Abstract:
Provided is a low-reference-current generator that includes a circuit employing two feedback loops enabling it to operate even at a low voltage, has a high power supply rejection ratio (PSRR) to control power supply noise, and simply forms a voltage without a voltage-to-current converter used in a conventional general reference current generator. The reference current generator includes: a first voltage generator receiving a predetermined current and generating a first voltage that decreases as temperature increases; a second voltage generator generating a second voltage that increases as temperature increases; a first current generator generating a first current corresponding to the first voltage; a second current generator generating a second current corresponding to the second voltage; and a reference current generator receiving the first current and the second current and generating a reference current that is the sum of the first current and the second current.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and the benefit of Korean Patent Application No. 2004-104300, filed Dec. 10, 2004, and Korean Patent Application No. 2005-70624, filed Aug. 2, 2005, the disclosures of which are incorporated herein by reference in their entirety.  
       BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a reference current generator, and more particularly, to a reference current generator that sums up current sources having different temperature characteristics from each other at one node and generates a reference current.  
         [0004]     2. Discussion of Related Art  
         [0005]     In an integrated circuit (IC), a reference voltage and a reference current are used during an analog operation of an analog-to-digital converter and so forth, and are essential for reducing circuit variation resulting from process variation and helping the circuit to stably operate even within a wide temperature variation range. A typical example of a conventional reference voltage generation method uses a voltage of a diode (or only one junction of a transistor) biased at a uniform current, and a voltage VT of a thermal voltage generator.  
         [0006]      FIG. 1  is a circuit diagram of a conventional reference voltage generator. Referring to  FIG. 1 , the reference voltage generator comprises a voltage generator  10  including a voltage source that is proportional to temperature and another voltage source that is inversely proportional to temperature, a voltage former  20  forming a uniform voltage level using the voltage generated by the voltage generator  10 , and a voltage output  30  connected to the voltage former  20  and outputting a voltage corresponding to the voltage formed by the voltage former  20 .  
         [0007]     The voltage generator  10  receives a first power supply Vcc and a second power supply Vss, and includes a first line and a second line. The first line includes a first transistor T 1 , a second transistor T 2 , a third transistor T 3 , and a fourth transistor T 4  connected in series between the first power supply Vcc and the second power supply Vss, and the second line is connected between the first power supply Vcc and the second power supply Vss like the mirror image of the first line, and includes a fifth transistor T 5 , a sixth transistor T 6 , a seventh transistor T 7 , and an eighth transistor T 8  connected in series to one another. The first line is connected to the second power supply Vss through a first bipolar junction transistor Q 1 , and the second line is connected to the second power supply Vss through a resistor R 11  and a second bipolar junction transistor Q 2 . The first and second bipolar junction transistors Q 1  and Q 2  are diode-connected. The first, second, fifth, and sixth transistors T 1 , T 2 , T 5 , and T 6  are P-channel metal oxide semiconductor (PMOS) transistors, and the third, fourth, seventh, and eighth transistors T 3 , T 4 , T 7 , and T 8  are N-channel metal oxide semiconductor (NMOS) transistors.  
         [0008]     Gates of the first and second transistors T 1  and T 2  are connected to gates of the fifth and sixth transistors T 5  and T 6  respectively in the mirror configuration, and gates of the third and fourth transistors T 3  and T 4  are connected to gates of the seventh and eighth transistors T 7  and T 8  respectively in the mirror configuration. The third, fourth, fifth, and sixth transistors T 3 , T 4 , T 5 , and T 6  are diode-connected.  
         [0009]     When the fifth and sixth transistors T 5  and T 6  are turned on, since the first and second transistors T 1  and T 2  are connected to the fifth and sixth transistors T 5  and T 6  in the mirror configuration, the same current that flows through the fifth and sixth transistors T 5  and T 6  flows through the first and second transistors T 1  and T 2 . When the first and second transistors T 1  and T 2  are turned on, the third and fourth transistors T 3  and T 4  are turned on so that the same current that flows through the third and fourth transistors T 3  and T 4  flows through the seventh and eighth transistors T 7  and T 8 . Therefore, a first current I 1  and a second current I 2  of equal magnitude flow through the first line and the second line, respectively, due to mutual current mirror operation.  
