How do we create current sources?
Constant current sources ... there are no such devices in nature; almost all of sources are (constant) voltage sources. So we have to make them.


I think you are correct. Due to physics chemical batteries (DC) and electrol-magnetic (AC) almost always (wriggle room!) create voltage sources. Note, however, that if the load resistance is very low in comparison to the source, \$ R_L &lt;&lt; R_S \$, that the source now becomes a constant current source.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A voltage source with internal resistance acts like a current source when \$ R_L &lt;&lt; R_S \$.


  So what are the basic ideas behind current sources? How are they implemented in practice? Why are they implemented exactly in such a way? What do active elements in these circuits do? What functions do they perform?


An ideal voltage source has zero output impedance. An ideal current source has an infinite output impedance. The voltage across the current source or load is determined solely by the load.


  I would like not only to know specific circuit solutions... I want to understand them... to see the idea behind them... "the forest for the trees".




Figure 2. This switched constant-current driver provides a constant-current to the LED over a range of supply voltages and independent of the LED’s forward voltage. If switching is not required then connect In to Vbb. Source: Simple constant current driver.

The theory of operation is explained in my linked article. The basic idea is that Q1 is turned on by R1 but as the current through R2 exceeds 0.6 V or so that Q2 turns on and steals the bias from Q1 and the circuit stabilises when the current = 0.6 / R2. There will be some small errors due to current through R1 and Q2.

Integrated two-terminal current regulators are available such as these from Diodes Incorporated.



Figure 2. The AL5809 SMD package.



Figure 3. The AL5809 contains a factory-set constant current regulator with associated control circuitry. Image source: LEDnique.com.

A low drop-out (LDO) voltage regulator provides power for the internal circuitry. This will determine the minimum operating voltage of the device which is 2.5 V. This means that the minimum supply voltage required is the VF of the diodes + 2.5 V. ( VIN(min) = VLOAD + 2.5 V.) A fixed preset LED current setting resistor sets the reference current of the Current regulation block. The LED current setting resistor varies with each variant of the AL5809. An accurate current mirror within the Current regulation control block increases the reference current to the preset LED current of the AL5809. See the linked article for more.