[0001]CROSS-REFERENCE TO RELATED APPLICATION
[0002]This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-125291, filed on Jun. 14, 2013, the entire contents of which are incorporated herein by reference.
[0003]FIELD
[0004]The embodiments discussed herein are related to a variable inductor circuit and a high frequency circuit.
[0005]BACKGROUND
[0006]High frequency circuits used for radio communication and the like include matching circuits for realizing high gain at operating frequencies. The addition of a matching circuit enables impedance matching between a transistor and a load, so power reflection does not occur between them. As a result, good characteristics are obtained (in the case of an amplifier, high gain is obtained).
[0007]In addition to capacitors and resistors, matching circuits include transmission lines which realize inductance components. However, if a transmission line is used, it is not easy to adjust an inductance value (because an adjustment of an inductance value requires, for example, the process of performing design and trial manufacture again). Accordingly, a change in operating frequency causes a deterioration in performance.
[0008]Therefore, variable inductor circuits which can electrically adjust inductance values (which may also be referred to as active inductors) are used.
[0009]Japanese Laid-open Patent Publication No. 2012-165435
[0010]Japanese Laid-open Patent Publication No. 08-181571
[0011]Y. Yu et al., “A Compact Wideband CMOS Low Noise Amplifier With Gain Flatness Enhancement”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. 45, No. 3, pp. 502-509, 2010
[0012]With conventional variable inductor circuits, however, resonance occurs at high frequencies and it is difficult to adjust inductance values at high frequencies. With the conventional variable inductor circuits, for example, resonance occurs at frequencies higher than and equal to 10 GHz. It is difficult to realize a variable inductor circuit which operates even at comparatively high frequencies like those of millimeter waves higher than and equal to 30 GHz.
[0013]SUMMARY
[0014]According to an aspect, there is provided a variable inductor circuit including a first transistor and a second transistor cascade-connected, a wiring which connects a drain of the first transistor and a gate of the second transistor, a capacitor whose one terminal is connected between the first transistor and the second transistor cascade-connected and whose other terminal is grounded, and a control circuit which adjusts an inductance value by controlling a capacitance value of the capacitor or gate voltage of the first transistor or the second transistor.
[0015]The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
[0016]It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.
[0017]BRIEF DESCRIPTION OF DRAWINGS FIG. 1 illustrates an example of a variable inductor circuit; FIG. 2 illustrates an example of an equivalent circuit of the variable inductor circuit illustrated in FIG. 1 ; FIG. 3 indicates an example of a circuit simulation result indicative of the relationship between a frequency and an inductance value; FIG. 4 illustrates an example of a high frequency circuit; FIG. 5 illustrates a first example of a variable inductor circuit; FIG. 6 indicates an example of the relationship between control voltage and a capacitance value of a capacitor C 1 (varactor diode); FIG. 7 illustrates a second example of a variable inductor circuit; FIG. 8 indicates an example of the relationship between gate voltage and mutual conductance of a transistor; and FIG. 9 illustrates an example of a variable inductor circuit using p-channel MOSFETs.
[0018]DESCRIPTION OF EMBODIMENTS
[0019]Embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals refer to like elements throughout.
[0020]First Embodiment
[0021]FIG. 1 illustrates an example of a variable inductor circuit.
[0022]A variable inductor circuit 1 is used as a part of a matching circuit included in a high frequency circuit used for radio communication or the like. The high frequency circuit is an amplifying circuit, a frequency mixer circuit, or the like. In this specification description will be given with comparatively high frequencies higher than and equal to 30 GHz as high frequencies. However, high frequencies are not limited to frequencies in particular higher than and equal to 30 GHz.
[0023]The variable inductor circuit 1 includes transistors Tr 1 and Tr 2 , capacitors C 1 and C 2 , a control circuit 2 , and a signal propagation suppressor 3 . In the following description it is assumed that the transistors Tr 1 and Tr 2 are n-channel MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors). An example of a variable inductor circuit using p-channel MOSFETs will be described later.
[0024]The transistors Tr 1 and Tr 2 are cascade-connected. That is to say, a source of the transistor Tr 1 is connected to a drain of the transistor Tr 2 . Power supply voltage VDD is applied to a drain of the transistor Tr 1 via the signal propagation suppressor 3 . Furthermore, a terminal (or a node) P is connected to the drain of the transistor Tr 1 . The terminal P is connected to, for example, a node in a high frequency circuit described later. Direct-current bias voltage Vias is applied to a gate of the transistor Tr 1 .