         [0010]     Here, when the first current I 1  flows through the first bipolar transistor Q 1  to the second power supply Vss, a temperature around the first bipolar transistor Q 1  goes up so that a voltage corresponding to the first current I 1  decreases due to semiconductor characteristics of the first bipolar transistor Q 1 .  
         [0011]     When the second current I 2  flows through the first resistor R 11  and the second bipolar transistor Q 2  to the second power supply Vss, a predetermined voltage drop occurs across the first resistor R 11 .  
         [0012]     The voltage drop across the first resistor R 11  is described below. Since voltage levels of sources of the fourth and eighth transistors T 4  and T 8  are the same, voltages applied to the first and second bipolar junction transistors Q 1  and Q 2  and the first resistor R 11  are as shown in Formula 1 according to Kirchhoff&#39;s voltage law: 
 
 Vq 1 −Vq 2 −V   R11 =0  Formula 1 
 
         [0013]     Here, Vq 1  denotes a voltage across the first bipolar junction transistor Q 1 , Vq 2  denotes a voltage across the second bipolar junction transistor Q 2 , and V R11  denotes a voltage across the first resistor R 11 .  
         [0014]     Since the bipolar junction transistors are diode-connected, voltages formed at the bipolar junction transistors are as shown in Formula 2: 
 
 Vq=V   T  ln( Id/Is )  Formula 2 
 
         [0015]     Here, Is denotes a saturated current as a constant, and Id denotes a current flowing through the bipolar junction transistors.  
         [0016]     When Formula 2 is inserted into Formula 1, the voltage across the first resistor R 11  is given by Formula 3: 
 
 V   R11   =V   T  ln( N )  Formula 3 
 
         [0017]     Here, V R11  denotes the voltage of the first resistor R 11 , and V T  denotes a thermal voltage (kT/q), which is proportional to temperature and is about 25.6 mV at normal temperature. N denotes a size ratio of the first and second bipolar junction transistors Q 1  and Q 2 .  
         [0018]     Referring to Formula 3, the size ratio of the first and second bipolar junction transistors Q 1  and Q 2  is adjusted by the voltage applied to the first resistor R 11  so that the voltage across the first resistor R 11  generated by the second current I 2  can be adjusted. However, the voltage of the first resistor R 11  is proportional to temperature as shown in Formula 3.  
         [0019]     The voltage former  20  includes a third line that is supplied with power from the first power supply Vcc and the second power supply Vss, and has a ninth transistor T 9  and a tenth transistor T 10  connected in series to each other. In the third line, a third bipolar junction transistor Q 3  and a second resistor R 12  are connected between the tenth transistor T 10  and the second power supply Vss. The third bipolar junction transistor Q 3  is diode-connected. Also, a first node N 1  that is connected to the voltage output  30  is formed between the tenth transistor T 10  and the diode-connected third bipolar junction transistor Q 3 .  
         [0020]     The ninth and tenth transistors T 9  and T 10  are PMOS transistors. Gates of the ninth and tenth transistors T 9  and T 10  are connected to the gates of the fifth and sixth transistors T 5  and T 6  respectively in the mirror configuration so that a third current I 3  of the same magnitude as the current flowing through the fifth and sixth transistors T 5  and T 6  flows through the ninth and tenth transistors T 9  and T 11 .  
         [0021]     Here, the third current I 3  flows through the second resistor R 12  and the diode-connected third bipolar junction transistor Q 3  to the second power supply Vss, the second resistor R 12  mirrors the voltage of the first resistor R 11  in the second line, and the third bipolar junction transistor Q 3  closely mirrors the voltage applied to the first bipolar junction transistor Q 1  in the first line.  
