An amplifier system usually consists of several cascaded stages. Amplifiers are classified in many ways according to their frequency range, the method of operation, the ultimate use, the type of load, the method of inter-stage coupling, etc.
Linear Amplifiers
Linear amplifiers are classified according to their mode of operation. Various amplifier descriptions are based on the following factors:
Based on its input
(a) Small-signal amplifier
(b) Large-signal amplifier
Based on its frequency response
(a) Audio frequency (20 Hz to 20 kHz) amplifier
(b) Intermediate-frequency amplifier,
(c) Audio frequency (20kHz to hundreds of megahertz) amplifier
(d) Ultrahigh-frequency (hundreds of thousands of megahertz) amplifier.
Large signal amplifiers (power amplifiers) are classified according to the position of the quiescent point. Transistor power amplifiers handle large signals. Power amplifiers are also classified based on the transistor’s biasing condition and the input signal’s amplitude. It also gives an idea of the portion of the input cycle for which the transistor conducts.
Classifications of power amplifiers
On this basis, power amplifiers are classified as
1. Class A Power Amplifier
2. Class B Power Amplifier
3. Class AB Power Amplifier
4. Class C Power Amplifier
Class A Power Amplifier
Class A power amplifier is one in which the operating point and the input signal are such that the collector current in the output circuit flows for the full cycle, i.e.,360o. A class A amplifier operates over a linear portion of the characteristic. i.e., Q-point is located approximately at the center of the linear portion of the characteristic.
Class B Power Amplifier
A class B power amplifier is one in which the operating point is located at an extreme end (cut-off) of its characteristics. The output current flows only for half a cycle, i.e., 180o of the input signal. The current will be zero for the other half cycle. Because of the small power dissipation, efficiency is high.
Class AB Power Amplifier
A class AB amplifier chooses the operating point so that the output current flows for more than half the cycle and less than the full cycle of the input signal.
Class C Power Amplifier
A class C power amplifier chooses the operating point so that the output current flows for less than half the input signal. The current flows in the form of pulses. The efficiency is very high.
Class A, Class AB, and Class B operations use untuned amplifiers for audio frequencies, whereas Class C operation uses tuned radio frequency amplifiers.
Other Classification of Amplifiers
Class D Amplifier
Class D amplifier is fundamentally a non-linear switching amplifier. It is also called a PWM amplifier. Theoretically, a class D amplifier can achieve 100% efficiency since there is no period throughout a cycle where the voltage and current waveforms overlie as current is drawn only through the transistor that is on.
Class F Amplifier
Class F amplifiers can boost both the efficiency and output with the help of harmonic resonators in the output network to obtain the square waveform at the output. This class of amplifiers attains high efficiency (above 90%) if the infinite harmonic tune is used.
Class G Amplifier
It offers the enhancement design of basic class AB amplifier design. Class G uses several power supply rails of different voltages and automatically switches between these supply rails when there is a change in the input signal. This continuous switching reduces the normal power consumption. Thus power losses are caused by wasted heat.
Class I Amplifier
Class I amplifier has 2 sets of paired output switching devices arranged in an equivalent push-pull arrangement, with both sets of switching devices sampling the identical input waveform.
Class S Amplifier
Class S amplifier operation is similar to that of Class D amplifier. It converts the analog input signal into a square waveform with the help of a delta-sigma modulator. It amplifies them to increase the output power before finally being demodulated by a bandpass filter. The digital signals of this switching amplifier are constantly either fully “ON” or “OFF.”
Class T Amplifier
Class T amplifiers are another kind of digital switching amplifier. Now a day, Class T amplifiers are becoming more popular as an audio amplifier design due to the existence of digital signal processing chips and multi-channel surround sound amplifiers. It converts analog signals into digital pulse width modulated (PWM) signals for amplification and increases the amplifier’s efficiency. Class T amplifiers design combine the low distortion signal levels of a class AB amplifier and the power efficiency of a class D amplifier.
Amplifiers Class By Conduction Angle
Amplifier Class
Description
Conduction Angle
Class-A
Full cycle 360o of Conduction
θ = 2π
Class-B
Half cycle 180o of Conduction
θ = π
Class-AB
Slightly more than 180o of conduction
π < θ < 2π
Class-C
Slightly less than 180o of conduction
θ < π
Class-D to T
ON-OFF non-linear switching
θ = 0
Conclusion
In conclusion, the classification of amplifiers serves as a vital foundation for understanding the intricate world of electronic signal amplification. From the basic operation of Class A amplifiers to the efficiency of Class D amplifiers, each category offers distinct advantages and trade-offs. By grasping the nuances and applications of these amplifier classes, engineers, hobbyists, and audio enthusiasts can make well-informed choices to suit their specific needs. Whether it’s for high-fidelity audio reproduction or industrial power systems, a comprehensive understanding of amplifier classifications ensures optimal performance, efficiency, and reliability in a wide range of electronic applications.
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