Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
2001-01-17
2002-04-09
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S109000
Reexamination Certificate
active
06369655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a feedback circuit provided in a transistor and an amplifier and a mixer comprising the same.
2. Description of the Related Art
In recent years, radio waves having a great many frequencies have been required in order to establish communication with the rapid development of mobile communication. The frequencies of the radio waves used in the mobile communication have been shifted to a microwave band. A handy terminal for such mobile communication is provided with an amplifier using a field effect transistor (hereinafter referred to as “FET”). There is a method of providing a feedback circuit in one of the designs of the amplifier.
FIG. 26
is a circuit diagram of an amplifier comprising a conventional feedback circuit.
As shown in
FIG. 26
, a feedback circuit
200
comprising a resistor
101
and a capacitor
102
is connected between a drain electrode (an output terminal) and a gate electrode (an input terminal) of an FET
100
. A signal outputted from the drain electrode of the FET
100
is fed back to the gate electrode in reversed phase by the feedback circuit
200
. Consequently, the oscillation of the FET
100
is prevented by a negative feedback effect, and the gain of the FET
100
is adjusted.
On the other hand, in a multi-channel communication system, a wide band amplifier is used to simultaneously amplify a plurality of carrier waves. Generally, when the linearity of the amplifier is degraded, several harmonics such as a second harmonic (second order harmonic) and a third harmonic (third order harmonic) for a fundamental wave are generated. It has been known that the harmonics cause various disturbance waves such as intermodulation distortions.
In the intermodulation distortions, a second order distortion (IM
2
) is generated mainly as a result of the second harmonic, and third order distortion (IM
3
) is generated mainly as a result of the third harmonic. Further, in the multi-channel communication system, composite second order distortions (CSO) are generated by the second order distortion on a plurality of channels, and composite triple beat (CTB) and cross modulation (X-MOD) are generated by the third order distortion on the plurality of channels.
When a fundamental wave s
1
having a frequency f
1
and a fundamental wave s
2
having a frequency f
2
are inputted to the amplifier, as shown in
FIG. 27
, for example, disturbance waves im
2
respectively having a frequency (f
1
+f
2
) and a frequency (f
2
−f
1
) are generated by the composite second order distortions. Further, disturbance waves im
3
respectively having a frequency (2f
2
−f
1
) and a frequency (2f
1
−f
2
) are generated by the composite third order distortions.
In the case of a cable television system, 100 or more channels exist at spacing of 6 MHz in a band from approximately 100 to 800 MHz. When a fundamental wave having a frequency of 750 MHz (f
1
) on a certain channel and a fundamental wave having a frequency of 756 MHz (f
2
) on the other channel, for example, are inputted, disturbance waves are respectively generated at a frequency of 744 MHz (2f
1
−f
2
) and a frequency of 762 MHz (2f
2
−f
1
) in the band due to the composite third order distortions. The respective frequencies of the disturbance waves are the same as the frequency of the adjacent channel. Accordingly, the fundamental wave on the adjacent channel is disturbed by the disturbance waves.
When the fundamental wave having a frequency of 750 MHz (f
1
) on a certain channel and the fundamental wave having a frequency of 450 MHz (f
2
) on the other channel are inputted, a disturbance wave is generated at a frequency of 300 MHz (f
2
−f
1
) in the band as a result of the composite second order distortions. The frequency of the disturbance wave is the same as the frequency of the other channel. Accordingly, the fundamental wave of the other channel is disturbed.
Consequently, it is required that a wide band amplifier for simultaneously amplifying a plurality of carrier waves has high linearity in order to prevent multi-channel intermodulation distortions. For example, JP-A-10-290125 discloses a power amplifier having a nonlinear distortion compensating circuit of a feedforward type.
In the power amplifier, in the entire band, an inputted signal is distributed into signals. One of the signals obtained by the distribution is separated into bands requiring nonlinear distortion compensation. A distortion component and the amount of compensation in each of the bands are detected, to synthesize compensating signals. The compensating signal is used, to compensate for the other signal obtained by the distribution.
In the conventional amplifier shown in
FIG. 26
, the feedback circuit
200
is constituted by the resistor
101
and the capacitor
102
. Even when the resistance value of the resistor
101
and the capacitance value of the capacitor
102
are set such that a desired frequency is negatively fed back, therefore, not only the desired frequency but also all frequencies are negatively fed back. Particularly, the feedback circuit
200
has the capacitor
102
. When the frequency is increased, therefore, the impedance is decreased. Accordingly, the higher the order of a harmonic is than that of the second harmonic, the larger the amount of feedback thereof is. That is, it is impossible to perform setting such that a particular frequency is not fed back.
If the setting is performed such that a second harmonic having a frequency which is two times the frequency of the fundamental wave or a harmonic of a higher order than the second harmonic is fed back, the fundamental wave is fed back to some extent. As a result, the characteristics of the amplifier and particularly, the gain thereof is degraded. A matching circuit for impedance matching with a load circuit connected to the amplifier is difficult to design.
On the other hand, in the power amplifier disclosed in JP-A-10-290125, several circuits such as a circuit for distributing a signal in an input portion, a circuit for separating each of signals obtained by the distribution into bands, a circuit for detecting a distortion component and the amount of compensation in each of the bands, and a circuit for synthesizing compensating signals must be provided. Accordingly, the circuit scale is significantly increased, and the power consumption in the circuits is also increased.
It is necessary to determine a pass-band of a band-pass filter in conformity with the bandwidth of a channel in a communication system using the power amplifier. Consequently, the power amplifier designed for a certain communication system cannot be applied to another communication system which differs in the channel bandwidth.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
Another object of the present invention is to provide an amplifier comprising a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
Still another object of the present invention is to provide a mixer comprising a feedback circuit capable of reducing nonlinear distortion generated in a transistor on a small circuit scale.
A feedback circuit connected between an output terminal and an input terminal of a transistor according to one aspect of the present invention comprises at least one series connection circuit comprising a series connection of a capacitance and an inductance, the value of the capacitance and the value of the inductance of the at least one series connection circuit being set such that the at least one series connection circuit enters a substantially opened state with respect to a first frequency, and the at least one series connection circuit enters a substantially short-circuited state with respect to a second frequency.
In the feedback circuit, the series connection circuit including the series connection of the ca
Nishida Masao
Sawai Tetsuro
Armstrong Westerman & Hattori, LLP
Choe Henry
Pascal Robert
Sanyo Electric Co,. Ltd.
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