         [0022]     Therefore, the resulting voltage across the second resistor R 12  is increased by the surrounding temperature as shown in Formula 1, and the voltage across the third bipolar junction transistor Q 3  is decreased by the surrounding temperature like the first bipolar junction transistor Q 1 . When the voltage decrease and increase perfectly offset each other, voltage variation according to temperature can be reduced.  
         [0023]     The method described above is considered to be an effective method of reducing deviation of reference voltage and reference current in response to variation of temperature and process in an IC, and thus widely used. When deviation according to temperature characteristics of general PN junction and temperature of V T  are designed to offset one another, a reference voltage of the method has a value of about 1.26 V corresponding to the bandgap of silicon, and thus called a bandgap reference voltage.  
         [0024]     In an IC device, a metal-oxide semiconductor field-effect transistor (MOSFET) device has been continuously scaled down in order to improve operating speed, and thus a gate length of the MOSFET device reached 130 μm. Therefore, characteristics of the device are considerably improved, and a power supply voltage has been reduced to 1.2 V so that power consumption can be largely reduced. However, the power supply voltage of 1.2 V is lower than a conventional general reference voltage of 1.26 V. In addition, considering a margin of an operating point of a transistor to output a reference voltage, it is generally essential that the reference power supply voltage decreases to 1.0 V or below. However, with a conventional bandgap reference voltage generator, it is hard to reduce the reference voltage as described above.  
         [0025]     In addition, reduction of operating voltage due to scaling of devices causes a signal-to-noise ratio (SNR) of a signal to decrease. This is because noise does not largely decrease compared to an actual signal interval, but rather increases due to a high operating speed and so forth. Due to this effect, a louder noise source exists in a power supply line that supplies power to a circuit, and in result, output noise of the reference voltage generator also increases.  
       SUMMARY OF THE INVENTION  
       [0026]     The present invention is directed to a reference current generator that includes a circuit employing two feedback loops enabling it to operate even at a low voltage, has a high power supply rejection ratio (PSRR) to control power supply noise, and simply forms a voltage without a voltage-to-current converter used in a conventional general reference current generator.  
         [0027]     One aspect of the present invention provides a reference current generator comprising: a first voltage generator receiving a predetermined current and generating a first voltage that decreases as temperature increases; a second voltage generator generating a second voltage that increases as temperature increases; a first current generator generating a first current corresponding to the first voltage; a second current generator generating a second current corresponding to the second voltage; and a reference current generator receiving the first current and the second current and generating a reference current that is the sum of the first current and the second current.  
         [0028]     Another aspect of the present invention provides a reference current generator comprising: a first current generator receiving a predetermined current and generating a first current that increases as temperature increases; a second current generator receiving a predetermined current and generating a second current that decreases as temperature increases; and a reference current generator summing up the first current and the second current to generate a third current. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail preferred exemplary embodiments thereof with reference to the attached drawings in which:  
         [0030]      FIG. 1  is a circuit diagram of a conventional reference voltage generator;  
         [0031]      FIG. 2  is a circuit diagram of a first exemplary embodiment of a reference current generator according to the present invention;  
         [0032]      FIG. 3  is a circuit diagram of a second exemplary embodiment of a reference current generator according to the present invention;  
         [0033]      FIG. 4  is a circuit diagram of an initial driving circuit applied to the reference current generator shown in  FIG. 3 ; and  
         [0034]      FIG. 5  is a circuit diagram of an example of amplifiers shown in  FIGS. 2 and 3 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0035]     Hereinafter, an exemplary embodiment of the present invention will be described in detail. However, the present invention is not limited to the exemplary embodiments disclosed below, but can be implemented in various types. Therefore, the present exemplary embodiment is provided for complete disclosure of the present invention and to fully inform the scope of the present invention to those of ordinary skill in the art.  
