Patent Publication Number: US-6339362-B1

Title: Microwave amplifier optimized for stable operation

Description:
This application is a continuation of application Ser. No. 09/333,030 filed Jun. 15,1999, now U.S. Pat. No. 6,130,580. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to microwave amplifiers and, more particularly, to a microwave amplifier implemented by a transistor for amplifying signals having a millimeter-wave frequency or a microwave frequency. 
     2. Description of the Related Art 
     FIG. 4 is a circuit diagram of a microwave amplifier according to the related art disclosed, for example, in Japanese Laid-Open Patent Application No. 61-285811. Referring to FIG. 4, the microwave amplifier comprises a field-effect transistor  1  having a relatively high operating frequency, a grounding terminal  2  for the field-effect transistor  1 , a stabilized resistor  3  connected to the grounding terminal  2 , a grounding conductor pattern  4  having one end thereof connected to the stabilized resistor  3  and the other end grounded, and an open-circuit stub  5  having a length equal to ¼ of a wavelength at the operating frequency of the field-effect transistor  1  and connected to the grounding terminal  2  so as to be parallel with a series circuit formed of the stabilized resistor  3  and the grounding conductor pattern  4 . 
     A description will now be given of the operation of the microwave amplifier of FIG.  4 . 
     The microwave amplifier shown in FIG. 4 operates such that a drain current from the drain D to the source S of the field-effect transistor  1  is amplified in accordance with a gate voltage applied to the gate G. 
     The grounding conductor pattern  4  is a channel by which the grounding terminal  2  of the field-effect transistor  1  is grounded. In a low-frequency band, the inductance of the grounding conductor pattern  4  is negligible so that the field-effect transistor  1  is properly grounded. In a high-frequency band, the inductance of the grounding conductor pattern  4  is not negligible. The grounding conductor pattern  4  thus acts as a short-circuited stub having inductance, resulting in a loss of the gain of the field-effect transistor  1  due to the inductance of the grounding conductor pattern  4 . 
     Accordingly, the open-circuit stub  5  having a length equal to ¼ of the wavelength at the operating frequency of the microwave amplifier is connected to the grounding terminal  2  to provide high-frequency grounding of the grounding terminal  2  at the operating frequency. 
     However, at certain points in the high-frequency band including the operating frequency, the inductance of the grounding conductor pattern  4  and the capacitance of the open-circuit stub  5  produce parallel resonance, causing the reactance of the grounding terminal  2  to become infinite at the parallel resonance frequency. 
     The stabilized resistor  3  is connected between the grounding terminal  2  and the grounding conductor pattern  4  in order to suppress parallel resonance at the parallel resonance frequency. 
     FIG. 5 is a circuit diagram of a microwave amplifier according to the related art shown in TECHNICAL REPORT OF IEICE (THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS) MW 92-149 LOW-NOISE AMPLIFIER USING DIRECTLY COOLED HEMTs, FEBRUARY, 1993. Referring to FIG. 5, the microwave amplifier includes the field-effect transistor  1 , the grounding terminal  2  of the field-effect transistor  1 , and the inductor  7  having one end thereof connected to the grounding terminal  2  and the other end grounded. 
     A description will now be given of the operation of the microwave amplifier of FIG.  5 . 
     The microwave amplifier shown in FIG. 5 operates such that a drain current from the drain D to the source S of the field-effect transistor  1  is amplified in accordance with a gate voltage applied to the gate G. 
     The inductor  7  can be configured such that the impedance that minimizes the noise for the field-effect transistor  1  substantially matches the impedance that minimizes reflection. Thus, the noise characteristic and the reflection characteristic can be simultaneously improved. 
     The microwave amplifier of FIG. 4 has a drawback in that the stabilized resistor  3  produces a voltage drop when a bias is applied to the field-effect transistor  1 . This makes it difficult to use the construction as shown in FIG. 4 in a high-power amplifier having a large current consumption. 
     The microwave amplifier of FIG. 5 has a drawback in that when a member having inductance and capacitance is used to implement the inductor  7 , parallel resonance results at a certain frequency (parallel resonance frequency) so that the reactance of the grounding terminal  2  becomes infinity at the parallel resonance frequency. If this occurs, the operation of the microwave amplifier becomes unstable. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide microwave amplifiers in which the aforementioned drawbacks are eliminated. 
     Another and more specific object of the present invention is to provide a microwave amplifier optimized for stable operation. 
     The aforementioned objects can be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; a grounding conductor pattern having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a first open-circuit stub having a length equal to one quarter of a wavelength at an operating frequency of said transistor and connected to the grounding terminal of said transistor so as to be placed in parallel with said grounding conductor pattern; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said grounding conductor pattern; and a second open-circuit stub having a length equal to one quarter of a parallel resonance frequency of said grounding conductor pattern and said first open-circuit stub, and connected to the other end of said resistor. 
     Accordingly, it is not only possible to suppress undesirable parallel resonance caused by the inductance of the grounding conductor pattern and the capacitance of the first open-circuit stub, but also to prevent a voltage drop in the stabilized resistor when a bias is applied to a transistor. Thus, a microwave amplifier optimized for stable operation and applicable to a high-power amplifier having a large current consumption is obtained. 
