U.S. patent application number 09/750908 was filed with the patent office on 2001-07-05 for predistortion linearizer for power amplifier.
This patent application is currently assigned to LG Electronics, Inc.. Invention is credited to Lee, Jae Hyuk.
Application Number | 20010006354 09/750908 |
Document ID | / |
Family ID | 19634288 |
Filed Date | 2001-07-05 |
United States Patent
Application |
20010006354 |
Kind Code |
A1 |
Lee, Jae Hyuk |
July 5, 2001 |
Predistortion linearizer for power amplifier
Abstract
The present invention relates to a predistortion linearizer for
a power amplifier which is capable of improving nonlinear character
of a high power amplifier. The predistortion linearizer for the
power amplifier includes first and second envelope detectors, which
separately detect an envelope of input signals and output signals,
respectively, a DSP, which adjusts a tally of a work function by
comparing the output signals of the first and the second envelope
detector, and a work function generator, which generates a work
function from an envelope of input signals to output gain and phase
control voltage in accordance with work function tally inputted
from the DSP. It also includes a vector modulator, which
pre-distorts the input signal in accordance with the control
voltages inputted from the work function generator to output a
distortion compensation signal to the power amplifier.
Inventors: |
Lee, Jae Hyuk; (Seoul,
KR) |
Correspondence
Address: |
FLESHNER & KIM
P.O. Box 221200
Chantilly
VA
20153-1200
US
|
Assignee: |
LG Electronics, Inc.
|
Family ID: |
19634288 |
Appl. No.: |
09/750908 |
Filed: |
January 2, 2001 |
Current U.S.
Class: |
330/149 |
Current CPC
Class: |
H03F 1/3288
20130101 |
Class at
Publication: |
330/149 |
International
Class: |
H03F 001/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 1999 |
KR |
67170/1999 |
Claims
What is claimed is:
1. A predistortion linearizer for a power amplifier, comprising:
first and second envelope detector circuits configured to detect an
envelope of first and second signals, respectively; a digital
signal processor (DSP) coupled to compare output signals of the
first and second envelope detector circuits to adjust a tally of a
work function; a work function generator coupled to receive the
output signal of the first envelope detector circuit and the work
function tally received from the DSP to generate a work function
and output at least one control voltage; and a vector modulator
configured to distort the first signal in accordance with the at
least one control voltage received from the work function generator
and output a distortion compensation signal to a power
amplifier.
2. The predistortion linearizer of claim 1, wherein the distortion
compensation signal comprises third and fifth elements of
-bx.sup.3-cx.sup.5.
3. The predistortion linearizer of claim 1, wherein the work
function comprises a signal equal to bx.sup.2+cx.sup.4.
4. The predistortion linearizer of claim 1, wherein the first
envelope detector circuit comprises a directional coupler
configured to sample a portion of the first signal, a delay unit
configured to delay the sampled first signal for a prescribed
period of time, an envelope detector configured to detect an
envelope of the delayed first signal, and a filter configured to
pass only a prescribed frequency of the detected envelope as the
output signal.
5. The predistortion linearizer of claim 4, wherein the first
signal is an input signal to be amplified, and the filter comprises
a low-pass filter.
6. The predistortion linearizer or claim 1, wherein the second
envelope detector circuit comprises a directional coupler
configured to sample a portion of the second signal, a variable
attenuator configured to adjust an amplitude of the sampled second
signal, an envelope detector configured to detect an envelope of
the adjusted second signal, and a filter configured to pass only a
prescribed frequency of the detected envelope as the output
signal.
7. The predistortion linearizer of claim 6, wherein the second
signal is an output signal of the power amplifier and the filter
comprises a low-pass filter, and wherein the variable attenuator
adjusts the amplitude of the second signal to match an amplitude of
the first signal.
8. The predistortion linearizer of claim 1, wherein the DSP
comprises a subtracter configured to subtract output signals of the
first and second envelope detector circuits, a conjugator
configured to change a sign of an imaginary component of the output
signal of the first envelope detector, a multiplier coupled to
multiply an output of the subtracter by an output of the
conjugator, an inverter coupled to invert an output signal of the
multiplier, and an integrator coupled to integrate an output of the
inverter.