         [0036]      FIG. 2  is a conceptual circuit diagram of a reference current generator according to the present invention. Referring to  FIG. 2 , the reference current generator comprises a first current generator  100 , a second current generator  200 , and a first reference voltage generator  300 . According to an increase in temperature, the first current generator  100  reduces a current, and the second current generator  200  increases a current. The reference current generator sums the currents formed by the first and second current generators  100  and  200 , and thus generates a uniform current. The first reference voltage generator  300  generates a predetermined voltage using the sum of currents formed by the first and second current generators  100  and  200 .  
         [0037]     The first current generator  100  includes a first diode D 1 , a current source I D , a first amplifier  131 , a first transistor M 1 , and a second transistor M 2 . When the current source I D  allows a forward current to flow, a predetermined voltage is formed across the first diode D 1  due to diode characteristics irrespective of the uniform current flowing through the first diode. Here, the predetermined voltage that is formed across the first diode D 1  varies according to temperature, and decreases when the surrounding temperature increases.  
         [0038]     The first amplifier  131  is supplied with two input voltages and adjusts one output voltage level. In addition, the first diode D 1  is connected to one input terminal of the first amplifier  131 , and a first resistor R 1  through which a predetermined current flows is connected to the other input terminal. Therefore, the voltage formed across the first diode D 1  is applied to the former input terminal, and a voltage of the first resistor R 1  is applied to the latter input terminal. Hence, as the temperature increases, the voltage across the first diode D 1  decreases so that the voltage output from the first amplifier  131  decreases. In addition, the first amplifier  131  is an inverted amplifier, and thus has a negative voltage level. As a result, a voltage that is added to the first resistor R 1  by feedback becomes the same as the voltage of the first diode D 1 .  
         [0039]     Gates of the first and second transistors M 1  and M 2  are connected to each other in the mirror configuration. In addition, the gates are connected to an output terminal of the first amplifier  131  so that a predetermined current flows through the first and second transistors M 1  and M 2  according to the output voltage of the first amplifier  131 , and a current corresponding to a ratio of the first transistor M 1  and the second transistor M 2  flows through the second transistor M 2 . Here, the current flowing through the first transistor M 1  flows through the first resistor R 1  and thus allows a predetermined voltage to be applied to the first amplifier  131 . In addition, magnitudes of the currents flowing through the first and second transistors M 1  and M 2  are determined according to the output voltage of the first amplifier  131 , and the first amplifier  131  outputs a voltage that decreases as temperature is increased by the first diode D 1 . Therefore, the magnitudes of the currents flowing through the first and second transistors M 1  and M 2  decrease as the temperature increases. And, the current flowing through the second transistor M 2  flows through a second node N 2 .  
         [0040]     The second current generator  200  includes a third transistor M 3 , a fourth transistor M 4 , a fifth transistor M 5 , a sixth transistor M 6 , a second amplifier  231 , a third amplifier  232 , a second resistor R 2 , a first bipolar junction transistor Q 12 , and a second bipolar junction transistor Q 22 . The third and fourth transistors M 3  and M 4  are connected so as to mirror each other, and gates thereof are connected to an output terminal of the second amplifier  231 . Therefore, currents flowing through the third and fourth transistors M 3  and M 4  are determined according to an output voltage of the second amplifier  231 . In addition, the first and second bipolar junction transistors Q 12  and Q 22  are diode-connected.  
         [0041]     The output terminal of the second amplifier  231  is connected to the gates of the third and fourth transistors M 3  and M 4 . One input terminal of the second amplifier  231  is connected in parallel to the third transistor M 3  and the first bipolar junction transistor Q 12 , the other input terminal is connected in parallel to the fourth transistor M 4 , and the second resistor R 2  and the second bipolar junction transistor Q 22  connected in series. Therefore, the former input terminal is supplied with a voltage formed by the current flowing through the third transistor M 3  at the second bipolar junction transistor Q 12 , and the latter input terminal is supplied with a voltage across the second resistor R 2  and the second bipolar junction transistor Q 22 . Here, the voltage across the second resistor R 2  and the second bipolar junction transistor Q 22  corresponds to Formula 3 above and increases according to increase in temperature.  
