U.S. patent application number 16/195894 was filed with the patent office on 2019-09-26 for phased array antenna system.
The applicant listed for this patent is Qorvo US, Inc.. Invention is credited to Alexander Wayne Hietala, Nadim Khlat, Michael F. Zybura.
Application Number | 20190296697 16/195894 |
Document ID | / |
Family ID | 65011544 |
Filed Date | 2019-09-26 |
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United States Patent
Application |
20190296697 |
Kind Code |
A1 |
Hietala; Alexander Wayne ;
et al. |
September 26, 2019 |
PHASED ARRAY ANTENNA SYSTEM
Abstract
A phased array antenna system having a plurality of antenna
elements arranged into an array is disclosed. Each of a plurality
of amplifier circuitries has an output terminal coupled to a
corresponding one of the plurality of antenna elements and includes
a power amplifier having a control terminal coupled to an input
terminal. The power amplifier has a first current terminal coupled
to the output terminal and a second current terminal coupled to a
fixed voltage node. Further included in each of the plurality of
amplifier circuitries is a current limiter having a bias terminal
coupled to the control terminal of the power amplifier to adjust a
bias point of the power amplifier to limit current flowing through
the first current terminal and the second current terminal to
within a predetermined current range. Some embodiments also include
a voltage limiter to limit voltage amplitude at the output
terminal.
Inventors: |
Hietala; Alexander Wayne;
(Phoenix, AZ) ; Khlat; Nadim; (Cugnaux, FR)
; Zybura; Michael F.; (Scotts Valley, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Qorvo US, Inc. |
Greensboro |
NC |
US |
|
|
Family ID: |
65011544 |
Appl. No.: |
16/195894 |
Filed: |
November 20, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15935081 |
Mar 26, 2018 |
10187019 |
|
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16195894 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03F 2200/426 20130101;
H04B 7/00 20130101; H01Q 21/065 20130101; H03F 2200/129 20130101;
H01Q 1/245 20130101; H03F 1/52 20130101; H04B 1/04 20130101; H03F
2200/471 20130101; H01Q 3/2605 20130101; H01Q 21/061 20130101; H03F
2200/462 20130101; H01Q 1/243 20130101; H01Q 3/34 20130101; H03F
3/245 20130101; H03F 3/19 20130101; H03F 2200/451 20130101 |
International
Class: |
H03F 1/52 20060101
H03F001/52; H01Q 21/06 20060101 H01Q021/06; H03F 3/24 20060101
H03F003/24; H03F 3/19 20060101 H03F003/19; H04B 1/04 20060101
H04B001/04 |
Claims
1. A phased array antenna system comprising: a plurality of antenna
elements arranged into an array; and a plurality of amplifier
circuitries, each of the plurality of amplifier circuitries
comprising: an input terminal; an output terminal coupled to a
corresponding one of the plurality of antenna elements; a power
amplifier having a control terminal coupled to the input terminal
and a first current terminal coupled to the output terminal and a
second current terminal coupled to a fixed voltage node; and a
voltage limiter coupled between the output terminal and the fixed
voltage node to limit voltage amplitude at the output terminal to
within a predetermined voltage range.
2. The phased array antenna system of claim 1 wherein the voltage
limiter comprises a first branch of series-connected diodes coupled
in anti-parallel with a second branch of series-connected
diodes.
3. The phased array antenna system of claim 2 wherein the voltage
limiter further includes an overvoltage detector configured to
generate a positive overvoltage flag signal at a positive
overvoltage flag terminal and a negative overvoltage flag signal at
a negative overvoltage flag terminal for providing the negative
overvoltage flag signal and the positive overvoltage flag signal to
external circuitry.
4. The phased array antenna system of claim 1 further including a
current limiter having a bias terminal coupled to the control
terminal of the power amplifier to adjust a bias point of the power
amplifier to limit current flowing through the first current
terminal and the second current terminal to within a predetermined
current range.
5. The phased array antenna system of claim 4 wherein the current
limiter further includes an overcurrent detector configured to
generate an overcurrent flag signal at an overcurrent flag terminal
for providing the overcurrent flag signal to external
circuitry.
