The Classes and Classification of Amplifiers With Their Applications

In earlier days, before the invention of electronic amplifiers, the coupled carbon microphones are used as crude amplifiers in telephone repeaters. The first electronic device that practically amplifies was the Audion vacuum tube, invented by the Lee De Forest in the year 1906. The term amplifier and amplification are from Latin word amplificare to expand or enlarge. The vacuum tube is the only simplifying device for 40 years and dominated electronics up to 1947. When the first BJT was in the market it has created another revolution in the electronics and it is a first portable electronic device like transistor radio developed in the year 1954.  This article discusses  the classes and classification of amplifiers.

What is an Amplifier and Classification of Amplifiers?

Simply the amplifiers are called as an amp. The amplifier is an electronic device used to increase the signal of current, voltage and power. The function of the amplifier is by using the power from the power supply and longer altitude, it controls the output signal with the help of input signal. An amplifier modulates the out of the power supply is based on the properties of the input signal. The amplifier is quite opposite to an attenuator if the amplifier provides the gain, hence the attenuator provides the loss. The amplifier is also a discrete part of the electrical circuit which is continued with the other device.

Amplifier
Amplifier

An amplifier is used in all the electronic equipments. The amplifiers can be categorized in different types. The first one is by the frequency of the electronic signal being improved. The next one is the audio amplifier and amplifies the signal in the range of less than the 20 kHz and the RF amplifier amplifies the radio frequency ranges from 20 kHz to 300 KHz. The last one is current quality and voltage are being amplified

There are different types of amplifiers includes a current amplifier, a voltage amplifier or the transconductance amplifier and trans-resistance amplifier. Nowadays, most of the amplifiers used in the market are transistors but, vacuum tubes are also using in some of the applications.

Classification of Amplifiers

The classification of amplifiers is shown in the following

  • Input and output variable
  • Common terminal
  • Unilateral and bilateral
  • Inverting and Non-inverting
  • Interstage coupling method
  • Frequency range
  • Function

Input and Output Variable

The electronic amplifier uses only one variable i.e. current or voltage. It may be current or voltage can be used in the input or either in the output. There are four types of amplifiers and which are dependent on the source used as a linear analysis.

Input output Dependent source Amplifier type Gain Units

I

I

Current Controlled Current Source  CCCS Current Amplifier Unitless

I

V

Current Controlled Voltage Source  CCVS Trans resistance Amplifier Ohm

V

I

Voltage Controlled Current Source  VCCS Trans Conductance Amplifier Siemens

V

V

Voltage Controlled Voltage Source VCVS Voltage Amplifier Unitless

Common Terminal

The classification of the amplifier is based on the device terminal which is common to both input and output circuit. In the Bipolar Junction Transistor, there are three classes namely. a common emitter, common base, and common collector. In the case of Field Effect Transistor,  it has the corresponding configurations like common source, common gate, and a common drain. The common emitter is the most frequently to provide amplification of a voltage applied between base and emitter. The input signal is in between collector and emitter is inverted it is relative to the input. The common collector circuit is called as an emitter follower, source follower, and cathode follower.

Unilateral and Bilateral

The amplifier whose output displays no feedback to the input side is called as unilateral. The unilateral amplifier of the input impedance is independent of load and the output impedance is independent signal source impedance.

The amplifier which uses the feedback to connect part of the output back to the input is called as a bilateral amplifier. The input impedance of bilateral amplifier depends on the load and the output impedance of source impedance. The linear unilateral and bilateral amplifiers are denoted as two port networks.

Inverting and Non-Inverting

In this,  the classification of an amplifier uses the phase relationship of the input signal to the output signal. The inverting amplifier gives the output of 180 degrees out of phase with the input signal.

The non-inverting amplifier continuous the phase of the input signal waveforms and the emitter is a non-inverting amplifier. The voltage follower is called as non-inverting amplifier and it has unity gain.

Interstage coupling Method

This type of amplifier is classified by using the coupling method of the signal at the input, output and between the stages. There are different types of methods in the interstage coupling amplifier.

  • Resistive-capacitive coupling amplifier
  • Inductive-capacitive coupling amplifier
  • Transformed coupling amplifier
  • Direct coupling amplifier

Classes of Amplifiers

There are different types of classes of amplifiers mentioned in the following

  • Class A amplifier
  • Class B amplifier
  • Class C amplifier
  • Class D amplifier
  • Class AB amplifier
  • Class F amplifier
  • Class S amplifier
  • Class R amplifier

Class A Amplifier

The class A amplifiers are simple designed amplifiers and this amplifier is mostly commonly used amplifiers. Basically,  the class A amplifiers are the best class amplifiers because of their low distortion levels. This amplifier is the best in the audio sound system and in most of the sound system use the class A amplifier. The class A amplifiers are formed by the output stage devices which are biased for the class A operation. By comparing the other classes amplifiers to class A amplifier has the highest linearity.

Class A Amplifier
Class A Amplifier

To obtain high linearity and gain in class A amplifier the output of the class A amplifier should be biased ON for all times. Hence the amplifier is said to be as a class A amplifier. The zero signal ideal current in the output stage should be equal to or more than the maximum load current is required to produce more amount of signal.

Advantages

  • It eliminates Non-linear distortion
  • It has low ripple voltage
  • It does not require any frequency compensation
  • There is no cross and switching distortions
  • There is low harmonic distortion in the voltage and current amplifier

Disadvantages

  • The transformers used in this amplifier are bulk and they are high cost
  • Its requirement of two identical transistors

Class B Amplifier

The class B amplifiers are the positive and negative halves of the signals, that are allocated to the different parts of the circuits and the output device switched ON and OFF continuously. The basic class B amplifiers are used in two complementary transistors which are FET and bipolar. These two transistors of each half of the waveform with its output are configured in a push-pull type arrangement. Hence the each amplifier only half of the output waveform.

