Arrange the following amplifiers as per their efficiency from highest to lowest A. class B amplifier B. class C amplifier C. class A amplifier D. class AB amplifier E. class D amplifier Choose the correct answer from the options given below :
E, B, A, D, C
What sets amplifier efficiency. Efficiency is the ratio of a.c. power delivered to the load to d.c. power drawn from the supply,
\(\eta=\dfrac{P_{ac}}{P_{dc}}\times 100\%\)
and it is governed almost entirely by the conduction angle — how much of each input cycle the active device conducts. The longer a transistor conducts while carrying both voltage and current, the more heat it wastes.
Class-by-class figures.
| Class | Conduction angle | Max. efficiency | Distortion / use |
|---|---|---|---|
| D (E) | switching (on/off) | ~90–100 % | audio, power stages |
| C (B) | < 180° | ~80–90 % | RF tuned amplifiers only |
| B (A) | 180° | 78.5 % (π/4) | push-pull; crossover distortion |
| AB (D) | slightly > 180° | ~50–78 % | audio push-pull, low distortion |
| A (C) | 360° | 25 % (50 % transformer-coupled) | lowest distortion, small signal |
Reasoning through the extremes. A class-D stage never sits in the linear region — its devices are either fully ON (low voltage) or fully OFF (no current), so \(P = VI\) is small in both states and the efficiency approaches 100 %. A class-A stage is the opposite: it conducts the full 360° and draws its quiescent current even with no input signal, dissipating maximum power at zero output, which caps efficiency at 25 %.
Arranging highest to lowest: class D > class C > class B > class AB > class A, i.e.
\(E \gt B \gt A \gt D \gt C\)
The trade-off worth remembering. Efficiency and linearity run in opposite directions: class C is very efficient but so distorted that it is usable only with a tuned output circuit to restore the sine wave, while class A is the most faithful and the most wasteful. Class AB is the practical audio compromise, keeping a small quiescent current to remove class B's crossover distortion.
Hence, the order is E, B, A, D, C.
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