Current source: ideal and real

The current source (IT) can be regarded as an electronic device that feeds an electrical current into the external circuit , independent of the voltage on the elements of the circuit and on it itself.

A distinctive property of IT is its large (infinitely large in the ideal) internal resistance R _vn . Why is that?

Imagine that we want to transfer 100% of the power from the power source to the load. This is the transfer of energy.

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.

Current always follows the shortest path, choosing a route with the least resistance. Therefore, in our case, we must organize the source and load in such a way that the former has a much higher resistance than the second.

This is a guarantee that the current will flow 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 the flow of current from the IT along the shortest path, that is, through the load.

Since R _{b 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 a load having a relatively low resistance. This shows the output current curve of an ideal source.

With an infinitely large internal resistance of the IT, any changes in the value of the load resistance have no effect on the magnitude of the current flowing in the external circuit of the ideal source.

Infinite resistance is dominant in the circuit and does not allow changing current (despite fluctuations in 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 tends to find itself the path of least resistance, which is the 8Ω load. All current from the current source (100 mA) flows through the load resistor 8Ω. This ideal case is an example of 100% energy efficiency.

Now let's look at the schema 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 100% of its power.

This is because the internal resistance of the source will pick up 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 source and 8Ω load.

A simple calculation can determine which part of the current flows through the load resistance 8Ω

I = 100 mA -100 mA (8x10 ^-6 MΩ / 10MΩ) = 99.99mA.

Although physically ideal sources of current do not exist, they serve as a model for building real IT, close to them in terms of their characteristics.

In practice, various types of current sources are used, differing in circuitry solutions. The simplest IT can be a voltage source circuit with a resistor connected to it. This option is called resistive.

A source of very good quality can be built on a transistor. There is also a cheap serial current source on the field-effect transistor, which is just a PT with a pn junction and a gate connected to the source.