Threshold voltage generating circuit
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a threshold voltage generating circuit for generating threshold voltages, which are employed for discriminating signals in a semiconductor integrated circuit. 
     2. Description of the Background Art 
     FIG. 1 is a circuit diagram showing a conventional threshold voltage generating circuit. Referring to FIG. 1, a differential amplifier formed by npn transistors Q1 and Q2 is provided in a semiconductor integrated circuit 1, which is formed on a semiconductor substrate. The emitters of the transistors Q1 and Q2 are connected to one end of a constant current source 4 through resistors 2 and 3, respectively, while the other end of the constant current source 4 is grounded. The collector of the transistor Q1 is connected to a power source V CC , while the collector of the transistor Q2 is connected to the power source V CC  through a resistor 5. A voltage obtained by dividing an external reference voltage 6 by external resistors 7 and 8 is applied to the base of the transistor Q1, while an internal reference voltage 9 is applied to the base of the transistor Q2. The external reference voltage 6 may be replaced by a voltage source 10 provided in the semiconductor integrated circuit 1, as shown by dotted lines in FIG. 1. 
     In operation, a current which is responsive to the base voltage difference between the transistors Q1 and Q2 flows to the resistor 5. A voltage drop V a  is developed in the resistor 5 by this current, and is derived as a threshold voltage. The threshold voltage V a  can be changed by adjusting the voltage dividing ratio between the external resistors 7 and 8. 
     FIG. 2 is a circuit diagram showing another conventional threshold voltage generating circuit. Referring to FIG. 2, a semiconductor integrated circuit 1 is provided therein with a voltage-to-current conversion circuit, which is formed by an operational amplifier 11, an npn transistor Q3 and a resistor 12. The output of the operational amplifier 11 is oonnected to the base of the transistor Q3. The emitter of the transistor Q3 is connected to a negative input of the operational amplifier 11, while being grounded through the resistor 12. The collector of the transistor Q3 is connected to a voltage source 14 through a resistor 13. A voltage obtained by dividing an external reference voltage 6 by external resistors 7 and 8 is applied to a positive input of the operational amplifier 11. 
     In operation, a current which is responsive to the voltage applied to the positive input of the operational amplifier 11 flows to the transistor Q3. This current also flows to the resistor 13, so that a voltage drop V a  developed in the resistor 13 is used as a threshold voltage. Similarly to the circuit shown in FIG. a desired threshold voltage V a  can be obtained by adjusting the voltage dividing ratio between the external resistors 7 and 8. 
     The conventional threshold voltage generating circuits have the aforementioned structures, each adapted to generate a voltage which is responsive to the reference voltage supplied from the exterior of the semiconductor integrated circuit through the amplifier provided in the semiconductor integrated circuit, to use this voltage as a threshold voltage within the semiconductor integrated circuit. Thus, the circuit is complicated in structure, and dispersion of threshold voltages is increased due to manufacturing dispersion of such integrated circuits. Because of a large number of components. The circuit is further complicated when a plurality of threshold voltages are generated. 
     SUMMARY OF THE INVENTION 
     A threshold voltage generating circuit in accordance with the present invention comprises a current mirror circuit including a first transistor serving as a reference transistor and a second transistor which are formed in a semiconductor integrated circuit to have a common control electrode, a current source formed in the semiconductor integrated circuit and connected to one electrode of the first transistor, an internal resistor formed in the semiconductor integrated circuit and connected to one electrode of the second transistor for generating a threshold voltage responsive to a current flowing therethrough, and first and second external resistors provided in an exterior of the semiconductor integrated circuit and connected to other electrodes of the first and second transistors, respectively, for setting the threshold voltage by the ratio therebetween. 
     According to the present invention, a reference current flows to a first transistor from a current source, while a current which is responsive to the ratio of a first external resistor to a second external resistor with respect to the reference current flows to a second transistor. The current flowing to the second transistor also flows to an internal resistor, which in turn generates a threshold voltage in response to this current. The threshold voltage is arbitrarily determined in response to the ratio of the first external resistor to the second external resistor. The number of such threshold voltages can be increased by increasing the number of second transistors in a current mirror circuit. Further, manufacturing dispersion of an integrated circuit is cancelled when a current from the current source provided in the integrated circuit is converted to a voltage by the internal resistor. 
     Accordingly, an object of the present invention is to provide a threshold voltage generating circuit, which can accurately generate a desired number of threshold voltages at desired values with a simple circuit structure. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are circuit diagrams showing conventional threshold voltage generating circuits; 
     FIG. 3 is a circuit diagram showing an embodiment of a threshold voltage generating circuit according to the present invention; 
     FIG. 4 is a circuit diagram showing another embodiment of a threshold voltage generating circuit according to the present invention; and 
     FIG. 5 is a circuit diagram showing still another embodiment of a threshold voltage generating circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a circuit diagram showing an embodiment of a threshold voltage generating circuit according to the present Invention. Referring to FIG. 3, a semiconductor integrated circuit 1 formed on a semiconductor substrate is provided therein with a current mirror circuit, which is formed by npn transistors Q4, Q5 and Q6. The bases of the transistors Q4, Q5 and Q6 are connected in common. The collector of the transistor Q4, which forms the basis of the current mirror circuit, is connected to a power source V CC  through a reference current source 15, as well as to the base of a transistor 07. The emitter of the transistor Q7 is connected to the base of the transistor Q4, and the collector thereof is connected to the power source V CC . The collectors of the transistors Q5 and Q6 are connected to arbitrary constant voltage sources 18 and 19 through internal resistors 16 and 17, respectively. The emitters of the transistors Q4, Q5 and Q6 are grounded through external resistors 20, 21 and 22, respectively. 
     In operation, base currents of the transistors Q4, Q5 and Q6 are supplied from the power source V CC  through the transistor Q7. Since the transistor Q7 has a large amplification facter, its base current is substantially negligible. If the base currents of the transistors Q4, Q5 and Q6 are not so large, the transistor Q7 may be omitted to directly connect the base and the collector of the transistor Q4 with each other. 
     It is assumed here that, when a current I ref  is supplied from the reference current source 15 to the transistor Q4, currents I a  and I b  flow to the transistors Q5 and Q6, respectively. It is further assumed that voltages V 1 , V 2  and V 3  are developed across the external resistors 20, 21 and 22, respectively. Since the bases of the transistors Q4, Q5 and Q6 are connected in common, the following equation holds: 
     
