In Sections 2.06 and 2.14 we described some methods for generating constant currents within a circuit, including voltage-programmed currents with floating or grounded loads and various forms of current mirrors. In Section 3.06 we showed how to use FETs to construct some simple current-source circuits, including "current-regulator diodes" (a JFET with gate tied to source) such as the 1N5283 series. In Section 4.07 we showed how to get improved performance (at low frequencies, anyway) by using op-amps to construct current sources. And in Section 6.15 we mentioned the convenient LM334 3-terminal current source IC. There is often a need, however, for a flexible constant-current supply, which can supply substantial voltage and current, as a complete instrument. In this section we will look at some of the more successful circuit techniques.