[0025]The signal propagation suppressor 3 which suppresses the propagation of a high-frequency signal is placed between the drain of the transistor Tr 1 which the high-frequency signal is inputted to or outputted from and power supply. This curbs the influence of the high-frequency signal on the power supply. A transmission line whose length is one fourth of a wavelength of the high-frequency signal may be used as the signal propagation suppressor 3 . However, in order to accommodate signals at different frequencies, it is desirable to use, for example, a RF (Radio Frequency) choke L 1 as illustrated in FIG. 1 . The RF choke L 1 suppresses the propagation of signals at frequencies higher than a certain frequency.
[0026]In addition, the variable inductor circuit 1 includes a wiring ln 1 which connects the drain of the transistor Tr 1 and a gate of the transistor Tr 2 . In the example of FIG. 1 , the drain of the transistor Tr 1 and the gate of the transistor Tr 2 are connected by the wiring ln 1 via the capacitor C 2 used for removing direct-current voltage. However, the capacitor C 2 may not be used.
[0027]One terminal of the capacitor C 1 is connected between the transistors Tr 1 and Tr 2 cascade-connected and the other terminal of the capacitor C 1 is grounded.
[0028]The control circuit 2 controls a capacitance value of the capacitor C 1 or gate voltage of the transistor Tr 1 or Tr 2 .
[0029]By connecting the transistors Tr 1 and Tr 2 and the capacitor C 1 in the above way, the variable inductor circuit 1 has a function as an inductor circuit. Furthermore, by controlling a capacitance value of the capacitor C 1 or gate voltage of the transistor Tr 1 or Tr 2 , an inductance value can be changed. The reason for this is as follows.
[0030]FIG. 2 illustrates an example of an equivalent circuit of the variable inductor circuit illustrated in FIG. 1 .
[0031]FIG. 2 illustrates an example of an equivalent circuit of a circuit including the transistors Tr 1 and Tr 2 and the capacitor C 1 illustrated in FIG. 1 . The transistor Tr 1 is replaced with a current source 10 and the transistor Tr 2 is replaced with a current source 11 . g m1 and g m2 indicate mutual conductance of the transistors Tr 1 and Tr 2 , respectively, and are parameters indicative of amplification degrees. V gs1 and V gs2 indicate gate-source voltage of the transistors Tr 1 and Tr 2 , respectively, and are influenced by gate-source parasitic capacitance.
[0032]Current I which flows in from the terminal P is expressed as I=g m1 V gs1 (1)
[0033]Furthermore, the following relationship holds in the equivalent circuit illustrated in FIG. 2 . g m1 V gs1 =g m2 V gs2 +jωC (− V gs1 ) (2) where C is a capacitance value of the capacitor C 1 and ω is angular velocity of the current I.
[0034]Equation (2) can be changed into ( g m1 +jωC ) V gs1 =g m2 V gs2 (3)
[0035]From equations (1) and (3), the current I is expressed as I=g m1 ( g m2 V gs2 /( g m1 +jωC )) (4)
[0036]By the way, impedance Z from the terminal P of the equivalent circuit is expressed as Z=V gs2 /I (5)
[0037]When equation (4) is substituted in equation (5), Z =(1/ g m2 )+( jωC/g m1 g m2 ) (6)
[0038]An imaginary part of the impedance Z corresponds to an inductance value L, so an inductance value L is equivalently expressed as L=C/g m1 g m2 (7)
[0039]Therefore, the inductance value L can be changed by adjusting the capacitance value C of the capacitor C 1 or the mutual conductance g m1 or g m2 of the transistor Tr 1 or Tr 2 .
[0040]If the inductance value L is changed by adjusting the capacitance value C, then the capacitor C 1 is considered as a variable capacitance element and the control circuit 2 adjusts bias voltage (control voltage) applied to the variable capacitance element. Alternatively, the control circuit 2 may change the inductance value L by adjusting gate voltage of the transistor Tr 1 or Tr 2 and thus adjusting the mutual conductance g m1 or g m2 .
