There are various types of amplifiers that differ in their technology and application:

• Tube amplifiers: Use vacuum tubes for amplification and are known for their warm, harmonically rich sound.
• Transistor amplifiers: Use transistors for amplification and often offer a cleaner, clearer sound.
• FET amplifier (Field Effect Transistor): Provide a characteristic, slightly distorted sound that is often described as musical.
• Hybrid amplifiers: Combine tube and transistor technology to utilize the advantages of both approaches.
• Class A:
  • How it works: The amplifier conducts current continuously, even if no input signal is present. The transistor or the tube system is always in operation.
  • Advantages: Very low distortion, high sound quality.
  • Disadvantages: Low efficiency (typically 20-30 %), high heat generation, high power consumption.
  • Application: High-quality audio equipment, high-end amplifiers.
• Class B:
  • How it works: Two transistors each amplify one half-wave of the signal (one for the positive, one for the negative half-wave).
  • Advantages: Higher efficiency (approx. 70 %) than Class A.
  • Disadvantages: Cross-over distortion at the transition point between the half-waves, which can impair the sound quality.
  • Application: Less common due to cross-over distortion, but used in some analog amplifiers.
• Class AB:
  • How it works: A combination of Class A and Class B. The amplifier operates in Class A mode at low power levels and in Class B mode at higher power levels.
  • Advantages: Provides a good balance between sound quality and efficiency, less distortion than Class B.
  • Disadvantages: Complex design, still higher power consumption than modern switching amplifiers.
  • Application: Very common in hi-fi amplifiers and professional audio amplifiers.
• Class D:
  • How it works: A switching amplifier that digitally processes the signal by converting it into a series of high frequency pulse width modulations (PWM). These are then filtered to generate the output signal.
  • Advantages: Very high efficiency (often over 90 %), low heat generation, compact design.
  • Disadvantages: Potentially higher distortion and noise, although modern Class-D amplifiers have largely solved these problems.
  • Application: Widely used in subwoofers, mobile devices, home theater systems and modern audio devices.
• Class G:
  • How it works: A variant of Class AB that uses several supply voltage stages.
    The amplifier switches between the stages depending on the power requirement.
  • Advantages: Higher efficiency than Class AB at higher output powers.
  • Disadvantages: Complexity in design, possible switch-on noise or distortion when switching between voltage levels.
  • Application: High-quality audio amplifiers, professional audio equipment.
• Class H:
  • How it works: Similar to Class G, but with a continuously variable supply voltage that adapts dynamically to the input signal.
  • Advantages: Very high efficiency, especially with high power requirements.
  • Disadvantages: Complex design, expensive to manufacture.
  • Application: High power amplifier, professional audio applications.