Abstract:
A short-stub matching circuit connected to a signal transfer line, includes at least one resistor element having a distributed constant effect which is inserted in a transfer path between the signal transfer line and a grounding conductor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-frequency matching circuit, and, more particularly, to a short-stub matching circuit in a monolithic integrated circuit, which is used in the millimeter wave band and microwave band. 
     2. Description of the Related Art 
     In designing millimeter wave/microwave amplifiers and making their prototypes, parasitic oscillation generally often worries the designers. This is because a highfrequency transistor to be used as an amplifier element. e.g., a heterojunction FET, has a power gain of greater than 0 dB over a wide frequency range from DC (direct current) to a microwave/millimeter wave band, the circuit meets the oscillation condition in some frequency band. 
     The stability condition for a transistor is such that with a variable load impedance Z 1  connected to the output terminal of the transistor, the absolute value of a reflection coefficient in on the input side is smaller than “1” even when the variable load impedance Z 1  lies anywhere within the Smith chart. In other words, no oscillation occurs when the absolute values of the input reflection coefficient in and an output reflection coefficient out are both smaller than “1.” 
     In general, a stability coefficient K, an index, is used to determine whether or not oscillation will occur. Stability can be discriminated by checking if the value of the stability coefficient K is greater than “1.” The stability coefficient X is given from the following equation using S parameters of the circuit. 
     
       
         K=(1−|S 11 | 2 −|S 22 | 2 +|Δ| 2 )/2·|S 12 ·S 21 |Δ=S 11 ·S 22 −S 12 ·S  22 −S 12 ·S 21 · 
       
     
     The condition for absolute stability is K&gt;1 in which case no oscillation occurs with respect to every passive load impedance Z 1  unless a feedback circuit Is externally added. 
     In Japanese Patent Unexamined Publication (Kokai) No. 7-240369, a matching circuit has been disclosed which is designed in consideration of the low-frequency stability in an amplifier circuit. This matching circuit is connected to a radio-frequency (RF) signal transfer line which is connected to the gate electrode of an FET (field effect transistor), and serves as both a bias circuit and a stabilizing circuit. 
     More specifically, the matching circuit has a λ/4 transfer line connected to the RF signal transfer line, and the λ/4 transfer line is grounded via a first MIM (Metal-Insulator-Metal) capacitor, and is further grounded via a second MIM capacitor and a resistor connected in series, where λ is a wave length. That is, the first MIM capacitor and the second MIM capacitor and the resistor form a parallel circuit. 
     The capacitance of the first HIM capacitor is set in such a way that the first MIM capacitor becomes nearly short-circuited at the use frequency and becomes nearly an open state at a low frequency outside the use frequency. The capacitance of the second MIM capacitor is set in such a way teat it is greater than the capacitance of the first MIM capacitor and the second MIM capacitor becomes nearly short-circuited at a low frequency outside the use frequency. Therefore, the matching circuit serves as a bias circuit with the λ/4 transfer line grounded via the first MIM capacitor at the use frequency, and serves as a stabilizing circuit for prevention of oscillation, with the λ/4 transfer line grounded via the series-connected second MIM capacitor and resistor at a low frequency outside the use frequency. 
     In Japanese Patent Unexamined Publication (Kokai) No. 1-233812, another matching circuit has been disclosed which has an oscillation preventing resistor inserted on the opposite side to the short-circuit side of a short-stub for matching. This matching circuit aims at decreasing the number of elements in an oscillation preventing circuit in a monolithic integrated circuit (IC), thereby reducing the chip area. To achieve this purpose, the oscillation preventing resistor is directly formed in the RF signal transfer line. 
     In the matching circuit disclosed in Unexamined Patent Publication (Kokai) No. 7-240369, however, as shown in FIG. 14, there is an area  1  where the stability coefficient K becomes K&lt;1 at a low frequency near 10 GHz. This matching circuit thus does not satisfy the complete stability condition. 
