Abstract:
There is provided a current source circuit in which a outflow current of an output terminal is equal to an inflow current thereof. The current source circuit includes a first transistor group converting a reference current from a reference current source into a voltage and a first transistor having a current mirror relationship with the first transistor group, and allowing an output current to flow therethrough. An error amplifier compares a voltage generated in the first transistor group and supplied to one input terminal with a voltage supplied to the other input terminal. A second transistor is driven with an output voltage of the error amplifier. A third transistor is driven with the output voltage of the error amplifier, and allows an output current to flow therethrough in a direction opposite to the output current of the first transistor with respect to an output terminal. A second transistor group converts a current flowing through the second transistor into a voltage to supply the voltage to the other input terminal of the error amplifier.

Description:
This application is a Division of application Ser. No. 10/730,839, filed Dec. 8, 2003, which application is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a current source circuit used in electronic equipment and a semiconductor integrated circuit, and an amplifier using the current source circuit. 
   2. Description of the Related Art 
   Conventionally, a current source circuit used in electronic equipment and a semiconductor integrated circuit is disclosed as a current mirror circuit, for example, in JP 2(1990)-124609 A, and Semiconductor Circuit Design Technology (Nikkei Business Publishers Inc., edited by T. Tamai, 1st edition, p. 302). 
     FIG. 20  is a circuit diagram showing an exemplary configuration of a conventional current source circuit. In  FIG. 20 , reference numeral  1  denotes a power supply terminal for supplying a voltage for operating a circuit,  2  denotes a reference current source for supplying a reference current,  4  denotes an output terminal through which a current flows out,  5  denotes an output terminal through which a current flows in, M 2 , M 12 , and M 7  denote n-channel MOS transistors, and M 6  and M 20  denote p-channel MOS transistors. M 2 , M 12 , and M 7  constitute a current mirror circuit, and M 6  and M 20  also constitute a current mirror circuit. 
   Next, the operation of the current source circuit thus configured will be described. The current that flows in from the reference current source  2  is received by the n-channel MOS transistor M 2 , and inverted by the n-channel MOS transistors M 7  and M 12 , respectively. The current inverted by the n-channel MOS transistor M 7  is drawn in through the output terminal  5 . The current inverted by the n-channel MOS transistor M 12  is received by the p-channel MOS transistor M 20 , and further inverted by the p-channel MOS transistor M 6  to flow out through the output terminal  4 . 
     FIG. 21  is a circuit diagram showing an exemplary configuration of a current source circuit configured in the same way as in  FIG. 20 , which includes the reference current source  2  through which a current flows out, the p-channel MOS transistors M 2 , M 12 , and M 7 , and the n-channel MOS transistors M 6  and M 20 . 
   Furthermore, a common feedback circuit for setting an operation point of an amplifier using the current source circuit shown in  FIG. 20  is disclosed, for example, in “CMOS Analog Circuit Design Second Edition” (p. 196, published by OXFORD, Phillip E. Allen, Douglas R. Holberg).  FIG. 22  shows the configuration of this amplifier. 
   In  FIG. 22 , reference numeral  6  denotes a voltage source,  8  and  9  denote input terminals of the amplifier,  11  and  12  denote loads,  13  and  14  denote output terminals of the amplifier, M 10 , M 11 , M 18 , and M 19  denote n-channel MOS transistors, and M 6   a , M 6   b , M 8 , and M 9  denote p-channel MOS transistors. 
   Next, the operation of the amplifier thus configured will be described. Signals input from the input terminals  8  and  9  of the amplifier are converted into currents by the n-channel MOS transistors M 18  and M 19  constituting a differential amplifier, and formed into amplified voltages by the loads  11  and  12  to be taken out from the output terminals  13  and  14  of the amplifier. In order to determine an operation point of the amplifier, the voltage at a connection point between the loads  11  and  12  is compared with the voltage of the voltage source  6  by the n-channel MOS transistors M 10  and M 11  constituting the differential amplifier (error amplifier), whereby currents flowing through the current mirror circuits M 8 , M 6   a , and M 6   b  are adjusted. As a result, the operation points of the loads  11  and  12  are set to be the voltage of the voltage source  6 . 
   Conventionally, in the case where an inflow current and an outflow current are used simultaneously in a current source circuit of electronic equipment and a semiconductor integrated circuit and a current source circuit used in an amplifier, there is a problem that these currents are not equal to each other. 
   In MOS transistor properties, a current Ids is represented by the following expression:
 
