Abstract:
A circuit includes a first node, a second node, a third node between the first and second nodes, a first field effect transistor coupled between the first and third nodes, a second field effect transistor coupled to the third node including a second gate terminal coupled to a second resistor, a third field effect transistor coupled to the third node including a third gate terminal coupled to a third resistor, a first capacitor coupled to the second field effect transistor, a second capacitor coupled to the third field effect transistor, a third capacitor coupled between the second and third nodes, and a fourth field effect transistor coupled between the second and third nodes.

Description:
The present application is a Divisional Application of U.S. patent application Ser. No. 13/495,977, filed on Jun. 13, 2012, which is a Continuation Application of U.S. patent application Ser. No. 13/360,584, filed on Jan. 27, 2012, which is a Continuation Application of U.S. patent application Ser. No. 13/137,055, filed on Jul. 18, 2011 now U.S. Pat. No. 8,203,397, which is a Divisional Application of U.S. patent application Ser. No. 12/929,328 filed Jan. 14, 2011, now U.S. Pat. No. 8,031,031, which is a Divisional Application of U.S. patent application Ser. No. 12/289,823, filed on Nov. 5, 2008, now abandoned, which is based on and claims priority from Japanese Patent Application No. 2007-305830, filed on Nov. 27, 2007, the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an attenuator, and more particularly relates to an attenuator having a circuit element that adjusts the attenuation characteristic of the attenuator. 
     2. Description of the Related Art 
     An attenuator has been known as a circuit having a function to attenuate a gain of an input signal, and is used for a cellular phone, for example. In the cellular phone, for example, an attenuator is provided between an antenna that receives a signal and a low noise amplifier that adjusts a gain of the received signal. In this case, the attenuator has a role to attenuate the gain of the input signal so that the gain of the received signal may not exceed the dynamic range of the low noise amplifier. Recently, it has been required to perform communications by using a high frequency signal in wideband such as ultra wideband communications systems. Thus, an attenuator capable of handling wideband high-frequency signals as been required. 
     However, in some cases, the attenuation characteristic of an attenuator may be sharply changed depending on the frequency of an input signal under the influence of a parasitic element component of a circuit element which constitutes the attenuator. This is because the impedance of the parasitic element component changes relative to the frequency of the input signal. As the frequency of the input signal increases, a change in the attenuation characteristic of the attenuator becomes large. If the amount of attenuation of the gain of the input signal changes greatly depending on the frequency of the input signal, the low noise amplifier that receives an output signal from the attenuator has to have a complicated circuit configuration such that the amplifier can correspond to the change in the gain of the output signal of the attenuator. Thus, it is important to adjust the attenuation characteristic of the attenuator and to design the attenuator capable of handling wideband high-frequency signals. 
     There are mainly two kinds of attenuator configurations. One is a Π-type attenuator and the other is a T-type attenuator. In the Π-type attenuator, circuit elements are connected in the form of a Π-type circuit in a two terminal pair network. Meanwhile, in the T-type attenuator, circuit elements are connected in the form of a T-type circuit in a two terminal pair network. Japanese Patent Application Publication No. 2000-286659 (Patent Document 1) discloses a technique related to an attenuator in which the Π-type attenuator is combined with the additional T-type attenuator.  FIG. 11  shows the attenuator described in Patent Document 1. This attenuator adjusts the amount of attenuation of an input signal by adjusting a value of a control voltage applied to a control terminal  1008  and a value of a bias voltage applied to a bias terminal  1021 . For example, Patent Document 1 describes an attenuation technique in which while the bias voltage is applied to the bias terminal  1021  to drive PIN diodes  1004 ,  1006 ,  1010 ,  1012 , and  1014 , the value of the control voltage applied to the control terminal  1008  is increased. Thereby, internal resistances in the PIN diodes  1010 ,  1012 , and  1014  are decreased, while internal resistances in the PIN diodes  1004  and  1006  are increased, so that the amount of attenuation of the gain of the input signal received at an input terminal is increased. In other words, in the technique described in Patent Document 1, the attenuation characteristic of an attenuator  1000  is adjusted by adjusting the balance between the control voltage to be applied to the control terminal  1008  and the bias voltage to be applied to the bias terminal  1021 . 