         [0042]     The fifth and sixth transistors M 5  and M 6  are connected so as to mirror each other and thus gates thereof are connected to each other. The gates of the fifth and sixth transistors M 5  and M 6  are connected to an output terminal of the third amplifier  232 . Therefore, currents according to an output voltage of the third amplifier  232  flow through the fifth and sixth transistors M 5  and M 6 , and a ratio of the currents flowing through the fifth and sixth transistors M 5  and M 6  is determined according to sizes of the fifth and sixth transistors M 5  and M 6 . In addition, one input terminal of the third amplifier  232  is connected to the second resistor R 2 , and thus the voltage level increases as temperature increases so that the output voltage of the third amplifier  232  increases according to increase in temperature. Therefore, the currents flowing through the fifth and sixth transistors M 5  and M 6  increase as the temperature increases. And, the current flowing through the sixth transistor M 6  is supplied to the second node N 2 , and thus added to the current flowing through the second transistor M 2 .  
         [0043]     Here, sizes of the first and second transistors M 1  and M 2  and the fifth and sixth transistors M 5  and M 6  are adjusted, and thus the magnitudes of currents flowing through the second and sixth transistors M 2  and M 6  are adjusted so that a current sum at the second node N 2  remains constant irrespective of a change in temperature.  
         [0044]     The first reference voltage generator  300  includes a reference resistor Rref, and supplies the reference resistor Rref with a uniform voltage irrespective of change in temperature using the current flowing through the second node N 2  as a source current.  
         [0045]      FIG. 3  is a circuit diagram of a second exemplary embodiment of a reference current generator according to the present invention. Referring to  FIG. 3 , the reference current generator comprises a third current generator  400 , a fourth current generator  500 , and a second reference voltage generator  600 . As temperature increases, the third current generator  400  increases a current and the fourth current generator  500  decreases a current. The reference current generator sums up the currents generated by the third and fourth current generators  400  and  500  to form a uniform current. The second reference voltage generator  600  generates a predetermined voltage using the uniform current resulting from summing the currents formed by the third and fourth current generators  400  and  500 .  
         [0046]     The third current generator  400  includes a first transistor M 11 , a second transistor M 12 , a third transistor M 13 , a fourth transistor M 14 , a fifth transistor M 15 , a sixth transistor M 16 , a first amplifier  431 , a first bipolar junction transistor Q 13 , a first resistor Ra, a third resistor Rc, and a capacitor Cc. The fourth current generator  500  includes a seventh transistor M 21 , an eighth transistor M 22 , a ninth transistor M 23 , a tenth transistor M 24 , an eleventh transistor M 25 , a twelfth transistor M 26 , a second amplifier  531 , a second bipolar junction transistor Q 23 , a third bipolar junction transistor Q 33 , and a second resistor Rb.  
         [0047]     The first and second transistors M 11  and M 12  and the fifth transistor M 15 , the third and fourth transistors M 13  and M 14  and the sixth transistor M 16 , the seventh and eighth transistors M 21  and M 22  and the eleventh transistor M 25 , and the ninth and tenth transistors M 23  and M 24  and the twelfth transistor M 26  are connected to mirror each other, respectively. And the first, second, and third bipolar junction transistors Q 13 , Q 23 , and Q 33  are diode-connected.  
         [0048]     The first bipolar junction transistor Q 13  is connected to a drain of the third transistor M 13  through a first node N 1 . The first resistor Ra is connected to a drain of the fourth transistor M 4  through a second node N 2 . The second resistor Rb and the second bipolar junction transistor Q 23  are connected in series to a drain of the ninth transistor M 23  through a third node N 3 . The third bipolar junction transistor Q 33  is connected to a drain of the tenth transistor M 24  through a fourth node N 4 .  
         [0049]     Gates of the first, second, and fifth transistors M 11 , M 12 , and M 15  are connected to an output terminal of the first amplifier  431 . A voltage of the first node N 1  is supplied to one input terminal of the first amplifier  431 , and a voltage of the second node N 2  is supplied to the other input terminal.  