 Ids=k× ( Vgs−Vt ) 2 ×(1+λ× Vds )
 
where Ids is a current of a MOS transistor, k is an amplification ratio, Vgs is a gate—source voltage, Vt is a threshold voltage, λ is a channel length modulation coefficient, and Vds is a drain—source voltage. A supplied current is influenced by a channel modulation effect every time it passes through a MOS transistor. Assuming that the sizes of the transistors are designed to be equal to each other, Vds is set to be substantially the same, and λ of the n-channel is substantially the same as that of the p-channel, a current ratio of an inflow current I 5  flowing through the output terminal  5  to an outflow current I 4  flowing through the output terminal  4  in  FIG. 20  is approximated as follows:
 
                   I4   /   I5     =         (     1   +     λ   ×   Vds       )     2     /     (     1   +     λ   ×   Vds       )                   =     (     1   +     λ   ×   Vds       )                 
and the current ratio is not 1. For example, when λ=0.05 and Vds=1.5 V, an error of 7.5% occurs, and thus, an outflow current is larger than an inflow current.
 
   Similarly, even in the common feedback circuit shown in  FIG. 22 , a similar error occurs. However, this error further can be reduced by a loop gain A 1  determined by the n-channel MOS transistors M 10 , M 11  constituting a differential amplifier (error amplifier), the current mirrors M 8 , M 6   a , M 6   b , and the loads  11  and  12 . It should be noted that the loop gain A 1  cannot be set to be large in order to prevent oscillation, and can be set to be at most  10  times. Thus, the error is reduced to 1/10, and 0.75% error remains. Furthermore, the loads  11  and  12  are placed in a loop of the common feedback circuit, so that they cannot take large values in order to prevent oscillation. Consequently, the gain of the differential amplifier composed of the n-channel MOS transistors M 18  and M 19  cannot be set to be large. 
   SUMMARY OF THE INVENTION 
   Therefore, with the foregoing in mind, it is an object of the present invention to provide a current source circuit capable of prescribing an outflow current to be equal to an inflow current. 
   Furthermore, it is another object of the present invention to provide an amplifier capable of setting a gain to be large while ensuring a stable operation point. 
   In order to achieve the above-mentioned object, a first current source circuit according to the present invention includes: a reference current source supplying a reference current; a first transistor group M 1 , M 2 ) connected in series to the reference current source, and converting the reference current into a voltage. A first transistor (M 7 ) has a current mirror relationship with the first transistor group, and allows an output current to flow therethrough. An error amplifier (Op. Amp) receives a voltage generated in the first transistor group at one input terminal, and compares the voltage at the one input terminal with a voltage supplied to the other input terminal. A second transistor (M 5 ) is driven with an output voltage of the error amplifier. A third transistor (M 6 ) is driven with the output voltage of the error amplifier, and allows an output current to flow therethrough in a direction opposite to the output current of the first transistor with respect to an output terminal. A second transistor group (M 3 , M 4 ) is connected in series to the second transistor, and converts a current flowing through the second transistor into a voltage to supply the voltage to the other input terminal of the error amplifier. 
   Furthermore, in order to achieve the above-mentioned object, a second current source circuit according to the present invention includes: a reference current source supplying a reference current and a first transistor (M 2 ) connected in series to the reference current source, and converting the reference current into a voltage. A second transistor (M 4 ) has a current mirror relationship with the first transistor, and converts a current into a voltage. A third transistor (M 7 ) has a current mirror relationship with the first transistor, and allows an output current to flow therethrough. An error amplifier (Op. Amp) receives a voltage generated in the second transistor at one input terminal, and compares the voltage at the one input terminal with a voltage supplied to the other input terminal to output an error voltage. A voltage source supplies a voltage to the other input terminal of the error amplifier. A fourth transistor (M 5 ) is connected in series to the second transistor, and is driven with an output voltage of the error amplifier. A fifth transistor (M 6 ) is driven with the output voltage of the error amplifier, and allows an output current to flow therethrough in a direction opposite to the output current of the third transistor with respect to an output terminal. 