[0041]Furthermore, as stated above, the variable inductor circuit 1 functions as an inductor circuit by the two transistors Tr 1 and Tr 2 , that is to say, by a small number of transistors. As a result, parasitic capacitance (gate-source capacitance) can be reduced. If parasitic capacitance is high, the inductance value L is large at high frequencies and resonance occurs. With the variable inductor circuit 1 according to the first embodiment, however, parasitic capacitance is low, so resonance is prevented. Therefore, it is possible to make the variable inductor circuit 1 function as an inductor circuit even at comparatively high frequencies.
[0042]FIG. 3 indicates an example of a circuit simulation result indicative of the relationship between a frequency and an inductance value. In FIG. 3 , a horizontal axis indicates a frequency (GHz) and a vertical axis indicates an inductance value (pH).
[0043]The following transistors are used as the transistors Tr 1 and Tr 2 included in the variable inductor circuit 1 illustrated in FIG. 1 for doing a circuit simulation. The values of the mutual conductance g m1 and g m2 are 58 mS and gate-source capacitance (parasitic capacitance) is 20 fF. In addition, the capacitance value C of the capacitor C 1 is 150 fF.
[0044]As can be seen from FIG. 3 , the variable inductor circuit 1 functions as an inductor circuit even at a frequency of 100 GHz.
[0045]As has been described, the variable inductor circuit 1 according to the first embodiment functions as a variable inductor even at high frequencies like those of millimeter waves higher than and equal to 30 GHz.
[0046]Second Embodiment
[0047]FIG. 4 illustrates an example of a high frequency circuit.
[0048]A high frequency circuit 20 is connected to a transistor Tr 3 which receives a high-frequency signal, and includes a matching circuit 21 which performs impedance matching. The transistor Tr 3 is, for example, an n-channel MOSFET. A high-frequency (millimeter-wave, for example) signal is inputted to a gate of the transistor Tr 3 via an input terminal IN. A source of the transistor Tr 3 is grounded and the matching circuit 21 is connected between a drain of the transistor Tr 3 and an output terminal OUT. The transistor Tr 3 may be a p-channel MOSFET.
[0049]The matching circuit 21 includes a variable inductor circuit 22 and a capacitor 23 . The capacitor 23 is connected between the transistor Tr 3 and the output terminal OUT. The variable inductor circuit 22 is connected to a node Nd between the transistor Tr 3 and the capacitor 23 .
[0050]Two examples of the variable inductor circuit 22 will now be described. A first example of the variable inductor circuit 22 will be referred to as a variable inductor circuit 22 a and a second example of the variable inductor circuit 22 will be referred to as a variable inductor circuit 22 b.
[0051](Example 1 of Variable Inductor Circuit)
[0052]FIG. 5 illustrates a first example of the variable inductor circuit. Components in FIG. 5 which are the same as those included in the variable inductor circuit 1 illustrated in FIG. 1 are marked with the same numerals.
[0053]The variable inductor circuit 22 a is almost the same as the variable inductor circuit 1 . However, a control circuit 30 controls a capacitance value of a capacitor C 1 . The capacitor C 1 is a variable capacitance element and is, for example, a varactor diode (variable capacitance diode). Furthermore, the capacitor C 1 is connected between transistors Tr 1 and Tr 2 via a capacitor C 3 used for removing direct-current voltage. In addition, a terminal P is connected to the node Nd of the matching circuit 21 illustrated in FIG. 4 . Direct-current voltage is applied to the capacitor C 1 . Accordingly, the use of the capacitor C 3 curbs the influence of this direct-current voltage on drain voltage of the transistor Tr 2 .
[0054]The control circuit 30 includes a resistor 31 and power supply 32 . Control voltage generated by the power supply 32 is applied to the capacitor C 1 via the resistor 31 . A capacitance value of the capacitor C 3 is significantly small compared with a capacitance value of the capacitor C 1 . Accordingly, a large part of a capacitance value of the whole of the capacitors C 1 and C 3 connected in series is the capacitance value of the capacitor C 1 . Therefore, the capacitance value C in equation (7) is determined on the basis of the capacitance value of the capacitor C 1 .
