What are electronic components? Passive and Active Components

Electronic Circuits are designed to obtain a particular function. For this, a number of electronic components are suitably connected. Although we shall study the function of various components in detail, however, we shall study in general about these components.

Electronic Components

All the electronic circuits (simple to complex) contain a few basic components such as resistors, capacitors, inductors, tube devices and semiconductor devices. The first three components resistors, capacitors and inductors are called the passive components, whereas, the remaining two components tube devices (such as vaccum diode, vaccum triode, vaccum pentode, gas diode, thyratron etc.) and semiconductor devices (such as junction diode, zener diode, transistor, field-effect transistor, unijunction transistor, silicon control rectifier, tunnel diode etc.) are called the active components.

Passive Components

The electronic components which are not capable of amplifying or processing an electrical signal are called passive components such as resistors, capacitors and inductors.

However, in electronic devices, these components are as important as active components. Without the aid of these components, the active components can not process the electrical signal e.g. a transistor cannot be made to amplify signal without the aid of passive components (resistors, capacitors and inductors).

Resistors

An electronic component which is used to limit the amount of current or divide the voltage in an electronic circuit is called resistor. Resistors have ability to oppose the current, that ability of a resistor to resist the current is called resistance. The unit of resistance (R) is ohm which is denoted by the symbol 𝛀 (the capital Greek letter called omega).

Capacitors

The two conducting plates separated by an insulating material (called dielectric) forms a capacitor. The basic purpose of a capacitor is to store the charge. The capacity of a capacitor to store charge per unit potential difference is called its capacitance. The unit of capacitance is farads (F). However, the unit farad being too large, the capacitors are specified practically in microfarads (uF) or picofarads (pF).

A capacitor may be a component which offers low impedance to a.c. but very high impedance (resistance)  to d.c. In most of the electronic circuits, a capacitor  has d.c. voltage applied, combined with a much smaller ac. signal voltage. The usual function of the capacitor is to block the d.c. voltage but pass the a.c. signal voltage, by means of charging and discharging. These  applications include coupling, bypassing and filtering for a.c. signal.

Inductors

The electronic component (mostly a coil) which opposes the change of current in circuit is named an inductor. The ability of the coil or inductor thanks to which it opposes the change of current flowing through it's referred to as its inductance. The inductance of an inductor can also be defined because the ability of an inductor (or coil) to supply induced voltage when the present varies through it. The unit of inductance is henry which is named after Joseph Henry.

An inductor may be a component which offers high impedance to a.c. but very low impedance (opposition) to d.c. The usual function of an inductor, in the electronic circuit, is to block a.c. signal but to pass d.c. signal (or voltage). The variable inductors are used in tuning circuits (for changing the resonant frequency) for radio frequencies. These are also used for impedance matching.

Active Components

The electronic components which are capable of amplifying or processing an electrical signal are called active components such as vaccum tubes, gas tubes and semiconductor (solid state) devices.  

Although, there are many active components used in electronic circuits. However, all the active  components can be broadly classified into tube-type and semiconductor (solid state) type. 

The tube-type  components can be further classified into two types ; vacuum tubes and gas tubes. Since semiconductor components (devices) have some advantages over tube-type components, therefore, semiconductor devices  are replacing tube-type devices very rapidly. Some of the commonly used active components with their  schematic symbols are given below.

1. Tube Devices

(a) Vaccum tubes

(i) Vaccum diode: It is used as rectifier and detector.

(ii) Vaccum triode : It is used as amplifier and oscillator.

(iii) Vaccum pentode : It is used as amplifier and oscillator.

(b) Gas Tubes

(i) Gas diode: It is used as voltage regulator and in neon signs.

(ii) Thyratron: It is used as controlled rectifier.

2. Semiconductor Devices

(i) Junction diode : It is used as rectifier, detector and in switching circuits.

(ii) Zener diode: It is used as voltage regulator. 

(iii) Tunnel diode: It is used in oscillators.

(iv) Varactor diode: In reverse bias condition, it is used as a variable capacitor in the electronic circuits.

(v) Light-emitting diode (LED): It emits visible light and is used in instrument displays, digital watches, calculators etc.

(vi) Transistor (BJT): It is used as amplifier and oscillator.

(vii) Field-effect transistor (FET): It is used as amplifier and oscillator.

(viii) Unijunction transistor (UJT): It is used in power controls and switching circuits.

(ix) Silicon controlled rectifier (SCR) : It is used for speed control of motors and power controls. 

(x) Triac: It is a bidirectional device and is used to obtain regulated a.c. at the output.

(xi) Diac. It is also a bidirectional device with two terminals. Generally it is used to give a pulse to the gate of triac.

(xii) Visual display devices: The major visual display device is Cathode Ray Tube (CRT). The solid-state types of visual display include light-emitting diode (LED) and liquid-crystal diode (LCD). For example, electronic calculators and digital watches use LEDs or LCDs for the digital display.