 112 reaches a level that
is the NMOS threshold voltage above the voltage at the drain of
transistor 106. In some embodiments, this point may be designed to
be when Vout reaches a level that is delta V below supply voltage
140. At Vout equal to supply voltage 140, current mirror 191B may
be isolated from the amplifier output and current mirror 191C may
assume the role of sole provider of amplifier output current as
Vout rises to its maximum value.
[0038] As stated previously, the methods described above for
smoothly transferring the load from one output stage to another
dependent on the value of the amplifier output voltage are readily
extensible to embodiments of the disclosed amplifier including any
number of output stages, even though FIG. 1A depicts a particular
embodiment with three output stages.
[0039] FIG. 2 is a flowchart of a method for operating a class G
amplifier with three output stages, according to one embodiment. At
200, if the amplifier output voltage, Vout, is less than the power
supply voltage, V1, associated with the lowest voltage output
stage, then output current may be sourced from the lowest voltage
power supply. For example, if the lowest voltage power supply is
2.5 volts, then the output stage associated with this power supply
may contribute output current until the output voltage reaches 2.5
volts. As shown at decision block 220, if the amplifier output
voltage rises to within a small voltage difference, d, of the first
output stage supply voltage, V1, then the second output stage
associated with power supply V2 may start to contribute current to
the amplifier output, as indicated by block 230.
[0040] During the time that the amplifier output is between V1-d
and V1, both the first and second output stages may contribute
current to the amplifier output. By the time the amplifier output
reaches V1, the second output stage associated with power supply V2
may be supplying the total output current for the amplifier.
Further increases in Vout may be brought about by corresponding
increases in current output to the amplifier load from power supply
V2.
[0041] When the amplifier output reaches a level of V2-d, a similar
transitioning of the load current may take place between the second
and third output stages associated with power supplies V2 and V3
respectively. As shown at blocks 240 and 250, when Vout rises to
V2-d, power supply V3 may begin to contribute current to the output
of the amplifier and this contribution may increase relative to the
contribution of power supply V2 until V3 is supplying all of the
amplifier output current when the amplifier output voltage reaches
V2. When Vout exceeds V2, the third output stage may supply all of
the amplifier output current. Note that this method may be extended
to operate an amplifier with more than three output stages.
[0042] FIG. 3 illustrates a mechanism for protecting the power
supply of an output stage of a class G amplifier from reverse
current flow, according to one embodiment. Voltage-controlled
current sink 305 may draw current proportional to the amplifier
input voltage signal. The current through sink 305, or a portion
thereof, may be drawn from power supply 325 with voltage Vn through
transistor 315. Transistor 320 may source current to the output of
the amplifier 330 in proportion to the current through transistor
315. This current may be combined with the output currents from the
other output stages of the amplifier 335 to form the complete
output current for the amplifier.
[0043] As the current sourced by this stage and the other output
stages 335 of the amplifier increases, the amplifier output voltage
330 may increase. The level of the amplifier output voltage 330 is
monitored with respect to the power supply voltage, Vn, for output
stage, n, by comparator 300. When Vout 330 is less than Vn 325,
comparator 300 may output a high voltage level to the gate of
transistor 310. This high voltage level may keep transistor 310
turned off isolating Vout 330 from the gates of transistors 315 and
320. When Vout 330 is equal to or exceeds Vn 325, comparator 300
may output a low voltage level to the gate of transistor 310. This
low voltage level may turn on transistor 310 and apply Vout to the
gates of transistors 315 and 320. When transistor 310 is on, the
gate of transistor 320 will be at a voltage level greater than or
equal to the voltage level at its source. Since transistor 320 is
an enhancement mode PMOSFET in FIG. 3, this bias condition may
prevent current from flowing into power supply 325 when Vout 330
exceeds Vn 325.
[0044] This mechanism is significant in that it allows the power
supply associated with an output stage to contribute current to the
amplifier output up until the point that the amplifier output
reaches the level of the power supply for that stage. In an output
stage of a typical class G amplifier, a diode is used to prevent
current from flowing from the amplifier output into the power
supply associated with the output stage when the amplifier output
exceeds the level of the supply. This limits the efficiency of the
typical class G output stage because all current output from the
stage must pass through the diode in addition to a gating device
such as a power transistor. The combined voltage drop from the
supply voltage to the amplifier output level is a minimum of about
one volt in the typical case, and the power wasted in the output
stage will be the one volt times the output current. For a typical
low-voltage amplifier where the largest supply voltage is, for
example, three volts, this wasted power may be nearly as great as
the power being supplied to the load. The disclosed mechanism of
FIG. 3 may reduce the power wasted in an output stage by nearly an
order of magnitude.