6. The phased array antenna system of claim 4 wherein the current
limiter comprises: current feedback circuitry having a current
sense input terminal coupled to the first current terminal and a
current sense output terminal, wherein the current feedback
circuitry is configured to generate a current sense signal that is
proportional to current flowing through the output terminal of the
power amplifier and provide the current sense signal at the current
sense output terminal; and bias circuitry having a current feedback
terminal coupled to the current sense output terminal, wherein the
bias circuitry is configured to adjust a bias point of the power
amplifier to limit current flowing through the first current
terminal to within the predetermined current range in response to
the current sense signal.
7. (canceled)
8. (canceled)
9. (canceled)
10. (canceled)
Description
RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 15/935,081, filed Mar. 26, 2018, the disclosure of which
is hereby incorporated herein by reference in its entirety.
[0002] This application is related to U.S. patent application Ser.
No. 14/933,552, filed Nov. 5, 2015, which claims priority to
provisional patent application Ser. No. 62/077,481, filed Nov. 10,
2014, the disclosures of which are hereby incorporated herein by
reference in their entireties.
FIELD OF THE DISCLOSURE
[0003] The present disclosure relates generally to protecting power
amplifiers that drive phased array antenna systems employed in
telecommunications equipment such as smartphones.
BACKGROUND
[0004] Development of fifth-generation (5G) wireless networks with
higher modulation bandwidth and data rates has increased demands on
cellular handset technology. For example, phased array antennas are
envisioned as key components needed to meet these increased
demands. However, a difficult problem has arisen in protecting
power amplifiers that drive phased array antennas. The problem is
at least caused in part by chaotic antenna impedance fluctuations
as a user varies the placement of his body relative to a phased
array antenna included in a cellular handset. The chaotic antenna
impedance fluctuations may in turn cause voltage standing wave
ratios (VSWRs) that are high enough to damage radio frequency power
amplifiers driving the phased array antenna. What is needed is a
phased array antenna system that protects radio frequency power
amplifiers that drive phased array antennas from damage that may
result from high VSWRs.
SUMMARY
[0005] A phased array antenna system having a plurality of antenna
elements arranged into an array is disclosed. Each of a plurality
of amplifier circuitries has an output terminal coupled to a
corresponding one of the plurality of antenna elements and includes
a power amplifier having a control terminal coupled to an input
terminal. The power amplifier has a first current terminal coupled
to the output terminal and a second current terminal coupled to a
fixed voltage node. Further included in each of the plurality of
amplifier circuitries is a current limiter having a bias terminal
coupled to the control terminal of the power amplifier to adjust a
bias point of the power amplifier to limit current flowing through
the first current terminal and the second current terminal to
within a predetermined current range. Some embodiments also include
a voltage limiter to limit voltage amplitude at the output
terminal.
[0006] Those skilled in the art will appreciate the scope of the
present disclosure and realize additional aspects thereof after
reading the following detailed description of the preferred
embodiments in association with the accompanying drawing
figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0007] The accompanying drawing figures incorporated in and forming
a part of this specification illustrate several aspects of the
disclosure and, together with the description, serve to explain the
principles of the disclosure.
[0008] FIG. 1 is a diagram of a related-art phased array antenna
that may be included in a fifth-generation cellular handset.
[0009] FIG. 2 is a diagram of an embodiment of a phased array
antenna system 16 that protects power amplifiers driving the
plurality of antenna elements 14 from damage that may result from
standing wave ratios having high values.
[0010] FIG. 3 is a schematic of a first embodiment of a
single-ended version of each one of the plurality of amplifier
circuitries having a current limiter and a voltage limiter in
accordance with the present disclosure.
[0011] FIG. 4 is a schematic of a second embodiment of a
single-ended version of each one of the plurality of amplifier
circuitries that further includes an overcurrent detector and an
overvoltage detector in accordance with the present disclosure.
[0012] FIG. 5 is a diagram of an exemplary cellular handset having
a controller that is configured to switch between a first phased
array antenna system and a second phased array antenna system based
upon flags generated by the overcurrent detector and/or the
overvoltage detector depicted in FIG. 4.
[0013] FIG. 6 is a schematic of an embodiment of a differential
version of each one of the plurality of amplifier circuitries in
accordance with the present disclosure.
DETAILED DESCRIPTION
[0014] The embodiments set forth below represent the necessary
information to enable those skilled in the art to practice the
embodiments and illustrate the best mode of practicing the
embodiments. Upon reading the following description in light of the
accompanying drawing figures, those skilled in the art will
understand the concepts of the disclosure and will recognize
applications of these concepts not particularly addressed herein.