   According to the above-mentioned first and second current source circuits, the outflow current of the output terminal can be set to be equal to the inflow current thereof. 
   Furthermore, in order to achieve the above-mentioned object, a first amplifier according to the present invention includes a reference current source supplying a reference current and a first transistor (M 2 ) connected in series to the reference current source, and converting the reference current into a voltage. A second transistor (M 4 ) has a current mirror relationship with the first transistor, and converts a current into a voltage. A third transistor (M 7 ) has a current mirror relationship with the first transistor, and allows a first current to pass therethrough. An error amplifier (Op. Amp) receives a voltage generated in the second transistor at one input terminal, and compares the voltage at the one input terminal with a voltage supplied to the other input terminal to output an error voltage. A voltage source supplies a voltage to the other input terminal of the error amplifier. A fourth transistor (M 5 ) is connected in series to the second transistor and is driven with an output voltage of the error amplifier. A fifth transistor (M 6 ) is driven with the output voltage of the error amplifier and allows a second current to flow therethrough. A differential amplifier (Diff. Amp) is operated using the first current flowing through the third transistor as one supply current and using the second current flowing through the fifth transistor as the other supply current, and amplifies a voltage supplied to an input terminal. 
   In the first amplifier, the reference voltage at an operation point of the differential amplifier is set to be the voltage of the voltage source. 
   In order to achieve the above-mentioned object, a second amplifier according to the present invention includes a reference current source supplying a reference current and a first transistor (M 2 ) connected in series to the reference current source, and converting the reference current into a voltage. A second transistor (M 4 ) has a current mirror relationship with the first transistor, and allows a first current to pass therethrough. A third transistor (M 7 ) has a current mirror relationship with the first transistor, and allows a second current to pass therethrough. A first differential amplifier (1st Diff. Amp) is operated using the first current flowing through the second transistor as one supply current, and receives a voltage supplied to an input terminal. An error amplifier (Op. Amp) receives an output voltage of the first differential amplifier at the one input terminal, and compares the voltage at the one input terminal with a voltage supplied to the other input terminal to output an error voltage. A voltage source supplies a voltage to the other input terminal of the error amplifier. A fourth transistor (M 5 ) operates the first differential amplifier, using a third current driven to flow with an output voltage of the error amplifier as the other supply current. A fifth transistor (M 6 ) is driven with the output voltage of the error amplifier, and allows a fourth current to pass therethrough. A second differential amplifier (2nd Diff. Amp) is operated using the second current flowing through the third transistor as one supply current and using the fourth current flowing through the fifth transistor as the other supply current, and amplifies a voltage supplied to the input terminal. 
   In the second amplifier, a reference voltage at an operation point of the second differential amplifier is set at a voltage of the voltage source. 
   According to the above-mentioned first and second amplifiers, a gain can be set to be large while a stable operation point is ensured. 
   These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing an exemplary configuration of a current source circuit according to Embodiment 1 of the present invention. 
       FIG. 2  is a circuit diagram showing a first specific example of the current source circuit shown in  FIG. 1 . 
       FIG. 3  is a circuit diagram showing a second specific example of the current source circuit shown in  FIG. 1 . 
       FIG. 4  is a circuit diagram showing a modified example of the current source circuit shown in  FIG. 1 . 
       FIG. 5  is a circuit diagram showing a first specific example of the current source circuit shown in  FIG. 4 . 
       FIG. 6  is a circuit diagram showing a second specific example the current source circuit shown in  FIG. 4 . 
