An ideal current source generates a current that is independent of the voltage changes across it. An ideal current source is a mathematical model, which real devices can approach very closely. If the current through an ideal current source can be specified independently of any other variable in a circuit, it is called an independent current source. 

Conversely, if the current through an ideal current source is determined by some other voltage or current in a circuit, it is called a dependent or controlled current source. There are different kinds of sources available like voltage sources, current sources, independent sources, and dependent sources. Out of these, the ideal current source is one of the most fundamental sources used across various applications. 

In this article, we will discuss everything about ideal current sources including their definition, characteristics, internal resistance, different types, and applications.

Energy Sources

All electrical energy sources can be broadly classified into two categories:-

(An ideal voltage source and ideal current source)

• Voltage sources 
• And current sources.

 A voltage source supplies power by maintaining a set voltage across its terminals while a current source provides power through a constant current flow. Some common examples of voltage sources include batteries, solar panels, and alternators in cars. They maintain the voltage and provide power as long as the current drawn from them is within limits. 

On the other hand, current sources like photovoltaic cells and transistor collector currents function by supplying a fixed current flow independent of voltage fluctuations.

An ideal current source is defined as a two-terminal circuit element that supplies a constant current regardless of the voltage across it. It has infinite resistance.

It delivers the same value of current to any external resistive load connected to its terminals. The current provided by an ideal current source is independent of the voltage and does not vary with changes in the external circuit. This makes ideal current sources very useful building blocks for developing different kinds of circuits in electronics and power applications.

Mathematically, it can be represented as a controlled current source with a controlling voltage equal to zero. 

Fig- Ideal Current Source Circuit

Some key characteristics of an ideal current source include:

• The current output (I) is independent of the load connected and remains constant for any external resistance.
• The internal resistance (Rint) of an ideal current source is infinite which allows it to control the current perfectly without being affected by voltage.
• The voltage across an ideal current source is adjustable and depends entirely on the external load. It rises infinitely high to maintain a constant current in an open circuit.
• An ideal current source delivering zero current acts as an open circuit while one providing current without resistance resembles a short circuit.
• No physical current source can perfectly mimic the behavior of an ideal current source due to non-zero internal resistance. However certain implementations provide close approximations.

I‑V Characteristic of Ideal Current Source

The I‑V characteristic helps us understand how an ideal current source behaves when connected to different loads. In simple words, it shows the relationship between the current delivered and the voltage across the source. For an ideal current source, this graph looks completely different from that of a voltage source.

In an ideal current source, the current remains fixed even if the voltage changes. This means, no matter how the load tries to increase or decrease the voltage, the current remains the same. This is why we draw a vertical line on the I‑V graph to represent it.

Here’s a comparison of the I‑V characteristics of different sources:

Types of Current Sources

Current sources can be mainly categorized as ideal and practical based on how accurately they emulate the characteristics of an ideal current source.

Ideal Current Source

As defined above, it supplies a constant preset current regardless of voltage without any internal resistance. The concept of internal resistance holds significant importance. While ideal current sources are envisaged to produce a constant current, they are typically accompanied by an internal resistance. This internal resistance, although assumed to be zero in ideal cases, affects the practical implementation and behavior of the current source.

Practical Current Source

All real current sources have small but finite internal resistance. Their output current varies slightly with changes in external voltage depending on the resistance. In contrast to ideal current sources, practical current sources exhibit certain characteristics that deviate from the ideal model. 

These characteristics include:

• Non-zero internal resistance: Practical current sources possess a finite internal resistance, leading to variations in output current based on load conditions.
• Limited output range: Unlike ideal current sources, practical sources may have limitations in terms of the maximum and minimum current they can provide.
• Sensitivity to external factors: Practical current sources may be sensitive to changes in temperature, humidity, and other environmental factors, affecting their performance.

Independent Sources Vs Dependent Sources

Current sources can also be classified as independent sources or dependent sources based on what controls the output current.