It should be understood that these concepts and applications fall
within the scope of the disclosure and the accompanying claims.
[0015] It will be understood that, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. For example, a first
element could be termed a second element, and, similarly, a second
element could be termed a first element, without departing from the
scope of the present disclosure. As used herein, the term "and/or"
includes any and all combinations of one or more of the associated
listed items.
[0016] It will be understood that when an element such as a layer,
region, or substrate is referred to as being "on" or extending
"onto" another element, it can be directly on or extend directly
onto the other element or intervening elements may also be present.
In contrast, when an element is referred to as being "directly on"
or extending "directly onto" another element, there are no
intervening elements present. Likewise, it will be understood that
when an element such as a layer, region, or substrate is referred
to as being "over" or extending "over" another element, it can be
directly over or extend directly over the other element or
intervening elements may also be present. In contrast, when an
element is referred to as being "directly over" or extending
"directly over" another element, there are no intervening elements
present. It will also be understood that when an element is
referred to as being "connected" or "coupled" to another element,
it can be directly connected or coupled to the other element or
intervening elements may be present. In contrast, when an element
is referred to as being "directly connected" or "directly coupled"
to another element, there are no intervening elements present.
[0017] Relative terms such as "below" or "above" or "upper" or
"lower" or "horizontal" or "vertical" may be used herein to
describe a relationship of one element, layer, or region to another
element, layer, or region as illustrated in the Figures. It will be
understood that these terms and those discussed above are intended
to encompass different orientations of the device in addition to
the orientation depicted in the Figures.
[0018] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the disclosure. As used herein, the singular forms "a," "an," and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. It will be further understood
that the terms "comprises," "comprising," "includes," and/or
"including" when used herein specify the presence of stated
features, integers, steps, operations, elements, and/or components,
but do not preclude the presence or addition of one or more other
features, integers, steps, operations, elements, components, and/or
groups thereof.
[0019] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
disclosure belongs. It will be further understood that terms used
herein should be interpreted as having a meaning that is consistent
with their meaning in the context of this specification and the
relevant art and will not be interpreted in an idealized or overly
formal sense unless expressly so defined herein.
[0020] FIG. 1 is a diagram of a related-art phased array antenna 10
that may be included in a fifth-generation (5G) cellular handset.
The related-art phased array antenna 10 has a substrate 12 onto
which a plurality of antenna elements 14 are arranged into a
rectangular array. Centers of the antenna elements 14 are spaced
apart by fractions of a wavelength A of a desired emission from the
related-art phased array antenna 10. In this case, the related-art
phased array antenna 10 is a 5G millimeter-wave array. Compared
with a typical 4G antenna system, there is relatively little loss
of power between a power amplifier driving the related-art phased
array antenna 10 and the related-art phased array antenna 10
because there are typically no intervening lossy elements present.
As a result, power amplifiers driving the plurality of antenna
elements 14 are vulnerable to damage from excessive current and
voltage when the plurality of antenna elements 14 is obstructed
such that a poor (i.e., high) standing wave ratio (SWR) is
experienced by the related-art phased array antenna 10. Therefore,
a phased array antenna system that protects power amplifiers
driving the plurality of antenna elements 14 from damage that may
result from high SWRs is needed.
[0021] In this regard, FIG. 2 is a diagram of an embodiment of a
phased array antenna system 16 that protects power amplifiers
driving a plurality of antenna elements 14 (FIG. 1) from damage
that may result from high SWRs. The phased array antenna system 16
has an array of antenna elements 18 and a plurality of amplifier
circuitries 20. Each of the plurality of amplifier circuitries 20
has a power amplifier 22 coupled between an input terminal 24 for
receiving a radio frequency (RF) signal to be amplified and an
output terminal 26 coupled to a corresponding one of the array of
antenna elements 18 for transmitting an amplified version of the RF
signal from the corresponding one of the array of antenna elements
18.
[0022] Further included in each of the plurality of amplifier
circuitries 20 is a current limiter 28 having a bias terminal 30
coupled to the power amplifier 22. The current limiter 28 may be
supplied from a supply voltage node 32 labeled VCC that provides
power to the power amplifier 22. In general, the current limiter 28
is configured to actively lower current flowing through the power
amplifier 22 whenever the current limiter 28 senses that current
flowing through the power amplifier 22 exceeds a predetermined
current range. For example, in at least one embodiment the
predetermined current range is from 5 A to 5.5 A. However, it is to
be understood that the predetermined current range can be different
for different power amplifier technologies.