9. The predistortion linearizer of claim 1, wherein the work
function generator comprises a first squaring circuit configured to
square the output signal of the first envelope detector, a second
squaring circuit, coupled to square an output of the first squaring
circuit, a first control voltage generator configured to generate a
first control voltage based on outputs of the first and second
squaring circuits and the tally received from the DSP, and a second
control voltage generator configured to generate a second control
voltage based on outputs of the first and second squaring circuits
and the tally received from the DSP.
10. The predistortion linearizer of claim 9, wherein the first
control voltage controls a gain distortion and the second control
voltage controls a phase distortion.
11. The predistortion linearizer of claim 9, wherein the first and
second control voltage generator comprise a first multiplier
configured to multiply the output of the first squaring circuit
unit by the tally, a second multiplier configured to multiply the
output of the second squaring unit by the tally, and an adder
configured to add outputs of the first and second multiplier.
12. The predistortion linearizer of claim 1, wherein the first
signal is an input signal to be amplified and the second signal is
an output signal from the power amplifier.
13. A predistortion linearizer for a power amplifier, comprising:
first and second envelope detectors configured to detect an
envelope of an input signal and an output signal, respectively; a
digital signal processor (DSP) coupled to receive an output of the
first and second envelope detectors, adjust a tally of a work
function by comparing the output signals, and generate the work
function using an output of the first envelope detector and the
work function tally to output a gain control voltage and a phase
control voltage; and a vector modulator coupled to receive the gain
and phase control voltages and distort the input signal in
accordance with the gain and phase control voltages to output a
distortion compensation signal to a power amplifier.
14. The predistortion linearizer of claim 13, wherein the DSP
comprises a first squaring circuit coupled to square the output of
the first envelope detector, a second squaring circuit coupled to
square an output of the first squaring circuit, a first control
voltage generator coupled to receive outputs of the first and
second squaring circuits and the work function tally to generate
the gain control voltage, and a second control voltage generator
coupled to receive outputs of the first and second squaring
circuits and the work function tally to generate the phase control
voltage.
15. The predistortion linearizer of claim 14, wherein the
distortion compensation signal comprises a signal having third and
fifth elements of -bx.sup.3-cx.sup.5.
16. The predistortion linearizer of claim 14, wherein the work
function WF is represented by the equation
WF=bx.sup.2+cx.sup.4.
17. The predistortion linearizer of claim 14, wherein the first
envelope detector comprises a directional coupler, which samples
the input signal, a delay circuit, which delays the input signal
for a prescribed period of time, an envelope detection device,
which detects the envelope of the delayed input signal, and a
low-pass filter, which passes a low frequency component of the
output of the envelope detector.
18. The predistortion linearizer of claim 14, wherein the second
envelope detector comprises a directional coupler, which samples
the output signal, a variable attenuator, which adjusts an
amplitude of the sampled output signal to equal an amplitude of the
input signal, an envelope detection device, which detects the
envelope of the adjusted output signal, and an low-pass filter,
which passes a low frequency component of the output of the
envelope detector.
19. The predistortion linearizer of claim 14, wherein the DSP
further comprises a subtracter, which subtracts envelope signals
detected by the first and second envelope detectors, a conjugator,
which changes a sign of an imaginary component of the envelope
signal detected by the first envelope detector, a multiplier, which
multiplies an output of the subtracter by an output of the
conjugator, an inverter, which inverts an output signal of the
multiplier, and an integrator, which integrates an output of the
inverter to provide the work function tally.
20. A predistortion linearizer for a power amplifier, comprising: a
first squaring circuit, which squares a first envelope signal; a
second squaring circuit, which squares an output of the first
squaring circuit; a first control voltage generator, which
generates a first control voltage based on an output of the first
squaring circuit, an output of the second squaring circuit, and a
tally signal; a second control voltage generator, which generates a
second control voltage based on the output of the first squaring
circuit, the output of the second squaring circuit, and the tally
signal; and a vector modulator, which distorts an input signal in
accordance with the first and second control voltages to provide a
distortion signal as an input to a power amplifier.
