An ideal current source is a fundamental concept in electrical engineering and circuit analysis. It represents a theoretical component that is capable of supplying a constant electric current to any connected load, regardless of the voltage that develops across its terminals. This means its output current remains unchanged even if the load resistance or the voltage across it varies.

Source Resistance and Ideal Current Sources

The term source resistance refers to the internal resistance of a current source. This internal resistance affects how the current source behaves when a load is connected. For a current source to be considered "ideal," it must deliver its specified current without any loss or diversion due to its own internal properties.

Current Source Internal Resistance Explained

Let's delve into why an ideal current source must have infinite source resistance:

• An ideal current source aims to provide a fixed current, say \(I_S\), to the external circuit or load.
• Conceptually, a real current source can be modeled as an ideal current source in parallel with its internal source resistance, \(R_{source}\). This is known as the Norton equivalent circuit.
• If this internal source resistance \(R_{source}\) were finite (a specific, measurable value), then when a load is connected, the current \(I_S\) would have two paths to flow: through the load and through its own internal resistance \(R_{source}\).
• According to Kirchhoff's Current Law, the total current \(I_S\) would split between the load and the internal resistance. This means the current delivered to the load would not be the constant \(I_S\), but rather a fraction of it, which would also vary if the load changes.
• To ensure that all of the current \(I_S\) flows exclusively through the external load and none is diverted through its internal path, the internal source resistance \(R_{source}\) must be infinitely large. An infinite resistance acts like an open circuit, meaning no current can flow through it, thus forcing all the current to go to the load.
• In essence, an ideal current source acts as a perfect "current forcing" device, and to achieve this, it must present an infinitely high impedance to ensure no internal current leakage or diversion.

Comparison with Ideal Voltage Sources

It's helpful to contrast an ideal current source with an ideal voltage source:

This comparison highlights that for an ideal source to deliver its intended output (current or voltage) perfectly, its internal resistance must be either infinite (for current sources, to prevent current diversion) or zero (for voltage sources, to prevent voltage drop).

Therefore, an ideal current source has infinite source resistance.