Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-04-25
2004-05-25
Mottola, Steven J. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S132000, C330S136000
Reexamination Certificate
active
06741128
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a distortion canceling circuit for use in an amplifier; and, more particularly, to a distortion canceling circuit for reducing an unbalance between an upper 3rd-order distortion and a lower 3rd-order distortion generated by the amplifier.
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, there 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. 11
illustrates, as an amplifying device using a distortion canceling method of the prior art, an example of an amplifying device which cancels a distortion by using a distortion canceling circuit adopting a feed-forward method.
In the amplifying device shown in
FIG. 11
, an input signal (a main signal) is distributed in two ways by a distributor
81
. A distributed signal is amplified by an amplifier (a main amplifier)
82
to thereby be inputted into a subtractor
84
, and another distributed signal is inputted into the substractor
84
through a delay line
83
. In the subtractor
84
, a distortion component is extracted by deducting the signal inputted from the delay line
83
from the amplified signal inputted from the main amplifier
82
. The distortion component is inputted therefrom into a distortion amplifier
85
and the amplified signal, which includes the distortion component, inputted from the main amplifier
82
is outputted to another subtractor
87
through another delay line
86
. Further, the distortion component, which is extracted by the subtractor
84
, is amplified by the distortion amplifier
85
to thereby be outputted to the subtractor
87
. The subtractor
87
generates a final amplified signal without a distortion by deducting the amplified distortion component inputted from the distortion amplifier
85
from the amplified signal, which includes the distortion component, inputted from the delay line
86
.
Here, the signal inputted from the delay line
86
to the subtractor
87
is generated by amplifying the input signal in the main amplifier
82
and the signal includes a distortion caused by the main amplifier
82
. Further, the signal inputted from the distortion amplifier
85
to the subtractor
87
is generated by amplifying the distortion. Therefore, the output signal from the subtractor
87
is considered to be a signal generated by canceling the distortion caused by the main amplifier
82
, i.e., deducting the distortion from the amplified signal generated by the main amplifier
82
. Further, each of the distributor
81
and the subtractors
84
and
87
comprises, e.g., a directional coupler.
However, in such an amplifying device as described above, there is a problem of poor efficiency in the main amplifier
82
. This is caused by a fact that an amplified signal outputted from the main amplifier
82
is attenuated while passing through the subtractor
84
, the delay line
86
or subtractor
87
, which requires an increase of an output level of the main amplifier
82
in accordance with a required output level of the amplifying device.
Meanwhile,
FIG. 12
exhibits an example of an amplifying device having a distortion canceling circuit adopting a pre-distortion method.
The amplifying device of
FIG. 12
has a pre-distortion circuit
91
coupled to an input of a main amplifier
92
. The pre-distortion circuit
91
generates a distortion before a main signal being generated, a phase of the 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
92
. Therefore, the distortion caused by the main amplifier
92
is canceled by the distortion generated by the pre-distortion circuit
91
.
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
92
. However, in this case, the distortion generated by the pre-distortion circuit
91
should have same characteristic as that generated by the main amplifier
92
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. 13A
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. 13A
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. 13A
, 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. 13B
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. 13B
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. 13B
, the ideal gain characteristic (P
1
) can be obtained by combining those (P
2
and P
3
) of the main amplifier and the pre-distortion circuit.
However, as shown in
FIGS. 13A and 13B
, characteristics of the AM—AM and the AM-PM conversion are so complicated that characteristic of the pre-distortion circuit must have a complicated function to implement an amplifying device having the above described ideal characteristics. Therefore, it is so difficult to calculate coefficients of the characteristic functions by using an analog approach.
Accordingly, as an alternative amplifying device having a distortion canceling circuit adopting the pre-distortion method, there has been proposed an amplifying device as shown in FIG.
14
.
In the amplifying device of
FIG. 14
, an input signal, e.g., an RF (radio frequency) signal, is branched by a branch circuit
101
. A branch signal is outputted from the branch circuit
101
to an amplitude/phase circuit
107
through a delay circuit
102
. Another branch signal is outputted from the branch circuit
101
to an amplitude detector (envelope detector)
103
.
The amplitude detector
103
detects an amplitude level (envelope level) of the inputted branch signal. And then, the detected amplitude level is converted into a digital signal by an A/D (analog to digital) converter
104
. The digital signal is inputted to a table
105
.
The table
105
stores data for the correction of an amplitude and a phase of a signal together with a corresponding amplitude level of the signal. Therefore, when the digitized amplitude level outputted from the A/D converter
104
is inputted to the table
105
, corresponding data for amplitude and phase correction are read from the table
105
to thereby be outputted to a D/A (digital to analog) converter
106
. The D/A converter
106
converts the data for amplitude and phase correction into an analog signal, the analog signal being inputted to the amplitude/phase circuit
107
.
The branch signal, which is delivered from the branch circuit
101
to the amplitude/phase circuit
107
through the delay circuit
102
, is synchronized with the data for amplitude and phase correction from the D/A converter
106
.
Accordingly, in the amplitude/phase circuit
107
, the delayed branch signal inputted from the delay circuit
102
is distorted in its amplitude and phase by using the data for amplitude and phase correction from the D/A converter
106
. The amplitude and phase distortion imposed on the delayed branch signal by the am
Funada Kiyoshi
Hongo Naoki
Horaguchi Masato
Okubo Yoichi
Suto Masaki
Bacon & Thomas PLLC
Hitachi Kokusai Electric Inc.
Mottola Steven J.
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