• Independent Current Source: The output current is predetermined and not dependent on any other parameter. Ideal current sources fall in this category.
• Dependent Current Source: The output current is controlled by and proportional to some other quantity like voltage or current in the external circuit.

Symbol of Ideal Current Source

To represent an ideal current source in circuit diagrams, we use a unique symbol. This symbol helps us quickly identify the type of source and understand the direction of current flow. It’s important for students to be familiar with this, especially for solving circuit problems in exams.

The symbol of an ideal current source consists of a circle with an arrow inside. The arrow shows the direction of the current. This symbol is different from that of a voltage source, which uses a plus and minus sign to show polarity.

Let’s compare the symbols of different sources below:

Independent Sources

Independent sources are not impacted or controlled by other circuit elements and components. Their operation is self-governed to deliver a preset output.

the independent current source stands as a steadfast provider of consistent current, regardless of the surrounding circuit variables. It remains unwavering in its commitment to supplying specified current levels, ensuring a constant flow of electricity to the components it serves.

• Direct Current Source (DC Source): Direct current sources, often referred to as DC sources, epitomize the essence of independence by furnishing a steady current output at all times. Regardless of fluctuations in the circuit or external factors, these sources maintain a constant flow of current, offering reliability and stability in electrical systems.
• Alternating Current Source (AC Source): An alternating current source, also known as an AC source, introduces a dynamic element into the equation. Unlike its direct current counterpart, an AC source generates an oscillating current that periodically reverses direction, fluctuating both in magnitude and polarity over time. This versatility makes AC sources indispensable in applications requiring varying current profiles and power delivery.

Ideal Current Source

As discussed earlier, an ideal current source perfectly emulates the behavior of an independent current source by delivering a fixed current that remains constant irrespective of the external conditions. In the case of ideal current sources, they have infinite resistance.

Some additional points about ideal current sources:

• They are represented by a horizontal line with an arrow in the direction of conventional current flow in circuit diagrams.
• Delivering zero or infinite current, ideal current sources can replace open and short circuits respectively in any network.
• They form the basis for the analysis of more complex circuits using techniques like Kirchhoff's laws and network theorems.
• No physical device can accurately behave as an ideal current source due to non-zero internal resistance in reality. However certain implementations provide a close approximation.

Internal Resistance of Ideal Current Source

One of the key characteristics that differentiates an ideal component from its practical counterpart is internal resistance. For an ideal current source:

• The internal resistance (Rint) is infinite which means it can control the current flawlessly without being impacted by voltage changes across it.
• An infinite resistance implies the current source can deliver the rated current even when extremely high voltages are produced across it to retain constant current flow.
• No power is dissipated inside an ideal current source since the internal resistance through which the current passes is infinite. All the supplied power is transferred to the external load.
• During circuit analysis and simulations, an ideal current source is modeled as a voltage-controlled current source with zero controlling voltage to emulate an infinite internal resistance condition.

Fig- Ideal Current Source Vs Practical Current Source

Characteristics of Practical Current Source

While ideal current sources are useful conceptual models, all real-world sources have non-zero internal resistance. Here are some properties of practical current sources:

• The internal resistance (Rint) is small but finite. It causes the output current to vary slightly from the rated value with changes in the voltage across the source.
• The higher the internal resistance, the lesser the deviation in output current and the closer the performance to that of an ideal source.
• No physical source can supply infinite current into a short circuit or maintain rated current when open circuit voltage exceeds the maximum voltage rating.
• The maximum current is limited by the voltage compliance of the source. Beyond this, it no longer behaves as a constant current element.
• Practical sources lose regulation and deviate from ideal characteristics with temperature changes affecting internal resistance values.

