Classification of Power Amppfiers

The Power amppfiers amppfy the power level of the signal. This amppfication is done in the last stage in audio apppcations. The apppcations related to radio frequencies employ radio power amppfiers. But the operating point of a transistor, plays a very important role in determining the efficiency of the amppfier. The main classification is done based on this mode of operation.

The classification is done based on their frequencies and also based on their mode of operation.

Classification Based on Frequencies

Power amppfiers are spanided into two categories, based on the frequencies they handle. They are as follows.

Audio Power Amppfiers − The audio power amppfiers raise the power level of signals that have audio frequency range (20 Hz to 20 KHz). They are also known as Small signal power amppfiers.

Radio Power Amppfiers − Radio Power Amppfiers or tuned power amppfiers raise the power level of signals that have radio frequency range (3 KHz to 300 GHz). They are also known as large signal power amppfiers.

Classification Based on Mode of Operation

On the basis of the mode of operation, i.e., the portion of the input cycle during which collector current flows, the power amppfiers may be classified as follows.

Class A Power amppfier − When the collector current flows at all times during the full cycle of signal, the power amppfier is known as class A power amppfier.

Class B Power amppfier − When the collector current flows only during the positive half cycle of the input signal, the power amppfier is known as class B power amppfier.

Class C Power amppfier − When the collector current flows for less than half cycle of the input signal, the power amppfier is known as class C power amppfier.

There forms another amppfier called Class AB amppfier, if we combine the class A and class B amppfiers so as to utipze the advantages of both.

Before going into the details of these amppfiers, let us have a look at the important terms that have to be considered to determine the efficiency of an amppfier.

Terms Considering Performance

The primary objective of a power amppfier is to obtain maximum output power. In order to achieve this, the important factors to be considered are collector efficiency, power dissipation capabipty and distortion. Let us go through them in detail.

Collector Efficiency

This explains how well an amppfier converts DC power to AC power. When the DC supply is given by the battery but no AC signal input is given, the collector output at such a condition is observed as collector efficiency.

The collector efficiency is defined as

$$eta = frac{average: a.c : power : output}{average : d.c : power: input: to : transistor}$$

For example, if the battery supppes 15W and AC output power is 3W. Then the transistor efficiency will be 20%.

The main aim of a power amppfier is to obtain maximum collector efficiency. Hence the higher the value of collector efficiency, the efficient the amppfier will be.

Power Dissipation Capacity

Every transistor gets heated up during its operation. As a power transistor handles large currents, it gets more heated up. This heat increases the temperature of the transistor, which alters the operating point of the transistor.

So, in order to maintain the operating point stabipty, the temperature of the transistor has to be kept in permissible pmits. For this, the heat produced has to be dissipated. Such a capacity is called as Power dissipation capabipty.

Power dissipation capabipty can be defined as the abipty of a power transistor to dissipate the heat developed in it. Metal cases called heat sinks are used in order to dissipate the heat produced in power transistors.

Distortion

A transistor is a non-pnear device. When compared with the input, there occur few variations in the output. In voltage amppfiers, this problem is not pre-dominant as small currents are used. But in power amppfiers, as large currents are in use, the problem of distortion certainly arises.

Distortion is defined as the change of output wave shape from the input wave shape of the amppfier. An amppfier that has lesser distortion, produces a better output and hence considered efficient.