We are now in a position to learn a practical way of implementing current sources. The circuit schematic below (which has been entered into the pSpice circuit simulator) shows a simple way of doing this using a JFET (see Lecture Notes page 86). Note that the symbol used for the JFET, type 2N3819, is not quite the same as the more widely accepted one we have encountered!
The negative terminal of the supply V1 (battery symbol) is connected to Ground, also known as Node 0, and represents the reference point for any measurements we might make. Although the battery voltage is indicated as 5V, pSpice will let us vary this experimentally over any required range. The current developed by the circuit flows in the drain circuit, and is in fact ID. For this application a light-emitting diode LED is shown as the load, in which the constant current flows. This is a very typical usage for this kind of circuit; the LED works most consistently (giving a steady light output) if the current passing through it can be maintained constant, even if the supply voltage changes – for example, as the battery discharges.
Read More Source by David Holburn