Class B Amplifier
Class B Amplifier

In the Class B amplifier, if the input signal is positive, then the positively biased transistor conduct and the negative transistor is switched OFF. If the input signal is negative, then the positive transistor switches OFF and negative biased transistor turn ON. Hence the transistor conduct half of the time whatever it may be like positive or negative half cycle of the input signal.

Advantages

  • Some amount of distortion in the circuit gives the more output per device because of  there is no presence of the even harmonics
  • The use of push-pull system in the class B amplifier eliminates the even harmonic

Disadvantages

  • In the class B amplifier, there is high harmonic distortion
  • In this amplifier, there is no need for self bias

Applications

  • The class B amplifiers are used in low-cost design
  • This amplifier is more significant than the class A amplifier
  • The class B amplifier suffers from the bad distortion if the signal level is low

Class AB Amplifier

The class AB is the combination of class A and class B amplifier. The class AB amplifiers are using commonly in the audio power amplifiers. From the diagram the two transistors have the small amount of voltage which is 5 to 10% of the quiescent current and the bias the transistor just above the cutoff point. Then the device may be FET or bipolar will be ON for more than the one-half of the cycle, but it is less than the one full cycle of the input signal. Hence, in the class AB amplifier design each of the push-pull transistors is conducting slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A.

Class AB Amplifier
Class AB Amplifier

The conduction angle of Class AB amplifier is in between 1800 to 3600 which is depending on the bias point. The advantage of the small bias voltage is to give in series resistance and diode.

Advantages

  • The class AB has a linear behavior
  • The design of this amplifier is very simple
  • The distortion of this amplifier is less than 0.1%
  • The sound quality of this sound is very high

Disadvantages

  • The power dissipation of this amplifier generates the heat and requires large amount of heat sink
  • This amplifier has low power efficiency and the average efficiency is less than the 50%

Applications

The class AB amplifiers are used in hi-fi systems.

Class C Amplifier

The design of class C amplifier has a great efficiency and poor linearity. In the previous amplifiers, we have discussed the class A, B and AB are the linear amplifiers. The class C amplifier is a deeply biased hence the output current is zero for more than the one-half of the input signal and the transistor idling at the cut off point. Because of the serious audio distortion, the class C amplifiers are high-frequency sine wave oscillation.

Class C Amplifier
Class C Amplifier

Advantages

  • The efficiency of Class C amplifier is high
  • In class C amplifier the physical size is low for the given o/p power

Disadvantages

  • The linearity of Class C amplifier is low
  • The class C amplifiers are not used in the audio amplifiers
  • The dynamic range of the class c amplifier is decreased
  • The class C amplifier will produce more RF interfaces

Applications

This amplifier is used in the RF amplifiers

Class D Amplifier

The class D amplifier is non-linear switching amplifiers or PWM amplifiers. This amplifier can reach 100% efficiency in theoretically and there is no period during the cycle. The voltage and the current waveforms overlap current is drawn only with the help of transistor which is in ON state. These amplifiers are also called as the digital amplifiers.

Class D Amplifier
Class D Amplifier

Advantages

  • The class D amplifier has more efficiency that is more than 90%
  • In the class D amplifiers, there is a low power dissipation

Disadvantages

The design of the class D amplifier is more complex than the class AB amplifier.

Applications

  • This amplifier is used in the sound cards of the mobile devices and personal computers
  • These amplifiers are used in cars of audio subwoofer amplifiers.
  • Nowadays, in most of the applications, these amplifiers are using.

Class F Amplifier

The F amplifiers are used to increase the efficiency and output by the harmonic resonators in the form of output network and to shape the output waveform in a square wave. The class F amplifiers have more than 90% of efficiency if the infinite harmonic tuning is used.

Class F Amplifier
Class F Amplifier

Class S Amplifier

The class S amplifiers are similar operations to the class D amplifiers. These amplifiers are Non-linear switching mode amplifiers. It converts the analog input signals to the digital square wave pulses by using the delta-sigma modulations. It amplifies them in to increase the output power by the help of band pass filter. The digital signal of the switching amplifier is fully in ON or OFF state and its efficiency can reach 100%.

Class S Amplifier
Class S Amplifier

Class T Amplifier

The class T amplifiers are designed with a type of digital switching amplifiers. Nowadays these amplifiers became more popular as an audio amplifier design because of the extension of the DSP chip and multi-channel sound amplifier. This amplifier converts the signal from analog signal to the digital pulse width modulation signal and the amplification increase the efficiency of an amplifiers. The class T amplifiers are the combination of low distortion signal of class AB amplifier and the other one is the efficiency of class D amplifier.

Class T Amplifier
Class T Amplifier

Class G Amplifier

The enhancement of class G Amplifier is the basic of Class AB Amplifier. The class G amplifier used in the multiple power supply rails of different voltages. Automatically switches between the supply rails as the input signal changes. The contact switching decreases the average power consumption hence, the power loss is produced by the wasted heat. The below circuit diagram shows the class G amplifier.

Class G Amplifier
Class G Amplifier

This article describes the classification of amplifiers. Furthermore any queries, felt anything missed, you want to know any info about any particular topic, please let me know by commenting in the comment section below. Here is a question for you, What are the functions of different types of amplifiers?

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6 Comments

  1. good, but I need the explanation of class e power amplifier !!!! Found it missing. If possible can you tell where can I get it?

    1. Tarun Agarwal says:

      Hi Abbas
      We are very sorry for the inconvenience.

  2. Very Nice explanation, please keep it up

  3. Cool… . information is gud when we are away from books…and how to quick search your pages……???? Any URL

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