[0023] Each of the plurality of amplifier circuitries 20 may also
include a voltage limiter 34 coupled between the output terminal 26
and ground. In general, the voltage limiter 34 is configured to
limit voltage across the power amplifier 22 whenever the voltage
across the power amplifier 22 exceeds a predetermined voltage
level.
[0024] FIG. 3 is a schematic of a first embodiment of a
single-ended version of each one of the plurality of amplifier
circuitries 20 detailing versions of the power amplifier 22, the
current limiter 28, and the voltage limiter 34 in accordance with
the present disclosure. In this exemplary embodiment, the power
amplifier 22 is made up of a plurality of bipolar junction
transistors Q1, Q2, Q3, through QN coupled in parallel and
configured to amplify RF signals having wavelengths between 3
centimeters and 15 centimeters. The power amplifier 22 has a
control terminal 38 coupled to the bias terminal 30. The control
terminal 38, in this exemplary embodiment, is also coupled to the
input terminal 24 through a first capacitor C1 and a first inductor
L1 coupled in series with first capacitor C1 thereby forming a
resonant circuit. The power amplifier 22 has a first current
terminal 40 coupled to the output terminal 26 and a second current
terminal 42 coupled to the fixed voltage node 36, which in this
exemplary embodiment is at ground potential. The output terminal 26
is in turn coupled to a corresponding one of the array of antenna
elements 18. In this exemplary embodiment, the first current
terminal 40 is also coupled to the supply voltage node 32 through
an RF choke inductor L2.
[0025] In the exemplary embodiment of FIG. 3, the current limiter
28 includes bias circuitry 44 that provides an operating point bias
for the power amplifier 22. In this example, a first current source
46 is configured to generate a mirror voltage V.sub.MIR at a first
reference node 48 of a double diode stack made up of a first
diode-connected transistor 50 coupled in series with a second
diode-connected transistor 52. During operation, the mirror voltage
V.sub.MIR is produced as a portion of a first reference current
I.sub.REF1 flows through the first diode-connected transistor 50
and the second-diode connected transistor 52. A bias transistor 54
has a first base terminal 56 coupled to the first reference node
48, a first collector terminal 58 coupled to the supply voltage
node 32, and a first emitter terminal 60 coupled to the bias
terminal 30. In operation, a direct current (DC) bias current
I.sub.BIAS that is emitted from the first emitter terminal 60
establishes an operating point for the power amplifier 22.
[0026] The current limiter 28 of the exemplary embodiment of FIG. 3
also includes a sense transistor 62 having a second base terminal
64 coupled to the control terminal 38 of the power amplifier 22. A
second emitter terminal 66 is coupled to the fixed voltage node 36,
which in this case is at ground potential. A second collector
terminal 68 is coupled through a first resistor R1 to the first
current terminal 40 of the power amplifier 22.
[0027] Further included is a first comparator 70 having a feedback
output terminal 72, an inverting input terminal 74, and a
non-inverting input terminal 76. A second resistor R2 couples the
inverting input terminal 74 to the second collector terminal 68 of
the power amplifier 22. A second capacitor C2 couples between the
inverting input terminal 74 and ground. The second resistor R2 and
the second capacitor C2 realize a low-pass filter that during
operation provides a sense voltage V.sub.SEN at the inverting input
terminal 74. During operation, the sense voltage V.sub.SEN is a DC
voltage that is proportional to the RF signal voltage at the first
current terminal 40 with respect to the fixed voltage node 36.
[0028] Also, in the exemplary embodiment, a second current source
78 is coupled between the non-inverting input terminal 76 and
ground. A third resistor R3 couples the non-inverting input
terminal 76 and second current source 78 to the supply voltage node
32. During operation a first reference voltage V.sub.REF1 is
applied to the non-inverting input terminal 76 as a second
reference current I.sub.REF2 flows through the third resistor
R3.
[0029] Further included is a third current source 80 that is
coupled between the first reference node 48 and ground. The third
current source 80 is controlled by a feedback signal output from
the feedback output terminal 72. The feedback signal may be either
current or voltage depending on the type of controlled current
source the third current source 80 is, which may be either a
current-controlled type or a voltage-controlled type. Either type
of controlled current source is employable with the present
disclosure.