21. The predistortion linearizer of claim 20, wherein the
distortion compensation signal comprises third and fifth order
elements of -bx.sup.3-cx.sup.5.
22. The predistortion linearizer of claim 21, wherein the first and
second control voltage generators each comprise a first multiplier
to multiply the output of the first squaring circuit by the tally
signal, a second multiplier to multiply the output of the second
squaring circuit by the tally signal, and an adder to add outputs
of the first and second multipliers.
23. The predistortion linearizer of claim 20, further comprising a
digital signal processor (DSP), which adjusts a tally of a work
function by comparing the first envelope signal to a second
envelope signal, and outputs the tally signal.
24. The predistortion linearizer of claim 23, wherein the DSP
comprises a subtracter to subtract the first envelope signal from
the second envelope signal, a conjugator to reverses a sign of an
imaginary part of the first envelope signal, a multiplier to
multiply an output of the subtracter by an output of the
conjugator, an inverter to reverse an output signal of the
multiplier, and an integrator to integrate an output of the
inverter.
25. The predistortion linearizer of claim 24, wherein the first
envelope signal is an envelope of a signal to be amplified by the
power amplifier, and the second envelope signal is an envelope of
an output signal of the power amplifier.
26. The predistortion linearizer of claim 20, further comprising: a
first envelope detector configured to detect an envelope of an
input signal and output the first envelope signal; a second
envelope detector configured to detect an envelope of an output
signal of the power amplifier and output the second envelope
signal; and a digital signal processor, configured to receive the
first and second envelope signals to generate the tally signal.
27. The predistortion linearizer of claim 26, wherein the input
signal is a signal to be amplified by the power amplifier, and the
output signal is an amplified input signal.
28. The predistortion linearizer of claim 20, wherein the first
control voltage controls a gain distortion of the power amplifier,
and the second control voltage controls a phase distortion of the
power amplifier.
29. The predistortion linearizer of claim 20, wherein the tally
signal comprises an integrated inverse signal, wherein the inverse
signal comprises a negative of a product of a difference of the
first envelope signal minus the second envelope signal multiplied
by a conjugated first envelope signal.
30. A method of generating a predistortion signal for a power
amplifier, comprising: detecting an envelope of first and second
signals; comparing a first detected envelope with a second detected
envelope to generate a tally of a work function; generating a work
function using the output of the first envelope detector and the
work function tally; generating first and second control voltages
in accordance with the work function; distorting the first signal
by vector modulating the first signal with the first and second
control voltages.
31. The method of claim 30, wherein the first control voltage
controls a gain distortion of the power amplifier, and wherein the
second control voltage controls a phase distortion of the power
amplifier.
32. The method of claim 30, wherein the first signal is an input
signal to be amplified by the power amplifier, and wherein the
second signal is an amplified input signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a high power amplifier, in
particular to a predistortion linearizer for a high power amplifier
that is capable of improving a nonlinear character of a high power
amplifier.
[0003] 2. Background of the Related Art
[0004] In general, a high power amplifier is used to increase a
power of an input RF signal, and an ideal high power amplifier
increases size of the input RF signal without distorting it.
[0005] Because the high power amplifier comprises active elements
having nonlinear characteristics, however, distortion components
are inevitably included in the output of the high power
amplifier.
[0006] There have been many linearization technologies and
algorithms developed to improve the nonlinear characteristics
issues of high power amplifiers. Among them, a predistortion
method, an envelope feedback method, and a feedforward method are
well-known.
[0007] Recently the predistortion method has been widely used in
mobile communication base stations because its structure is simple,
and its efficiency is superior as compared to the feedforward
method. Additionally, unlike the envelope feedback method, the
predistortion method has no limit on bandwidth.
[0008] The basic principle of the predistortion method is to
improve the linearity of a high power amplifier by distorting an
input signal in advance contrary to nonlinear distortion character
of the high power amplifier, and providing the predistorted signal
to the power amplifier as an input. Thus, the amplifiers distortion
neutralizes the predistortion, leaving an undistorted amplified
signal.
[0009] FIG. 1 illustrates a related art predistortion linearizer
for a high power amplifier.