[0055]FIG. 6 indicates an example of the relationship between control voltage and the capacitance value of the capacitor C 1 (varactor diode). In FIG. 6 , a horizontal axis indicates control voltage (V) applied to the capacitor C 1 and a vertical axis indicates the capacitance value (pF) of the capacitor C 1 .
[0056]As can be seen from FIG. 6 , an increase in control voltage causes a decrease in the capacitance value of the capacitor C 1 and a decrease in control voltage causes an increase in the capacitance value of the capacitor C 1 . From equation (7), a small inductance value is obtained by making a capacitance value small. Therefore, the power supply 32 increases control voltage. On the other hand, a large inductance value is obtained by making a capacitance value large. Therefore, the power supply 32 decreases control voltage.
[0057]With the variable inductor circuit 22 a illustrated in FIG. 5 , as has been described, an inductance value can be adjusted. As a result, if the variable inductor circuit 22 a is used in the matching circuit 21 illustrated in FIG. 4 , a matching frequency can be adjusted. Accordingly, applications which differ in frequency can be dealt with by the use of the same circuit.
[0058]Furthermore, as stated above, the variable inductor circuit 22 a functions as an inductor circuit by the two transistors Tr 1 and Tr 2 , that is to say, by a small number of transistors. As a result, parasitic capacitance can be reduced and resonance is prevented. Therefore, as indicated in FIG. 3 , the variable inductor circuit 22 a functions as an inductor circuit even at high frequencies.
[0059]In addition, with the variable inductor circuit 22 a power supply is connected via a RF choke L 1 to a drain of the transistor Tr 1 connected to the terminal P, and power supply voltage VDD is applied to the drain of the transistor Tr 1 . The drain of the transistor Tr 1 is connected to the transistor Tr 3 of the high frequency circuit 20 via the terminal P, so bias voltage is supplied to the transistor Tr 3 . As a result, there is no need to add a bias supply circuit for the transistor Tr 3 . This checks an increase in circuit area.
[0060](Example 2 of Variable Inductor Circuit)
[0061]FIG. 7 illustrates a second example of the variable inductor circuit. Components in FIG. 7 which are the same as those included in the variable inductor circuit 1 illustrated in FIG. 1 are marked with the same numerals.
[0062]The variable inductor circuit 22 b is almost the same as the variable inductor circuit 1 . However, a control circuit 40 controls gate voltage of a transistor Tr 2 . In addition, a terminal P is connected to the node Nd of the matching circuit 21 illustrated in FIG. 4 .
[0063]The control circuit 40 includes a resistor 41 and power supply 42 . Control voltage generated by the power supply 42 is applied to a gate of the transistor Tr 2 via the resistor 41 . By adjusting gate voltage of the transistor Tr 2 , mutual conductance g m2 of the transistor Tr 2 changes. Accordingly, from equation (7), an inductance value L can be adjusted.
[0064]FIG. 8 indicates an example of the relationship between gate voltage and mutual conductance of the transistor. In FIG. 8 , a horizontal axis indicates gate voltage V g (V) and a vertical axis indicates the mutual conductance g m2 (S/mm) of the transistor Tr 2 . In the example of FIG. 8 , calculations are performed with gate width of the transistor Tr 2 as 80 μm.
[0065]As can be seen from FIG. 8 , an increase in the gate voltage V g causes an increase in the mutual conductance g m2 and a decrease in the gate voltage V g causes a decrease in the mutual conductance g m2 . From equation (7), a small inductance value L is obtained by making the mutual conductance g m2 large. Therefore, the power supply 42 increases the gate voltage V g . On the other hand, a large inductance value L is obtained by making the mutual conductance g m2 small. Therefore, the power supply 42 decreases the gate voltage V g .
[0066]With the variable inductor circuit 22 b illustrated in FIG. 7 , as has been described, an inductance value L can also be adjusted. The same effect that is achieved by the variable inductor circuit 22 a illustrated in FIG. 5 is obtained.
[0067]In the above example an inductance value L is adjusted by changing gate voltage of the transistor Tr 2 . However, gate voltage of the transistor Tr 1 may be changed. When gate voltage of the transistor Tr 1 is changed, mutual conductance g m1 of the transistor Tr 1 changes the same as the mutual conductance g m2 indicated in FIG. 8 changes. Accordingly, an inductance value L is adjusted on the basis of equation (7).