Series and Parallel Combination of Current Sources In real circuits, we often use more than one current source. Understanding how current sources behave in series and parallel connections is important while designing or analyzing circuits. The rules are different compared to voltage sources. Here’s what happens in each case: Parallel Connection: Current sources can be connected in parallel only if they have the same current value. If different values are connected, it can cause conflict. Series Connection: Ideal current sources cannot be connected in series. This is because the current would have to be the same, but the voltages could be undefined or unstable. Let’s look at this in a table:

Norton and Thevenin Equivalents

Norton’s and Thevenin’s theorems are useful tools in circuit analysis. They help us simplify complex circuits into simpler forms. For ideal current sources, the Norton equivalent is especially important, as it directly uses a current source and parallel resistance.

The Norton equivalent represents a circuit using an ideal current source in parallel with a resistor. On the other hand, the Thevenin equivalent uses a voltage source in series with a resistor. You can convert one form into another easily using Ohm’s Law.

Here’s a simple comparison:

Applications of Ideal Current Source

Despite being an idealized concept, the ideal current source model finds numerous applications in electronics, instrumentation, and power systems due to its simplicity. 

Here are some examples:

• Basic elements in circuit analysis using techniques like Kirchhoff's laws and network theorems.
• Biasing current sources for transistors in active filter design, oscillator circuits, and other signal processing applications.
• The constant current drive for LEDs, optical sensors, and other similar loads requires regulated current.
• Test & measurement instruments like current calibrators, and constant current power supplies for labs.
• Battery chargers and power management circuits where constant charging/load currents are essential.
• Analog computing, specifically in current-mode circuits used for mathematical functions.
• Emulation of wire connections and current probing in SPICE simulations of large circuits.

Compliance Voltage of Practical Current Source

Although ideal current sources maintain constant current under any condition, practical current sources cannot do this forever. They can only maintain their current up to a certain voltage limit. This voltage is known as compliance voltage. Compliance voltage is the maximum voltage a current source can handle across its terminals while still keeping the current constant. If the voltage exceeds this limit, the current source will no longer behave like a constant source, and the current may drop.

Current Source in Electronic Devices

Semiconductor active devices like transistors, JFETs, and thyristors are frequently employed as current sources in modern electronics. Some applications:

• BJT & MOSFET current mirrors: Used as scalable current sources by mirroring a bias current onto multiple legs through duplicate transistor configurations.
• JFET constant current source: JFET's ohmic region above pinch-off can act as a stable current source by fixing its gate-source voltage.
• LED & laser drivers: JFET/BJT/MOSFET circuits maintain tight control over light source current for photometric needs.
• Voltage references: Bandgap references and Zener diodes generate fixed voltage drops to realize high-precision current sources.
• Current regulators/limiters: Circuits limit excessive device/power rail/line currents within safe operating limits.

Current Source in Power Supplies

Ideal current sources play a significant role in the design of efficient switch-mode and linear power supplies:

• Switch-mode converter controller ICs: Internal oscillator/ramp generator circuits deliver stable bias currents.
• Bulk capacitor charging: Pre-charge/soft-start circuits use current limiting to prevent inrush transients.
• Linear regulator reference: Bandgap or Zener diode reference powers feedback amplifier as current source.
• Current mode control: Switching converter control loop senses inductor current as feedback.
• LED driver stages: Constant current is crucial for lighting and backlight applications.
• Battery chargers: Preset termination currents avoid overcharging while ensuring fast charge times.

Conclusion

In conclusion, the concept of an ideal current source forms the foundation for analysis and design across many domains of electrical and electronics engineering. While no physical implementation can perfectly emulate its characteristics of infinite internal resistance and output invariance, certain circuits provide close approximations making them highly useful. Understanding ideal sources aids in solving complex circuits and designing robust current-critical applications.

This article summarises all the information related to Ideal Current Source, which helps in propelling your preparation for various AE/JE and ESE examinations. You can visit the Testbook app to keep yourself updated with all the exam-oriented information related to the upcoming examination, like SSC JE, GATE, ESE, RRB JE, and state AE/JE Exams.