       FIG. 7  is a circuit diagram showing an exemplary configuration of a current source circuit according to Embodiment 2 of the present invention. 
       FIG. 8  is a circuit diagram showing an exemplary configuration of an amplifier according to a third embodiment of the present invention. 
       FIG. 9  is a circuit diagram showing a first specific example of an amplifier shown in  FIG. 8 . 
       FIG. 10  is a circuit diagram showing a second specific example of the amplifier shown in  FIG. 8 . 
       FIG. 11  is a circuit diagram showing a third specific example of the amplifier shown in  FIG. 8 . 
       FIG. 12  is a circuit diagram showing a fourth specific example of the amplifier shown in  FIG. 8 . 
       FIG. 13  is a circuit diagram showing a modified example of the amplifier shown in  FIG. 8 . 
       FIG. 14  is a circuit diagram showing an exemplary configuration of an amplifier according to Embodiment 4 of the present invention. 
       FIG. 15  is a circuit diagram showing a first specific example of the amplifier shown in  FIG. 14 . 
       FIG. 16  is a circuit diagram showing a second specific example of the amplifier shown in  FIG. 14 . 
       FIG. 17  is a circuit diagram showing a third specific example of the amplifier shown in  FIG. 14 . 
       FIG. 18  is a circuit diagram showing a fourth specific example of the amplifier shown in  FIG. 14 . 
       FIG. 19  is a circuit diagram showing a modified example of the amplifier shown in  FIG. 14 . 
       FIG. 20  is a circuit diagram showing an exemplary configuration of a conventional current source circuit. 
       FIG. 21  is a circuit diagram showing a modified example of a conventional current source circuit. 
       FIG. 22  is a circuit diagram showing an exemplary configuration of a conventional amplifier. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the present invention will be described by way of preferred embodiments with reference to the drawings. 
   Embodiment 1 
     FIG. 1  is a circuit diagram showing an exemplary configuration of a current source circuit according to Embodiment 1 of the present invention. In  FIG. 1 , reference numeral  3  denotes an error amplifier (Op. Amp) composed of an operational amplifier, M 1 , M 2 , M 3 , M 4 , M 7  denote n-channel MOS transistors, and M 5 , M 6  denote p-channel MOS transistors. The n-channel MOS transistors M 1  and M 2 , which are included in a first transistor group, constitute a converter for converting a current of a reference current source  2  into a voltage. Furthermore, the n-channel MOS transistors (first transistors) M 2  and M 7  constitute a current mirror circuit. The p-channel MOS transistor (second transistor) M 5  and the p-channel MOS transistor (third transistor) M 6  constitute a current source driven with an output voltage of the error amplifier  3 . Furthermore, the n-channel MOS transistors M 3  and M 4 , which are included in a second transistor group, constitute a converter for converting a current of the p-channel MOS transistor into a voltage. 
     FIG. 2  is a circuit diagram showing a first specific example of the current source circuit shown in  FIG. 1 . In  FIG. 2 , M 8 , M 9  denote p-channel MOS transistors, M 10 , M 11 , M 12  denote n-channel MOS transistors, and C denotes a capacitor. The n-channel MOS transistor M 12  functions as a current source, the n-channel MOS transistors M 10  and M 11  function as a differential transistor pair, and the p-channel MOS transistors M 8  and M 9  function as a current mirror that is an active load of the differential transistor pair M 10  and M 1 . The transistors M 8  to M 12  and the capacitor C constitute the error amplifier  3 . 
   Next, the operation of the current source circuit according to Embodiment 1 configured as above will be described. 
   A voltage generated in the first transistor group (M 1 , M 2 ) and a voltage generated in the second transistor group (M 3 , M 4 ) are input to the error amplifier  3 , and a gate voltage of the p-channel MOS transistor M 5  is adjusted so that these voltages are equal to each other. Thus, a current flowing from the p-channel MOS transistor M 5  is equal to that of the reference current source  2 , and a gate of the p-channel MOS transistor M 6  is driven with the same voltage as the gate voltage of the p-channel MOS transistor M 5 . Therefore, a current of the p-channel MOS transistor M 6  also is substantially equal to that of the reference current source  2 . When a current ratio of an inflow current I 5  of an output terminal  5  to an outflow current I 4  of an output terminal  4  is calculated by adopting the same approximation as that in the conventional example, the following result is obtained:. 
   