[0030] During operation, excessive current flow through the power
amplifier 22 causes collector voltage of the sense transistor 62 to
drop and result in the sense voltage V.sub.SEN dropping below the
first reference voltage V.sub.REF1. In response, the feedback
signal output from the feedback output terminal 72 transitions from
a lower value to a higher value. The higher value of the feedback
signal causes a control current I.sub.CON1 that flows through the
third current source 80 to increase. In turn, the mirror voltage
V.sub.MIR decreases as a smaller portion of the first reference
current I.sub.REF1 flows through the first diode-connected
transistor 50 and the second diode-connected transistor 52. In
response to the lower mirror voltage V.sub.MIR, the bias transistor
54 lowers the bias current I.sub.BIAS, which lowers the operating
point of the power amplifier 22 to limit the current flowing
through the power amplifier 22 to a predetermined safe level. The
value to which the current flowing through the power amplifier 22
is limited is relatively easily set by the resistance value chosen
for the third resistor R3.
[0031] In the exemplary embodiment of FIG. 3, the voltage limiter
34 is made up of a first branch 82 of series-connected diodes
coupled in anti-parallel with a second branch 84 of
series-connected diodes. Each diode provides from 0.5 V to 0.7 V of
voltage drop. In at least one embodiment, a number of diodes
selected for each of the first branch 82 and the second branch 84
is nine diodes. Therefore, during operation, in that particular
embodiment, a maximum voltage across the power amplifier 22 from
the first current terminal 40 to the second current terminal 42 is
limited to from 4.5 V to 6.3 V.
[0032] FIG. 4 is a schematic of a second embodiment of a
single-ended version of each one of the plurality of amplifier
circuitries 20 that further include an overcurrent detector 86 and
an overvoltage detector 88. In the exemplary embodiment of FIG. 4,
the overcurrent detector 86 includes a second comparator 90 having
a second non-inverting input terminal 92 coupled to the feedback
output terminal 72 and a second inverting input terminal 94 that
receives a second reference voltage V.sub.REF2 and an overcurrent
flag terminal 96. The second reference voltage V.sub.REF2 is a
fixed voltage having a voltage level set to a value that causes the
second comparator 90 to output a high current flag signal to
external circuitry if current flowing through the power amplifier
22 becomes excessive during operation.
[0033] In the exemplary embodiment of FIG. 4, the overvoltage
detector 88 includes a fourth resistor R4 coupled between the
voltage limiter 34 and ground. A relatively small sample of RF
voltage appearing at the output terminal 26 appears across the
fourth resistor R4 during operation if excessive voltage is limited
by the voltage limiter 34. A negative rectification circuit 98 and
a positive rectification circuit 100 are coupled from a node
between the fourth resistor R4 and the voltage limiter 34 to
ground. In the exemplary embodiment of FIG. 4, the negative
rectification circuit 98 is made up of a first rectifier diode 102
coupled in series with a third capacitor C3. A high negative
voltage flag signal V.sub.HIGHN is output from negative flag
terminal 104 at a node between the first rectifier diode 102 and
the third capacitor C3. During operation a negative DC voltage
appears across the third capacitor C3 and thus appears at the
negative flag terminal 104 that is available to external circuitry.
Moreover, the positive rectification circuit 100 is made up of a
second rectifier diode 106 coupled in series with a fourth
capacitor C4. A high positive voltage flag signal V.sub.HIGHP is
output from positive flag terminal 108 at a node between the second
rectifier diode 106 and the fourth capacitor C4. During operation a
positive DC voltage appears across the fourth capacitor C4 and thus
appears at the positive flag terminal 108. External circuitry may
respond to any or all of the high current signal flag, the high
negative voltage flag signal V.sub.HIGHN, and the high positive
voltage flag signal V.sub.HIGHP to take further action to protect
the power amplifier 22 and/or take some other action that enhances
performance of a wireless communication device employing one or
more of the phased array antenna systems 16.
[0034] In this regard, FIG. 5 is a diagram of an exemplary
embodiment of a wireless communication device 110 having a
controller 112 and memory 114 that is configured to switch between
a first phased array antenna system 16A and a second phased array
antenna system 16B based upon flags generated by the overcurrent
detector 86 and/or the overvoltage detector 88 depicted in FIG. 4.
