Abstract:
For realizing a stabilized current source circuit providing a stabilized current which is insensitive to a change in the threshold voltage of a MOS transistor, a gate electrode of a first MOS transistor feeding a drain current as a constant current output is supplied with the sum of the gate-source voltage of a second MOS transistor and the potential of a stabilized voltage source.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a stabilized current source circuit and in particular to a current source circuit employing MOS transistors which supplies a constant current irrespective of the threshold voltage of the MOS transistors. 
     Various circuits employing MOS transistors have been made in the form of integrated circuits. Among these circuits, filters and integrators need precise current sources as described in &#34;MOS integrated PLL loop filter&#34;, 1980 National Conference Record on Communications, The Institute of Electronics and Communication Engineers of Japan, No. 85, for example. In a simple current mitter circuit which has been widely used as the current source circuit, however, the current value unadvantageously varies largely because of nonuniformity of MOS transistor characteristics caused by the fabrication process and because of variation in temperature and in power source. 
     FIG. 2 shows the principle for configuring a constant current circuit employing MOS transistors. Assuming that an n-channel MOS transistor is used, bias voltage from a bias voltage source 2 is supplied between a gate electrode of an n-channel MOS transistor 1 and a source electrode thereof. As a result, a drain current I D1  flows through the MOS transistor 1 and the value of the drain current I D1  is represented as 
     
         I.sub.D1 =βA(V.sub.GS1 -V.sub.TH).sup.2,              (1) 
    
     where: 
     β=μCo/2, A=W 1  /L 1 , 
     μ: channel mobility, 
     Co: gate capacitance, 
     W 1  : channel width, 
     L 1  : channel length, 
     V TH  : threshold voltage. 
     In the above equation, the nonuniformity of Co, W 1  and L 1  can be limited to ten and several % by sufficiently managing the fabrication process. And the nonuniformity of V GS1  can also be limited to ten and several % by using a well-known band-gap reference circuit. The variation in each of parameters Co, W 1 , L 1  and V GS1  due to the ambient temperature is negligible. However, the mobility μ varies in proportion to the minus one and a half power of the absolute temperature. And the threshold voltage V TH  has fabrication nonuniformity as much as ±50% and varies as much as ±20% for a temperature change of ±50° C. Therefore, the drain current I D1  largely varies due to the nonuniformity caused by the fabrication process and due to changes in temperature. Furthermore, the variation in power source may cause additional current change. The ratio between the maximum value of the drain current I D1  and the minimum value thereof amounts to 5 or 6. As a result, it becomes difficult to realize an analog circuit needing a precise current source. In addition, the power dissipation of the circuit varies largely. These are primary factors hampering improvement of analog MOS integrated circuits. 
     As a stabilized current source circuit employing MOS transistors which is less sensitive to a change in the voltage source and a change in V TH , a circuit comprising a combination of MOS transistors of different types, i.e., a depletion MOS transistor and an enhancement MOS transistor is known as described in &#34;Constant Current Circuit&#34;, Japanese Patent Unexamined Publication No. 51-138848. Since the MOS transistors of different types must be combined, the fabrication process of the circuit becomes complicated. In addition, the relation between magnitudes of currents flowing through three transistors must be set as predetermined. And the gate voltage of a specific transistor must be set to a point where the temperature coefficient is zero. Thus the circuit is subjected to many constraints in its fabrication and design. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to realize a current source circuit which can be easily fabricated (that is to say, which is formed by combining the same kind of MOS transistors and which is relatively simple in circuit configuration) and which supplies a current less sensitive to a change in the threshold voltage V TH  of the MOS transistor. 
     Another object of the present invention is to provide a stabilized current source circuit which is suitable to integrated circuits comprising MOS transistors. 
     In accordance with one aspect of the present invention, in a circuit comprising a first MOS transistor for supplying a constant current output and a second MOS transistor having a gate electrode connected to a gate electrode of the first MOS transistor and having a drain electrode connected to a current source, a stabilized voltage source having suitable magnitude and polarity is connected to a source electrode of the second MOS transistor, whereby the gate-source voltage of the second MOS transistor has a value which is sufficiently smaller than the gate-source voltage of the first MOS transistor and which is close to the threshold voltage V TH . As a result, the voltage which is one of the factors defining the current flowing through the first MOS transistor is substantially only the voltage of the above described stabilized voltage source. And a stabilized voltage source can be realized with relative ease. Thus it becomes possible to eliminate the influence of the threshold voltage which varies most largely. In addition, the fabrication process is simple since transistors of the same type are used as the first and second MOS transistors. 
     The above and other objects and features of the present invention will become apparent from the description made in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION ,OF DRAWINGS 
     FIG. 1 is a circuit diagram of an embodiment of a stabilized current source circuit according to the present invention. 
     FIG. 2 is a circuit diagram for illustrating the principle of a current source circuit of the prior art. 
     FIG. 3 is a specific circuit diagram of the circuit illustrated in FIG. 2. 
     FIG. 4 is a circuit diagram of another embodiment of a stabilized current source circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a circuit diagram for illustrating the principle of an embodiment of a stabilized current source circuit according to the present invention. An n-channel MOS transistor 1 is a current output stage. A gate electrode of the MOS transistor 1 is directly connected to a gate electrode of an n-channel MOS transistor 3. A stabilized voltage source 4 for supplying a voltage value V 1  (approximately 200 to 300 mV) is connected to a source electrode of the transistor 3. The polarity of the stabilized voltage source 4 viewed from the reference point (earth) is the same as the sense of the gate-source voltage of the transistor 3. That is to say, the potential of the gate G is higher than the potential of the source S in this embodiment. A current source 5 is connected to a drain electrode of the transistor 3. The gate electrode of the transistor 3 needs not be connected to the drain electrode thereof. The drain current I D1  of the transistor 1 to be stabilized, the gate-source voltage V GS1  of the transistor 1, the drain current I D2  of the transistor 3, and the gate-source voltage V GS2  of the transistor 3 can be expressed as 
     