     The present inventor has found that the conventional technique according to Patent Document 1 has the following problem. As described above, it is important to adjust the attenuation characteristic of an attenuator, and to design the attenuator capable of handling wideband high-frequency signals. However, in the technique described in Patent Document 1, the attenuation characteristic of the attenuator is adjusted by the values of the voltages to be applied to the control terminal and the bias terminal. In this case, for example, a step-down circuit for adjusting the value of the voltage to be applied to the bias terminal is required, so that the circuit scale of the attenuator is increased. In addition, since the attenuation characteristic of the attenuator is adjusted by the voltage to be applied to the terminal in Patent Document 1, a thermal noise and a shot noise may in some cases be mixed in an output signal of the attenuator, the thermal noise and the shot noise being turbulence of a voltage signal caused by the random motion of electric charges to be superimposed on the applied voltage. Since a receiving circuit of communications equipment is not to handle a signal having a high gain, the noise component has a great influence on a signal. For this reason, it is required that the receiving circuit have a circuit configuration of not generating a noise as much as possible. 
     SUMMARY 
     An attenuator according to the present invention includes: a T-type two terminal pair network including first and second terminals, first, second and third circuits, wherein said first terminal receives an input signal to be attenuated, wherein said first circuit is connected between said first and second terminals, wherein said second circuit is connected between said first circuit and said second terminal and is connected to said first via a node, wherein said third circuit is connected to said node; and 
     a capacitor connected to said node, wherein an amount of attenuation of said input signal is adjusted by a capacitance value of said capacitor. This capacitor is a shunt capacitor. 
     This shunt capacitor shunts an input signal of the attenuator. A current component to be shunted increases or decreases depending on a capacitance value of the shunt capacitor, since the amount of current flowing through the shunt capacitor is proportional to the capacitance value of the shunt capacitor. If a shunt capacitor having a large capacitance value is connected, the current component to be shunted becomes large, and accordingly a current component that flows out from an output terminal of the attenuator decreases. If the current component that flows out from the output terminal of the attenuator decreases, the amount of attenuation of the gain of the input signal increases since a gain of an output signal of the attenuator decreases. Conversely, if a shunt capacitance having a small capacitance value is connected, the current component to be shunted decreases, and a current component that flows out from the output terminal of the attenuator increases. If the current component that flows out from the output terminal of the attenuator increases, the amount of attenuation of the gain of the input signal decreases since a gain of an output signal of the attenuator increases. In this manner, according to the present invention, the attenuation characteristic of the attenuator is adjusted by using the capacitance component. Since the scale of the capacitance element is small as compared with a step-down circuit, the present invention can also prevent the circuit scale of the attenuator from being increased. Furthermore, in the present invention, since the attenuation characteristic of the attenuator is not adjusted by applying a voltage itself to a terminal of the attenuator, the attenuator can prevent the output signal of the attenuator from being mixed with a thermal noise and a shot noise. 
     The present invention exhibits the following effects: an attenuator that adjusts the attenuation characteristics of the attenuator and has a good attenuation characteristic can be designed; the circuit scale of the attenuator can be prevented from being increased; and an unnecessary noise component can be prevented from being mixed with an output anal of the attenuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows an attenuator according to a first embodiment of the present invention; 
         FIG. 2  is a graph for explaining an example of an attenuation characteristic of the attenuator; 
         FIG. 3  shows a high-frequency equivalent circuit of a MOS transistor; 
         FIG. 4  shows an attenuator according to a second embodiment of the present invention: 
         FIG. 5  shows a simulation result of the attenuation characteristic of the attenuator; 
         FIG. 6  shows an attenuator according to a third embodiment of the present invention; 
         FIG. 7  is a diagram for explaining a parasitic capacitance generated in a gate connection; 
         FIG. 8  shows an attenuator according to a fourth embodiment of the present invention: 
         FIG. 9  shows an attenuator according to a fifth embodiment of the present invention; 
         FIG. 10  shows a simulation result of an attenuator according to a sixth embodiment of the present invention; 
         FIG. 11  shows a conventional attenuator; and 
         FIG. 12  shows an attenuator relating to the attenuator according to the fourth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposes. 
