Class C Amplifiers, Physics tutorial

Class C Amplifiers:

Class C amplifiers conduct less than 50% of input signal, permitting it to reach 90% efficiency but resulting in high distortion at output. Therefore, in Class C amplifier, output current (or voltage) is zero for more than 50% of input waveform cycle. In some appliances like megaphones, high distortion of Class C amplifiers can be tolerated. Likewise, Class C amplifiers can be utilized in tuned Radio Frequency applications as the distortion of class C amplifier can be considerably decreased by tuned loads.

In Radio Frequency and Tuned Load applications, input signal is utilized to roughly switch amplifying device on and off that causes pulses of current to flow through tuned circuit.

Class C amplifiers are like Class B in that output stage has zero idle bias current. Though, Class C amplifiers have the region of zero idle current that is more than 50% of total supply voltage. Drawbacks of Class B amplifiers are even clearer in Class C amplifiers, so Class C is similarly not practical for audio applications.

Class C applications are mainly radio frequency applications where highly distorted output of amplifier can be filtered out by tuned circuit.

Applications like FM transmission where linearity is not significant often apply class C power output amplifiers stages.

Modes of class c amplifier:

Class C amplifiers have two modes of operation which are tuned mode and the untuned mode of operation. In untuned operation, waveform analysis will demonstrate massive distortion in signal and when proper load like pure inductive-capacitive filter is utilized, two things will take place. First is that output's bias level is clamped, so that output variation is centered at one-half of supply voltage. This is why tuned operation is at times known as clamper operation. This action of inspiring bias level permits waveform to be restored to proper shape, permitting complete waveform to be re-established despite having only one-polarity supply. The second thing to take place is that waveform on centre frequency becomes much less distorted and distortion which is present is dependent on bandwidth of tuned load, with centre frequency experiencing very little distortion, but experiencing greater attenuation the farther from tuned frequency that the signal gets.

The tuned circuit will only resonate at specific frequencies, and so unwanted frequencies are noticeably suppressed, and wanted full signal (sine wave) will be extracted by tuned load provided the transmitter is not needed to operate over very extensive band of frequencies. Tuned mode Class C amplifier works very well and other residual harmonics can be removed using filter.

Output characteristics of Class C power amplifier:

1190_Output characteristics of Class C power amplifier.jpg

In the above figure operating point is placed some way below cut-off point in DC load-line and so only fraction of input waveform is available at output.

Class C power amplifier circuit:

1756_Class C power amplifier circuit diagram.jpg

Biasing resistor Rb pulls base of Q1 further downwards and Q-point will be set some way below cut-off point in DC load line. Thus transistor will start conducting only after input signal amplitude has risen above base emitter voltage (Vbe~0.7V) plus downward bias voltage caused by Rb. That is the reason why major portion of input signal is absent in output signal.

Inductor L1 and capacitor C1 forms the tank circuit that helps in extraction of required signal from pulsed output of transistor. Actual job of active element (transistor) here is to generate the series of current pulses according to input and make it flow through resonant circuit. Values of L1 and C1 are so selected that resonant circuit oscillates in frequency of input signal. As resonant circuit oscillates in one frequency (usually carrier frequency) all other frequencies are attenuated and needed frequency can be squeezed out using the properly tuned load.  Harmonics or noise present in output signal can be eliminated utilizing extra filters. The coupling transformer can be utilized for transferring power to load.

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