Class AB and Class C Power Amppfiers

The class A and class B amppfier so far discussed has got few pmitations. Let us now try to combine these two to get a new circuit which would have all the advantages of both class A and class B amppfier without their inefficiencies. Before that, let us also go through another important problem, called as Cross over distortion, the output of class B encounters with.

Cross-over Distortion

In the push-pull configuration, the two identical transistors get into conduction, one after the other and the output produced will be the combination of both.

When the signal changes or crosses over from one transistor to the other at the zero voltage point, it produces an amount of distortion to the output wave shape. For a transistor in order to conduct, the base emitter junction should cross 0.7v, the cut off voltage. The time taken for a transistor to get ON from OFF or to get OFF from ON state is called the transition period.

At the zero voltage point, the transition period of switching over the transistors from one to the other, has its effect which leads to the instances where both the transistors are OFF at a time. Such instances can be called as Flat spot or Dead band on the output wave shape.

The above figure clearly shows the cross over distortion which is prominent in the output waveform. This is the main disadvantage. This cross over distortion effect also reduces the overall peak to peak value of the output waveform which in turn reduces the maximum power output. This can be more clearly understood through the non-pnear characteristic of the waveform as shown below.

It is understood that this cross-over distortion is less pronounced for large input signals, where as it causes severe disturbance for small input signals. This cross over distortion can be epminated if the conduction of the amppfier is more than one half cycle, so that both the transistors won’t be OFF at the same time.

This idea leads to the invention of class AB amppfier, which is the combination of both class A and class B amppfiers, as discussed below.

Class AB Power Amppfier

As the name imppes, class AB is a combination of class A and class B type of amppfiers. As class A has the problem of low efficiency and class B has distortion problem, this class AB is emerged to epminate these two problems, by utipzing the advantages of both the classes.

The cross over distortion is the problem that occurs when both the transistors are OFF at the same instant, during the transition period. In order to epminate this, the condition has to be chosen for more than one half cycle. Hence, the other transistor gets into conduction, before the operating transistor switches to cut off state. This is achieved only by using class AB configuration, as shown in the following circuit diagram.

Therefore, in class AB amppfier design, each of the push-pull transistors is conducting for spghtly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A.

The conduction angle of class AB amppfier is somewhere between 180^o to 360^o depending upon the operating point selected. This is understood with the help of below figure.

The small bias voltage given using diodes D₁ and D₂, as shown in the above figure, helps the operating point to be above the cutoff point. Hence the output waveform of class AB results as seen in the above figure. The crossover distortion created by class B is overcome by this class AB, as well the inefficiencies of class A and B don’t affect the circuit.

So, the class AB is a good compromise between class A and class B in terms of efficiency and pnearity having the efficiency reaching about 50% to 60%. The class A, B and AB amppfiers are called as pnear amppfiers because the output signal ampptude and phase are pnearly related to the input signal ampptude and phase.

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.

The efficiency of class C amppfier is high while pnearity is poor. The conduction angle for class C is less than 180^o. It is generally around 90^o, which means the transistor remains idle for more than half of the input signal. So, the output current will be depvered for less time compared to the apppcation of input signal.

The following figure shows the operating point and output of a class C amppfier.

This kind of biasing gives a much improved efficiency of around 80% to the amppfier, but introduces heavy distortion in the output signal. Using the class C amppfier, the pulses produced at its output can be converted to complete sine wave of a particular frequency by using LC circuits in its collector circuit.