[0010] As shown in FIG. 1, the related art predistortion linearizer
comprises a first directional coupler 1, which samples some of an
input signal, a phase shifter 2, which changes a phase of the input
signal, and a variable attenuator 3, which changes a gain of the
input signal. The related art device also includes a power
amplifier 4, which amplifies an output of the variable attenuator
3, and a second directional coupler 5, which samples the output of
the power amplifier. Finally, a comparison unit 6 is provided to
control the phase shifter 2 and the variable attenuator 3 by
comparing the output of the first and second directional couplers
1, 5.
[0011] An operation of the related art predistortion linearizer for
the power amplifier will now be described.
[0012] The power amplifier 4 has nonlinear characteristics. In
other words, its gain decreases and its phase is delayed in
accordance with an increase of an input signal. Accordingly the
nonlinear character of the power amplifier 4 can be improved by
changing an input signal, so as to be an inverse of the gain
decrease and phase change of the power amplifier 4. This is done
using the phase shifter 2 and the variable attenuator 3.
[0013] When the phase shifter 2 and the variable attenuator 3
operate normally and the sampling rate is adjusted, signals
outputted from the first and second directional coupler 1, 5 are
equivalent. Herein, the sampling rate is determined so as to make
both signals equivalent, and the sampling is performed in
accordance with the output signal of the power amplifier 4.
[0014] The related art predistortion linearizer has various
problems. For example, most of the related art phase shifters and
variable attenuators are fabricated using FETs or diodes, and it is
very difficult to fashion the nonlinear characteristics of the FET
or diode to be accurately inverse to nonlinear characteristics of
the power amplifier.
[0015] Accordingly, a difference of the signals outputted from the
first and second directional coupler 1, 5, namely, the error value,
is measured by the comparison unit 6, which includes an OP amp.
Thus the phase shifter 2 and the variable attenuator 3 are
controlled in accordance with the measured error value, and the
nonlinear characteristics of the power amplifier 4 are
compensated.
[0016] Using this method, however, the related art predistortion
linearizer of the power amplifier cannot properly compensate when
the nonlinear characteristics of the power amplifier change in
accordance with time or external circumstances. Accordingly, the
linearity of the power amplifier is reduced and remains
uncompensated.
[0017] Specifically, as recited above, the phase shifter and the
variable attenuator of the related art predistortion linearizer are
diodes, and the comparison unit comprises OP-amps. As such, these
analog circuits are greatly influenced by external circumstances
(such as temperature and noise), and their accuracy is thus lower
than a digital circuit.
[0018] In addition, an analog circuit can not accurately transmit a
control voltage, and can not follow a response speed when the
bandwidth of an input signal is wide. Accordingly, the linearity of
the power amplifier lowers.
[0019] The above references are incorporated by reference herein
where appropriate for appropriate teachings of additional or
alternative details, features and/or technical background.
SUMMARY OF THE INVENTION
[0020] An object of the invention is to solve at least the above
problems and/or disadvantages and to provide at least the
advantages described hereinafter.
[0021] An object of the present invention is to provide a
predistortion linearizer for a power amplifier, which substantially
obviates problems due to limitations and disadvantages of the
related art.
[0022] Another object of the present invention is to provide a
predistortion linearizer for a power amplifier, which is capable of
improving nonlinear character of the power amplifier quickly and
accurately.
[0023] Another object of the present invention is to provide a
predistortion linearizer for a power amplifier, which can maintain
the linearity of the power amplifier when the nonlinear
characteristics of the power amplifier change in accordance with
external circumstances.
[0024] To achieve at least the above objects, in whole or in part,
there is provided a predistortion linearizer for a power amplifier
having a first and a second envelope detector which separately
detect an envelope of input signals and output signals, a DSP which
adjusts tally of a work function by comparing output signals of the
first and the second envelope detectors, a work function generator,
which generates a work function from the envelope of input signals
and outputs gain and phase control voltages in accordance with the
work function tally inputted from the DSP, and a vector modulator,
which distorts input signals in accordance with the control voltage
inputted from the work function generator and outputs the
distortion compensation signal to the power amplifier.
