Abstract:
A linearizer includes: an input terminal; an output terminal; a connection point connected between the input terminal and the output terminal; a diode connected to the connection point; a voltage terminal; and a resistor connected between the voltage terminal and the connection point, wherein 0 V is applied to the voltage terminal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a diode linearizer that improves distortion characteristics of a high-frequency amplifier. 
         [0003]    2. Background Art 
         [0004]    In recent years, research and development into transistors using a nitride semiconductor (e.g., GaN) are being actively carried out, and an application example thereof is a high output amplifier for communication. Amplifiers using GaN are enabled to have higher output than conventional compound semiconductors (e.g., GaAs). However, there is a problem that particularly an AMAM characteristic deteriorates due to GaN-specific soft compression in which a gain slowly decreases from low input power. 
         [0005]    There are linearizers using a diode as analog predistortion that compensates for the AMAM characteristic. Among such linearizers, parallel diode linearizers have a simple circuit configuration, are smaller and less power consuming (e.g., see Kazuhisa Yamauchi, Kazutomi Mori, Masatoshi Nakayama, Yasuo Mitsui, and Tadashi Takagi, “A Microwave Miniaturized Linearizer Using a Parallel Diode with a Bias Feed Resistance,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 45, NO. 12, DECEMBER 1997 and Kazuhisa Yamauchi, Masatoshi Nakayama, Yukio Ikeda, Hiromasa Nakaguro, Naoto Kadowaki, and Takahiko Araki, “AN 18 GHZ-BAND MMIC LINEARIZER USING A PARALLEL DIODE WITH A BIAS FEED RESISTANCE AND A PARALLEL CAPACITOR,” 2000 IEEE). 
         [0006]    When a linearizer is incorporated in a GaN-based amplifier MMIC using a SiC substrate, a smallest possible circuit is preferred because epitaxial substrates are expensive. Since parallel diode linearizers require only a simple configuration, they are circuits best suited to a GaN-MMIC. 
         [0007]    Parallel diode linearizers are normally designed to have reverse characteristics of AMAM and AMPM of amplifiers connected upstream or downstream thereof. The AMAM and AMPM characteristics of a linearizer vary depending on a voltage applied to a diode of the linearizer. For this reason, the linearizer is provided with a control terminal that controls AMAM and AMPM, and controls and/or adjusts an applied voltage of the linearizer according to AMAM and AMPM of the amplifier. 
       SUMMARY OF THE INVENTION 
       [0008]    When a bias is applied to the linearizer, a current flows through an anode electrode of the diode. For this reason, migration occurs in an anode electrode metal, which causes deterioration of reliability. Preventing the migration requires the anode current value to be limited, which restrains a degree of freedom in designing the linearizer. 
         [0009]    The present invention has been implemented to solve the above-described problems and it is an object of the present invention to provide a linearizer capable of improving reliability and increasing the degree of freedom of the design thereof. 
         [0010]    According to the present invention, a linearizer includes: an input terminal; an output terminal; a connection point connected between the input terminal and the output terminal; a diode connected to the connection point; a voltage terminal; and a resistor connected between the voltage terminal and the connection point, wherein 0 V is applied to the voltage terminal. 
         [0011]    In the present invention, only a low anode current of the diode D is required by applying 0 V to the voltage terminal of the linearizer, and it is thereby possible to prevent migration from occurring in the anode electrode metal and improve reliability. Elimination of the need to limit the anode current value allows the degree of freedom in designing the linearizer to improve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a circuit diagram illustrating a linearizer according to a first embodiment of the present invention. 
           [0013]      FIG. 2  is a diagram illustrating an equivalent circuit of the diode. 
           [0014]      FIG. 3  is a diagram illustrating an AMAM characteristic of the linearizer according to the first embodiment of the present invention. 
           [0015]      FIG. 4  is a diagram illustrating an AMPM characteristic of the linearizer according to the first embodiment of the present invention. 
           [0016]      FIG. 5  is a circuit diagram illustrating a linearizer according to a second embodiment of the present invention. 
           [0017]      FIG. 6  is a diagram illustrating a relationship between a capacity of the diode and an anode voltage. 
           [0018]      FIG. 7  is a circuit diagram illustrating a linearizer according to a third embodiment of the present invention. 
           [0019]      FIG. 8  is diagram illustrating AMAM and AMPM characteristics of an amplifier whose distortion characteristic is compensated. 
           [0020]      FIG. 9  is a diagram illustrating AMAM and AMPM characteristics of the linearizer according to the third embodiment of the present invention. 
           [0021]      FIG. 10  is a block diagram illustrating a linearizer according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    A linearizer according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
       First Embodiment 
       [0023]      FIG. 1  is a circuit diagram illustrating a linearizer according to a first embodiment of the present invention. A high-frequency signal of a Ku-band is inputted from an input terminal IN. A connection point T 1  is connected between the input terminal IN and an output terminal OUT. A capacitor C 1  and a matching circuit M 1  are connected between the input terminal IN and the connection point T 1 . A capacitor C 2  and a matching circuit M 2  are connected between the connection point T 1  and the output terminal OUT. The capacitors C 1  and C 2  are used to cut a DC component. A diode D is connected between the connection point T 1  and a grounding point. A resistor R is connected between a voltage terminal T 2  and the connection point T 1 . A resistance value of the resistor R is 2000Ω. 0 V is applied to the voltage terminal T 2 . 
