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 design, 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 distortion in between the two signals. Both transistors used are of NPN transistor type where their emitter terminals are connected together [2].

discusses the mechanism in which the circuit works. Primarily, when no input signal is present, both transistors are biased at cutoff. When a signal is present during the positive half cycle, the transistors base inputs are in anti-phase to each other which causes transistor TR1to become biased above cutoff due to the positive base input that drives a large collector current, while TR2 is still biased at cutoff as the base current will go negative, making the collector current decrease by an equal amount of the increase in the collector current in TR1. This will result in an amplified positive-output half cycle. When the input signal is in the negative half cycle, the same procedure is done but in opposite, resulting in TR1 returning to the cutoff state while TR2 start building until it becomes biased above cutoff. This will result in an amplified negative-output half cycle. The half cycles combine to produce an output amplified 360 degrees signal with distortion. This effect is the push-pull effect.

Advantages and Disadvantages of Class B power amplifier:

Class B amplifiers have an advantage over class A amplifiers which is that the collector current is zero when the input signal to amplifier is zero that cause no power dissipation in the quiescent condition which leads to higher efficiency [1]. The main goal of using power amplifiers is to generate maximum AC power to the load, while consuming the minimum DC power possible from the supply.

The efficiency of class B amplifiers in all types may reach up to 75% which is much higher than class A amplifiers [2]. The equation shown above is used to calculate the percentage efficiency of all power amplifiers. Pout is the power delivered to the load and PDC is the power taken from the supply.

Class B amplifiers has one main disadvantage which is the high distortion that occurs in the output signal. This type of distortion is called the crossover distortion [2]. This distortion occurs when the transistors are switching over from one to another. Each transistor requires minimum voltage VBE which is about 0.7V to conduct. When one transistor is turning off by having VBE less than 0.7V and the other transistor is turning on but still the input voltage is not greater than VBE, the resulting is a zero voltage “flat spot” on the output wave shape as it crosses over from one half of the waveform to the other [2, 3].According to [2], the crossover distortion cause a reduction in the overall peak to peak value of the output waveform causing the maximum power output to be reduced. Hence, class B power amplifiers today are not used in many applications; it is only used as an audio amplifier. There are many ways to get rid of crossover distortion effect, but the most common one is to bias both the transistors at a point slightly above their cut-off point [1, 2]. This will lead us to class AB Amplifier circuit which is out of this paper scope.

Application:

The cost of class B amplifier is not high; therefore it is used in low budget designs. Furthermore, it is used in designs where the quality of the sound is not important because of the distortion it provides [3]. However, class B amplifier is preferred in designing audio power amplifiers [3]. [3] reveals that most of the time the music will be low so the signal will act as if it is in class A region. This means that the distortion will not be that bad. Moreover, the distortion can be minimized by using a negative feedback loop [3]. [3] also emphasizes that the sound power produced by this amplifier is huge and it can be run without heat sinks.

Conclusion:

This research was conducted in order to investigate the properties of class B power amplifier. First an overview about power amplifiers was considered in order to develop a better understanding about the reasons behind classifying them to many types. It was found that each class has its own application which it suits for. This is determined according to many aspects such as linearity and efficiency which are inversely proportional. Class B was found to be higher in efficiency compared to class A but less in linearity.

Looking into the configuration of class B, it was found that class B amplifier has two possible configurations, one with a transformer and the other without the transformer. Both configurations works with two transistors where each one conducts for a half cycle (180 degrees), which gives the opportunity to the other transistor to cool. However, the configuration where the amplifier does not have a transformer is more efficient because less energy is lost in elements as a result of eliminating the transformer core.

Class B amplifier was found to be used in low cost designs and designs where the clarity of the voice is not important. The crossover distortion can be removed by using a negative feedback loop or bias both transistors at a point slightly above their cut-off point, which is the idea of class AB power amplifier.