[0025] Additional advantages, objects, and features of the
invention will be set forth in part in the description which
follows and in part will become apparent to those having ordinary
skill in the art upon examination of the following or may be
learned from practice of the invention. The objects and advantages
of the invention may be realized and attained as particularly
pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention will be described in detail with reference to
the following drawings in which like reference numerals refer to
like elements wherein:
[0027] FIG. 1 is a circuit diagram of a related art predistortion
linearizer for a power amplifier.
[0028] FIG. 2 is a circuit diagram of a predistortion linearizer
for a power amplifier according to a preferred embodiment of the
present invention.
[0029] FIG. 3 is a circuit diagram of a DSP of FIG. 2.
[0030] FIG. 4 is a circuit diagram of a work function generator of
FIG. 2.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] The preferred embodiment of a predistortion linearizer for a
power amplifier of the present invention will now be described with
reference to accompanying drawings.
[0032] Referring to FIG. 2, the predistortion linearizer preferably
includes a first directional coupler 110, which extracts input
signals, a vector modulator 111, which changes gain and phase of
the extracted input signals, and a high power amplifier 112, which
amplifies the output of the vector modulator 111. The predistortion
linearizer also preferably includes a second directional coupler
113, which extracts output signals, and first and second envelope
detector circuits 114, 115, which separately detect envelopes of
the input and output signals, respectively. Next, a digital signal
processor (DSP) 116 preferably compares the extracted envelope
signals E1, E2, and outputs tally signals C1, C2 of a work
function, and a work function generator 117 generates control
voltages V1, V2 to control the gain and phase of the vector
modulator 111. Thus the gain and phase are controlled according to
the output of the first envelope detector 114 and the tally C1,
C2.
[0033] The first envelope detector circuit 114 preferably includes
a delay unit 10, which delays the input signals for a prescribed
period of time, an envelope detector 11, which detects the envelope
of the delayed input signals, and low-pass filter 12, which detects
low frequency elements of the detected envelope signal to output a
first extracted envelope signal E1.
[0034] The second envelope detector circuit 115 preferably includes
a variable attenuator 13, which adjusts a size (gain) of an output
signal to be the same as the size of an input signal, an envelope
detector 14, which detects the envelope of the adjusted output
signal, and a low-pass filter 15, which detects low frequency
elements of the detected envelope.
[0035] As shown in FIG. 3, the DSP 116 preferably includes a
subtracter 20, which subtracts the envelope signals E1, E2 detected
by the first and the second envelope detector circuits 114, 115,
and a conjugator 114, which changes a code of an imaginary number
part of the first envelope signal El detected by the first envelope
detector circuit 114. A multiplier 22 preferably multiplies the
output of the subtracter 20 by the output of the conjugator 21, and
an inverter 23 reverses an output signal of the multiplier 22.
Finally, an integrator 24 integrates the output of the inverter 23
to provide tally signals C1, C2.
[0036] As shown in FIG. 4, the work function generator 117
preferably includes a first square unit 31, which squares the first
envelope signal E1 outputted from the first envelope detector
circuit 114, a second square unit 32, which squares an output of
the first square unit 31, and first and second control voltage
generators 33, 34. The first control voltage generates the first
control voltage V1 based on the output of the first and the second
square units 31, 32 and the tally signals C1, C2 of the DSP so as
to control a gain distortion. The second control voltage generator
34 generates the second control voltage V2 based on the output of
the first and second square units 31, 32 and the tally signals C1,
C2 of the DSP 116 so as to control a phase distortion.
[0037] The first and second control voltage generators 33, 34 each
preferably has a first multiplier 56, which multiplies an output of
the first square unit 31 by the first tally C1, a second multiplier
57, which multiplies output of the second square unit 32 by the
second tally C2, and an adder 58, which adds output of the first
and the second multiplier 56, 57.
[0038] An operation of the predistortion linearizer for the power
amplifier of the preferred embodiment will now be described with
reference to the accompanying drawings.
[0039] Initially, it should be noted that a distortion signal due
to nonlinearity of the power amplifier is typically generated by
third and fifth elements of the power amplifier. In other words,
when an input signal is y and y=x, the output of the power
amplifier is p=x+bx.sup.3+cx.sup.5, and bx.sup.3+cx.sup.5 generates
the distortion signal.