     The aforementioned objects can also be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; an inductor having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said inductor; and an open-circuit stub having a length equal to a quarter of a parallel resonance frequency of an inductance of said inductor and a capatitance inherent in a member constituting an inductor, and connected to the other end of said resistor. 
     Accordingly, it is not only possible to suppress undesirable parallel resonance caused by the inductance of the grounding conductor pattern and the capacitance of the first open-circuit stub, but also to prevent a voltage drop in the stabilized resistor when a bias is applied to a transistor. Thus, a microwave amplifier optimized for stable operation and applicable to a high-power amplifier having a large current consumption is obtained. 
     The aforementioned objects can also be achieved by a microwave amplifier comprising: a transistor for amplifying an input current; an inductor having one end thereof connected to a grounding terminal of said transistor and the other end grounded; a resistor having one end thereof connected to the grounding terminal of said transistor so as to be placed in parallel with said inductor; and an open-circuit stub having a length equal to a quarter of a parallel resonance frequency of an inductance of said inductor and a capatitance inherent in a member constituting an inductor, and connected to the other end of said resistor. 
     Accordingly, undesirable parallel resonance caused by inductance and capacitance of a member constituting an inductor is suppressed so that the stable operation of a microwave amplifier is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a microwave amplifier according to a first embodiment of the present invention; 
     FIG. 2 is a circuit diagram of a microwave amplifier according to a second embodiment of the present invention; 
     FIG. 3 is a circuit diagram of a microwave amplifier according to a third embodiment; 
     FIG. 4 is a circuit diagram of a microwave amplifier according to the related art; 
     FIG. 5 is a circuit diagram of another microwave amplifier according to the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 1 is a circuit diagram of a microwave amplifier according to the first embodiment. Referring to FIG. 1, the microwave amplifier comprises a field-effect transistor  1 , a grounding terminal  2  for the field-effect transistor  1 , a first open-circuit stub  5  having a length equal to ¼ of a wavelength at the operating frequency of the field-effect transistor  1 . The components listed above are the same as the corresponding components of FIG. 4 
     The microwave amplifier of FIG. 1 further comprises a grounding conductor pattern  11  having one end thereof connected to the grounding terminal  2  and the other end grounded, a stabilized resistor  12  having one end thereof connected to the grounding terminal  2  so as to be parallel with the grounding conductor pattern  11 , and a second open-circuit stub  13  having a length equal to ¼ of a wavelength at a parallel resonance frequency of the grounding conductor pattern  11  and the first open-circuit stub  5 , and connected to a junction  14  at the other end of the stabilized resistor  12 . 
     A description will now be given of the operation of the microwave amplifier. 
     The microwave amplifier shown in FIG. 1 operates such that a drain current from the drain D to the source S of the field-effect transistor  1  is amplified in accordance with a gate voltage applied to the gate G. 
     The grounding conductor pattern  11  is a channel by which the grounding terminal  2  is grounded. In a low-frequency band, the inductance of the grounding conductor pattern  11  is negligible so that the field-effect transistor  1  is properly grounded. In a high-frequency band, the inductance of the grounding conductor pattern  11  is not negligible. The grounding conductor pattern  11  thus acts as a short-circuited stub, resulting in a loss of the gain of the field-effect transistor  1  due to the inductance of the grounding conductor pattern  11 . 
     Accordingly, the open-circuit stub  5  having a length equal to ¼ of the wavelength at the operating frequency of the microwave amplifier is connected to the grounding terminal  2  to provide high-frequency grounding of the grounding terminal  2  at the operating frequency. 
     However, at certain points in the high-frequency band including the operating frequency, the inductance of the grounding conductor pattern  11  and the capacitance of the open-circuit stub  5  produce parallel resonance, causing the reactance of the grounding terminal  2  to become infinite at the parallel resonance frequency. As a result, the field-effect transistor  1  becomes unstable in the neighborhood of the parallel resonance frequency, producing an undesirable oscillation. 
     The first embodiment resolves this drawback by connecting a series circuit including the stabilized resistor  12  and the open-circuit stub  13  to the grounding terminal  2 . Since the open-circuit stub  13  has a length equal to ¼ of the wavelength at the parallel resonance frequency, the series circuit provides high-frequency grounding of the junction  14  at the parallel resonance frequency. Since the junction  14  is provided with high-frequency grounding at the parallel resonance frequency, connection of the stabilized resistor  12  to the parallel circuit comprising the grounding conductor pattern  11  and the first open-circuit stub  5  prevents undesirable resonance at the parallel resonance frequency. 
     Thus, the construction according to the first embodiment prevents undesirable parallel resonance caused by the inductance of the grounding conductor pattern  11  and the capacitance of the first open-circuit stub  5 . Added to this is an effect of elimination of a voltage drop across, the stabilized resistor  3  of the related art. Thereby, a stable operation of the microwave amplifier results. As such, the construction of the first embodiment is applicable to a high-power amplifier with a large current consumption. 
