- Definition of Circuit Components: Active components supply energy, while passive components receive and store or dissipate energy.
- Active Circuit Elements: These include voltage sources, current sources, transistors, and diodes, which control electron flow and amplify signals.
- Passive Circuit Elements: These include resistors, inductors, capacitors, and transformers, which store or dissipate energy without amplification.
- Voltage and Current Sources: Voltage sources and current sources are key active components, providing energy and controlling charge flow in circuits.
- Differences Between Active and Passive Components: Active components can amplify and control energy, while passive components cannot generate or amplify energy, only store or dissipate it.
What are Active and Passive Circuit Elements (Components)?
Types of Electronic Components
Electronic elements that make up a circuit are connected together by conductors to form a complete circuit. If these connecting conductors are ideal conductors (i.e. they have no resistance) then all parts of the circuit can be classified into two main categories depending on whether they deliver or absorb energy from the circuit:
- Active components
- Passive components
Electrical symbols are used to represent both active and passive components. An example of a basic circuit made up of two electronic elements has been illustrated below:
Active Components
An active component is defined as an electronic component that supplies energy to a circuit and can control electron flow. All electronic circuits must contain at least one active component.
Common examples of active components include:
- Voltage sources
- Current sources (e.g. DC current source)
- Generators (such as alternators and DC generators)
- All different types of transistors (such as bipolar junction transistors, MOSFETS, FETs and JFET)
- Diodes (such as Zener diodes, photodiodes, Schottky diodes and LEDs). Diodes control current direction rather than providing power gain, so some references treat them as passive, yet their rectifying and controlling behaviour usually places them among active devices.
Voltage Sources
A voltage source is defined as an active component because it supplies energy to a circuit. When current leaves the positive terminal, it provides energy to the circuit. A battery is also considered an active element since it continuously delivers energy during discharge.
Current Sources
A current source is defined as an active component because it supplies constant current regardless of circuit voltage. Since it controls charge flow, it is classified as an active element.
Transistors
Although not as obvious as a current or voltage source, transistors are also active circuit components because they can amplify the power of a signal (see our article on transistors as an amplifier).
Because this amplification essentially controls the flow of charge, transistors are classified as active components.
Passive Components
A passive component is an electronic component which can only receive energy, which it then either dissipates or absorbs, or stores in an electric field or a magnetic field. Passive elements do not need any form of electrical power to operate.
As the name ‘passive’ suggests, passive devices provide no gain or amplification. They cannot amplify, oscillate or generate an electrical signal.
Common examples of passive components include:
Resistors
A resistor is defined as a passive element because it cannot deliver energy to a circuit. Instead, it receives energy and dissipates it as heat when current flows through it.
Inductors
An inductor is also considered a passive element of a circuit because it can store energy as a magnetic field and can return that energy to the circuit, though not continuously. The energy absorbing and delivering capacity of an inductor is limited and transient in nature. That is why an inductor is taken as a passive element of a circuit.
Capacitors
A capacitor is defined as a passive element because it stores energy in an electric field. Its energy storage is limited and temporary, meaning it does not supply energy but stores it for later use.
It is therefore not considered an active component, since no energy is supplied or amplified.
Transformers
A transformer is also a passive electronic component. This can seem surprising because transformers are often used to raise voltage levels, but remember that the power stays constant apart from small losses.
When transformers step up (or step down) voltage, the power delivered by the secondary side equals the power received by the primary side. Since no energy is actually amplified, a transformer is classified as a passive element.
Bilateral Elements
Conduction of current in both directions through a circuit element with the same magnitude is termed a bilateral circuit element. It offers the same resistance to current flow in both directions.
Examples: Resistors, inductors, capacitors etc.
This figure shows that a bilateral circuit element can conduct from both sides and offers the same resistance for current from either side.
Unilateral Elements
The unilateral circuit element does not offer the same resistance to current in either direction. The resistance of the unilateral circuit element is different for forward current than for reverse current.
Examples: diode, transistor etc.
The figure above shows a diode as a unilateral circuit element. When the diode is forward biased, it offers very small resistance and conducts; when it is reverse biased, it offers very high resistance and does not conduct. The circuit element can be categorized in another manner, such as lumped and distributed circuit elements.
Lumped Elements
When the voltage across and the current through an element do not vary with the dimensions of the element, it is called a lumped circuit element.
Examples: Resistor connected in any electrical circuit.
Distributed Elements
When the voltage across and the current through an element change with the dimensions of the element, it is called a distributed circuit element.
Examples: Resistance of a transmission line. It varies with the length of the line.
