The present invention relates to a low-value current source circuit for providing a low-value output current.
There is known, as a bipolar integrated circuit arranged to provide a low-value current, a circuit as shown in FIG. 1 and disclosed in U.S. Pat. No. 3,320,439 to Widlar. In this circuit, if it is assumed that an input current I1 is 100 .mu.A and an output current I2 is 0.1 .mu.A, the value of a resistor R is given by V.sub.T /I2 ln I1/I2=1.8 M.OMEGA.. At the present stage of technology in this field, it is impossible to fabricate a resistor of 1 M.OMEGA. or more at a high level of accuracy.
A circuit using a base current of a transistor as a low-value current, as shown in FIG. 2, has also been known. In the circuit, when the emitter current I is 100 .mu.A and the common emitter current amplification factor .beta. is 100, the base current I.sub.B (=I/.beta.) of 1 .mu.A is obtained. This base current depends largely on the amplification factor .beta., so that its accuracy is poor. With present bipolar integrated circuits, the amplification factor .beta. of a transistor will vary from 100 to 500. In the present bipolar integrated circuits, it is very difficult to fabricate current source circuits arranged to provide a very small current on the order of a .mu.A or less.