[0040] Accordingly, if the distortion signal of the third and fifth
elements having an opposite value, namely, -bx.sup.2-cx.sup.5 is
generated using a predistortion circuit and is then inputted to the
power amplifier 112, a compensated output signal p, free of
distortion, is generated by the power amplifier 112.
[0041] The work function generator 117 generates the negative
distortion signal -bx.sup.2-cx.sup.4 of the second and fourth
elements.
[0042] Referring to FIG. 4, when the envelope signal E1 is X, and
X=1+jQ, the output of the first square unit 31 is X.sup.2, and
output of the second square unit 32 is x.sup.4.
[0043] Accordingly, when the output of the first and second square
units passes through the first and second control voltage
generators 33, 34, V1 and V2 are C1x.sup.2+C2x.sup.4. After that,
the vector modulator 111 multiplies input signal y=x by distortion
signal -bx.sup.2-cx.sup.4 generated by the work function generator
117, and the negative distortion signal having third and fifth
elements -bx.sup.3-cx.sup.5 is generated.
[0044] The DSP 116 determines the tally signals b, c of the
distortion signal -bx.sup.2-cx.sup.4. The subtracter 20 of the DSP
116 subtracts the envelope signals E1, E2 and outputs error value.
Thus, when the envelope signal E1 is x, and x=1+jQ, the conjugator
21 conjugates the envelope signal E1=x=1+jQ, its result is 1-jQ,
and the multiplier 22 multiplies the error value by 1-jQ.
[0045] The output of the multiplier 22 is then reversed by the
inverter 23 (-1 +jQ), and the integrator 24 outputs the tally
signals C1, C2 by integrating the output of the inverter 23.
[0046] Thus, non-linear characteristics of the power amplifier 112
are measured accurately, and distortion elements of third and fifth
are detected. When the distortion elements are detected, tally
signals b, c of second and fourth elements of the work function
generator 117 are determined using the DSP 116.
[0047] Accordingly, the vector modulator 111 controls gain
distortion and phase distortion in accordance with control voltage
V1, V2 of the work function generator 117, and generates a
pre-distortion signal.
[0048] When nonlinear characteristics of the power amplifier change
in accordance with time and external circumstances (for example,
temperature, noise, or others), predistortion characteristics of
the input signal have to be changed to ensure linearity of the
power amplifier 112.
[0049] When nonlinear character of the power amplifier 112 changes
in accordance with time and external circumstances, the DSP 116
compares the envelope signals E1, E2 of the input and output signal
detected on the first and the second envelope detector circuits
114, 115, and adjusts tally values b, c of the second and fourth
elements, respectively, using the work function generator 117.
[0050] The predistortion linearizer as broadly described herein has
many advantages. For example, although a characteristic of the
power amplifier changes in accordance with temperature, noise, or
input voltage, the linearity of the power amplifier is
maintained.
[0051] Additionally, the adjustment operation is performed more
quickly and accurately using the DSP.
[0052] Additionally, the predistortion linearizer of the power
amplifier of the preferred embodiment generates a distortion signal
that is the inverse of a distortion element of an output signal
using the work function generator, and the DSP adjusts a tally of
the work function by comparing the envelope of the input and output
signals.
[0053] Accordingly, the predistortion linearizer of the power
amplifier of the preferred embodiment is capable of improving
nonlinearity of the power amplifier, and in particular, compensates
for external circumstances and performs more accurately by
implementing the DSP.
[0054] The predistortion linearizer of the power amplifier of the
present invention is thus capable of maintaining linearity of the
power amplifier when nonlinear characteristics of the power
amplifier change in accordance with time or external circumstances,
such as temperature or noise.
[0055] The foregoing embodiments and advantages are merely
exemplary and are not to be construed as limiting the present
invention. The present teaching can be readily applied to other
types of apparatuses. The description of the present invention is
intended to be illustrative, and not to limit the scope of the
claims. Many alternatives, modifications, and variations will be
apparent to those skilled in the art. In the claims,
means-plus-function clauses are intended to cover the structures
described herein as performing the recited function and not only
structural equivalents but also equivalent structures.
* * * * *