     First Example 
     Embodiments of the present invention will be described below with reference to the accompanying drawings.  FIG. 1  shows an attenuator  100  according to an embodiment of the present invention. The attenuator  100  is a two terminal pair network (two-port circuit) that includes a circuit element between ports composed of terminals  101  and  103 , and ports composed of terminals  102  and  104 . The attenuator  100  includes Metal-Oxide-Semiconductor (MOS) transistors  105  and  106  which are a kind of a field-effect transistor. The transistors  105  and  106  are connected in series between the terminal  101  and the terminal  103 , as examples of a first circuit and a second circuit respectively. The attenuator  100  further includes a MOS transistor  107  connected in shunt between the MOS transistors  105  and  106 . A circuit  109  formed of the MOS transistors  105  to  107  is a typical T-type attenuator formed of the MOS transistors. Further, a capacitance element  108  is connected in shunt between the MOS transistors  105  and  106 , which are in the T-type attenuator. Since the MOS transistor  107  and the capacitance element  108  are shunt components in the two terminal pair network, they are also connected to an interconnection that connects between the terminal  103  and the terminal  104 . A value of the capacitance element  108  can be set to 20 [fF], for example. These MOS transistors are typically n-type MOS transistors, however, the MOS transistors may be p-type MOS transistors. Each of the MOS transistors  105  to  107  includes a gate terminal. 
       FIG. 2  shows the attenuation characteristic of the attenuator  100  in  FIG. 1  and that of the circuit  109 , i.e., the attenuator  100  in  FIG. 1  from which the capacitance element  108  is removed. “A” in  FIG. 2  denotes the attenuation characteristic of the attenuator (circuit  109 ), i.e., the attenuator  100  described in  FIG. 1  from which the capacitance element is removed. “B” in  FIG. 2  denotes the attenuation characteristic of the attenuator  100 . The vertical axis shows an amount of attenuation, i.e., the ratio of a gain of an output signal to a gain of an input signal of the attenuator (represented by decibel (dB)). The horizontal axis shows frequencies of the input signals to the respective attenuators. As apparent from  FIG. 2 , the attenuator  100  having the attenuation characteristic B has a larger amount of attenuation than the circuit  109  having the attenuation characteristic A. This is because the input signal is shunted via the capacitance element  108 , so that a current value of the output signal is lowered in the attenuator  100 . In this manner, the attenuator  100 , in which the capacitance element  108  is added to the T-type attenuator of the circuit  109 , can adjust the attenuation characteristic. Furthermore, the attenuator  100  having the attenuation characteristic B is capable of keeping the amount of attenuation at an approximate constant value even if the frequency of the input signal increases. On the other hand, as the frequency of the input signal increases, the amount of attenuation of the gain of the input signal largely decreases in the circuit  109  having the attenuation characteristic A. A “to-be-used frequency band” shown in  FIG. 2  shows the frequency range of signals to be received in a receiving circuit relating to communications that use a wide-band high-frequency signal, such as a UWB communications system. To take a specific example, assume a lower limit of the to-be-used frequency band is 3 GHz and an upper limit thereof is 5 GHz. In this case, the attenuator  100  having the attenuation characteristic B keeps the amount of attenuation at an approximate constant value regardless of the frequency of the input signal in the to-be-used frequency band, while the circuit  109  having the attenuation characteristic A has large decreases in the amount of attenuation in the to-be-used frequency hand as the frequency of the input signal increases. For this reason, when the circuit  109  having the attenuation characteristic A receives a signal in the “to-be-used frequency band” in  FIG. 2 , the amount of attenuation largely varies depending on the frequency of the received signal. Thus, in the communications using the wide band frequency signals, such as the UWB communications, the circuit configuration of a low noise amplifier that is located in the latter part of the attenuator becomes complicated. In this manner, the attenuator  100  in which the capacitance element  108  is added to the T-type attenuator of the circuit  109 , can adjust the attenuation characteristic and improve the attenuation characteristic. 
     Here, the reason why the amount of attenuation of the attenuator changes depending on the frequency of the input signal will be described.  FIG. 3  shows a high-frequency equivalent circuit of the MOS transistor that is one component of the attenuator  100  according to  FIG. 1 . The MOS transistor includes three electrodes of a source (S), a drain (D), and a gate (G), and among terminals of the electrodes, a parasitic capacitance component is present. For example, a capacitor  301  is a parasitic capacitance generated between the gate electrode and the source electrode of the MOS transistor; a capacitor  302 , between the gate electrode and a substrate; a capacitor  303 , between the gate electrode and the drain electrode; a capacitor  304 , between the source electrode and the substrate; a capacitor  306 , between the drain electrode and the substrate. Furthermore, a resistance  305  is a resistance component between the source electrode and the drain electrode. As described above, there are multiple, unavoidable parasitic capacitance components in the MOS transistor. Here, the attenuator  100  includes multiple MOS transistors as its circuit configuration. The attenuation characteristic of the attenuator  100  shown in  FIG. 1  can be obtained by analyzing a frequency characteristic of S 12  or S 21  that is a diagonal element of a scattering matrix (S matrix). The values of these S 12  and S 21  change depending on the parasitic capacitance generated in the circuit and the frequency. As a result, the attenuation characteristic of the attenuator obtained by the S 12  or the S 21  also changes depending on the frequency of the input signal. 