         I.sub.D1 =βA.sub.1 (V.sub.GS1 -V.sub.TH).sup.2        (2) 
    
     
         I.sub.D2 =βA.sub.2 (V.sub.GS2 -V.sub.TH).sup.2        (3) 
    
     
         V.sub.GS1 =V.sub.GS2 +V.sub.1                              (4) 
    
     where: 
     
         A.sub.1 =W.sub.1 /L.sub.1, A.sub.2 =W.sub.2 /L.sub.2 
    
     β and V TH  : the same as those in expression (1). 
     When I D2  is much smaller than I D1  (I D1  /I D2  approximately 100 to 10) and A 1  is nearly equal to A 2 , it follows that 
     
         V.sub.GS1 -V.sub.TH &gt;V.sub.GS2 -V.sub.TH 
    
     Thus, V GS2  can be approximated as 
     
         V.sub.GS2 ≃V.sub.TH                          (5) 
    
     From equations (4) and (5), V GS1  can be written as 
     
         V.sub.GS1 ≃V.sub.TH +V.sub.1                 (6) 
    
     By substituting equation (6) into equation (2), we get 
     
         I.sub.D1 =βA.sub.1 (V.sub.1).sup.2                    (7) 
    
     That is to say, the drain current I D1  of the transistor 1 is hardly affected by a change in the threshold voltage V TH  of each transistor. By using a well-known band-gap reference circuit, for example, for supplying the voltage V 1 , it becomes possible to realize a stabilized current source circuit which is sufficiently high in precision and stability. And the precision of A 1  depends on that of the mask used in the fabrication process. It is not very difficult to obtain a sufficiently high value of the precision A 1 . The remaining problem is β defined by the channel mobility and the gate capacitance. It is expected that the nonuniformity of β caused by the fabrication process can be limited to approximately ±10%. And the change of the channel mobility caused by a change in temperature is ±20 to 30% for a range of ±50° C. Accordingly, the change range is significantly narrowed as compared with the circuit of the prior art in which the ratio of the maximum value of the current I D1  to the minimum value thereof amounts to 5 to 6. 
     FIG. 3 shows a specific example of the prior art circuit realized according to the principle illustrated in FIG. 2. In this circuit, the voltage source 2 of FIG. 2 is constituted of a p-channel MOS transistor 6 and an n-channel MOS transistor 7. The circuit of FIG. 3 is a conventional so-called current mitter circuit. Values of W and L illustrated in FIG. 3 represent channel dimensions of respective transistors optimized so as to minimize the change in the current value. 
     FIG. 4 shows an example of a specific circuit which is another embodiment of a stabilized current source circuit according to the present invention. The current source 5 illustrated in FIG. 1 is realized as a current mitter circuit comprising p-channel MOS transistors 8, 10 and an n-channel MOS transistor 9. The value of the voltage source 4 has been chosen to be 0.27 V. Values of W and L in FIG. 4 represent channel dimensions optimized so as to minimize the change in the current value. 
     The change in the output current I D  was measured while the temperature, source voltage and the threshold voltage V TH  were being changed in each of circuits illustrated in FIGS. 3 and 4. The result of measurement is shown in a table below. Each of % values in the table represent a change in I D  with respect to a state B. 
     
         ______________________________________Comparison of Current StabilityState       A          B        C______________________________________Temperature -30° C.                  27° C.                           70° C.Source voltage       5.5 V      5.0 V    4.5 VV.sub.TH    0.4 V      0.7 V    1.1 V       Minimum    Standard MaximumFIG. 3      +97%       0%       -50%FIG. 4      +58.8%     0%       -30.9%______________________________________ 
    
     The above table indicates that the change range of the current I D  in the circuit of FIG. 4 (the present invention) is nearly reduced by half as compared with the circuit of FIG. 3 (prior art). 
     Although the foregoing description has been made for an n-channel MOS transistor, it also holds true for a p-channel MOS transistor. Further, although a band-gap reference circuit is ideal for the stabilized voltage source, even a simple voltage source such as a resistive voltage divider for stepping down the source voltage provides sufficient stability for some application.