An amplifiers 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 are classified according to their mode of operation. Various Amplifier descriptions are based on the following factors:
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As based on its input
(a) Small-signal amplifier
(b) Large-signal amplifier
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As based on its output
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As 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.
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As based on its biasing conditions, classification of amplifiers are
(a) Class A amplifier
(b) Class AB amplifier
(c) Class B Amplifier
(d) Class C amplifier
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As based on its transistor configuration
(a) Common-base amplifier
(b) Common-emitter amplifier
(c) Common-Collector 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 on the basis of biasing condition of the transistor and amplitude of the input signal. It also gives an idea that the portion of the input cycle for which the transistor conducts.
On the 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 centre on 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 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 is one in which the operating point is chosen 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 is one in which the operating point is chosen 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, Class B operations are used with untuned amplifiers for audio frequencies, whereas Class C operation is used with tuned radio frequency amplifies.
Other Classification of Amplifiers
Class D Amplifier
Class D amplifier is fundamentally a non linear switching amplifier. It is also called as PWM amplifier. Theoretically class D amplifier can achieve 100% efficiency, since there is no period throughout a cycle were 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 of enhancement design of basic class AB amplifier design. Class G uses several power supply rails of different voltage and automatically switches between these supply rails when there is a change in input signal. This continuous switching reduces the normal power consumption. Thus power losses caused by wasted heat.
Class I Amplifier
Class I amplifier has 2 set of paired output switching device arranged in a 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 the square waveform with the help of delta sigma modulator, and amplifies them to increases the output power before finally being demodulated by a band pass 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 amplifiers. Now a day the 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 analogue signals into digital pulse width modulated (PWM) signals for amplification and increasing the efficiency of amplifier. Class T amplifiers design are the combination of both the low distortion signal levels of 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 |
| Class-C | Slightly less than 180o of conduction | θ |
| Class-D to T | ON-OFF non-linear switching | θ = 0 |
