Class C Amplifier

  The class C power amplifier is a one kind of amplifier where the transistor conduct for less than 180° (one half cycle of the input signal) and its typical value is 80° to 120°. The reduced conduction angle progresses the efficiency to a great expand, but roots a lot of distortion. The maximum theoretical efficiency of a Class-C amplifier is around 90%. This type of amplifier is not used in audio amplifiers because of the vast amounts of distortion. The applications of the class c amplifier mainly involve in radio frequency circuits like RF amplifier, RF oscillator, etc.Where there are extra tuned circuits for regaining the original i/p signal from the pulsed o/p of the amplifier. So the distortion which is caused by the class c amplifier has little effect on the final o/p. The i/p and o/p waveforms of a typical class-c power amplifier is shown in the below figure. By observing the following waveforms, we can conclude that half of the i/p signal is missing in the o/p waveform that is shown below. Input and Output Signals of Class-C Power Amplifier Class C Power Amplifier Circuit The circuit diagram of the class c power amplifier circuit is shown above.In the above circuit, biasing resistor’ Rb’ is used to pull the base terminal of the Q1 transistor further downwards.The ‘Q’ point will be fixed to below the cutoff point in the DC load line. As a result,the Q1 transistor will start conducting only after the i/p signal amplitude has increased above the base emitter (BE) voltage plus the downward bias voltage caused by biasing resistor. This is the reason why the main portion of the i/p signal is not present in the o/p signal. Class C Amplifier Circuit In the above circuit, tank circuit can be formed by using a capacitor ‘C1’ and an Inductor ‘L1’ which help in the removal of the necessary signal from the pulsed o/p of the transistor.Here, the main function of the transistor is to generate a current pulse in series according to the i/p and make it flow through the resonant circuit. The values of the capacitor and inductor are selected so that the resonant circuit oscillates at the frequency of the i/p signal. Since the resonant circuit oscillates in the carrier frequency, then all other frequencies are attenuated & the values of L1 and C1 are so selected that the resonant circuit oscillates in the frequency of the input signal. Since the resonant circuit oscillates in one frequency (generally the carrier frequency) all the necessary frequency can be pushed out using a duly tuned load. Harmonics in the o/p signal can be removed using an extra filter. A coupling transformer is used to transfer the power to the load. Advantages and Disadvantages of Class C Amplifier The advantages of class c amplifier include · Efficiency is high · Used in RF applications · Physical size is low for a given power o/p The disadvantages of class c amplifier include · Linearity is low · Not fit in audio applications. · Creates a lot of RF interference. · It is difficult to obtain coupling transformers and ideal inductors. · Dynamic range will be reduced.