     
       
         
           
             
               
                 
                   I4 
                   / 
                   I5 
                 
                 = 
                 
                   
                     ( 
                     
                       1 
                       + 
                       
                         λ 
                         × 
                         Vds 
                       
                     
                     ) 
                   
                   / 
                   
                     ( 
                     
                       1 
                       + 
                       
                         λ 
                         × 
                         Vds 
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 = 
                 1 
               
             
           
         
       
     
   
   As described above according to the present embodiment, by providing the first transistor group (M 1 , M 2 ) for converting a current into a voltage, the second transistor group for converting a current into a voltage, the error amplifier for amplifying the difference of the respective converted voltages (error voltage), and the p-channel MOS transistors M 5  and M 6  driven by the error amplifier, an outflow current of the output terminal  4  can be set to be equal to an inflow current of the output terminal  5 . 
   In the present embodiment, the transistors that are stacked in series in two stages are used as a converter. However, as shown in  FIG. 3 , the current source of the error amplifier  3  is omitted, and a one-stage transistor may be used as a converter. 
   Furthermore, in the present embodiment, the current source circuit is configured using the n-channel MOS transistors as a converter. However, as shown in  FIGS. 4 ,  5 , and  6 , a current source circuit may be configured using the p-channel MOS transistors as a converter. 
   Furthermore, in the present embodiment, a current source circuit is configured using the MOS transistors. However, a current source circuit may be configured using bipolar transistors. 
   Embodiment 2 
     FIG. 7  is a circuit diagram showing an exemplary configuration of a current source circuit according to Embodiment 2 of the present invention. In  FIG. 7 , the components having the same configurations and functions as those in Embodiment 1 are denoted with the same reference numerals as those therein, and their description will be omitted here. In  FIG. 7 , reference numeral  6  denotes a voltage source for determining an operation point of an amplifier. 
   Next, the operation of the current source circuit according to Embodiment 2 configured as above will be described. 
   In  FIG. 7 , gate voltages of the p-channel MOS transistors M 5  and M 6  are adjusted so that a voltage at a connection point between a drain of the transistor M 4  and a drain of the transistor M 5  is equal to a voltage of the voltage source  6 . In this case, a current ratio of the inflow current I 5  flowing through the output terminal  5  to the outflow current I 4  flowing through the output terminal  4  is obtained as follows: 
   
     
       
         
           
             
               
                 
                   I4 
                   / 
                   I5 
                 
                 = 
                 
                   
                     ( 
                     
                       1 
                       + 
                       
                         λ 
                         × 
                         Vds 
                       
                     
                     ) 
                   
                   / 
                   
                     ( 
                     
                       1 
                       + 
                       
                         λ 
                         × 
                         Vds 
                       
                     
                     ) 
                   
                 
               
             
           
           
             
               
                 = 
                 1 
               
             
           
         
       
     
   
