Is a current source a single component or the same as a voltage source?
Whether something is acting as a voltage source or current source is only determined by what parameter they try to keep constant at the output, but both will deliver current as well as voltage (Ohm's law!).

In a voltage source, the voltage at the output is kept at a constant level and the source will (ideally) deliver as much current as is needed to drop that voltage over the load resistance. If there is an open circuit, the voltage source will deliver no current at all, but if there is load with the resistance R, than the current will be \$ I = \frac{U}{R} \$.

In a current source, the current at the output is kept at a constant level and the source will (again, ideally) supply whatever voltage is needed to keep that current flowing. With a very small load resistance the output voltage will be very small, with a higher resistance the voltage has to increase to keep the current flowing (again, \$ I = \frac{U}{R} \$).

In reality both sources obviously are limited. With increasing load resistance a current source will reach the maximum voltage at some place and than the current will not be kept constant anymore. The opposite is true for voltage sources: With very low load resistances the source won't be able to supply as much current as is needed to keep the voltage constant.

Circuit wise, you can look at a current source like a voltage source with indefinite (or really large) internal resistance and an indefinite (or really large) voltage. This will drop almost all voltage on the internal resistance and whether or not the load resistance is changing does not really change anything in regards of the current flow.
See on the left a load of 1 Ohm and on the right a resistance of 1kOhm. How much the current of the current source is changing with a change in resistance determines the quality of the source.







simulate this circuit &ndash; Schematic created using CircuitLab

In practise this is not done with ridiculously large component values, but with an active control of the output (a transistor at the output of the supply is controlling the output resistance).