Power Amplifier Classes Explained

Power amplifiers are essential components in electronics, designed to increase the power of a signal. They are classified into different classes based on how the amplifying device (like a transistor) operates within the input signal cycle. The choice of class significantly impacts the amplifier's performance, particularly its power efficiency.

Power efficiency ($ \eta $) is defined as the ratio of the output signal power ($ P_{out} $) to the DC input power ($ P_{DC} $):

$ \eta = \frac{P_{out}}{P_{DC}} \times 100\% $

The question asks us to arrange the given power amplifier configurations in order of increasing power efficiency. The options use labels A, B, C, D, E. Based on common conventions and the inclusion of Class AB, we can map these labels as follows:

• A = Class A
• B = Class AB
• C = Class B
• D = Class C
• E = Class D

Efficiency Analysis of Amplifier Classes

Let's examine the typical power efficiency characteristics of each class:

• Class A (Label A): Operates the amplifying device in the linear region for the entire input cycle ($ 360^\circ $). This provides excellent linearity but results in the lowest efficiency. The theoretical maximum efficiency is limited to $ 25\% $. Practical efficiencies are often much lower, typically around $ 10\% $ to $ 20\% $.
• Class AB (Label B): A hybrid class combining features of Class A and Class B. The device conducts for slightly more than $ 180^\circ $ but less than $ 360^\circ $. It offers a balance between efficiency and linearity, significantly better than Class A. Practical efficiencies generally range from $ 50\% $ to $ 75\% $.
• Class B (Label C): The amplifying device conducts for $ 180^\circ $ of the input cycle. This offers much higher efficiency than Class A. The theoretical maximum efficiency is $ 78.5\% $. Practical efficiencies are typically in the $ 50\% $ to $ 70\% $ range.
• Class C (Label D): The amplifying device conducts for significantly less than $ 180^\circ $. This leads to very high efficiency, theoretically exceeding $ 90\% $. However, Class C amplifiers produce highly distorted output waveforms and are generally unsuitable for audio amplification, being used mainly in RF power amplification.
• Class D (Label E): These are switching amplifiers, operating the device either fully ON or fully OFF. They achieve extremely high efficiencies, often exceeding $ 90\% $. Widely used in modern high-fidelity audio systems and power supplies due to their efficiency.

Ordering Classes by Increasing Efficiency

Comparing the efficiency ranges:

• Class A (A): ~$ 10\% - 25\% $
• Class AB (B): ~$ 50\% - 75\% $
• Class B (C): ~$ 50\% - 78.5\% $
• Class C (D): ~$ > 90\% $
• Class D (E): ~$ > 90\% $

Arranging these in order of increasing power efficiency:

1. Class A (A)
2. Class AB (B)
3. Class B (C)
4. Class C (D) and Class D (E) have the highest efficiencies. While their exact order can depend on specific design implementation, they are grouped together at the top. Conventionally, Class C is often considered slightly more efficient in specific RF contexts, but Class D is generally highly efficient across applications. For the purpose of this ordering, Class C (D) and Class D (E) represent the highest efficiency tier.

Thus, the required order is A, B, C, D, E.

Amplifier Class Efficiency Summary

Based on this comparison, the order of increasing power efficiency is Class A, Class AB, Class B, Class C, Class D, which corresponds to the sequence A, B, C, D, E.