Classification of Power Amplifiers

Classification of Power Amplifiers

AU : May-17, Dec.-17

• The position of the quiescent point on the load line decides the class of operation of the power amplifier. The various classes of the power amplifiers are :

i) Class A ii) Class B iii) Class C iv) Class AB v) Class D.

1. Class A Amplifiers

• The power amplifier is said to be class A amplifier if the Q point and the input signal are selected such that the output signal is obtained for a full input cycle.

Important Concept

For this class, position of the Q point is approximately at the midpoint of the load line.

• For all values of input signal, the transistor remains in the active region and never enters into cut-off or saturation region.

• The collector current flows for 360° (full cycle) of the input signal. In other words, the angle of the collector current flow is 360° i.e. one full cycle.

• The current and voltage waveforms for a class A operation are shown with the help of output characteristics and the load line, in the Fig. 8.15.1.

• As shown in the Fig. 8.15.1, for full input cycle, a full output cycle is obtained. Here signal is faithfully reproduced, at the output, without any distortion. This is an important feature of a class A operation.

• The efficiency of class A operation is very small, lies between 25 % to 50 %.

2. Class B Amplifiers

• The power amplifier is said to be class B amplifier if the Q point and the input signal are selected, such that the output signal is obtained only for one half cycle for a full input cycle.

Important Concept

For this operation, the Q point is shifted on X-axis i.e. transistor is biased to cut-off.

• Due to the selection of Q point on the X-axis, the transistor remains, in the active region, only for positive half cycle of the input signal. Hence this half cycle is reproduced at the output.

• But in a negative half cycle of the input signal, the transistor enters into a cut-off region and no signal is produced at the output.

• The collector current flows only for 180° (half cycle) of the input signal. In other words, the angle of the collector current flow is 180° i.e. one half cycle.

• The current and voltage waveforms for a class B operation are shown in the Fig. 8.15.2.

• As only a half cycle is obtained at the output, for full input cycle, the output signal is distorted in this mode of operation.

• To eliminate this distortion, practically two transistors are used in the alternate half cycles of the input signal. Thus overall a full cycle of output signal is obtained across the load. Each transistor conducts only for a half cycle of the input signal.

• The efficiency of class B operation is much higher than the class A operation, upto 78.5 %.

3. Class C Amplifiers

• The power amplifiers is said to be class C amplifier, if the Q point and the input signal are selected such that the output signal is obtained for less than a half cycle, for a full input cycle.

Important Concept

For this operation, the Q point is to be shifted below X-axis.

• Due to such a selection of the Q point, transistor remains active, for less than a half cycle. Hence only that much part is reproduced at the output.

• For remaining cycle of the input cycle, the transistor remains cut-off and no signal is produced at the output.

• The angle of the collector current flow is less than 180°.

• The current and voltage waveforms for a class C amplifier operation are shown in the Fig. 8.15.3.

Important Concept

In class C operation, the transistor is biased well beyond cut-off. As the collector current flows for less than 180°, the output is much more distorted and hence the class C mode is never used for A.F. power amplifiers.

• But the efficiency of this class of operation is much higher and can reach very close to 100 %.

• The class C amplifiers are used in tuned circuits which are used in communication areas and radio frequency circuits. These are also used in mixers used in radio receivers and wireless communication circuits.

4. Class AB Amplifiers

• The power amplifier is said to be class AB amplifier, if the Q point and the input signal are selected such that the output signal is obtained for more than 180° but less than 360°, for a full input cycle.

Important Concept

The Q point position is above X-axis but below the midpoint of a load line.

• The current and voltage waveforms for a class AB operation, are shown in the Fig. 8.15.4.

• The transistor conducts for full positive half cycle and part of negative half cycle.

• The output is distorted but distortion can be eliminated.

• The efficiency is more than class A but less than class B operation.

• The class AB operation is important to eliminate cross-over distortion.

• As it eliminates cross-over distortion, used in most of the audio systems, radio receivers, T.V. receivers etc.

Review Questions

1. Give the classification of large signal amplifiers.

2. Why class C amplifiers are not used as the output stage in audio amplifiers ?

3. Describe the working of class A and class C power amplifier in details with relevant diagrams.

AU May-17, Marks 13

4. Explain the working of class C power amplifier.

AU: Dec.-17, Marks 7