Distortion-compensated amplifying circuit

Details Distortion-compensated amplifying circuit Distortion-compensated amplifying circuit

2002-12-20

2004-04-13

Shingleton, Michael B (Department: 2817)

Amplifiers

Hum or noise or distortion bucking introduced into signal...

C330S151000

Reexamination Certificate

active

06720829

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to distortion-compensated amplifying circuits. More specifically, the present invention relates to a distortion-compensated amplifying circuit that is used typically in a mobile communications base station for cellular phones and is capable of amplifying a signal and compensating for distortion that occurs at the time of amplification.
2. Description of the Background Art
In recent years, a transmission apparatus at a base station for mobile communications devices collectively amplifies a large number of signal channels, requiring a highly-efficient, more linear power amplifier. To increase the linearity of such a power amplifier, an amplifying circuit capable of compensating for distortion has to be adopted.
FIG. 17
shows one exemplary configuration of a conventional distortion-compensated amplifying circuit. In
FIG. 17
, the conventional distortion-compensated amplifying circuit includes an input terminal
601
, an output terminal
602
, a power distributor
603
, a delay circuit
604
, a distortion generating circuit
605
, a variable attenuator
606
, a phase changer
607
, a power combiner
608
, a power amplifier
609
, a directional coupler
610
, and a controller
611
.
In the above-structured distortion-compensated amplifying circuit, a carrier signal supplied from the input terminal
601
is distributed into two by the power distributor
603
. Based on one of these two carrier signals obtained by distribution, the distortion generating circuit
605
generates a distortion signal. This distortion signal is adjusted in amplitude and phase by the variable attenuator
606
and the phase changer
607
, respectively, and is then given to the power combiner
608
. The other one of the two carrier signals obtained by distribution is delayed by the delay circuit
604
, and is then forwarded to the power combiner
608
. The power combiner
608
combines the distortion signal and the carrier signal with each other for input to the power amplifier
609
. The power amplifier
609
amplifies the received signal for output to the output terminal
602
.
Provided between the power amplifier
609
and the output terminal
602
is the directional coupler
610
, where part of the signal supplied by the power amplifier
609
is branched to the controller
611
. The controller
611
controls the variable attenuator
606
and the phase changer
607
so that the distortion signal supplied to the power combiner
608
becomes equal in amplitude and opposite in phase to intermodulation distortion (hereinafter simply referred to as “distortion”) that occurs when the power amplifier
609
amplifies the carrier signal.
As described above, in the distortion-compensated amplifying circuit of
FIG. 17
, a distortion signal is generated by the power amplifier
609
so as to be equal in amplitude and opposite in phase to distortion that may occur when the power amplifier
609
amplifies the carrier signal. The distortion signal is then added in advance to the carrier signal to be supplied to the power amplifier
609
. This means that a distortion component equal in amplitude and opposite in phase to possible distortion is supplied to an input side of the amplifier. With this, distortion that occurs in the power amplifier
609
is reduced. Such a distortion compensation scheme is called a pre-distortion technique, which is disclosed in Japanese Patent Laid-Open Publication No. 2000-261252, for example.
However, the conventional distortion-compensated amplifying circuit adopting the pre-distortion technique as disclosed in
FIG. 17
has the following problems. The distortion signal to be supplied from the distortion generating circuit
605
via the variable attenuator
606
and the phase changer
607
to the power amplifier
609
includes a carrier component as well as a distortion component. This carrier component is also adjusted in amplitude and phase in the variable attenuator
606
and the phase changer
607
. Therefore, the power combiner
608
will receive a carrier component opposite in phase to the carrier signal received via the delay circuit
604
. Therefore, part of the carrier signal is cancelled by the carrier component opposite in phase thereto, thereby causing attenuation in carrier level. Such attenuation in carrier level requires an additional amplifier in order to compensate for lost power to satisfy a desired carrier level.
Moreover, in the circuit as illustrated in
FIG. 17
, amplification of the received distortion component and suppression of the distortion are simultaneously carried out at the power amplifier
609
. Therefore, in practice, it is very difficult to obtain a sufficiently large amount of distortion.
SUMMARY OF THE INVENTION
The present invention is made in order to solve the above problems. An object of the present invention is to provide a distortion-compensated amplifying circuit capable of suppressing a larger amount of distortion, compared with conventional distortion-compensated amplifying circuits, without attenuation in level of a carrier component.
The present invention has the following features to attain the object mentioned above.
A first aspect of the present invention is directed to a distortion-compensated amplifying circuit for amplifying a signal and compensating for distortion that occurs at a time of amplifying the signal, including:
two amplifying sections placed in parallel each for performing a signal amplifying process;
a first combining and distributing section supplied with an original signal including a carrier component and a first distortion signal having a frequency equal to a frequency of distortion that occurs when the original signal is amplified by each of the two amplifying sections, the first combining and distribution section for combining the original signal and the first distortion signal together and then distributing the combined signal into two signals for output to the two amplifying sections;
a second combining and distributing section for combining the two signals supplied from the two amplifying sections and then distributing the combined signal into two signals; and
a combining section for combining the two signals supplied from the second combining and distributing section for output, wherein
each of the first and second combining and distributing sections is a hybrid circuit that outputs two signals with a predetermined phase difference.
In the above first aspect, two amplifying sections are placed between first and second hybrid circuits to form balanced circuitry. When the first hybrid circuit is supplied with the original signal and the distortion signal, the second hybrid circuit outputs two signals: a signal including only a distortion component delayed in phase with respect to the received distortion signal, and a signal including a carrier component delayed in phase with respect to the received original signal and a distortion component advanced in phase with respect to the received distortion signal. When these output signals are combined, at least part of the distortion components included therein are canceled with each other because these components are out of phase. Consequently, distortion included in the combined signal is suppressed (this suppression is an effect of the post-distortion technique). In this case, the carrier component is not cancelled because it is included in only one of the two output signals. Therefore, attenuation in carrier level can be prevented.
This distortion suppression effect achieved by the post-distortion technique becomes larger, as the distortion components included in the two signals produced from the second hybrid circuit are closer to being opposite in phase (having a phase difference of 180 degrees) and to being equal in amplitude to each other. When these distortion components become opposite in phase and equal in amplitude to each other, a maximum suppression effect can be obtained. Therefore, as described below in a fourth aspect, when 90-degree hybrid circuits are used, the distortion

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