Transistor Amplifier | It’s Important types | Variant such as BJT, FET

In this article, different transistor types will be discussed, primarily related to bipolar junction transistor (BJT) and field-effect transistor (FET) and their characteristics. However, transistors have been used as an amplifier in different circuits and various stages, mode, configurations, etc. That will also be discussed.

Although there are diverse classification of the amplifier as per different parameters are as follows:

transistor amplifier
Image credit :Filip Dominec, Transistor amplifier designCC BY-SA 3.0

Transistor Amplifier Classification

Transistor Amplifier Class: as per the number of stages

As per the number of stage of amplification, there are two class is available in transistor amplifiers, are

Single-stage Amplifier

− The circuit comprising one transistor circuitry for only step of amplification.

Multi-stage Amplifier

– This circuitry has multiple transistor circuits that be responsible for multi-stage amplification in the course of operation.

Transistor Amplifier Class: as per the input signal

As per the input signal’s level the categorization are as follows:

Small signal Amplifier

− If the input signal is very weak to generate minor or insignificant fluctuations in the collector current than quiescent value, then it’s termed as a small-signal amplifier circuit.

Large signal amplifier

− If the fluctuations existing in collector current are to be high enough, then it’s is termed as a large-signal amplifier circuit.

Class as per its output

If the output is considered as parameters, then the amplifier can be of two types. They are – Voltage Amplifiers and Power Amplifiers.

Voltage Amplifier

− It is the amplifier circuit that increases the input signal’s voltage level (V0) is called a Voltage amplifier.

Power Amplifier

− It is the amplifier circuit that increases the input signal’s power level (P0) is called a Power amplifier.

Transistor Amplifier Class: as per the frequency range

As per the signals freq. range, there are two types of an audio amplifier and radio amplifier.

Audio-Amplifier

− The audio amplifier circuit capable of amplifying the input signal in the range marked for audio signals, i.e., Frequency Range: from 20Hz to 20 kHz range.

Radio-Amplifier

−The radio amplifier capable of amplifying the input signal in the radio frequency range or lie in a very high-freq. range.

Transistor Amplifier Class: as per Biasing and mode

As per the biasing and mode of operation, classifications are class A, class B, class C, and Class AB type transistor amplifiers. The condition is as follows:

Class-A Amplifier

− The collector current carried through for the entire cycle (One Cycle) of applied alternative current signal.

Class-B Amplifier

− The collector current pass through for half-cycle (equal to 0.5 Cycle)  of applied input alternative current signal.

Class-C Amplifier

− The collector current carried for the less than half the cycle (< 0.5 Cycle) of applied input alternative current signal.

Class AB amplifiers

− Class AB amplifiers: Class AB amplifiers are formed by combining A and B classes. It helps to achieve all the gains as well as it eliminates the negatives.

Transistor Amplifier Class: Based on Configuration

Transistor Amplifier Classes: There are three types on the basis of configurations. They are – Common Emitter, Common Collector and Common Base types. 

C E or Common Emitter Amplifier Configuration

− The amplifier circuit formed using a Common Emitter configured transistor combination is called a CE amplifier.

C B or Common Base Amplifier Configuration

− The amplifier circuit formed using a Common Base configured transistor combination is called a CB amplifier.

CC or Common Collector Amplifier Configuration

− The amplifier circuit formed using a Common Collector configured transistor combination is called a CC amplifier.

Transistor Amplifier Class: Based on Coupling method

There are three types on the basis of Method of coupling. They are – Resistor-Capacitor Coupled, Transformer Coupled, and the last one is the Direct Coupled.

Direct-Coupled Amplifier

− If a multi-stage amplifier is coupled directly to the subsequent stage.

RC-Coupled Amplifier

− A Multi-stage amplifier coupled to the subsequent stage using a resistive and capacitive (RC) element via a combination circuit then it is termed as an RC coupled amplifier.

Transformer-coupled Amplifier

− A Multi-stage amplifier coupled to the subsequent stage by means of a transformer based circuit, then it is a transformer coupled amplifier.

The Types of Transistors:

There are several transistors available in the market as per different applications. The important types are as follows.

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Transistor Type

Bipolar Junction Transistor (BJT)

A BJT is a type of transistor, has both electrons and holes. Electrons, as well as the holes, act here as charge carriers.”

  • Bipolar junction transistor is a current controlled device.
  • A Bipolar junction transistor (BJT) has two PN junctions for its functioning.
  • There are two types of standard transistors, which are bipolar; PNP & NPN.
  • There are three leads in a transistor, labeled as Base (B), Collector(C), and Emitter (E).

PNP Transistor:

In P-N-P transistors, two types of diodes are assembled here. They are P-N and N-P.

The transistor consists of three section-

  • − Base
  • − Collector
  • − Emitter

In the PNP configuration, the transistor P junction has many holes, and the intermediate junction called N has efficiency and electrons. Now, the EB junction becomes the reverse biased and the CB junction becomes revere bias.

Due to the connection the bias formed and the holes started flowing from P junction. After that, the flow continues towards the N region. Here recombination takes place. The rest of the holes again flow towards the N. Now, the current through the emitter is known as Emitter Current which goes into two side. One is the Base Current another is the Collector current.

IE=IB+IC

But 2% of the total current flows in the IB, so IB is negligible.

Hence, IE=IC

NPN Transistor:

In the NPN configuration of a transistor, two types of diodes are used: N-P & P-N.

As mentioned earlier, a transistor has three Terminals. They are – Collector, Emitter and Base.

Due to the connection the bias formed and the holes started flowing from N junction. After that, the flow continues towards the P region. Here recombination takes place. The rest of the holes again flow towards the P. Now, the current through the emitter is known as Emitter Current which goes into two side. One is the Base Current another is the Collector current.

IE=IB+IC

Field Effect Transistor (FET):

In a field-effect transistor, only an electrical field is used to control the flow of current. They have three terminals, which are Source, Drain, and Gate. FETs are unipolar transistors.

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