         V.sub.1 +V.sub.BE4 =V.sub.2 +V.sub.BE5 =V.sub.3 +V.sub.BE6 . . . (1) 
    
     where V BE4 , V BE5  and V BE6  represent base-to-emitter voltages of the transistors Q4, Q5 and Q6, respectively. Assuming that R 20 , R 21  and R 22  represent resistance values of the external resistors 20, 21 and 22, respectively, 
     
         V.sub.1 =I.sub.ref ·R.sub.20                      . . . (2) 
    
     
         V.sub.2 =I.sub.a ·R.sub.21                        . . . (3) 
    
     
         V.sub.3 =I.sub.b ·R.sub.22                        . . . (4) 
    
     These equations (2), (3) and (4) are substituted in the equation (1), to attain the following equation (5): ##EQU1## Hence, ##EQU2## 
     (V BE5  -V BE4 ) and (V BE6  -V BE4 ) are about 0 to 20 mV and R 20  ·I ref  is about 0.3 to 1 V, such that: 
     
         V.sub.BE5 -V.sub.BE4 &lt;&lt;R.sub.20 ·I.sub.ref        . . . (8) 
    
     
         V.sub.BE6 -V.sub.BE4 &lt;&lt;R.sub.20 ·I.sub.ref        . . . (9) 
    
     Hence, the equations (6) and (7) can be transformed as follows: ##EQU3## 
     Thus, the collector currents I a  and I b  of the transistors Q5 and Q6 are expressed as follows: ##EQU4## 
     In general, a current I ref  of a current source, such as the reference current source 15, formed in an integrated circuit is expressed as follows: 
     
         I.sub.ref =A/R.sub.0                                       . . . (14) 
    
     where A represents a constant, and R 0  represents internal resistance in relation to the reference current source 15. Thus, the equation (14) is substituted in the equations (12) and (13) to attain: ##EQU5## 
     Hence, assuming that R 16  and R 17  represent resistance values of the internal resistors 16 and 17, the threshold voltages V a  and V b  developed across the internal resistors 16 and 17 are expressed as follows: ##EQU6## 
     Thus, the threshold voltages V a  and V b  are determined by the products of the ratios (R 20  /R 21  and R 20  /R 22 ) between the external resistors and the ratios (R 16  /R 0  and R 17  R 0 ) between the internal resistors. Since the external resistors are discrete components whose resistance values are correct, the ratios therebetween are also correct. Further, dispersion of resistance values caused by manufacturing dispersion of the integrated circuit 1 is cancelled by the ratios between the internal resistors. Therefore, it is possible to sufficiently correctly set the threshold voltages V a  and V b . Further, the threshold voltages V a  and V b  can be set at desired values by changing the ratios (R 20  /R 21  and R 20  /R 22 ) between the external resistors. 
     FIG. 4 is a circuit diagram showing another embodiment of a threshold voltage generating circuit according to the present invention. In this embodiment, the number of transistors forming a current mirror circuit is increased as compared with the embodiment shown in FIG. 3, in order to generate n threshold voltages V a , V b , . . ., V n . Referring to FIG. 4, a transistor Q8 typically represents the increased transistors. Similarly to transistors Q5 and Q6, the collector of the transistor Q8 is connected to an arbitrary constant voltage source 24 through an internal resistor 23, and the emitter thereof is grounded through an external resistor 25. 
     Through operation similar to the above, a threshold voltage V n  expressed as follows is developed across the internal resistor 23: ##EQU7## Thus, the number of threshold voltages can be easily increased by increasing the number of transistors forming a current mirror circuit. 
     FIG. 5 is a circuit diagram showing still another embodiment of a threshold voltage generating circuit according to the present invention. In this embodiment, the transistors Q4 to Q8 shown in FIG. 4 are replaced by pnp transistors, and the level of a power source V CC  and the ground level are inverted. In this case, threshold voltages V a , V b , . . . V n  are set from the ground level. The operation of this embodiment is similar to those of the aforementioned embodiments. Thus, it is possible to accurately set threshold voltages in this embodiment similarly to the aforementioned embodiments. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention should be limited only by the terms of the appended claims.