     While it has been assumed that the microwave amplifier uses a field-effect transistor  1 , the benefit of the first embodiment is also available when a bipolar transistor is used. 
     Embodiment 2 
     FIG. 2 is a circuit diagram of the microwave amplifier according to the second embodiment. Referring to FIG. 2, the microwave amplifier includes a capacitor  21  having one end thereof connected to the stabilized resistor  12  and the other end grounded. 
     The other aspects of the illustrated construction are the same as the corresponding aspects shown in FIG. 1, and the description thereof is omitted. 
     A description will now be given of the operation. 
     In the first embodiment, the second open-circuit stub  13  having a length equal to ¼ of the parallel resonance frequency of the grounding conductor pattern  11  and the first open-circuit stub  5  is used to provide grounding of the junction  14  connected to the stabilized resistor  12 . In the second embodiment, the capacitor  21  is used to ground the junction  14  connected to the stabilized resistor  12 . 
     The requirement for the capacitor  21  is that it is capable of storing a sufficient amount of charge at the parallel resonance frequency. That is, it is required that the capacitance of the capacitor  21  be larger than a level sufficient to provide simulated grounding of the junction  14 . 
     More specifically, the requirement is represented as 
     
       
         1/j2πf 0 C&lt;&lt;R 
       
     
     where f 0  indicates a parallel resonance frequency, C indicates a capacitance of the capacitor  21  and R indicates a resistance of the stabilized resistor  12 . 
     With the junction  14  grounded by the capacitor  21  at the parallel resonance frequency, the stabilized resistor  12  is connected to a parallel circuit comprising the grounding conductor pattern  11  and the first open-circuit stub  5  so that undesirable parallel resonance at the parallel resonance frequency is suppressed. 
     Thus, like the construction according to the first embodiment, the construction according to the second embodiment suppresses undesirable parallel resonance caused by the inductance of the grounding conductor pattern  11  and the capacitance of the first open-circuit stub  5 . Added to this is an effect of elimination of a voltage drop across the stabilized resistor  3  of the related art when a bias is applied to the field-effect transistor  1 . Thereby, stable operation of the microwave amplifier results. As such, the construction of the second embodiment is applicable to a high-power amplifier with a large current consumption. 
     By constructing the capacitor  21  using a chip capacitor or a MIM capacitor, the size of the microwave amplifier is reduced. 
     Embodiment 3 
     FIG. 3 is a circuit diagram of the microwave amplifier according to the third embodiment. Referring to FIG. 3, the microwave amplifier comprises a field-effect transistor  1 , a grounding terminal  2  for the field-effect transistor  1 , and an inductor  7  having one end thereof connected to the grounding terminal  2  and the other end grounded. The components listed above are the same as the corresponding components shown in FIG. 5 
     The microwave amplifier according to the third embodiment also comprises a stabilized resistor  31  having one end thereof connected to the grounding terminal  2  so as to be parallel with the inductor  7 , an open-circuit stub  32  having a length equal to ¼ of a wavelength at a parallel resonance frequency of the inductor  7  and the capacitance inherent in a member implementing the inductor  7  and connected to a junction  33  at the other end of the stabilized resistor  31 . 
     A description will now be given of the operation of the microwave amplifier. 
     The microwave amplifier shown in FIG. 3 operates such that a drain current from the drain D to the source S of the field-effect transistor  1  is amplified in accordance with a gate voltage applied to the gate G. 
     The inductor  7  can be configured such that the impedance that minimizes the noise for the field-effect transistor  1  substantially matches the impedance that minimizes reflection. Thus, the noise characteristic and the reflection characteristic can be simultaneously improved. 
     The inductor  7  is implemented by a distributed constant element such as an open-circuit stub or a spiral inductor, or a lumped constant element such as a chip inductor. At certain points in a frequency band that includes the operating frequency of the microwave amplifier, the inductance of the inductor  7  and the parasitic capacitance inherent in the member implementing the inductor  7  produce parallel resonance. As a result, the reactance of the grounding terminal  2  becomes infinite at the parallel resonance frequency. If this occurs, the operation of the field-effect transistor  1  becomes unstable in the neighborhood of the parallel resonance frequency, causing undesirable oscillation. 
     Accordingly, a series circuit comprising the stabilized resistor  31  and the open-circuit stub  32  is connected to the grounding terminal  2 . Since the open-circuit stub  32  has a length equal to ¼ of the wavelength at the parallel resonance frequency, the open-circuit stub  32  provides grounding of the junction  33  between the stabilized resistor  31  and the open-circuit stub  32 . By grounding the junction  33  at the parallel resonance frequency, the stabilized resistor  31  is connected to a parallel circuit that includes the inductance of the inductor  7  and the associated capacitance. Therefore, undesirable parallel resonance at the parallel resonance frequency is suppressed. 
     With the benefit of suppressed parallel resonance caused by the inductance of the inductor  7  and the associated capacitance, a stable operation of the microwave amplifier results. 
     While it has been assumed that the microwave amplifier uses a field-effect transistor  1 , the benefit of the third embodiment is also available when a bipolar transistor is used. 
     The present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the present invention.