Class-C Power Amplifier Circuit and Tutorial

An amplifier is one kind of electronic device used to increase the strength of a signal. It can be either a different device or an electrical circuit in any electronic device. Amplifiers are widely used in all electronic equipment to generate high power to drive some output devices. The output power range of the audio amplifier may be less than 1 watt to 100 watts. Amplifiers are categorized into different types such as voltage amplifiers, power amplifiers, linear amplifiers, current amplifiers, non-linear amplifiers, trans resistance, and transconductance and amplifiers. In fact, these amplifiers are used in various applications. RF amplifiers are used in transmitters to generate 1000 kilowatts of output power. Whereas, DC amplifiers are used in control systems to drive motors and actuators. This article gives an overview of the Class C Power Amplifier and its tutorial.

Class-C Power Amplifier
Class-C Power Amplifier

What is a Power Amplifier?

Power amplifiers are used to receive the amplified i/p signal from a series of voltage amplifiers, and then supply sufficient power to drive loudspeakers. In a power amplifier, the power (product of the V and I) at the output is greater than the power at the input. The main components of the power amplifier include the i/p stage, the o/p stage, and the power supply.

Power Amplifier
Power Amplifier

The power supply receives AC (alternating current) from an electrical outlet and changes it to DC (direct current). The input stage in the power amplifier receives a DC signal from the power supply, where it is prepared for the output stage and then transferred to the o/p stage. The output stage is connected to the speaker.

Power amplifiers are categorized into different types such as an audio power amplifier, an RF power amplifier, vacuum tube power amplifiers, transistor, FM power amplifier, stereo power amplifier, and a class-A, class-B, class-C, class-D & class AB power amplifiers. The different types of amplifiers are used to amplify the output signals with weak input signals.

Class C Power Amplifier

The class C power amplifier is 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
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, the 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 a 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
Class C Amplifier Circuit

In the above circuit, a 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.
  • It creates a lot of RF interference.
  • It is difficult to obtain coupling transformers and ideal inductors.
  • The dynamic range will be reduced.

Thus, this article discusses class C power amplifier tutorial which includes what is a power amplifier, Class C power amplifier circuit. The applications of the Class C power amplifier circuit mainly involve in RF oscillators, RF amplifiers, FM transmitters, Booster amplifiers, High-frequency repeaters. Tuned amplifiers, etc. We hope that you have got a better understanding of this concept. Furthermore, any queries regarding this concept or electrical and electronics projects please give your feedback in the comment section below. Here is a question for you, what is the main function of an amplifier?

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