# Classification of Circuit Elements

A circuit element is the basic building block of an electric circuit. Basically, a circuit element is a mathematical model of an electrical device which is characterized by the relationship between voltage and current. The circuit element is the most elementary part of a circuit, therefore it cannot be further divided into sub-components. The common examples of circuit elements are resistors, inductors, capacitors, energy sources, etc.

## Types of Circuit Elements

Depending on the behavior in the circuit, the electric circuit elements can be classified into the following types −

• Active Elements
• Passive Elements
• Unilateral Elements
• Bilateral Elements
• Linear Elements
• Non-Linear Elements
• Lumped Elements
• Distributed Elements

## Active Elements

The types of circuit elements that deliver power to a circuit or provide power gain in the circuit are called active elements. Energy sources like voltage sources or current sources are the examples of active elements. Semiconductor devices like transistors, Op-Amps, etc. are also considered as active circuit elements because these devices can provide power gain in the circuit. In other words, an active element is one whose i-v characteristic curve has a negative slope, i.e. the power expression of the element has a negative sign. Figure-1 shows the i-v curve of the active elements.

## Passive Elements

An electric circuit element that absorbs electrical energy, and cannot supply energy to the circuit is called a passive element. In other words, a circuit element is said to be a passive element if its i-v curve has positive slope. Figure-2 shows the i-v curve of passive circuit elements. A passive circuit can either transform or store the electrical energy supplied to it. Examples of passive elements are resistor, inductor, capacitor, transformer, etc.

## Unilateral Elements

A circuit element for which the relationship between voltage and current is different for different directions of current is called a unilateral element. Figure-3 shows the i-v characteristic of unilateral circuit elements. Semiconductor diode, LEDs, etc. are the examples of unilateral circuit elements. Hence, for unilateral elements the response to excitation relationship depends on the direction of applied excitation, which they behave differently in different directions.

## Bilateral Elements

A circuit element for which the voltage-current relationship is the same for current flowing in different directions is known as a bilateral element. Thus, a bilateral element has a relationship between response to excitation which is independent of the direction of the applied excitation. Therefore, bilateral elements behave equally in either directions of current flow. For example, if a voltage of V volts is applied to a resistor, then the current through the resistor would always be V/R irrespective of the direction of the applied voltage. Thus, examples of bilateral elements are resistors, inductors, and capacitors.

## Linear Elements

A circuit element is a linear element if it follows both homogeneity and additivity properties for the relationship between voltage and current. For a linear element, the response is directly proportional to the applied excitation. Which means, if excitation of a linear element is doubled, the response must also be doubled. Mathematically, a linear circuit element is one whose response to excitation relationship can be expressed by using a linear differential equation. Examples of linear circuit elements are pure resistor, inductor and capacitor.

## Non-Linear Elements

A circuit element which does not follow homogeneity and additivity properties for the relationship between excitation and response is called a non-linear element. Thus, in simple terms, a circuit element which is not linear is a non-linear circuit element. In case of a non-linear circuit element, the response is no directly proportional to the applied excitation. Examples of non-linear circuit elements are diodes and other semiconductor devices. For a diode, when the applied voltage is less than the cut-in voltage, the current through the diode is practically zero. Though, if the applied voltage is increased above the cut in voltage, the current through the diode becomes excessively high. Thus, the diode is a non-linear circuit element.

## Lumped Elements

When the physical size of the circuit element is small with respect to the signal wavelength is called a lumped element. Thus, the lumped elements are the elements which are pure and physically separable. Pure resistors, inductors and capacitors are the examples of lumped elements.

## Distributed Elements

When the physical size of the circuit element is of the order of single wavelength, it is called distributed element. Therefore, distributed elements are not pure and physically separable, instead these are distributed over the entire length of the circuit. Transmission lines are the examples of distributed elements.

## Conclusion

In the above sections of this article, we discussed the different types of circuit elements. This classification of elements is based on their behavior in the circuit. From the above discussion we can point out some important conclusions, which are as follows −

• Every linear element must exhibit bidirectional property.

• If the slope of i-v curve at any point is negative, then the circuit element is an activeelement otherwise it is a passive element.

• If the characteristic curve of an element is similar in opposite quadrants, then the element is bidirectional otherwise it is a unidirectional element.