Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-11-26
2004-03-30
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000, C375S297000
Reexamination Certificate
active
06714073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a distortion compensation amplification apparatus of a predistortion type.
BACKGROUND OF THE INVENTION
A communications apparatus using W(Wideband)-CDMA(Code Division Multiple Access) signal and multi-carrier signal has a problem of distortion generation occurring when amplifing a signal. In order to solve the problem, a distortion compensation amplification apparatus is generally employed.
Referring to
FIG. 5
, there is illustrated a conventional feed forward type distortion compensation amplification apparatus. As shown in
FIG. 5
, the amplification apparatus includes a distributor
51
, a primary amplifier
52
, a delay line
53
, a subtractor
54
, a distortion amplifier
55
, a delay line
56
, and a subtractor
57
. The distributor
51
and the subtractors
54
and
57
can be implemented by, e.g., a directional coupler.
First, an input signal is distributed by the distributor
51
. One output of the distributor
51
is amplified by the primary amplifier
52
and then provided to the subtractor
54
. The output of the primary amplifier
52
includes a distortion component that is generated at the time the primary amplifier
52
amplifies the input signal. The other output of the distributor
51
is fed to the subtractor
54
via the delay line
53
. The subtractor
54
obtains the distortion component by subtracting the output of the delay line
53
from the output of the primary amplifier
52
. The obtained distortion component is then fed to the distortion amplifier
55
; and simultaneously, the output of the primary amplifier
52
is provided to the delay line
56
as an amplified signal. The distortion component is amplified by the distortion amplifier
55
and then fed to the subtractor
57
. The amplified signal is delayed by the delay line
56
, and subsequently fed to the subtractor
57
. The subractor
57
removes the distortion component of the amplified signal by subtracting the output of the distortion amplifier
55
from the output of the delay line
56
, after which the output of the subtractor
57
is provided to, e.g., a transmitter (not shown) for the transmission thereof.
The drawback of the above-mentioned distortion compensation amplification apparatus employing the feed forward method is a loss in the amplified signal outputted from the primary amplifier
52
. Such loss occurs when the amplified signal passes through the subtractor
54
, the delay line
56
, and the subtractor
57
. Thus generated loss necessitates the output level of the primary amplifier
52
to be controlled in order to meet the output higher than actually required by the amplification apparatus, resulting in the low efficiency of the primary amplifier
52
.
The above-mentioned problem of the feed forward type distortion compensation amplification apparatus can be solved by using a predistortion type distortion compensation amplification apparatus. The conventional amplification apparatus for compensating distortion using the predistortion method is explained briefly with reference to
FIG. 6
hereinbelow. As shown in
FIG. 6
, the amplification apparatus for compensating distortion includes a predistortion(PD) circuit
61
and a primary amplifier
62
.
At the PD circuit
61
, a predistortion component is added to an input signal to be provided to the primary amplifier
62
. The amplitude of the predistortion component is the same as that of a distortion component to be generated by the primary amplifier
62
but the phases thereof are different by 180°. Thus, the distortion generated at the primary amplifier
62
is cancelled out by the predistortion. Consequently, an amplified signal with no distortion is generated as an output of the primary amplifier
62
. As described, since the distortion-removed amplified signal is obtained without employing any additional circuits at the output side of the primary amplifier
62
, the loss in the amplified signal can be prevented and thus the amplification efficiency can be improved.
In the predistortion type amplification apparatus, however, the predistortion provided by the PD circuit
61
should be adaptively adjusted to the distortion (generated by the primary amplifier
62
) varying according to the frequency characteristic thereof and the variation of the input signal.
Distortion in an output of an amplifier will now be explained. The distortion in an output of an amplifier is originated from a gain characteristic and a phase characteristic of output of the amplifier.
FIG. 7A
depicts a plot of the gain characteristic versus an input level of a conventional amplifier. Herein, the x-axis represents the input level of the amplifier and the y-axis represents the gain of the amplifier. In
FIG. 7A
, G
1
represents an ideal gain characteristic of amplifier and G
2
represents an actual gain characteristic of the amplifier. That is, a predistortion circuit should be adjusted such that a combined result of the gain characteristic of the predistortion circuit and the gain characteristic G
2
of the amplifier yields the ideal characteristic G
1
.
FIG. 7B
depicts a plot of a phase characteristic versus an input level of the conventional amplifier. Herein, the x-axis represents the input level and the y-axis represents the phase of an output of the amplifier. In
FIG. 7B
, P
1
represents the ideal phase characteristic of an ideal amplifier and P
2
represents the actual phase characteristic of the amplifier. That is, the predistortion circuit should be adjusted such that a combining result of the phase characteristic of the predistortion circuit and the phase characteristic P
2
of the amplifier yield the ideal characteristic P
1
.
The nature of the gain characteristic and the phase characteristic shown in FIG.
7
A and
FIG. 7B
is very complex. Therefore, the characteristic of a predistortion circuit which gives rise to the ideal characteristic of an amplifier to realize a distortion compensation amplification apparatus outputting distortion-free amplified signals, can only be represented by a very complex function. Thus, it is virtually impossible to realize the ideal predistortion circuit by determining the coefficients of a characteristic curve by calculation in an analog manner.
FIG. 8
illustrates a block diagram of an exemplary predistortion type distortion compensation amplification apparatus. As shown in
FIG. 8
, the amplification apparatus includes a distributor
1
, a delay circuit
2
, a compensation circuit
200
, a predistortion circuit
3
, a primary amplifier
4
, a distributor
5
, a distortion detector circuit
10
, and a table update circuit
12
. Also, the compensation circuit
200
includes a power detector
6
, an A/D converter
7
, an amplitude compensation table
8
a,
a D/A converter
9
a,
an LPF(Low Pass Filter)
10
a,
a phase compensation table
8
b,
a D/A converter
9
b
and an LPF
10
b.
The predistortion circuit
3
includes an attenuator
31
and a phase shifter
32
.
An input signal is distributed by a distributor
1
. One output of the distributor
1
is fed to the delay line
2
, and the other output of the distributor
1
is provided to the compensation circuit
200
.
In the compensation circuit
200
, the power detector
6
detects instantaneous amplitude levels by executing an envelope detection of the output of the distributor
1
. The detected instantaneous amplitude levels are converted from an analog signal form into a digital signal form by the A/D converter
7
, and then are fed to the an amplitude compensation table
8
a
and the phase compensation table
8
b.
The amplitude compensation table
8
a
stores in a memory amplitude compensation data for each of various amplitude levels, the amplitude compensation data being used in compensating an amplitude of an input signal having a corresponding amplitude level. The amplitude compensation data corresponding to detected instantaneous amplitude level is read off the memory and sent to the D/A converter
9
a.
The amplitude compensation data is converted from a digital signal to an analog
Hongo Naoki
Okubo Yoichi
Suto Masaki
Takada Toshio
Choe Henry
Hitachi Kokusai Electric Inc.
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