A current source (IT) can be considered as an electronic device that supplies an external circuit with an electric current that is independent of the voltage on the circuit elements and on it.
A distinctive feature of IT is its large (infinitely large ideally) internal resistance R ext . Why is that?
Imagine that we want to transfer 100% of the power from the power source to the load. This is energy transfer.
To deliver 100% of the power from the source to the load, it is necessary to distribute the resistance in the circuit so that the load receives this power. This process is called current splitting.
The current always goes along the shortest path, choosing a route with the least resistance. Therefore, in our case, we must organize the source and load so that the first has a much higher resistance than the second.
This ensures that current flows from the source to the load. That is why we use the ideal current source with infinite internal resistance in this example . This ensures that current flows from IT along the shortest path, that is, through the load.
Since R vn of the source is infinitely large, the output current from it will not change (despite the change in the value of the load resistance). Current will always tend to flow through the infinite resistance of the IT towards the load, which has a relatively low resistance. This shows a graph of the output current of an ideal source.
With an infinitely large internal IT resistance, any changes in the load resistance value do not have any effect on the magnitude of the current flowing in the external circuit of an ideal source.
Infinite resistance is dominant in the circuit and does not allow the current to change (despite fluctuations in the load resistance).
Let's look at the circuit with an ideal current source, shown below.
Since IT has infinite resistance, the current flowing from the source seeks to find the path of least resistance, which is the 8Ω load. All current from the current source (100 mA) flows through an 8Ω load resistor. This ideal case is an example of 100% energy efficiency.
Now let's look at a scheme with real IT (as shown below).
This source has a resistance of 10 MΩ, which is high enough to provide a current very close to the full value of the 100 mA source, but in this case, IT will not give up 100% of its power.
This is because the internal resistance of the source will draw some of the current, resulting in a certain leakage.
It can be calculated using a specific cleavage.
The source produces 100 mA. This current is then divided between the resistances of 10 MΩ of the source and 8Ω of the load.
With a simple calculation, you can determine how much current flows through the load resistance 8Ω
I = 100 mA -100 mA (8x10 -6 MΩ / 10MΩ) = 99.99mA.
Although physically ideal current sources do not exist, they serve as a model for building real IT, close to them in their characteristics.
In practice, various types of current sources are used, which differ in circuitry solutions. The simplest IT can be a voltage source circuit with a resistor connected to it. This option is called resistive.
A very good quality current source can be built on a transistor. There is also a cheap serial current source on a field effect transistor, which is just a PT with a pn junction and a gate connected to the source.