         [0050]     Gates of the seventh, eighth, and eleventh transistors M 21 , M 22 , and M 25  are connected to an output terminal of the second amplifier  531 . A voltage of the third node N 3  is supplied to one input terminal of the second amplifier  531 , and a voltage of the fourth node N 4  is supplied to the other input terminal.  
         [0051]     The fifth transistor M 15  is connected to the gates of the first and second transistors M 11  and M 12 , and thus supplies a current corresponding to a current flowing through the second transistor M 12  to the sixth transistor M 16 . The sixth transistor M 16  is connected to gates of the third and fourth transistors M 13  and M 14 , and to a reference resistor Rref through a fifth node N 5 .  
         [0052]     The eleventh transistor M 25  is connected to the gates of the seventh and eighth transistors M 21  and M 22 , and thus supplies a current corresponding to a current flowing through the seventh transistor M 21  to the twelfth transistor M 26 . The twelfth transistor M 26  is connected to gates of the ninth and tenth transistors M 23  and M 24 , and to the reference resistor Rref through the fifth node N 5 .  
         [0053]     Operation of the reference current generator will be now described below. First, a voltage allows the first, second, and fifth transistors M 11 , M 12 , and M 15  to generate predetermined currents, supplied from the output terminal of the first amplifier  431  to the gates of the transistors M 11 , M 12 , and M 15 . And, the third, fourth, and sixth transistors M 13 , M 14 , and M 16  are turned on by the voltage applied to gates thereof, and thus allow the currents formed by the first, second, and fifth transistors M 11 , M 12 , and M 15  to flow. The current formed by the first transistor M 11  is supplied to the first bipolar junction transistor Q 13 , and the first bipolar junction transistor Q 13  is connected in a forward bias direction and thus has a predetermined voltage level. Here, the level of the voltage across the first bipolar junction transistor Q 13  decreases when a surrounding temperature increases.  
         [0054]     Therefore, when the surrounding temperature increases, the voltage of the first node N 1  decreases, and thus the first, second, and fifth transistors M 11 , M 12 , and M 15  allow less current to flow. In addition, since the second node N 2  has a voltage applied to the first resistor Ra by a current flowing through the fourth transistor M 14 , the first amplifier  431  is supplied with a predetermined voltage by the current generated by the output terminal of the first amplifier  431 . In result, an output voltage of the first amplifier  431  is adjusted by the current flowing through the output terminal of the first amplifier  431 .  
         [0055]     Therefore, the fifth node N 5  that is connected to the fifth and sixth transistors M 15  and M 16  is supplied with the current that decreases when temperature increases.  
         [0056]     In addition, a voltage supplied from the output terminal of the second amplifier  531  to the gates of the seventh, eighth, and eleventh transistors M 21 , M 22 , and M 25  enables the transistors M 21 , M 22 , and M 25  to generate predetermined currents. And, the ninth, tenth, and twelfth transistors M 23 , M 24 , and M 26  are turned on by the voltage applied to gates thereof, and thus allow the currents formed by the seventh, eighth, and eleventh transistors M 21 , M 22 , and M 25  to flow. The current formed by the seventh transistor M 21  is supplied to the third node N 3 , and the current formed by the eighth transistor M 22  is supplied to the fourth node N 4 . Here, a voltage is formed at the third node N 3  according to Formula 3 described above, and thus increases when a surrounding temperature increases. Since the voltage of the third node N 3  that is input to the second amplifier  531  increases, the seventh, eighth, and eleventh transistors M 21 , M 22 , and M 25  allow larger currents to flow. Hence, the fifth node N 5  is supplied with a current that increases when the surrounding temperature increases.  
         [0057]     Therefore, the currents flowing through the fifth and eleventh transistors Ml 5  and M 25  are summed up and become a current Iref that is independent of temperature, the current Iref flowing through the fifth node N 5 . And, the current Iref that flows through the fifth node N 5  is supplied to the reference resistor Rref so that a uniform voltage which is temperature invariant is formed across the reference resistor Rref.  