     Second Example 
       FIG. 4  shows an attenuator  400  that employs a variable capacitance element  408  as the capacitance element of the attenuator  100  according to  FIG. 1 . Other circuit elements exclusive of the capacitance element  408  in the attenuator  400  have the same configuration as that of the attenuator  100 . 
       FIG. 5  shows a change in the attenuation characteristic of the attenuator  400  according to  FIG. 4  constituted of MOS transistors  405  and  406 , a MOS transistor  407 , and the variable capacitance element  408 , the change appearing when gate widths of the MOS transistors  405  and  406  are respectively set to 13.5 [μm], a gate width of the MOS transistor  407  is set to 18.24 [μm], and a capacitance value of the variable capacitance element  408  is varied among 0 [fF], 20 [fF], 50 [fF], and 100 [fF]. As apparent from  FIG. 5 , the capacitance value of the variable capacitance element  408  is varied so that the attenuation characteristic of the attenuator  400  can be adjusted. Referring to  FIG. 5 , when the value of the variable capacitance element  408  is set to 20 [fF] a flat attenuation characteristic can be achieved within the range of up to 5 GHz of the frequency of the input signal. 
     Third Example 
       FIG. 6  shows an attenuator  600  having a configuration in which two capacitance elements are further added to the attenuator  400  shown in  FIG. 4 . More specifically, the attenuator  600  includes MOS transistors  605 ,  606 , and  607  and a variable capacitance element  608  that are the same circuit elements as those in the attenuator  400  of  FIG. 4 , and further includes new capacitance elements  609  and  610 . The attenuator  600  includes the capacitance elements  609  and  610  in addition to the capacitance element  608 . Thereby, an influence caused when the capacitance values of these capacitance elements vary at the time of manufacturing the attenuator  600  can be reduced, the influence being applied to the attenuation characteristic. As described above, the attenuation characteristic of the attenuator can be obtained by analyzing the frequency characteristic of S 12  and S 21  that are diagonal elements of the scattering matrix. One of the parameters that exert a large influence on the values of these S 12  and S 21  is the capacitance value of the capacitance element. Thus, the attenuator  600  is provided with multiple capacitance elements in the circuit so that a change caused by variation in the capacitance elements of S 12  or S 21  that are the diagonal elements of the scattering matrix can be reduced. S 12  and S 21  are fractional parameters. Thus, by providing the multiple capacitance elements in the circuit, terms that change depending on the capacitance value are included in both a denominator and a numerator of S 12  or S 21 . Accordingly, even when the capacitance elements  608 ,  609 , and  610  vary at the time of manufacturing the respective capacitance values, the change in S 12  or S 21  is canceled out by a change in the denominator and a change in the numerator. As a consequence, the attenuation characteristic that can be obtained from S 12  or S 21  does not largely change by the variation of the capacitance values of the capacitance elements  608 ,  609 , and  610  at the time of manufacturing. 
