A power amplifier

A power amplifier Introduction: A power amplifier is an electronic device that changes the magnitude of a signal. It is usually used to transfer high power to a low output load such as speakers where there load is about 8 ohms [1]. The relationship between the output and input is called the transfer function. An important aspect of the amplifier is the gain. The gain is the ratio between input and output whether this input is current, voltage or power. Therefore, the gain does not have a unit. The quality of the amplifier is determined by many aspects, which are gain, bandwidth, efficiency, linearity, noise and others. As one of these characteristics increase the others may decrease. Therefore, the amplifier will be designed to meet the required specifications determined by the application for which it will be used for. The various designs of the amplifiers are classified into many types which include A,B,C,D and some other types derived by combining the basic ones such as class AB which is obtained from class A and class B pertaining to their efficiency and linearity [1]. Ideally, power amplifiers are supposed to deliver 100% of the power to the load. However, as some of the power dissipates in the components of the amplifier, hence we do not have ideal power amplifiers. It is found that the linearity has an inverse relationship to the efficiency of the amplifier. The classes A, B and C power amplifiers are considered inefficient but linear whereas the other types are considered efficient but more complex than the basic types. Class A power amplifiers are least efficient than the others. The efficiency of class B is much higher than class A, however, class C has the highest efficiency level compared to class A and B [2]. The scope of this paper is to discuss class B power amplifier in details. Class B power amplifier: This class of amplifier was developed in order to improve class A power amplifiers, which have low efficiency rating. [1-3] shows and explains the internal desig n, the efficiency, the advantages and disadvantages, and the applications of class B power amplifiers. Internal circuit and operation: To get a sufficiently good amplification of the input waveform in the output, a push-pull class B power amplifier configuration must be used. This configuration is based on two transistors. The term push-pull comes from the fact that two transistors in a class B amplifier conduct in alternating half-cycles of the input [1]. Moreover, there are two types of push-pull configurations, one with transformers and one without transformers [2]. A balanced centre tapped input transformer is used to split the incoming waveform signal into two equal cycles that are 180 degrees out of phase with each other. Another center-tapped output transformer is used to recombine the signals. If the transformer is ideal we can get two collector currents that flow in opposite directions, hence no magnetization of the transformer core takes place which in turn minimizes the d istortion in between the two signals. Both transistors used are of NPN transistor type where their emitter terminals are connected together [2].