         [0058]     The third resistor Rc and the capacitor Cc are connected in series to the gates of the first, second, and fifth transistors M 11 , M 12 , and M 15 . The first resistor Ra that passes a current by a diode voltage is driven by one cascade current mirror circuit. The cascade current mirror circuit is driven by a differential-input single-output amplifier. A high loop gain by the amplifier and cascade current mirror is not guaranteed to be stabilized by only the third resistor Rc, and thus is compensated by the structure having the third resistor Rc and the capacitor Cc connected in series. In order to sufficiently separate a power supply line and a signal line, a high power supply rejection ratio (PSRR) is required, and thus a high loop gain is needed.  
         [0059]      FIG. 4  is a circuit diagram of an initial driving circuit applied to the reference current generator shown in  FIG. 3 . Referring to  FIG. 4 , the initial driving circuit includes a thirteenth transistor MS 1 , a fourteenth transistor MS 2 , and a fifteenth transistor MS 3 . As for the thirteenth transistor MS 1 , a source is connected to a first power supply Vcc, a drain is connected to a gate of the fourteenth transistor MS 2 , and a gate is connected to a second power supply Vss. As for the fourteenth transistor MS 2 , a drain is connected to a predetermined terminal, a source is connected to the second power supply Vss, and the gate is connected to the drain of the thirteenth transistor MS 1  and a drain of the fifteenth transistor MS 3 . As for the fifteenth transistor MS 3 , a drain is connected to the gate of the fourteenth transistor MS 2 , a source is connected to the second power supply Vss, and a gate is connected to a predetermined terminal. The thirteenth transistor MS 1  is a P-channel metal oxide semiconductor (PMOS) transistor, and thus is turned on by a low voltage. One the contrary, the fourteenth and fifteenth transistors MS 2  and MS 3  are N-channel metal oxide semiconductor (NMOS) transistors, and thus are turned on by a high voltage. In addition, the second power supply Vss denotes a ground terminal.  
         [0060]     Operation of the initial driving circuit is described below. First, when a width-to-length ratio of the thirteenth transistor MS 1  is reduced so that the thirteenth transistor MS 1  has a large resistance, and the resistance is reduced below a resistance of the fifteenth transistor MS 3  in an off-state, a voltage at the drain of the thirteenth transistor MS 1  is maintained high. Therefore, a voltage at the gate of the fourteenth transistor MS 2  gradually increases and thus the fourteenth transistor MS 2  is turned on. When the fourteenth transistor MS 2  is turned on, the drain thereof is grounded. Here, the drain of the fourteenth transistor MS 2  is connected to the gates of the first and eighth transistors M 11  and M 22  shown in  FIG. 3 , and thus reduces voltage levels of the gates of the first and eighth transistors M 11  and M 22 . Therefore, the first and eighth transistors M 11  and M 22  allow predetermined currents to flow, and the predetermined currents allow predetermined voltages to be applied to the first and fourth nodes N 1  and N 4  shown in  FIG. 3 . When the fifteenth transistor MS 3  is turned on by the voltages of the first and fourth nodes N 1  and N 4  shown in  FIG. 3 , the gate voltage of the fourteenth transistor MS 2  decreases again. With the method described above, initial driving is performed.  
         [0061]     The initial driving circuit shown in  FIG. 4  has been described in relation to  FIG. 3 , but can equally be applied to the reference current generator shown in  FIG. 2 .  
         [0062]      FIG. 5  is a circuit diagram of an example of amplifiers shown in  FIGS. 2 and 3 .  
         [0063]     The reference current generators of the present invention can generate a reference current that can operate at a relatively low voltage because a reference power is formed by a current mode technique, have structures that can control noise existing in a power supply line, can reduce nonlinearity due to temperature dependence, and can be formed into a relatively simple circuit.  
         [0064]     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.