   As described above, according to the present embodiment, by providing the voltage source  6  for generating a reference voltage at an operation point, the error amplifier  3 , and the p-channel MOS transistors M 5  and M 6  driven by the error amplifier  3 , the outflow current of the output terminal  4  can be set to be equal to the inflow current of the output terminal  5 . 
   Embodiment 3 
     FIG. 8  is a circuit diagram showing an exemplary configuration of an amplifier according to Embodiment 3 of the present invention. The amplifier according to the present embodiment uses the current source circuit according to Embodiment 2. 
   In  FIG. 8 , reference numeral  10  denotes a differential amplifier (Diff. Amp) that includes input terminals  8 ,  9  and output terminal  13 ,  14 . A load  11  is connected between the output terminal  13  and the voltage source  6 , and a load  12  is connected between the output terminal  14  and the voltage source  6 . The differential amplifier  10  is driven using a current flowing through the transistors M 6  and M 7  as a supply current. 
     FIG. 9  is a circuit diagram showing a first specific example of the amplifier according to the present embodiment. In  FIG. 9  reference numeral  15  denotes a voltage source, M 16 , M 17  denote p-channel MOS transistors, and M 18 , M 19  denote n-channel MOS transistors. A current supplied from the p-channel MOS transistor M 6  is divided by the voltage source  6  and the p-channel MOS transistors M 16  and M 17 . Furthermore, currents from the n-channel MOS transistors M 18  and M 19  are supplied to the n-channel MOS transistor M 7  through the p-channel MOS transistors M 16  and M 17 , respectively. 
   Next, the operation of the amplifier according to Embodiment 3 configured as above will be described. 
   In  FIG. 9 , signals input to the differential transistor pair (M 18  and M 19 ) are amplified by the loads  11  and  12  to be output to the output terminals  13  and  14 . At this time, as a necessary condition for the operation, the operation center of the output terminals  13  and  14  must be operated with the voltage of the voltage source  6 . For this purpose, the following is required: the outflow current due to the p-channel MOS transistor M 6  is equal to the inflow current due to the n-channel MOS transistor M 7 ; the differential transistor pair (M 18 , M 19 ) equally distributes the current due to the n-channel MOS transistor M 7  at the operation center; and furthermore, the voltage source  15  and the p-channel MOS transistors M 16  and M 17  equally distribute a current due to the p-channel MOS transistor M 6 . 
   Consequently, the voltage at the operation center of the output terminals  13  and  14  is equal to the voltage at the connection point between the drain of the p-channel MOS transistor M 5  and the drain of the n-channel MOS transistor M 4 . This voltage is equal to that of the voltage source  6  because of the error amplifier  3 , and the voltage at the operation center of the output of the differential amplifier  10  also is equal to that of the voltage source  6 . 
   At this time, the loads  11  and  12  are not included in the loop of the error amplifier  3 . Therefore, the loads  11  and  12  of the differential amplifier  10  composed of the differential transistor pair (M 18 , M 19 ) can have a large resistance. Because of this, the gain of the amplifier can be increased. Furthermore, the loads  11  and  12  can be omitted, and an amplifier with a large gain set at an output impedance of the MOS transistor also can be configured. 
   As described above, according to the present embodiment, by providing the current source circuit according to Embodiment 2 and the differential amplifier for amplifying a signal, a gain can be set to be large while a stable operation point is ensured. 
   In the present embodiment, the voltage source  6  and the p-channel MOS transistors M 16  and M 17  are used as a current distributor. However, as shown in  FIG. 10 , the p-channel MOS transistor M 16  shown in  FIG. 9  is divided into M 6   a  and M 6   b , and the voltage source  15  and the p-channel MOS transistors M 16  and M 17  shown in  FIG. 9  may be omitted. 
   Furthermore, in the present embodiment, a signal is input to the n-channel MOS transistor. However, as shown in  FIG. 11 , a signal may be input to the p-channel MOS transistor. 
