Single op amp current source
Current souces ar e used in a variety of applications. They can be designed in a number of ways. One of the simplest is a common resistor, but this presupposes a constant load and a constant voltage supply. If those requirements are not met, the current will vary according to the circuit elements. A considerable improvement is achieved when an active element like a transistor is added to the resistor. Variations in the ambient temperature and in the threshold potential of the p-n junction then determine the accuracy of the source.