     Fourth Example 
       FIG. 7  shows an attenuator  700  in which a parasitic capacitor  711  is generated between a gate interconnection and a ground in the attenuator  600  shown in  FIG. 6 . In the attenuator, a parasitic capacitance is actually generated in the gate interconnection and the ground. Circuit elements exclusive of the parasitic capacitor  711  in the attenuator  700  are the same as those in the attenuator  600  shown in  FIG. 6 . The parasitic capacitor  711  exerts an influence on the attenuation characteristic of the attenuator  700 . First, a part of an input signal received at a terminal  701  flows to a gate interconnection of a MOS transistor  705  via a parasitic capacitance generated between a gate electrode and a source electrode of the MOS transistor  705  (see,  FIG. 3 ). Then, a leakage current that flows to the gate interconnection of the MOS transistor  705  flows toward the ground via the parasitic capacitor  711 . Since the shunt current component of the input signal received at the terminal  701  increases, the amount of attenuation of the gain of the input signal possibly increases when the frequency of the input signal is high. However, a change in the attenuation characteristic caused by the parasitic capacitance should be avoided as much as possible. Accordingly, as a fourth embodiment,  FIG. 8  shows an attenuator capable of reducing the leakage current component caused via the gate interconnection of the MOS transistor. In an attenuator  800  shown in  FIG. 8 , resistances  811 ,  812 , and  813  are respectively connected to gate electrodes of MOS transistors  805 ,  806 , and  807  that constitute the attenuator  800 . The resistance value of each of the resistances  811 ,  812 , and  813  is 1 [kΩ], for example. The resistance is connected to each of the gate electrodes of the MOS transistors  805 ,  806 , and  807  so that the input signal received at a terminal  801  can be prevented from being leaked via each of the gate interconnections of the MOS transistors  805 ,  806 , and  807 . Note that, the attenuator  800  shown in  FIG. 8  includes capacitance elements  809  and  810 . These capacitance elements  809  and  810  exhibit the same effect as that of the capacitance elements  609  and  610  included in the attenuator  600  according to  FIG. 6 , and are not essential components for preventing the input signal received at the terminal  801  from being leaked via each of the gate interconnections of the MOS transistors  805 ,  806 , and  807 . In other words, the configuration without including capacitance elements can also be employed, as shown in  FIG. 12 . 
     Fifth Example 
       FIG. 9  shows an example that realizes the function of a variable capacitance element  808  in the attenuator  800  according to  FIG. 8  by replacing it with the MOS transistor. An attenuator  900  shown in  FIG. 9  includes MOS transistors  907 ,  910 , and  913 , and capacitance elements  908 ,  911  and,  914  that are connected in series to these MOS transistors  907 ,  910 , and  913 , respectively. The voltage to be applied to each of the gate electrodes of the MOS transistors  907 ,  910 , and  913  is controlled to thereby change the number of the MOS transistors in which the source and the drain are conducted with each other, thus realizing the variable capacitance element. The large number of the MOS transistors among the MOS transistors  907 ,  910 , and  913  in which the source and the drain are conducted with each other, makes the input signal component to be shunted large. This means that the capacitance value of the variable capacitance element  808  in  FIG. 8  increases equivalently. On the contrary, the small number of the MOS transistors among the MOS transistors  907 ,  910 , and  913  in which the source and the drain are conducted with each other, makes the input signal component to be shunted small. This means that the capacitance value of the variable capacitance element  808  in  FIG. 8  decreases equivalently. Note that, in  FIG. 9 , the three MOS transistors  907 ,  910 , and  913  contribute to realize the equivalent variable capacitance element, and the three capacitance elements  908 ,  911  and,  914  contribute to realize the equivalent variable capacitance element. However, the number of the MOS transistors and that of the capacitance elements are not limited to three. For example, a circuit designer can select the number of the MOS transistors and that of the capacitance elements appropriately depending on the range of the capacitance value to be changed. Further, capacitance elements  916  and  917  exhibit the same effect as that of the capacitance elements  609  and  610  included in the attenuator  600  according to  FIG. 6 , and are not essential components to realize the equivalent variable capacitance. Furthermore, resistances  918  and  919  exhibit the same effect as that of the resistances  811  to  813  included in the attenuator  800  according to  FIG. 8 , and are not essential components to realize the equivalent variable capacitance. 
     Sixth Example 
     It has been described that the attenuation characteristic of the attenuator is adjusted by the variable capacitance element. Additionally, there is an alternative method for adjusting the attenuation characteristic of the attenuator. In this method, the gate voltage of the MOS transistor is adjusted so that the ON-resistance value of the MOS transistor can be adjusted, thereby adjusting the attenuation characteristic of the attenuator.  FIG. 10  shows how the attenuation characteristic of the T-type attenuator changes when the gate voltage of a MOS transistor that is connected in shunt in the MOS transistor that constitutes a T-type attenuator is changed. As shown in  FIG. 10 , the attenuation characteristic of the attenuator can also be adjusted by adjusting the ON-resistance of the MOS transistor. 
     Although it has been described by using the MOS transistor as the circuit element that constitutes an attenuator in the present embodiment described above, the attenuator can be formed of a circuit element other than the MOS transistor. Thus, the circuit element that constitutes the attenuator should not be limited to the MOS transistor. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention.