Amplification circuit

This application claims priority from Japanese Patent Application No. 2016-148985 filed on Jul. 28, 2016. The content of this application is incorporated herein by reference in its entirety.

The present disclosure relates to amplification circuits. Amplification circuits are used in wireless communication systems such as wireless local area networks (LANs) and cellular phones in order to amplify a signal. For example, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-512926 (Patent Document 1) discloses a low noise amplifier (LNA) that is used in a receiver of a wireless communication system.

It is sometimes desirable to make the gain of an amplification circuit constant regardless of the temperature. Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-512926 (Patent Document 1) discloses a configuration that adjusts an inductive load that is provided in parallel with an inductor, but does not disclose a configuration that makes the gain constant regardless of the temperature.

The present disclosure was made in light of the above-described circumstances, and the present disclosure makes it possible to adjust the gain of an amplification circuit in accordance with the temperature.

An embodiment of the present disclosure provides an amplification circuit that amplifies an input signal and outputs an amplified signal. The amplification circuit includes: an amplification element that outputs the amplified signal from an output terminal thereof; an inductor having one end to which a power supply voltage is supplied and another end that is connected to the output terminal of the amplification element; a variable resistor that is connected in parallel with the inductor; and a resistance value adjusting circuit that adjusts a resistance value of the variable resistor in accordance with temperature.

According to the embodiment of the present disclosure, the gain of the amplification circuit can be adjusted in accordance with the temperature.

Other features, elements, characteristics and advantages of the present disclosure will become more apparent from the following detailed description of embodiments of the present disclosure with reference to the attached drawings.

FIG. 1 illustrates the configuration of an amplification circuit according to an embodiment of the present disclosure;

FIG. 2 illustrates the configuration of an amplification circuit, which is an example of the amplification circuit illustrated in FIG. 1 ;

FIG. 3 illustrates the configuration of an amplification circuit, which is an example of the amplification circuit illustrated in FIG. 1 ;

FIG. 4A illustrates simulation results of operation of the amplification circuit of FIG. 3 ;

FIG. 4B illustrates simulation results of operation of the amplification circuit of FIG. 3 ;

FIG. 5 illustrates the configuration of an amplification circuit, which is an example of the amplification circuit illustrated in FIG. 1 ;

FIG. 6A illustrates simulation results of operation of the amplification circuit of FIG. 5 ;

FIG. 6B illustrates simulation results of operation of the amplification circuit of FIG. 5 ;

FIG. 6C illustrates simulation results of operation of the amplification circuit of FIG. 5 ; and

FIG. 7 illustrates the configuration of an amplification circuit, which is an example of the amplification circuit illustrated in FIG. 1 .

Hereafter, embodiments of the present disclosure will be described while referring to the drawings. FIG. 1 illustrates the configuration of an amplification circuit 100 according to an embodiment of the present disclosure. The amplification circuit 100 amplifies an input signal Pin and outputs an amplified signal Pout. For example, the amplification circuit 100 is an LNA that is used in a receiver of a wireless communication system. As illustrated in FIG. 1 , the amplification circuit 100 includes N-channel MOSFETs N 1 and N 2 , a P-channel MOSFET P 1 , a resistor R 1 , a capacitor C 1 , inductors L 1 and L 2 , matching networks (MNs) 110 and 111 , and a resistance value adjusting circuit 120 .

The N-channel MOSFETs N 1 and N 2 form a cascode-connected amplifier.

The source of the N-channel MOSFET N 1 is connected to one end of the inductor L 1 and the other end of the inductor L 1 is grounded. The input signal Pin is supplied to the gate of the N-channel MOSFET N 1 via the matching network 110 . In addition, a bias voltage BIAS 1 is supplied to the gate of the N-channel MOSFET N 1 via the resistor R 1 .

The source of the N-channel MOSFET N 2 is connected to the drain of the N-channel MOSFET N 1 . A power supply voltage Vdd is supplied to one end of the inductor L 2 and the other end of the inductor L 2 is connected to the drain of the N-channel MOSFET N 2 . A bias voltage BIAS 2 is supplied to the gate of the N-channel MOSFET N 2 . In addition, the gate of the N-channel MOSFET N 2 is connected to one end of the capacitor C 1 and the other end of the capacitor C 1 is grounded.

The amplified signal Pout is output from the drain (output terminal) of the N-channel MOSFET N 2 (amplification element) of the cascode-connected amplifier via the matching network 111 . In FIG. 1 , a cascode-connected amplifier is illustrated, but the amplifier is not limited to this configuration. For example, the amplifier may be formed of a single MOSFET. In addition, the amplification element used in the amplifier may be a bipolar transistor and the amplifier may consist of three or more cascode-connected stages.

The P-channel MOSFET P 1 (first FET) is connected in parallel with the inductor L 2 . Specifically, the source of the P-channel MOSFET P 1 is connected to one end of the inductor L 2 and the drain of the P-channel MOSFET P 1 is connected to the other end of the inductor L 2 . The gate of the P-channel MOSFET P 1 is connected to the resistance value adjusting circuit 120 . The P-channel MOSFET P 1 functions as a variable resistor having a resistance value that corresponds to a gate voltage Vg 1 of the P-channel MOSFET P 1 . Specifically, the gate-source voltage of the P-channel MOSFET P 1 increases and the resistance value of the P-channel MOSFET P 1 decreases as the gate voltage Vg 1 falls. The variable resistor is not limited to this configuration. For example, an N-channel MOSFET or a plurality of MOSFETs connected in series or parallel with each other may be used as the variable resistor. In the case of an N-channel MOSFET, the gate-source voltage increases and the resistance value thereof decreases as the gate voltage rises. In addition, a variable resistor other than a MOSFET may be used.

The resistance value adjusting circuit 120 adjusts the resistance value of the P-channel MOSFET P 1 in accordance with the temperature. If the P-channel MOSFET P 1 were not provided, since a mutual conductance gm of a MOSFET typically decreases as the temperature rises, the gain of the amplification circuit 100 would decrease as the temperature rises. Accordingly, the resistance value adjusting circuit 120 , for example, increases the gate voltage Vg 1 as the temperature rises in order to suppress such a decrease in gain. The gate-source voltage decreases as the gate voltage Vg 1 rises in the P-channel MOSFET P 1 . Therefore, the current that flows to the P-channel MOSFET P 1 from the power supply voltage Vdd decreases and the loss due to this current decreases, and as a result, the gain of the amplification circuit 100 increases. In addition, the resistance value adjusting circuit 120 decreases the gate voltage Vg 1 as the temperature falls. The gate-source voltage increases as the gate voltage Vg 1 falls in the P-channel MOSFET P 1 . Therefore, the current that flows to the P-channel MOSFET P 1 from the power supply voltage Vdd increases and the loss due to this current increases, and as a result, the gain of the amplification circuit 100 decreases.

Thus, in the amplification circuit 100 , the resistance value adjusting circuit 120 adjusts the resistance value of the P-channel MOSFET P 1 by controlling the gate voltage Vg 1 in accordance with the temperature, and as a result, the gain of the amplification circuit 100 can be adjusted in accordance with the temperature.

In addition, when the gain of the amplification circuit 100 is reduced by reducing the resistance value of the P-channel MOSFET P 1 , there is a benefit in terms of distortion characteristics since the amplitude of the voltage of the output terminal of the N-channel MOSFET N 2 decreases.

Furthermore, since there is no need to change the input impedance of the input signal Pin and the operation currents of the N-channel MOSFETs N 1 and N 2 when the gain of the amplification circuit 100 is adjusted by changing the resistance value of the P-channel MOSFET P 1 , there is little effect on the noise figure. In addition, a large current would have to be supplied to the N-channel MOSFET N 1 when the temperature is high in the case where the gain of the amplification circuit 100 is controlled to be constant by changing the bias voltage BIAS 1 . In contrast, since the gain is adjusted by changing the resistance value of the P-channel MOSFET P 1 in the amplification circuit 100 , an increase in current in the amplification circuit 100 that occurs when the temperature is high can be suppressed.

FIG. 2 illustrates the configuration of an amplification circuit 100 A, which is an example of the amplification circuit 100 . A resistance value adjusting circuit 120 A, which is an example of the resistance value adjusting circuit 120 , is illustrated in FIG. 2 . The resistance value adjusting circuit 120 A includes a P-channel MOSFET P 2 , a diode D 1 and a current source 200 .

The power supply voltage Vdd is supplied to the source of the P-channel MOSFET P 2 (second FET) and the gate and the drain of the P-channel MOSFET P 2 are connected to each other. Thus, the P-channel MOSFET P 2 is diode connected. An anode of the diode D 1 (first voltage generating circuit) is connected to the drain of the P-channel MOSFET P 2 and a cathode of the diode D 1 is connected to an input terminal of the current source 200 . The current source 200 (first current generating circuit) is a current source that generates a constant current or a variable current (first current).

In the resistance value adjusting circuit 120 A, a current generated by the current source 200 flows through the P-channel MOSFET P 2 . First, considering the configuration while ignoring the diode D 1 , the P-channel MOSFETs P 1 and P 2 are current mirror connected with respect to each other. Therefore, the resistance value of the P-channel MOSFET P 1 can be adjusted without necessarily being affected by the power supply voltage Vdd or the threshold voltage of the P-channel MOSFET.

In the amplification circuit 100 A, the voltage of the cathode of the diode D 1 of the resistance value adjusting circuit 120 A (first voltage) is the gate voltage Vg 1 of the P-channel MOSFET P 1 . Here, the voltage of the cathode of the diode D 1 is a voltage that is lower than the gate voltage of the P-channel MOSFET P 2 by an amount equal to the forward voltage of the diode D 1 . The P-channel MOSFET P 2 has a characteristic that the threshold voltage thereof decreases as the temperature rises (for example, −1 mV/° C.). In addition, the diode D 1 has a characteristic that the forward voltage thereof decreases as the temperature rises (for example, −2 mV/° C.). Therefore, the resistance value adjusting circuit 120 A increases the gate voltage Vg 1 of the P-channel MOSFET P 1 as the temperature rises (for example, 3 mV/° C.). As a result of the gate voltage Vg 1 of the P-channel MOSFET P 1 increasing by an amount equal to the change in the forward voltage of the diode D 1 with temperature and the change in the threshold voltage of the P-channel MOSFET P 1 with temperature, the resistance value of the P-channel MOSFET P 1 increases and the gain of the amplification circuit 100 A increases. Therefore, a reduction in gain that occurs as the temperature increases can be suppressed in the amplification circuit 100 A.

The resistance value adjusting circuit 120 A includes one P-channel MOSFET and one diode, but the numbers of these elements are not limited to this. For example, for each of the P-channel MOSFET and the diode, the number of elements connected in series may be appropriately adjusted in accordance with the range over which the gate voltage Vg 1 of the P-channel MOSFET P 1 is to be adjusted.

In addition, the resistance value adjusting circuit 120 A may adjust the resistance value of the P-channel MOSFET P 1 by adjusting the current generated by the current source 200 in accordance with the temperature. This is also the case in other embodiments described below.

FIG. 3 illustrates the configuration of an amplification circuit 100 B, which is an example of the amplification circuit 100 . A resistance value adjusting circuit 120 B, which is an example of the resistance value adjusting circuit 120 , is illustrated in FIG. 3 . Elements of the resistance value adjusting circuit 120 B that are the same as those of the resistance value adjusting circuit 120 A illustrated in FIG. 2 are denoted by the same symbols and detailed description thereof is omitted. The resistance value adjusting circuit 120 B includes the P-channel MOSFET P 2 , the diode D 1 , current sources 200 and 310 to 312 , an operational amplifier 300 , and resistors R 31 and R 32 .

The non-inverting input terminal of the operational amplifier 300 (second voltage generating circuit) is connected to the cathode of the diode D 1 . The output terminal of the operational amplifier 300 is connected to the inverting input terminal of the operational amplifier 300 via the resistor R 31 . The output terminal of the operational amplifier 300 is connected to the gate of the P-channel MOSFET P 1 via the resistor R 32 . The inverting input terminal of the operational amplifier 300 is connected to an input terminal of the current source 310 .

The current source 310 (second current generating circuit) generates a current Iptat (second current) having a magnitude that changes in accordance with the temperature. The current Iptat can be a current that is directly proportional to the temperature, for example. The current source 310 is not limited to having these temperature characteristics.

The current source 311 generates a current I 31 that is a constant current. An output terminal of the current source 311 is connected to the input terminal of the current source 310 .

The current source 312 generates a current I 32 that is a constant current. An input terminal of the current source 312 is connected between the resistor R 32 and the gate of the P-channel MOSFET P 1 .

Operation of the resistance value adjusting circuit 120 B will be described next. Here, Vref represents a voltage that is supplied to the non-inverting input terminal of the operational amplifier 300 , V 31 represents a voltage of the output terminal of the operational amplifier 300 (second voltage), and R 31 and R 32 respectively represent the resistance values of the resistors R 31 and R 32 .

The voltage of the inverting input terminal of the operational amplifier 300 is equal to Vref due to imaginary short circuiting of the operational amplifier 300 . In addition, the size of the current that flows through the resistor R 31 is equal to Iptat−I 31 . Therefore, V 31 =Vref+(Iptat−I 31 )×R 31 . Vg 1 =V 31 −I 32 ×R 32 =Vref+(Iptat−I 31 )×R 31 −I 32 ×R 32 due to a voltage drop from the current I 32 and the resistor R 32 .

In the resistance value adjusting circuit 120 B, a voltage Vref also changes in accordance with the temperature due to the temperature characteristics of the P-channel MOSFET P 2 and the diode D 1 . In addition, in the resistance value adjusting circuit 120 B, the size of the current Iptat changes in accordance with the temperature. Therefore, the resistance value adjusting circuit 120 B can adjust the resistance value of the P-channel MOSFET P 1 by adjusting the gate voltage Vg 1 of the P-channel MOSFET P 1 in accordance with the temperature. In the resistance value adjusting circuit 120 B, since the change in the gate voltage Vg 1 with temperature can be adjusted using the current Iptat, which changes with temperature, and the resistor R 31 , it is possible to improve the freedom with which the resistance value is adjusted compared with the resistance value adjusting circuit 120 A illustrated in FIG. 2 .

The P-channel MOSFET P 2 and the diode D 1 do not necessarily have to be provided in the resistance value adjusting circuit 120 B. In addition, in the case where the voltage V 31 is equal to the gate voltage Vg 1 , the resistor R 32 and the current source 312 do not have to be provided in the resistance value adjusting circuit 120 B.

FIGS. 4A and 4B illustrates simulation results of operation of the amplification circuit 100 B.

In FIG. 4A , the horizontal axis represents temperature (° C.), the vertical axis on the left side represents voltage (V) and the vertical axis on the right side represents current (μA). In FIG. 4A , the power supply voltage Vdd, the voltage Vref applied to the non-inverting input terminal of the operational amplifier 300 , the current Iptat having a size that changes in accordance with the temperature, and the gate voltage Vg 1 of the P-channel MOSFET P 1 are illustrated. As illustrated in FIG. 4A , the power supply voltage Vdd is constant at around 1.8 V. The voltage Vref is generated by adjusting a voltage using a constant gradient resulting from the temperature characteristics of the P-channel MOSFET P 2 and the diode D 1 .

In FIG. 4B , the horizontal axis represents temperature (° C.) and the vertical axis represents the gain (dB) of the amplification circuit 100 B. In the amplification circuit 100 B, the gain decreases as the temperature rises in the case where there is no P-channel MOSFET P 1 (without adjustment of resistance value). In the amplification circuit 100 B, as illustrated in FIG. 4A , the gate voltage Vg 1 increases as the temperature rises. Thus, as illustrated in FIG. 4B , in the amplification circuit 100 B, the decrease in the gain that occurs as the temperature rises can be suppressed and the gain can be kept substantially constant regardless of the temperature.

FIG. 5 illustrates the configuration of an amplification circuit 100 C, which is an example of the amplification circuit 100 . A resistance value adjusting circuit 120 C, which is an example of the resistance value adjusting circuit 120 , is illustrated in FIG. 5 . Elements of the resistance value adjusting circuit 120 C that are the same as those of the resistance value adjusting circuit 120 B illustrated in FIG. 3 are denoted by the same symbols and detailed description thereof is omitted. The resistance value adjusting circuit 120 C includes P-channel MOSFETs P 2 , P 51 and P 52 , N-channel MOSFETs N 51 to N 54 , the diode D 1 , current sources 310 , 500 and 501 , the operational amplifier 300 and the resistor R 31 . The P-channel MOSFETs P 51 and P 52 , the N-channel MOSFETs N 51 to N 54 and the current sources 310 , 500 and 501 constitute the second current generating circuit.

The current source 500 generates a current I 51 , which is a constant current. The current source 501 generates a current Iref that is a constant current.

The P-channel MOSFET P 51 is diode connected and the drain thereof is connected to the input terminal of the current source 310 . Therefore, the current Iptat flows to the P-channel MOSFET P 51 .

The P-channel MOSFET P 52 is current mirror connected to the P-channel MOSFET P 51 . Assuming the ratio between the sizes of the P-channel MOSFETs P 51 and P 52 is 1:M 1 , a current Iptat 2 that flows to the P-channel MOSFET P 52 is Iptat 2 =Iptat 1 ×M 1 .

The N-channel MOSFET N 51 is diode connected and the drain thereof is connected to the output terminal of the current source 500 . Therefore, a current I 51 flows to the N-channel MOSFET N 51 .

The N-channel MOSFET N 52 is current mirror connected to the N-channel MOSFET N 51 . In addition, the drain of the N-channel MOSFET N 52 is connected to the drain of the P-channel MOSFET P 52 . Assuming the ratio between the sizes of the N-channel MOSFETs N 51 and N 52 is 1:M 2 , a current I 52 that flows to the N-channel MOSFET N 52 is I 52 =Iptat 2 with an upper limit of I 51 ×M 2 .

The N-channel MOSFET N 53 is diode connected. In addition, the drain of the N-channel MOSFET N 53 is connected to the drain of the P-channel MOSFET P 52 . Therefore, a current I 53 that flows through the N-channel MOSFET N 53 is I 53 ≈0 A in the case where I 51 ×M 2 >Iptat×M 1 . In addition, I 53 =Iptat 2 −I 52 in the case where I 51 ×M 2 Iptat×M 1 .

The N-channel MOSFET N 54 is current mirror connected to the N-channel MOSFET N 53 . Assuming the ratio between the sizes of the N-channel MOSFETs N 53 and N 54 is 1:M 3 , a current I 54 that flows to the N-channel MOSFET N 54 is I 54 =I 53 ×M 3 =(Iptat 2 −I 52 )×M 3 in the case where I 51 ×M 2 ≤Iptat×M 1 .

The drain of the N-channel MOSFET N 54 and the input terminal of the current source 501 are connected to the inverting input terminal of the operational amplifier 300 . Therefore, a current Itc (second current) that flows to the N-channel MOSFET N 54 and the current source 501 via the resistor R 31 from the output terminal of the operational amplifier 300 is Itc=I 54 +Iref=(Iptat 2 −I 52 )×M 3 +Iref. The gate voltage Vg 1 of the P-channel MOSFET P 1 is adjusted to Vref+Itc×R 31 by the current Itc.

FIGS. 6A to 6C illustrates simulation results of operation of the amplification circuit 100 C. The operation of adjusting the gain of the amplification circuit 100 C will be described while referring to FIGS. 6A to 6C .

In FIG. 6A , the horizontal axis represents temperature (° C.), the vertical axis on the left side represents voltage (V) and the vertical axis on the right side represents current (μA). The current Iptat 2 having a size that changes with temperature, the current I 52 that flows through the N-channel MOSFET N 52 and the current Itc that flows through the resistor R 31 are illustrated in FIG. 6A . The current Iptat 2 increases as the temperature rises. The current I 52 changes in a similar manner to the current Iptat 2 as the temperature rises, but is constant after reaching the upper limit (I 52 ×M 2 ). In the simulation illustrated in FIG. 6A , the current I 52 becomes constant at around 25° C. Since the current Itc=(Iptat 2 −I 52 )×M 3 +Iref, Itc=Iref (around 2.0 μA) at around 25° C. or lower (first temperature region). At a temperature of around 25° or higher (second temperature region), the current Itc increases as the temperature rises.

In FIG. 6B , the horizontal axis represents temperature (° C.) and the vertical axis represents voltage (V). In FIG. 6B , the power supply voltage Vdd, the voltage Vref that is applied to the non-inverting input terminal of the operational amplifier 300 and the gate voltage Vg 1 of the P-channel MOSFET P 1 are illustrated. As illustrated in FIG. 6B , the power supply voltage Vdd is constant at around 1.8 V. The voltage Vref can change with the temperature because of the temperature characteristics of the P-channel MOSFET P 2 and the diode D 1 .

In FIG. 6C , the horizontal axis represents temperature (° C.) and the vertical axis represents the gain (dB) of the amplification circuit 100 C. As illustrated in FIG. 6B , the gradient of the gate voltage Vg 1 changes at around 25° C. Thus, the gain can be changed at around 25° C. in the amplification circuit 100 C, as illustrated in FIG. 6C .

The current source 501 is provided in the resistance value adjusting circuit 120 C illustrated in FIG. 5 in order to adjust the lowest values of the current Itc (for example, around 2.0 μA), but the current source 501 does not have to be provided.

FIG. 7 illustrates the configuration of an amplification circuit 100 D, which is an example of the amplification circuit 100 . A resistance value adjusting circuit 120 D, which is an example of the resistance value adjusting circuit 120 , is illustrated in FIG. 7 . Elements of the resistance value adjusting circuit 120 D that are the same as those of the resistance value adjusting circuit 120 B illustrated in FIG. 3 are denoted by the same symbols and detailed description thereof is omitted. The resistance value adjusting circuit 120 D includes the P-channel MOSFET P 2 , the diode D 1 , current sources 700 and 701 , operational amplifiers 710 and 711 and resistors R 71 to R 78 .

The current source 700 is a current source that generates a constant current or a variable current. The P-channel MOSFET P 2 , which is diode connected, is connected to an input terminal of the current source 700 .

The current source 701 is a current source that generates a constant current or a variable current. An output terminal of the current source 701 is connected to the anode of the diode D 1 . The cathode of the diode D 1 is grounded.

The non-inverting input terminal of the operational amplifier 710 is connected to the gate of the P-channel MOSFET P 2 via the resistor R 71 . One end of the resistor R 72 is connected between the non-inverting input terminal of the operational amplifier 710 and the resistor R 71 , and the other end of the resistor R 72 is grounded. The output terminal of the operational amplifier 710 is connected to the inverting input terminal of the operational amplifier 710 via the resistor R 73 . The output terminal of the operational amplifier 710 is connected to the gate of the P-channel MOSFET P 1 . The inverting input terminal of the operational amplifier 710 is connected to the operational amplifier 711 via the resistor R 74 . A gate voltage Vg 2 of the P-channel MOSFET P 2 changes in accordance with variations in the power supply voltage Vdd or the threshold voltage of the P-channel MOSFET. The gate voltage Vg 1 of the P-channel MOSFET P 1 changes so as to cancel out the effects of variations in the power supply voltage Vdd or the threshold voltage of the P-channel MOSFET that occur with changes in the gate voltage Vg 2 .

The non-inverting input terminal of the operational amplifier 711 is connected to the anode of the diode D 1 via the resistor R 75 . One end of the resistor R 76 is connected between the non-inverting input terminal of the operational amplifier 711 and the resistor R 75 , and the other end of the resistor R 76 is grounded. The output terminal of the operational amplifier 711 is connected to the inverting input terminal of the operational amplifier 711 via the resistor R 77 . An offset voltage Vos is supplied to one end of the resistor R 78 and the other end of the resistor R 78 is connected to the inverting input terminal of the operational amplifier 711 . A voltage Vf 1 of the anode of the diode D 1 changes in accordance with the temperature characteristics of the diode D 1 . Therefore, a voltage Vtemp output from the operational amplifier 711 also changes in accordance with the temperature. The changes that occur in the voltage Vtemp with temperature correspond to changes that occur in the gain of the operational amplifier 711 with temperature.

With this configuration, similarly to as in the amplification circuit 100 B, the effect of variations in the power supply voltage Vdd or the threshold voltage of the P-channel MOSFET is suppressed in the amplification circuit 100 D, and the gain of the amplification circuit 100 D can be adjusted in accordance with the temperature.

Exemplary embodiments of the present disclosure have been described above. According to an embodiment, the resistance value adjusting circuit 120 adjusts the resistance value of a variable resistor in accordance with the temperature, and as a result, the gain of the amplification circuit 100 can be adjusted in accordance with the temperature. For example, as illustrated in FIG. 4B , the gain of the amplification circuit 100 can be made substantially constant regardless of the temperature.

In addition, according to an embodiment, a variable resistor can be realized using a MOSFET.

In addition, according to an embodiment, a voltage (for example, Vref illustrated in FIG. 3 ) that changes in accordance with the gate voltage of the P-channel MOSFET P 2 can be generated and the gate voltage Vg 1 of the P-channel MOSFET P 1 can be adjusted on the basis of this voltage. Thus, the resistance value of the P-channel MOSFET P 1 can be adjusted without necessarily being affected by variations in the power supply voltage Vdd or a P-channel MOSFET threshold voltage.

In addition, according to an embodiment, a voltage (for example, Vref illustrated in FIG. 3 ) that changes in accordance with the gate voltage of the P-channel MOSFET P 2 can be realized using a diode.

In addition, according to an embodiment, the gate voltage Vg 1 of the P-channel MOSFET P 1 can be adjusted on the basis of a current that changes in accordance with the temperature (for example, Iptat illustrated in FIG. 3 ). Thus, for example, compared with the case where the gate voltage Vg 1 is adjusted using just the diode D 1 , it is possible to make the degree of freedom when adjusting the gate voltage of the P-channel MOSFET P 1 comparatively high.

Furthermore, according to an embodiment, as illustrated in FIG. 6A , for example, the current Itc, which has a different rate of change depending on the temperature, can be generated and the gate voltage of the P-channel MOSFET P 1 can be adjusted on the basis of the current Itc. Thus, for example, as illustrated in FIG. 6C , the gain of the amplification circuit can be changed depending on the temperature.

The purpose of the embodiments described above is to enable easy understanding of the present invention and the embodiments are not to be interpreted as limiting the present invention. The present invention can be changed or improved without departing from the gist of the invention and equivalents to the present invention are also included in the present invention. In other words, appropriate design changes made to the embodiments by one skilled in the art are included in the scope of the present invention so long as the changes have the characteristics of the present invention. For example, the elements included in the embodiment and the arrangements, materials, conditions, shapes, sizes and so forth of the elements are not limited to those exemplified in the embodiments and can be appropriately changed. In addition, the elements included in the embodiments can be combined as much as technically possible and such combined elements are also included in the scope of the present invention so long as the combined elements have the characteristics of the present invention.

While embodiments of the invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the invention. The scope of the invention, therefore, is to be determined solely by the following claims.