The controller 112 may be a digital baseband processor typically
found in cellular handsets. Moreover, the controller 112 may
communicate with the memory 114, the first phased array antenna
system 16A and the second phased array antenna system 16B over a
communication bus 116 such as a radio frequency front-end bus.
[0035] In this example, a person's hand is hovering over the first
phased array antenna system 16A, which causes a poor standing wave
ratio to occur for at least some of the plurality of amplifier
circuitries 20. As a result, each power amplifier 22 (FIG. 3)
included in the first phased array antenna system 16A may
experience excess voltage and excess current. During operation, the
controller 112 polls states for high current signal flag signals,
high negative voltage flag signals V.sub.HIGHN, and high positive
voltage flag signals V.sub.HIGHP for each of the plurality of
amplifier circuitries 20 of each of the first phased array antenna
system 16A and the second phased array antenna system 16B. The
controller 112 stores the states in memory locations of the memory
114 corresponding to the first phased array antenna system 16A and
the second phased array antenna system 16B. Afterwards, the
controller 112 examines the states, which can be logic 0 for no
excess current and/or voltage or logic 1 for excessive current
and/or voltage and determine whether communication transmissions
should be switched from the first phased array antenna system 16A
to the second phased array antenna system 16B. The controller 112
continuously polls, stores, and determines from the states stored
in the memory 114 which of the first phased array antenna system
16A or the second phased array antenna system 16B will have a
better transmit performance and switches to the one determined to
have a better potential transmit performance. For example, the
controller 112 would switch back to the first phased array antenna
system 16A if the person's hand shifted from covering the first
phased array antenna system 16A to covering the second phased array
antenna system 16B.
[0036] FIG. 6 is a schematic of an exemplary embodiment of a
differential version of each one of the plurality of amplifier
circuitries 20 having the current limiter 28 and voltage limiter 34
and a voltage limiter 34' in accordance with the present
disclosure. Like elements depicted in FIG. 6 with element numbers
having a prime symbol are duplicates of elements having the same
element number without a prime symbol. For example, a power
amplifier 22' is a duplicate of the power amplifier 22. In the
exemplary embodiment of the differential version of each one of the
plurality of amplifier circuitries 20, a first RF transformer 118
couples the input terminal 24 to the control terminals 38 and 38'
of the power amplifiers 22 and 22', respectively. A second RF
transformer 120 couples the first current terminals 40 and 40' for
the power amplifiers 22 and 22' to the output terminal 26. A fifth
capacitor C5 couples from a center tap of a primary side of the
second RF transformer 120 to ground to provide filtering for the
second reference current I.sub.REF2.
[0037] During operation, excessive current flow through the power
amplifier 22 and/or the power amplifier 22' causes collector
voltage of the sense transistor 62 and/or a sense transistor 62' to
drop and results in the sense voltage V.sub.SEN dropping below the
first reference voltage V.sub.REF1. In response, the feedback
signal output from the feedback output terminal 72 transitions from
a lower value to a higher value. The higher value of the feedback
signal causes a control current I.sub.CON1 that flows through the
third current source 80 to increase. In turn, the mirror voltage
V.sub.MIR decreases as a smaller portion of the first reference
current I.sub.REF1 flows through the first diode-connected
transistor 50 and the second diode-connected transistor 52. In
response to the lower mirror voltage V.sub.MIR, the bias
transistors 54 and 54' lower the bias current I.sub.BIAS and a bias
current I.sub.BIAS', which lowers the operating point of the power
amplifier 22 and the power amplifier 22' to limit the current
flowing through the power amplifier 22 and the power amplifier 22'
to a predetermined safe level. The value to which the current
flowing through the power amplifier 22 and the power amplifier 22'
is limited is relatively easily set by the resistance value chosen
for the third resistor R3. A voltage limiter 34' operates identical
to the voltage limiter 34. Moreover, the differential version of
each one of the plurality of amplifier circuitries 20 may further
include the overcurrent detector 86 and the overvoltage detector 88
depicted in FIG. 4.
[0038] Those skilled in the art will recognize improvements and
modifications to the preferred embodiments of the present
disclosure. All such improvements and modifications are considered
within the scope of the concepts disclosed herein and the claims
that follow.
* * * * *