Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-09-25
2004-10-26
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000
Reexamination Certificate
active
06809588
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a distortion compensation circuit for use in an amplifying device; and, more particularly, to a distortion compensation circuit for reducing an unbalance between a higher 3rd order distortion and a lower 3rd order distortion generated by an amplifier of the amplifying device.
BACKGROUND OF THE INVENTION
In general, a distortion is generated in the course of amplifying a signal in an amplifier. Therefore, in a communications device which amplifies a communications signal by using an amplifier, it is needed to cancel a distortion caused in the course of amplifying a signal, e.g., a W-CDMA (wide-banded code division multiple access) signal or a multi-carrier signal, by using the amplifier.
FIG. 1
illustrates, as an amplifying device using a distortion compensation method of the prior art, an example of an amplifying device which cancels a distortion by using a distortion compensation circuit adopting a feed-forward method.
In the amplifying device shown in
FIG. 1
, an input signal (a main signal) is distributed into two ways at a distributor
41
. A distributed signal is amplified by an amplifier (a main amplifier)
42
to thereby be inputted into a subtractor
44
, and another distributed signal is inputted into the subtractor
44
through a delay line
43
. In the subtractor
44
, a distortion component is extracted by deducting the signal inputted from the delay line
43
from the amplified signal inputted from the main amplifier
42
. The distortion component is inputted therefrom into a distortion amplifier
45
and the amplified signal, which includes the distortion component, inputted from the main amplifier
42
is outputted to another subtractor
47
through another delay line
46
. Further, the distortion component, which is extracted at the subtractor
44
, is amplified at the distortion amplifier
45
to thereby be outputted to the subtractor
47
. The subtractor
47
generates a final amplified signal without a distortion by deducting the amplified distortion component inputted from the distortion amplifier
45
from the amplified signal, which includes the distortion component, inputted from the delay line
46
.
Here, the signal inputted from the delay line
46
to the subtractor
47
is generated by amplifying the input signal in the main amplifier
42
and the signal includes a distortion caused by the main amplifier
42
. Further, the signal inputted from the distortion amplifier
45
to the subtractor
47
is generated by amplifying the distortion. Therefore, the output signal from the subtractor
47
is considered to be a signal generated by canceling the distortion caused by the main amplifier
42
, i.e., deducting the distortion from the amplified signal generated by the main amplifier
42
. Further, each of the distributor
41
and the subtractors
44
and
47
comprises, e.g., a directional coupler.
However, in such an amplifying device as described above, the efficiency in the main amplifier
42
is known to be very poor, since an amplified signal outputted from the main amplifier
42
is attenuated while passing through the subtractor
44
, the delay line
46
or subtractor
47
, which requires an increase in an output level of the main amplifier
42
in accordance with a required output level of the amplifying device.
Meanwhile,
FIG. 2
exhibits an example of an amplifying device having a distortion compensation circuit adopting a pre-distortion method.
The amplifying device of
FIG. 2
has a pre-distortion circuit
51
coupled to an input of a main amplifier
52
. The pre-distortion circuit
51
generates a pre-distortion before a main signal being generated, a phase of the pre-distortion having a difference of 180° with respect to a distortion (i.e., an opposite phase) and a same amplitude as that of the distortion, the distortion being generated by the main amplifier
52
. Therefore, the distortion caused by the main amplifier
52
is canceled by the pre-distortion generated by the pre-distortion circuit
51
.
Such an amplifying device can be implemented to obtain a high efficiency since any other circuits are not coupled to an output of the main amplifier
52
. However, in this case, the distortion generated by the pre-distortion circuit
51
should have same characteristic as that generated by the main amplifier
52
regardless of the variation or frequency characteristics of an input signal.
Here, it is understood by those skilled in the art that the distortion caused by the main amplifier is due to an AM-AM (amplitude modulation-amplitude modulation) conversion or an AM-PM (amplitude modulation-phase modulation) conversion.
FIG. 3A
charts a graph showing an example of the AM-AM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and a gain of the main amplifier, respectively.
FIG. 3A
shows an ideal gain characteristic (G
1
) together with those (G
2
and G
3
) of the main amplifier and the pre-distortion circuit. As shown in
FIG. 3A
, the ideal gain characteristic (G
1
) can be obtained by combining those (G
2
and G
3
) of the main amplifier and the pre-distortion circuit.
Further,
FIG. 3B
exhibits a graph showing an example of the AM-PM conversion performed in the main amplifier. The horizontal and vertical axes of the graph represent an input level and an output phase of the main amplifier, respectively.
FIG. 3B
shows an ideal phase characteristic (P
1
) together with those (P
2
and P
3
) of the main amplifier and the pre-distortion circuit. As shown in
FIG. 3B
, the ideal phase characteristic (P
1
) can be obtained by combining those (P
2
and P
3
) of the main amplifier and the pre-distortion circuit.
Referring to
FIG. 2
, the underlying principle of the pre-distortion now will be described.
In
FIG. 2
, &agr; represents an instantaneous power of a signal inputted to the pre-distortion circuit
51
, &bgr; represents an instantaneous power of a signal outputted from the pre-distortion circuit
51
and inputted to the main amplifier
52
, and &ggr; represents an instantaneous power of a signal outputted from the main amplifier
52
.
An I/O characteristic of the main amplifier
52
can be expressed like as Equation (1) by using &bgr; and &ggr;. Here, A is a vector representing a gain and a phase of a small signal generated by the main amplifier
52
, B is a vector representing a gain and a phase of a 3rd order distortion generated by the main amplifier
52
, and C is a vector representing a gain and a phase of a 5th order distortion generated by the main amplifier
52
. Further, each of A, B, C, a, b, and c, which will be described later, is expressed by a vector, i.e., (gain coefficient, phase coefficient).
&ggr;=
A·&bgr;+B
·&bgr;
3
+C
·&bgr;
5
(1)
Likewise, an I/O characteristic of the pre-distortion circuit
51
can be expressed like as Equation (2) by using &agr; and &bgr;. Here, a is a vector representing a gain and a phase of a small signal generated by the pre-distortion circuit
51
, b is a vector representing a gain and a phase of a 3rd order distortion generated by the pre-distortion circuit
51
, and c is a vector representing a gain and a phase of a 5th order distortion generated by the pre-distortion circuit
51
.
&bgr;=
a·&agr;+b
·&agr;
3
+c
·&agr;
5
(2)
If &bgr; is eliminated by substituting Equation (2) for &bgr; in Equation (1), an equation, i.e., Equation (3), showing a relationship between &agr; and &ggr; can be obtained.
γ
=
A
·
a
·
α
+
(
A
·
b
+
B
·
a
3
)
·
α
3
+
(
A
·
c
+
3
·
B
·
a
2
·
b
+
C
·
a
5
)
·
α
5
+
⋯
Equation
(
3
)
In the amplifying device as shown in
FIG. 2
, a distortion canceling can be accomplished by setting each of coefficients for &agr;
3
and &agr;
5
in Equation (3) to zero, which is expressed by using Equations (4) and (5).
A·b+B·a
3
&equal
Hongo Naoki
Horaguchi Masato
Okubo Yoichi
Suto Masaki
Takada Toshio
Bacon & Thomas PLLC
Choe Henry
Hitachi Kokusai Electric Inc.
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