Most current-source and -sink circuits are either simple, precise, or low dropout. The circuits in Figure 1 have the advantage of all three features and are also switchable. Figure 1a was originally intended for the current-source side of an H-bridge drive for spacecraft torque rods. Thus, high-reliability and radiation-tolerant components are not necessary.
The circuits rely heavily on the LT1009 2.5V two-terminal shunt voltage regulator, which requires a minimum current of 0.4 mA. The 1N5290 constant-current diode in Figure 1a and R3 in Figure 1b assure this current. The LT1009 has an adjust input for a ±5% voltage adjustment. To use the adjust feature, the manufacturer recommends a 10-k(ohm) pot or appropriate fixed resistors across the device and the wiper connected to the adjust input. A disadvantage of using the adjust input is that the current through the 1N5290 or R3 would have to increase by 0.25 mA to maintain the 0.4-mA minimum through the regulator. If a lower control current or lower dropout is necessary, the AD1580 1.2V shunt reference is a good choice, provided greater temperature sensitivity is acceptable.
Q1A and Q1B have matched VBEs. To minimize differences in VBE between the two sides, the collector currents should be the same—nominally 0.47 mA. R2 sets this current. Theoretically, you should experimentally select R2's value in the fixed working circuit to reduce the TC even further or, if desired, to give the circuit a positive or negative TC that corrects a TC in a following stage. In Figure 1a, R4 speeds the turn-off of the circuit and reduces the off-state output current. The output current is nominally 2.5V/R1.
The current-source circuit in Figure 1a works optimally for an input voltage range of 24 to 36V and a load resistance of 200 to 650(ohm). The circuit is fairly immune to changes in output voltage, load resistance, and temperature. In the breadboard circuit, the desired output is 23.3 mA. The sensitivity of Figure 1a to input-voltage changes from 20 to 36V is approximately +0.03%/V. The sensitivity to load-resistor changes is a total 0.26% for a load change from 200 to 650(ohm). The sensitivity to temperature changes is approximately 35 ppm/°C for temperatures from 25 to 75°C. The dropout voltage is approximately 3.4V across Q1 and R1, where the output current drops about 0.64%. A dropout of 3.4V is low for constant-current applications. (DI #1889)