     Because the matching circuit disclosed in Unexamined Patent Publication (Kokai) No. 1-233812 has the oscillation preventing resistor directly formed as a semiconductor resistor in the RF signal transfer line, the degree of freedom at the time of implementing trimming in the fabrication process of a microwave monolithic IC (MMIC) is significantly limited. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a stable short-stub matching circuit which causes no parasitic oscillation and has an increased stability coefficient K of 1 or higher. 
     It is another object of this invention to provide a stable, parasitic-oscillation-free short-stub matching circuit whose fabrication is easily adjustable. 
     To achieve those objects, a short-stub matching circuit according to this invention includes at least one resistor element having a distributed constant effect that is inserted in a transfer path between said signal transfer line and a grounding conductor. The insertion of the resistor element having a distributed constant effect can permit the value of the stability coefficient K to be easily increased to 1 or more. In addition, as the resistor element having a distributed constant effect can be formed at a later stage of the fabrication process, the degree of freedom at the time of performing trimming becomes greater, thereby ensuring significantly easy adjustment of resistance value of the resistor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram illustrating the planar structure of an MMIC which uses a short-stub matching circuit according to a first embodiment of this invention; 
     FIG. 2 is a graph showing the stability characteristic of the short-stub matching circuit of the first embodiment; 
     FIG. 3 is a graph showing the stability characteristic of a comparative example (1) which does not have a resistor  102  having a distributed constant effect in the first embodiment; 
     FIG. 4 is a schematic diagram illustrating the planar structure of an MMIC which uses a short-stub matching circuit according to a second embodiment of this invention: 
     FIG. 5 is a graph showing the stability characteristic of the short-stub matching circuit of the second embodiment; 
     FIG. 6 is a graph showing the stability characteristic of a comparative example (2) which does not have a resistor  201  having a distributed constant effect in the second embodiment; 
     FIG. 7 is a schematic diagram illustrating the planar structure of an MMIC which uses a short-stub matching circuit according to a third embodiment of this invention; 
     FIG. 8 is a graph showing the stability characteristic of the short-stub matching circuit of the third embodiment; 
     FIG. 9 is a schematic diagram illustrating the planar structure of an MMIC which uses a short-stub matching circuit according to a fourth embodiment of this invention; 
     FIG. 10 is a graph showing the stability characteristic of the short-stub matching circuit of the fourth embodiment; 
     FIG. 11 is a schematic diagram illustrating the planar structure of an MMIC which uses a short-stub matching circuit according to a fifth embodiment of this invention; 
     FIG. 12 is a graph showing the stability characteristic of the short-stub matching circuit of the fifth embodiment; 
     FIG. 13A is a Smith chart showing reflection coefficients at a low-frequency area of the comparative example (1); 
     FIG. 13B is a Smith chart showing reflection coefficients at a low-frequency area of the first embodiment; and 
     FIG. 14 is a graph showing the stability characteristic of a conventional short-stub matching circuit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. 
     FIRST EMBODIMENT 
     The first embodiment has a microstrip line on a GaAs substrate and a resistor having a distributed constant effect, with the center frequency of 40 GHz. 
     As shown in FIG. 1, an RF signal transfer line  10  is connected to the gate electrode of an FET  11  formed on a GaAs substrate. The FET  11  has an AlGaAs/InGaAs-based heterojunction with a gate length of 0.18 μm, and a gate width Wg of 200 μm; the bias conditions are a drain bias Vd=4.0 V and a gate bias Vg=−0.6 V. The FET which is used here as an active element demonstrates such a characteristic that the maximum stable power gain (MSG) and the maximum available power gain (MAG) are switched from one to the other at near 60 GHz and the stability coefficient K at or below 60 GHz becomes equal to or smaller than “1” (unstable). 