[0045] FIG. 4 is a flowchart of a method for operating an output
stage of a class G amplifier, according to one embodiment. At block
400, the amplifier output voltage may be monitored with respect to
the power supply voltage, Vn, associated with a particular output
stage. As long as the amplifier output voltage, Vout, remains below
Vn, the output device for that output stage may be capable of
sourcing current to the amplifier load and may provide the load
current for amplifier output voltages in a range below Vn as
described previously and indicated by block 410. When Vout
surpasses Vn, the output stage's power supply may be isolated from
the output of the amplifier so that no current may flow from Vout
into the power supply, as illustrated at block 420. This may
prevent damage to the power supply for output stage, n, by
inhibiting reverse current flow into the supply.
[0046] FIG. 5 illustrates circuitry for limiting the total output
current of an amplifier as a function of the amplifier output
voltage, according to one embodiment. Voltage-controlled current
source 500 may output a current into resistor 520 that is inversely
proportional to the amplifier output voltage, Vout, 550.
Current-controlled current source 510 may also supply current to
resistor 520. Current source 510 may output a current that is a
particular fraction of the sum of the currents being output by all
output stages of the amplifier. Current sources 500 and 510 may be
designed such that the sum of their outputs delivered to resistor
520 is constant or nearly constant over the range of amplifier
output voltage.
[0047] The current through resistor 520 may produce a voltage at
the gate of transistor 530. The current through resistor 520 may be
set such that the voltage produced at the gate of transistor 530 is
insufficient to turn the transistor on under normal operating
conditions. If the current produced by the output stages should
rise disproportionately with respect to the amplifier output
voltage 550, the sum of the currents from sources 500 and 510 might
rise as well. This condition may be caused by a reduction in
amplifier load impedance, an extreme example of which might be a
short circuit of the amplifier output. Under these circumstances,
the current through source 510 may rise more rapidly than the
current from source 500 diminishes resulting in a net increase in
the current through resistor 520. This increase in current may
produce a corresponding increase in the voltage at the gate of
transistor 530.
[0048] As the current through resistor 520 increases, the
gate-to-source voltage of transistor 530 may also increase to the
point of turning the transistor on. When transistor 530 is off,
amplifier input signal 570 is applied to resistor 540 and a current
580 proportional to the input voltage is used to control the
current produced by the output stages of the amplifier as described
previously. When transistor 530 turns on, it shunts a portion of
the input current through resistor 540 to ground. The reduction in
control current 580 may produce a corresponding drop in the current
produced by the amplifier output stages. This mechanism may prevent
damage to the amplifier resulting from excessive output
current.
[0049] FIG. 6 is a flowchart of a method for providing output
current limiting in a class G amplifier, according to one
embodiment. At 600, a current corresponding to a fraction of the
output current from stage n of the amplifier may be summed with
similar fractional currents corresponding to each of the other n-1
output stages of the n-stage class G amplifier. This current is
added to a current that is inversely proportional to the amplifier
output voltage, Vout, to form the current Isum. As depicted in
decision block 610, Isum is compared to an output current limit,
Ilim. If Isum is less than or equal to Ilim, the amplifier output
current is within tolerance and no action is necessary. When Isum
exceeds Ilim, the current-representation of the input waveform Vin,
which is fed to the control section of each output stage may be
reduced, as shown at block 620. Reducing the control current to the
output stages may reduce the amplifier output current and eliminate
an over-current condition.
[0050] An exemplary application for the disclosed amplifier may be
to drive a cooling fan in a personal computer. Efficient
utilization of power may be of high importance to designers of
personal computers and particularly to those designing portable
computers powered from batteries. One well-known method of
conserving power in computers is variable speed fan control. This
method controls the speed of the cooling fan based on the
temperature of the air within the computer, the operational mode of
the processor, and/or other operational parameters such that more
cooling air is forced through the computer when more heat is being
generated by the internal components. If the fan is powered by a
fixed-voltage supply capable of operating the fan at its maximum
speed, then typically when it is desired to reduce the fan speed,
some portion of the supply power is diverted from the fan and
wasted as heat. If the fan spends the majority of its time
operating at speeds less than maximum, then the efficiency of the
fan's power supply my be less than desired.
[0051] The power supplies of personal computers generally provide
several output voltage levels. The processor and associated digital
components operate from low voltage levels that decrease as clock
speeds increase. Peripheral devices such as disk drives,
communications cards, and cooling devices may require power at
higher voltages. The availability of a variety of supply voltages
may allow for the implementation of a class G amplifier to provide
power to a cooling fan. The disclosed class G amplifier may be
implemented as an integrated circuit using CMOS technology to power
a cooling fan using the several voltages available from the
computer power supply. Since each output stage may supply power to
the fan until the amplifier output voltage reaches the level of the
supply associated with that output stage, as described above, the
efficiency of powering the fan may be very high (90% or more)
whenever the fan voltage is just below one of the supply
voltages.
[0052] Although some examples of the disclosed amplifier have
depicted a device with three output stages, it is noted that the
same inventive features may be applied to amplifiers with any
number of supply voltages and corresponding output stages greater
than or equal to two. Numerous variations and modifications will
become apparent to those skilled in the art once the above
disclosure is fully appreciated. It is intended that the following
claims be interpreted to embrace all such variations and
modifications.
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