A constant current source can be built based on one of the LM3900 or LM2900 amplifiers, as shown in the figure. A current of 1mA can be achieved in a load with resistance in the range 0 to 12 k ohms, starting from a 15 V power supply. The divider formed by R1 and R2 applies the reference voltage to the non-inverting input, that produces the desired current in the system. Note that 1 V over a 1k resistor corresponds exactly to a current of 1mA, since this current is derived from the emitter of the internal transistor Q5.
