When you see a "current source" symbol in a schematic, what is that exactly?

[JoeN]

I see these usually on schematics for ICs.  In real life, I am not sure what component is used here for a discrete version of the design, other than you can obviously use a resistor if you know your voltage.  What is used on an actual IC?  What should be used for a discrete design if you want the circuit to work for multiple voltages?  Thanks.  Example:

[Wim_L]

If you want to make these from discrete components, you need to keep in mind that it's essential to have nearly identical transistors. Not just the same model number, there can be too much variation between different batches of the same model. And worse, you need to keep them at the same die temperature too. For this reason, it's usually best to use a transistor array (IC package with multiple transistors) instead of discrete transistors for a circuit like this.

[SeanB]

As Blueskull says, they are a current mirror, and all are identical. In the LM358 given as an example all the positive current sources are supplied from a single diode connected top transistor, with the various currents being provided by making the current mirror transistors with different areas, so that the current ratios are all going to be very close. Thus the 4uA has an area of 2, the 6uA an area of 3, and the 100uA an area of 50 times the area of the diode connected transistor.

The current source is provided with startup current and bias with a jfet connected in pinch off, so that it gives a small well defined current. Smaller than a chip resistor of the same value by a large amount. The pull down is often implemented just by using a slightly larger jfet as a constant current sink, not too precise but will stay in the ballpark over operating range quite well.

[Zero999]

Yes the LM358 uses a current mirror with multi-collector transistors. See the detailed schematic.