         [0024]      FIG. 2  is a diagram illustrating an equivalent circuit of the diode. The diode D has a variable resistance component and a variable capacitance component.  FIG. 3  is a diagram illustrating an AMAM characteristic of the linearizer according to the first embodiment of the present invention.  FIG. 4  is a diagram illustrating an AMPM characteristic of the linearizer according to the first embodiment of the present invention. 
         [0025]    When a frequency of an input signal is as high as a Ku band, an impedance (1/ωC) of the capacitance component of the diode D decreases. For this reason, even when a potential of the connection point T 1  is 0 V and the impedance of the resistance component is high, the impedance of the diode D decreases. Therefore, since the impedance of the diode D in the case of a small signal is low, loss in the linearizer is large. 
         [0026]    On the other hand, in the case of a large signal, a current flows into the diode D in a forward direction, a potential of the connection point T 1  shifts from 0 V to a low voltage side, and therefore the impedance of the resistance component increases. When the potential is shifted toward the low voltage side (negative voltage side in the present embodiment), the capacitance component decreases. Therefore, the impedance of the diode D increases and the loss in the linearizer decreases. It is thereby possible to assure a dynamic range of AMAM as shown in  FIG. 3 . The same operation principles as those described in the aforementioned literatures apply to the AMPM characteristic. 
         [0027]    In the present embodiment, only a low anode current of the diode D is required by applying 0 V to the voltage terminal of the linearizer, and it is thereby possible to prevent migration from occurring in the anode electrode metal and improve reliability. Elimination of the need to limit the anode current value allows the degree of freedom in designing the linearizer to improve. In the present embodiment, the cathode of the diode D is grounded, but even when the anode is grounded, only the current direction changes and similar effects can be obtained. 
       Second Embodiment 
       [0028]      FIG. 5  is a circuit diagram illustrating a linearizer according to a second embodiment of the present invention. A voltage terminal T 2  of the linearizer is grounded. A diode D has a HEMT structure in which a drain and a source are short-circuited. The rest of the configuration is similar to that of the first embodiment. 
         [0029]    Since the voltage terminal T 2  is grounded, no power supply for the linearizer is necessary. A wiring layout in an MMIC is simplified. Therefore, it is possible to simplify and downsize the configuration of the amplifier module, and reduce the cost. 
         [0030]      FIG. 6  is a diagram illustrating a relationship between a capacity of the diode and an anode voltage. In the present embodiment, the potential is shifted from 0 V which is a bias point to a negative voltage direction. In contrast, with a normal diode, there is substantially no variation in the capacity in a negative voltage region. On the other hand, with a HEMT-based diode (diode whose gate is assumed to be an anode and whose short circuited drain and source are assumed to be a cathode), the capacitance variation is as large as a two-digit figure. Thus, the dynamic range of linearizer characteristics can be increased using an HEMT-based diode having a large variation of the capacitance component with respect to the voltage. Moreover, using an HEMT process diode allows the linearizer and the amplifier to be integrated on an MMIC. 
       Third Embodiment 
       [0031]      FIG. 7  is a circuit diagram illustrating a linearizer according to a third embodiment of the present invention. An inductor L is connected between a cathode of a diode D and a grounding point. The rest of the configuration is similar to that of the second embodiment. 
         [0032]      FIG. 8  is diagram illustrating AMAM and AMPM characteristics of an amplifier whose distortion characteristic is compensated. While AMAM decreases monotonously, AMPM increases once and then decreases.  FIG. 9  is a diagram illustrating AMAM and AMPM characteristics of the linearizer according to the third embodiment of the present invention. Insertion of the inductor L allows the AMPM characteristic of the linearizer to be inverted. Thus, the linearizer of the present embodiment allows the AMPM characteristic of the amplifier to be compensated up to a saturation region (region after AMPM is inverted). 
       Fourth Embodiment 
       [0033]      FIG. 10  is a block diagram illustrating a linearizer according to a fourth embodiment of the present invention. A driver amplifier  2  having a GaN-based transistor (e.g., GaN-HEMT) is provided upstream of the linearizer  1  according to any one of the first to third embodiments. A high output GaN single-stage amplifier  3  is provided downstream of the linearizer  1 . The driver amplifier  2  supplies an output signal to an input terminal IN of the linearizer  1 . Exciting the linearizer  1  by an output signal of the driver amplifier  2  allows the linearizer to operate. 
         [0034]    As shown in  FIG. 8 , although higher output of the GaN-based amplifier can be expected, the gain slowly deteriorates due to GaN-specific soft compression. The linearizer  1  is needed to compensate for this, but since the downstream stage has high output, the output of the linearizer  1  also needs to be kept high. For this purpose, a GaN-based diode D may be adopted. However, since a rising voltage of the GaN-based diode is approximately 1.0 V, which is by 0.3 V higher than 0.7 V of a conventional GaAs-based diode, large input power is necessary at the rise of the gain of the linearizer. Since GaN-HEMT has a higher power density than GaAs-HEMT, if a GaN-based transistor is used for the driver amplifier  2 , it is possible to sufficiently drive the linearizer  1 . 
         [0035]    Moreover, since the present embodiment can be implemented through an HEMT process without requiring any diode D dedicated process, it is possible to integrate the linearizer  1  and the driver amplifier  2  as an MMIC and reduce the circuit area. 
         [0036]    Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
         [0037]    The entire disclosure of Japanese Patent Application No. 2014-107276, filed on May 23, 2014 including specification, claims, drawings and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.