Classes of Amplifiers: A Guide to Electronic Amplification

Amplifier classes categorize electronic amplifiers based on their conduction angle—the portion of the input signal cycle during which the amplifying device (like a transistor or vacuum tube) is actively conducting current. The class you choose fundamentally dictates the tradeoff between linearity (how accurately the output mirrors the input) and efficiency (how effectively it converts DC power into useful AC output, rather than wasting it as heat).

The Analog Classes

These traditional classes operate by amplifying the continuous analog waveform.

Class A

In a Class A amplifier, the transistor is biased so that it remains constantly “ON” and conducts current through the entire 360 degrees of the input waveform.

• Pros: Exceptional linearity and extremely low distortion because the transistor never switches off.
• Cons: Terrible efficiency (typically 15% to 25% in practical circuits). Because current flows constantly, even when there is no input signal, the device generates a massive amount of heat.

Class B

To improve efficiency, a Class B amplifier uses two transistors in a “push-pull” arrangement. Each transistor only conducts for exactly half of the waveform (180 degrees)—one handles the positive half, the other the negative half.

• Pros: Much higher theoretical efficiency (up to 78.5%). It wastes no power when there is no input signal.
• Cons: It introduces crossover distortion. Because transistors require a small threshold voltage (e.g., 0.7V for silicon) to turn on, there is a brief dead zone where neither transistor is conducting as the signal crosses zero.

Class AB

Class AB is the industry-standard compromise for most high-fidelity analog applications. It operates like a Class B push-pull amplifier, but both transistors are biased to conduct just a little bit more than 180 degrees (usually around 190 to 200 degrees).

• Pros: By keeping both transistors slightly “ON” at the zero-crossing point, it entirely eliminates crossover distortion while maintaining a very respectable efficiency of around 50% to 70%.

Class C

Class C amplifiers are biased so that they conduct for significantly less than 180 degrees of the input cycle (often just brief pulses).

• Pros: Incredible efficiency (often 80% to 90%).
• Cons: Highly non-linear and produces massive distortion.
• Application: Class C is useless for audio, but it is heavily used in Radio Frequency (RF) transmission (like CW or FM). The distorted output pulses are fed into an LC (inductor-capacitor) tuned circuit, which acts like a flywheel to reconstruct the missing parts of the sine wave at a specific resonant frequency.

The Switching Classes

Modern electronics increasingly rely on switching amplifiers, which abandon the analog waveform entirely at the amplification stage.

Class D

Unlike analog classes that regulate current like a valve, a Class D amplifier operates its transistors strictly as high-speed switches—they are either fully “ON” or fully “OFF.” It converts the analog input into a high-frequency Pulse Width Modulation (PWM) signal, amplifies those digital pulses, and then uses a low-pass filter to smooth the output back into an analog wave before it hits the speaker.

• Pros: Near-perfect efficiency (often >90%), meaning they require almost no heat sinks. They dominate modern consumer audio, from smartphones to high-power PA systems.

(Note: Classes E, F, G, and H also exist. E and F are highly specialized switching RF amplifiers, while G and H are variations of Class AB that dynamically switch power supply voltages to save energy).

Summary Comparison

Class	Conduction Angle	Linearity	Typical Efficiency	Primary Use Case
A	360°	Excellent	~25%	Audiophile pre-amps, low-power stages
B	180°	Poor (crossover)	~78%	Rare (superseded by AB)
AB	~190° – 200°	Good	~60%	Traditional stereo amps, linear RF amps
C	< 180°	Very Poor	~85%	RF transmitters (CW, FM)
D	PWM (Switching)	Good	> 90%	Modern audio, Bluetooth speakers