     The RF signal transfer line  10  is connected to a bias circuit section  13  via a short-stub matching circuit  12  according to this embodiment. The short-stub matching circuit  12  Is comprised of a transfer line  101 . a resistor  102  having a distributed constant effect, a capacitor  103  and a grounding electrode  104 . The transfer line  101  has one end connected to the RF signal transfer line  10  and the other end connected to the resistor  102 . The resistor  102  is further connected to the grounding electrode  104  via the capacitor  103 , and is connected to the bias circuit section  13  via the capacitor  103  and a bias supply line  10 S 
     The transfer line  101 , the resistor  102  having a distributed constant effect and the capacitor  103  constitute the short-stub matching circuit with the center frequency of 40 GHz. This short-stub matching circuit serves as a matching circuit in the use frequency band and serves as a stabilizing circuit to prevent parasitic oscillation in a low-frequency band outside the use frequency. 
     Particularly, the resistor  102  having a distributed constant effect can be formed following a wiring step of forming a circuit pattern for the RF signal transfer line  10 , the transfer line  101  and the like. Specifically, after a wiring base film is formed by sputtering, gold wires are formed on the wiring base film and the resistor  102  made of metal resistor material with a distributed constant is further formed on the wiring base film. Because of the metal resistor, a post process like adjustment of the resistance by, for example, trimming becomes considerably easier. 
     For the resistor  102  to have a distributed constant effect, it is desirable that the resistor  102  should have length equal to or greater than approximately {fraction (1/16)} of the signal wavelength at the use frequency. In this example, the transfer line  101  has a length of 130 μm and a width of 20 μm, the resistor  102  having a distributed constant effect has a length of 50 μm, a width of 20 μm and a resistance 50Ω, and the capacitor  103  has a capacitance of 1 pF. 
     The influence of the matching circuit of this embodiment on the stability of active elements was checked. According to this embodiment, the short-stub matching circuit is comprised of only the transfer line  101 , the resistor  102  having a distributed constant effect and the capacitor  103 , and serves as a matching circuit at the use frequency and serves as a stabilizing circuit to prevent parasitic oscillation at a frequency outside the use frequency band due to the use of the resistor  10  having the distributed constant effect. This is because the power is attenuated by the resistor  102 , so that the reflection coefficient becomes smaller by a given amount regardless of the frequency. As a result, the input/output reflection coefficient on the Smith chart goes inward of the graph, making it difficult for the reflection coefficient to get greater than “1” even if an external impedance is changed. Despite the power attenuation, the gain drop at the use frequency is less than 1 dB. which is practically insignificant. 
     FIG. 2 shows the stability characteristic of the short-stub matching circuit of the first embodiment, and FIG. 3 shows the stability characteristic of a comparative example (1) which does not have the resistor  102  with a distributed constant effect as used in the first embodiment. 
     As apparent from FIG. 2, the reduced reflection coefficient causes the stability coefficient K to exceed “1” to in every frequency band in this embodiments, which proves that this invention dramatically improves the circuit stability. In the comparative example (1) shown in FIG. 3, by contrast, the stability coefficient K becomes smaller than “1” in an area as indicated by reference numeral 2 in the vicinity of 30 GHz. 
     The following will discuss some structures which are fundamentally the same as the structure of the first embodiment but are modified on the arrangement of the resistor. The circuits to be described below can be expected to have the some advantages as those of the first embodiment. Further, it was confirmed that an amplifier circuit with a bias circuit not connected to a short-stub circuit and an amplifier circuit using a short-stub circuit directly grounded via no capacitor had the same advantages as those of the first embodiment. 
     SECOND EMBODIMENT 
     The second embodiment, like the first embodiment, has a microstrip line on a GaAs substrate and a resistor having a distributed constant effect, with the center frequency of 76 GHz. To avoid the redundant description, like or same reference numerals are given to those components which are the same as the corresponding components of the first embodiment shown in FIG.  1 . 
     As shown in FIG. 4, the RF signal transfer line  10  is connected to the bias circuit section  13  via a short-stub matching circuit  20  according to the second embodiment. The short-stub matching circuit  20  is comprised of a resistor  201  having a distributed constant effect, a λ/4 transfer line  202 , a capacitor  203  and an grounding electrode  204 . The λ/4 transfer line  202  has one end connected via the resistor  201  to the RF signal transfer line  10  and the other end connected via the capacitor  203  to the grounding electrode  204 . The capacitor  203  is further connected to the bias circuit section  13  via a bias supply line  205 . 
