Compare class A, B and C amplifier.

Q.) Compare class A, B and C amplifier.

 Subject: Electronic Devices and Circuits

Class A Amplifier:

  • Operating Principle: In a class A amplifier, the output transistor conducts for the entire input signal cycle, resulting in a continuous flow of current through the output stage. This mode of operation provides excellent linearity and low distortion, making class A amplifiers ideal for applications where high-fidelity sound reproduction is required, such as audio amplifiers and high-quality music systems.
  • Characteristics:
    • Conduction Angle: 360 degrees
    • Efficiency: Typically low (around 25-35%) due to continuous current flow, even when no signal is present.
    • Linearity: Excellent linearity due to the absence of crossover distortion.
    • Distortion: Low distortion as the transistors operate in their active region for the entire signal cycle.
    • Advantages:
      • High-fidelity sound reproduction with minimal distortion
      • Excellent linearity
      • Good transient response
    • Disadvantages:
      • Low efficiency
      • High power dissipation
      • Requires large heat sinks for cooling

Class B Amplifier:

  • Operating Principle: In a class B amplifier, each half of the input signal is amplified by a separate transistor, with one transistor handling the positive half-cycle and the other handling the negative half-cycle. The transistors are biased just below the conduction threshold, so they only conduct when the input signal is present. This mode of operation results in higher efficiency compared to class A while maintaining good linearity and distortion performance.
  • Characteristics:
    • Conduction Angle: 180 degrees
    • Efficiency: Typically higher than class A (around 50-70%) due to reduced current flow during the non-conducting half-cycle.
    • Linearity: Good linearity, but not as good as class A due to the presence of crossover distortion at the transition between the two transistors.
    • Distortion: Moderate distortion due to crossover distortion and the non-linearity of the transistors near the cutoff region.
    • Advantages:
      • Higher efficiency compared to class A
      • Smaller heat sinks required
      • Good linearity and distortion performance
    • Disadvantages:
      • Crossover distortion
      • Reduced linearity compared to class A
      • More complex circuitry

Class C Amplifier:

  • Operating Principle: In a class C amplifier, the output transistor conducts for less than half of the input signal cycle, typically around 120 degrees. This mode of operation achieves very high efficiency, but at the cost of linearity and distortion. Class C amplifiers are commonly used in radio frequency (RF) power amplifiers, where efficiency is critical.
  • Characteristics:
    • Conduction Angle: Less than 180 degrees (typically around 120 degrees)
    • Efficiency: Very high (up to 90%) due to minimal current flow during the non-conducting period.
    • Linearity: Poor linearity due to the significant cutoff and saturation regions of operation.
    • Distortion: High distortion due to the non-linearity of the transistors and the large amount of harmonic content generated.
    • Advantages:
      • Very high efficiency
      • Small heat sinks required
    • Disadvantages:
      • Poor linearity
      • High distortion
      • Not suitable for audio applications

Comparison Table:

Feature Class A Class B Class C
Conduction Angle 360 degrees 180 degrees < 180 degrees
Efficiency Low (25-35%) Moderate (50-70%) High (80-90%)
Linearity Excellent Good Poor
Distortion Low Moderate High
Advantages High-fidelity sound reproduction, excellent linearity, good transient response Higher efficiency, smaller heat sinks Very high efficiency, small heat sinks
Disadvantages Low efficiency, high power dissipation, requires large heat sinks Crossover distortion, reduced linearity Poor linearity, high distortion, not suitable for audio applications

Applications:

  • Class A: Audio amplifiers, high-quality music systems, microphone preamps, guitar amplifiers
  • Class B: General-purpose audio amplifiers, power amplifiers, motor control, switching power supplies
  • Class C: Radio frequency (RF) power amplifiers, high-power transmitters, radar systems