Power Amplifiers

Power Amplifiers

April-03, May-03,12

In general, an amplifier receives an input signal from some transducer or other input source and provides a large amplified signal to some output device or another amplifier stage. The small signal amplifiers are basically voltage amplifiers, the voltage and current signal levels are small in such amplifiers. The output current capability of such amplifiers is limited. The amount of power handling capacity and power efficiency are of little concern for the small signal amplifiers.

The power amplifier is basically used to amplify an audio signal faithfully. The loads to such amplifiers are generally loud speakers and servomotors. Such loads require large current and sufficient power, typically few watts to tens of watts. Such power amplifiers develop and feed the sufficient power to the loads like speakers, motors etc. by handling the large signals hence these are also called as large signal amplifiers or power amplifiers. The low output impedance is another requirement of such loads which is to be satisfied by the power amplifiers.

Thus the main features of a large signal amplifier or an power amplifier are,

i) The large amount of power to be delivered to the load.

ii) The power efficiency.

 iii) The impedance matching to the output device.

The large signal amplifiers develop an a.c. power of the order of few watts. Similarly large power gets dissipated in the form of heat, at the junctions of the transistors used in the power amplifiers. Hence the transistors used in such amplifiers are power transistors with heat sinks.

1. Classification of Power Amplifiers

The power amplifiers are classified by their classes of operation. The class of operation represents the amount of the output signal varies over one cycle of operation for a full cycle of input signal.

For an amplifier, a quiescent operating point (Q point) is fixed by selecting the proper d.c. biasing to the transistors used. The position of the Q point on the load line decides the class of operation.

a. 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. For this, 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. When an a.c. input signal is applied, the collector voltage varies sinusoidally hence the collector current also varies sinusoidally. 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. 5.14.1.

As shown in the Fig. 5.14.1, 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.

b. Class B Amplifier

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. 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. 5.14.2.

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. 

c. Class C Amplifier

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. 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. 5.14.3.

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 %.

d. Class AB Amplifier

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. 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. 5.14.4.

The output signal is distorted in class AB operation. The efficiency is more than class A but less than class B operation. The class AB operation is important to eliminate cross over distortion. 

In general as the Q point moves away from the centre of the load line below towards the X-axis, the efficiency of class of operation increases.

2. Comparison of Amplifier Classes

The comparison of various amplifier classes is summarized in Table 5.14.1.

It is important to note that class C operation is never used for audio frequency amplifiers. This class is used in special areas of timed circuits, such as radio or communications.

The variety of monolithic as well as hybrid power amplifiers are commercially available and very commonly used now a days. Let us study some examples of IC audio power amplifiers, which are commercially available. 

Review Question

1. Write an explanatory note on power amplifier.