     The resistor  201  having the distributed constant effect, the λ/4 transfer line  202 , and the capacitor  203  constitute the short-stub matching circuit with the center frequency of 76 GHz. This short-stub matching circuit serves as a matching circuit in the use frequency band and serves as a stabilizing circuit to prevent parasitic oscillation in a low-frequency band outside the use frequency. Particularly, the resistor  201  with the distributed constant effect can be formed in a wiring step similar to the step of forming the RF signal transfer line  10  and the λ/4 transfer line  202 , as described in the first embodiment. For the resistor  201  to have a distributed constant effect. it is desirable that the resistor  201  should have a length equal to or greater than approximately {fraction (1/16)} of the signal wavelength at the use frequency. 
     FIG. 5 shows the stability characteristic of the short-stub matching circuit of the second embodiment, and FIG. 6 shows the stability characteristic of a comparative example (2) which does not have the resistor  201  with a distributed constant effect in the second embodiment. 
     As shown in FIG. 5, the stability coefficient K exceeds “1” in every band in this embodiments, which proves that this invention significantly improves the circuit stability. In the comparative example (2) shown in FIG. 6, by contrast. the stability coefficient K is smaller than “1” in a wide band of about 30 to 75 GHz. 
     THIRD EMBODIMENT 
     The third embodiment. like the first embodiment, has a microstrip line on a GaAs substrate and a resistor having a distributed constant effect, with the center frequency of 76 GHz. To avoid the redundant description, like or same reference numerals are given to those components which are the same as the corresponding components of the first embodiment shown in FIG.  1 . 
     As shown in FIG. 7, the RF signal transfer line  10  is connected to a short-stub matching circuit  30  according to the third embodiment. The short-stub matching circuit  30  is comprised of a transferline  301 , a resistor  302  having a distributed constant effect, a transfer line  303  and an grounding electrode  304 . The transfer line  301  has one end connected to the RF signal transfer line  10  and the other end connected via the resistor  302  and the transfer line  303  to the grounding electrode  304 . In this embodiment, the resistor  302  is grounded without capacitor intervention. 
     The transfer line  301 , the resistor  302  having a distributed constant effect and the transfer line  303  constitute the short-stub matching circuit with the center frequency of 76 GHz. This short-stub matching circuit serves as a matching circuit in the use frequency band and serves as a stabilizing circuit to prevent parasitic oscillation in a low-frequency band outside the use frequency. Particularly, the resistor  302  with a distributed constant effect can be formed in a wiring step similar to the step of forming the RF signal transfer line  10  and the transfer lines  301  and  303 , as described in the first embodiment. For the resistor  302  to have a distributed constant effect, it is desirable that the resistor  302  should have a length equal to or greater than approximately {fraction (1/16)} of the signal wavelength at the use frequency. 
     As shown in FIG. 8, the stability coefficient K exceeds “1” in every band in this embodiments, which proves that this invention significantly improves the circuit stability. 
     FOURTH EMBODIMENT 
     The fourth embodiment, like the first embodiment. has a microstrip line on a GaAs substrate and a resistor having a distributed constant effect, with the center frequency of 76 GHz. To avoid the redundant description, like or some reference numerals are given to those components which are the same as the corresponding components of the first embodiment shown in FIG.  1 . 
     As shown in FIG. 9, the RF signal transfer line  10  is connected to the bias circuit section  13  via a short-stub matching circuit  40  according to the fourth embodiment. The short-stub matching circuit  40  is comprised of a resistor  401  having a distributed constant effect, a λ/4 transfer line  402 . a resistor  403  having a distributed constant effect, a capacitor  404  and an grounding electrode  405 . The λ/4 transfer line  402  has one end connected via the resistor  401  to the RF signal transfer line  10  and the other end connected via the resistor  403  to the capacitor  404 . The capacitor  404  is further connected to the grounding electrode  405 , and to the bias circuit section  13  via a bias supply line  406 . 
