A current source (IT) can be considered as an electronic device that supplies an electric current to an external circuit, independent of the voltage on the circuit elements and on itself.
A distinctive property of IT is its large (infinitely large ideally) internal resistance Rext. Why is that?
Let's imagine that we want to transfer 100% of the power from the power supply to the load. It is a transfer of energy.
To deliver 100% 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.
Current always takes the shortest path, choosing the route with the least resistance. Therefore, in our case, we must organize the source and load in such a way that the first has a much higher resistance than the second.
This is to ensure that current flows from the source to the load. That's why we use in this example an ideal current source that has infinite internal resistance. This ensures that current flows from the IT along the shortest path, i.e. through the load.
BecauseRext of the source is infinitely large, the output current from it will not change (despite the change in the value of the load resistance). The current will always tend to flow through the infinite resistance of the IT towards the load with relatively low resistance. This shows the output current graph of an ideal source.
With an infinitely large IT internal resistance, any changes in the load resistance value have no effect on the amount of 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 load resistance fluctuations).
Let's look at the ideal current source circuit shown below.
Because IT has infinite resistance, the current flowing from the source tends to find its path of least resistance, which is an 8Ω load. All current from the current source (100mA) flows through the 8Ω pull-up resistor. This ideal case is an example of 100% energy efficiency.
Now let's look at the real IT circuit (as shown below).
This source has a resistance of 10 MΩ which is high enough to provide a current very close to the full 100 mA of the source, however in this case the IT will not deliver 100% of its power.
This is because the intern althe source resistance will take some of the current, resulting in a certain amount of leakage.
It can be calculated using a particular split.
Source delivers 100 mA. This current is then shared between the 10 MΩ source and 8Ω load.
With a simple calculation, you can determine what part of the current flows through the load resistance 8Ω
I=100mA -100mA (8x10-6 MΩ /10MΩ)=99.99mA.
Although physically ideal current sources do not exist, they serve as a model for building real ITs that are close in their characteristics.
In practice, various types of current sources are used, differing in circuit 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 commercial FET current source, which is just a FET with a p-n junction and a gate connected to the source.