[0068]By the way, in the above description the variable inductor circuits 22 a and 22 b each using the transistors Tr 1 and Tr 2 , which are n-channel MOSFETs, are taken as examples. However, even if p-channel MOSFETs are used, a variable inductor circuit is realized by adopting the same structure. A variable inductor circuit using p-channel MOSFETs will now be described as a modification.
[0069]Modification
[0070]FIG. 9 illustrates an example of a variable inductor circuit using p-channel MOSFETs. Components in FIG. 9 which are the same as those included in the variable inductor circuit 1 illustrated in FIG. 1 are marked with the same numerals.
[0071]A variable inductor circuit 1 a includes transistors Tr 4 and Tr 5 which are p-channel MOSFETs, a signal propagation suppressor 3 a (RF choke L 2 ), capacitors C 1 and C 2 , and a control circuit 2 a.
[0072]The transistors Tr 4 and Tr 5 are cascade-connected. That is to say, a drain of the transistor Tr 4 is connected to a source of the transistor Tr 5 . Power supply voltage VDD is applied to a source of the transistor Tr 4 . The signal propagation suppressor 3 a and a terminal (or a node) P are connected to a drain of the transistor Tr 5 . The terminal P is connected to, for example, the node Nd of the matching circuit 21 in the above high frequency circuit 20 . A gate of the transistor Tr 5 is grounded. The signal propagation suppressor 3 a is grounded. A high-frequency signal is inputted to or outputted from the drain of the transistor Tr 5 and the influence of the high-frequency signal on ground potential is curbed.
[0073]In addition, the variable inductor circuit 1 a includes a wiring 1 n 2 which connects the drain of the transistor Tr 5 and a gate of the transistor Tr 4 . In the example of FIG. 9 , the drain of the transistor Tr 5 and the gate of the transistor Tr 4 are connected by the wiring 1 n 2 via the capacitor C 2 used for removing direct-current voltage. However, the capacitor C 2 may not be used.
[0074]One terminal of the capacitor C 1 is connected between the transistors Tr 4 and Tr 5 cascade-connected and the other terminal of the capacitor C 1 is grounded.
[0075]The control circuit 2 a controls a capacitance value of the capacitor C 1 or gate voltage of the transistor Tr 4 or Tr 5 .
[0076]As has been described, even if p-channel MOSFETs are used, the transistors Tr 4 and Tr 5 and the capacitor C 1 are connected in the manner illustrated in FIG. 9 . By doing so, an inductance value L is expressed as L=C/g m4 g m5
[0077]where C is a capacitance value of the capacitor C 1 and g m4 and g m5 are mutual conductance of the transistors Tr 4 and Tr 5 respectively. This is the same with equation (7). That is to say, even if p-channel MOSFETs are used, the variable inductor circuit 1 a functions as an inductor circuit.
[0078]Furthermore, the control circuit 2 a changes a capacitance value of the capacitor C 1 . By doing so, an inductance value L is adjusted. In addition, the control circuit 2 a changes gate voltage of the transistor Tr 4 or Tr 5 . By doing so, the mutual conductance g m4 or g m5 of the transistor Tr 4 or Tr 5 changes and an inductance value L is adjusted.
[0079]Moreover, the variable inductor circuit 1 a functions as an inductor circuit by the two transistors Tr 4 and Tr 5 , that is to say, by a small number of transistors. As a result, parasitic capacitance can be reduced and resonance is prevented. Therefore, the variable inductor circuit 1 a functions as an inductor circuit even at high frequencies.
[0080]The control circuit 30 illustrated in FIG. 5 , the control circuit 40 illustrated in FIG. 7 , or the like may be used as the control circuit 2 a.
[0081]The variable inductor circuit and the high frequency circuit according to an aspect of the present invention have been described on the basis of the embodiments. However, these are simple examples and the present invention is not limited to the above description.
[0082]In the above description, for example, a case where an inductance value is adjusted by controlling gate voltage of a transistor and a case where an inductance value is adjusted by controlling a capacitance value of the capacitor C 1 are described separately. However, both of them may be controlled at the same time to adjust an inductance value.
[0083]According to the disclosed variable inductor circuit and high frequency circuit, an inductance value can be adjusted even at comparatively high frequencies.
[0084]All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