     The resistor  401  having a distributed constant effect, the λ/4 transfer line  402 , the resistor  403  having a distributed constant effect, and the capacitor  404  constitute the short-stub matching circuit with the center frequency of 76 GHz. This short-stub matching circuit serves as a matching circuit in the use frequency band and serves as a stabilizing circuit to prevent parasitic oscillation in a low-frequency band outside the use frequency. Particularly, the resistors  401  and  402  both having a distributed constant effect can be formed in a wiring step similar to the step of forming the circuit pattern for the RF signal transfer line  10  and the λ/4 transfer line  402 , as described in the first embodiment. For the resistors  401  and  403  to have a distributed constant effect, it is desirable that the resistors  401  and  403  should have lengths equal to or greater than approximately {fraction (1/16)} of the signal wavelength at the use frequency. 
     As shown in FIG.  10 . the stability coefficient K exceeds “1” in every band in this embodiments, which proves that this invention dramatically improves the circuit stability. 
     FIFTH EMBODIMENT 
     The fifth embodiment, as described in the first embodiment, has a microstrip line on a GaAs substrate and a resistor having a distributed constant effect, with the center frequency of 76 GHz. To avoid the redundant description, like or same reference numerals are given to those components which are the same as the corresponding components of the first embodiment shown in FIG.  1 . 
     As shown in FIG. 11, the RF signal transfer line  10  is connected to the bias circuit section  13  via a short-stub matching circuit  50  according to the fifth embodiment. The short-stub matching circuit  50  is comprised of a resistor  501  having a length of λ/4 and a distributed constant effect. a capacitor  502  and an grounding electrode  503 . The λ/4 resistor  501  has one end connected to the RF signal transfer line  10  and the other end connected via the capacitor  502  to the grounding electrode  503 . The capacitor  502  is also connected to the bias circuit section  13  via a bias supply line  504 . 
     The λ/4 resistor  501  having a distributed constant effect and the capacitor  502  constitute the short-stub matching circuit with the center frequency of 76 GHz. This short-stub matching circuit serves as a matching circuit in the use frequency band and serves as a stabilizing circuit to prevent parasitic oscillation in a low-frequency band outside the use frequency. Particularly, the λ/4 resistor  501  with a distributed constant effect can be formed in a wiring step similar to the step of forming the circuit pattern for the RF signal transfer line  10 , as per the first embodiment. This increases the degree of freedom at the time of trimming. 
     As shown in FIG. 12, the stability coefficient K exceeds “1” in every band in this embodiments, which proves that this invention significantly improves the circuit stability. 
     Although not illustrated, it was confirmed that a matching circuit with the capacitor  502  of FIG. 11 removed and having the λ/4 resistor  501  connected directly to the grounding electrode  503  also met the condition K&gt;1 in every band. 
     In FIGS. 13A and 13B, each curve indicated by box-like dot shows the reflection coefficients (S 11 ) as viewed from the input side, and each curve indicated by circle-like dot shows the reflection coefficients (S 22 ) as viewed from the output side. 
     The reflection coefficients in the first embodiment shown in FIG. 13B lie more inward of the Smith chart than those of the comparative example (1) depicted in FIG. 13A, and are thus smaller than the latter ones. This indicates a difficulty of the reflection coefficients to become greater than “1” even when the external impedance changes. 
     According to this invention, as described above, a stability coefficient K&gt;1 inside and outside the use frequency can be achieved and a stable MMIC can easily be realized by inserting a resistor having a distributed constant effect in a path through which the RF signal transfer line is grounded. Further, as a resistor having a distributed constant effect is used, this resistor can be formed at a later stage of the fabrication process, so that the degree of freedom at the time of executing trimming becomes greater, thereby ensuring considerably easy adjustment of resistance of the resistor. 
     Although five embodiments of the present invention have been described herein, it should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein. but may be modified within the scope of the appended claims.