A voltage is present across the terminals of a current source. Unlike a voltage source, the current source itself does not establish this voltage. The value of this potential difference is not a fixed characteristic of the source. Instead, it is a consequence of the complete electrical circuit to which the current source is connected. The source’s primary function is to maintain a constant flow of charge, and the resulting voltage across its terminals depends entirely on the external components.
Defining the Ideal Current Source
An ideal current source is a theoretical two-terminal component designed to deliver a perfectly constant flow of electrical current. This constant current is supplied to the external circuit regardless of the resistance encountered or the voltage that develops across its terminals. The ideal nature of this source means its output current remains fixed, even if the voltage across it changes drastically.
To achieve this theoretical independence from external conditions, an ideal current source is modeled as having infinite internal resistance. This infinite resistance ensures that none of the generated current is diverted internally, forcing the entire specified current to flow through the connected external circuit. While no physical device is perfectly ideal, this model is a fundamental concept used in circuit analysis to simplify complex calculations.
How Voltage is Determined Across the Source
The voltage across a current source is a dependent variable, determined by the external load connected to its terminals. The source supplies a fixed current (\(I\)), which is forced through the load resistance (\(R_{load}\)). The resulting voltage (\(V\)) across the load, and therefore across the source terminals, is a direct result of Ohm’s Law: \(V = I \times R_{load}\).
If the external load resistance is doubled, the voltage across the source must also double to maintain the fixed current output. Conversely, connecting a very small load resistance results in a correspondingly small voltage across the source. The source will theoretically produce any voltage necessary—even an infinite one in the case of an open circuit—to ensure the current remains at its specified value. The current source is the active controller of current, and the circuit’s resistance determines the resulting voltage.
Contrasting with Ideal Voltage Sources
The behavior of a current source is the inverse of an ideal voltage source, which helps clarify the difference between the two components. An ideal voltage source maintains a fixed voltage across its terminals, regardless of the current drawn. For a voltage source, the voltage (\(V\)) is the independent variable, and the current (\(I\)) changes based on the external load resistance (\(R_{load}\)) according to \(I = V/R_{load}\).
An ideal voltage source has zero internal resistance, allowing it to theoretically supply unlimited current to keep its voltage constant. The voltage source dictates the potential difference, and the load resistance determines the flow of charge. In contrast, the current source dictates the flow of charge, and the load resistance determines the potential difference.
Modeling Real World Current Sources
Real-world current sources, such as those built using transistors, cannot maintain a perfectly constant current indefinitely and possess physical limitations. To model these limitations, a practical current source is represented as an ideal current source with a large internal resistance (\(R_p\)) placed in parallel across its terminals. This parallel resistance is often called shunt resistance and is typically in the mega-ohm range.
The presence of this shunt resistance means a small fraction of the source’s current is diverted internally, preventing the entire current from reaching the load. As the external load resistance increases, the voltage across the source rises, and the current lost to the internal shunt resistance becomes more significant. This internal current loss causes the output current to the load to decrease slightly, especially when the required voltage approaches the source’s maximum operating limit, known as the compliance voltage.