   Furthermore, in the present embodiment, the voltage source  6  and the p-channel MOS transistors M 16  and M 17  are used as a current distributor. However, as shown in  FIG. 12 , the n-channel MOS transistor M 7  shown in  FIG. 9  is divided into M 7   a  and M 7   b , and the voltage source  15  and the p-channel MOS transistors M 16  and M 17  shown in  FIG. 9  may be omitted. 
   Furthermore, in the present embodiment, the amplifier is configured using the n-channel MOS transistors shown in  FIG. 8  as a current mirror. However, as shown in  FIG. 13 , the amplifier may be configured using the p-channel MOS transistor as a current mirror. 
   Furthermore, in the present embodiment, the amplifier is configured using the MOS transistors. However, the amplifier may be configured using bipolar transistors. 
   Embodiment 4 
     FIG. 14  is a circuit diagram showing an exemplary configuration of an amplifier according to Embodiment 4 of the present invention. In  FIG. 14 , reference numeral  7  denotes a first differential amplifier (1st Diff. Amp), and  10  denotes a second differential amplifier (2nd Diff. Amp) having a configuration equivalent to the first differential amplifier  7 . The other configuration is the same as that of Embodiment 3 shown in  FIG. 8 . 
     FIG. 15  is a circuit diagram showing a specific example of the amplifier shown in  FIG. 14 . In  FIG. 15 , n-channel MOS transistors M 13  and M 14  constitute a differential transistor pair, a p-channel MOS transistor M 15  constitutes a gate ground circuit, and the transistors M 13 , M 14 , and M 15  constitute a first differential amplifier  7  equivalent to the second differential amplifier  10 . 
   Next, the operation of the amplifier according to Embodiment 4 configured as above will be described. 
   In Embodiment 3, the channel modulation effect λ and Vds of the MOS transistor are approximated to be substantially constant. However, by providing the equivalent first differential amplifier  7 , the operation state of the MOS transistor of the first differential amplifier  7  becomes equal to the operation state of the MOS transistor of the second differential amplifier  10 , and an error ascribed to the current ratio of the inflow current of the output terminal  5  to the outflow current of the output terminal  4  is reduced further. 
   As described above, according to the present embodiment, by providing the differential amplifier  7  equivalent to the differential amplifier  10  in Embodiment 3, a gain is set to be large while ensuring a stable operation point, and an error can be reduced further. 
   In the present embodiment, the voltage source  6  and the transistors M 15 , M 16 , and M 17  shown in  FIG. 15  are used as a current distributor. However, as shown in  FIG. 16 , the transistor M 6  shown in  FIG. 15  is divided into M 6   a  and M 6   b , and the voltage source  15  and the transistors M 15 , M 16 , and M 17  shown in  FIG. 15  may be omitted. 
   Furthermore, in the present embodiment, a signal is input to the n-channel MOS transistor. However, as shown in  FIG. 17 , a signal may be input to a p-channel MOS transistor. 
   Furthermore, in the present embodiment, the voltage source  6  and the transistors M 15 , M 16 , and M 17  shown in  FIG. 15  are used as a current distributor. However, as shown in  FIG. 18 , the transistor M 7  shown in FIG.  15  is divided into M 7   a  and M 7   b , and the voltage source  15  and the transistors M 15 , M 16 , and M 17  shown in  FIG. 15  may be omitted. 
   Furthermore, in the present embodiment, the amplifier is configured using the n-channel MOS transistors as a current mirror. However, as shown in  FIG. 19 , the amplifier may be configured using the p-channel MOS transistors as a current mirror as shown in  FIG. 19 . 
   Furthermore, in the present embodiment, the amplifier is configured using the MOS transistors. However, the amplifier may be configured using bipolar transistors. 
   As described above, according to the present invention, an excellent current source circuit can be realized, which is capable of prescribing an inflow current to be equal to an outflow current of the output terminal. 
   Furthermore, an excellent amplifier can be realized, which is capable of setting a gain to be large while ensuring a stable operation point. 
   The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.