Opamp-Based Current Source

EE JULY/AUGUST 1987 A current source based on an operational amplifier alone is likely to be less known than the combination of an opamp and a transistor. This latter circuit can, however, only supply a unidi- rectional current, and must incorporate a stable reference capable of sourcing the re- quired current. The circuit pro- posed here is different from the usual design for a current source, because it has a real dif- ferential, high impedance, in- put. In spite of the small number of components in this circuit, its operation may not be apparent at a glance. An example calcu- lation example may help to clarify how the current source works. Assuming that 10 V is applied to input 2, and 4.5 V to the output, the voltage drop across R2 is 0.5 V, and that across R4 is 5 V. It will be recalled that the output voltage of a current source is H3 the output current of the circuit as follows. The amplification is 10 (R3/111), and the output voltage is available across R5, which therefore ...