Lecture - 32: Power Amplifier

Class C amplifier:

A class C amplifier can produce more power than a class B amplifier.

Consider the case of a radio transmitter in which the audio signals are raised in their frequency to the medium or short wave band to that they can be easily transmitted. The high frequency introduced is in radio frequency range and it serves as the carrier of the audio signal. The process of raising the audio signal to radio frequency called modulation.

The modulated wave has a relatively narrow band of frequencies centered around the carrier frequencies. At any instant, there are several transmitter transmitting programmes simultaneously. The radio receiver selects the signals of desired frequencies to which it is tuned, amplifies it and converts it back to audio range. Therefore, tuned voltage amplifiers are used. In short, the tuned voltage amplifiers selects the desired radio frequency signal out of a number of RF signals present at that instant and then amplifies the selected RF signal to the desired level as shown in fig. 1.

Fig. 1

Class C operation means that the collector current flows for less than 180° of the ac cycle. This implies that the collector current of a class C amplifier is highly non-sinusoidal because current flows in pulses. To avoid distortion, class C amplifier makes use of a resonant tank circuit. This results in a sinusoidal output voltage.

The resonant tank circuit is tuned to the frequency of the input signal. When the circuit has a high quality factor (Q) parallel resonance occurs at approximately

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