Lecture - 29: Power Amplifier

Power amplifier

The amplifiers in multistage amplifier near the load end in almost all-electronic system employ large signal amplifiers (Power amplifiers) and the purpose of these amplifiers is to obtain power again.

Consider the case of radio receiver, the purpose of a radio receiver is to produce the transmitted programmes with sufficient loudness. Since the radio signal received at the receiver output is of very low power, therefore, power amplifiers are used to put sufficient power into the signal. But these amplifier need large voltage input.

Therefore, it is necessary to amplify the magnitude of input signal by means of small amplifiers to a level that is sufficient to drive the power amplifier stages.

In multistage amplifier, the emphasis is on power gain in amplifier near the load. In these amplifies, the collector currents are much larger because the load resistances are small (i.e impedence of loud speaker is 3.2 ohm).

A power amplifier draws a large amount of dc power form dc source and convert it into signal power. Thus, a power amplifier does not truly amplify the signal power but converts the dc power into signal power.

DC and AC load lines:

Consider a CE amplifier as shown in fig. 1.

Fig. 1

The dc equivalent circuit gives the dc load line as shown in fig. 2.

Fig. 2

Q is the operative point. ICQ and V CEQ are quiescent current and voltage. The ac equivalent circuit is shown in fig. 3.

Fig. 3

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