Higher Efficiency Power Amplifiers: Class AB, B, and C

Power amplifiers are fundamental components in various electronic systems, particularly in RF and audio applications. These amplifiers boost the signal power to levels suitable for transmission or driving a load. Among the different classes of power amplifiers, Classes AB, B, and C are prominent due to their higher efficiency and distinct operating characteristics. In this blog, we will delve into the details of these amplifier classes, examining their waveforms, operating principles, and relevant equations to understand their advantages and trade-offs.

Class AB Amplifiers: The Balanced Approach

Class AB amplifiers are designed to overcome the inefficiencies of Class A amplifiers while avoiding the distortion issues found in Class B amplifiers. This class operates in a state where both transistors (in a push-pull configuration) conduct more than half of the input signal cycle but less than the entire cycle, which is typically achieved by slightly biasing the transistors above the threshold voltage.

Waveforms and Operation:

As depicted in the slides, the input signal Vg in a Class AB amplifier is biased slightly above the threshold voltage V_th​. The conduction angle α\alphaα is slightly greater than π radians but less than 2π radians, meaning each transistor conducts for more than half but less than the full cycle. The key advantage here is the reduction in crossover distortion compared to Class B amplifiers.

The corresponding current I_T exhibits a smooth transition, avoiding the sharp cutoff seen in Class B. This smooth transition results in a more linear amplification with better fidelity, albeit at the cost of reduced efficiency compared to Class B. The output voltage V_d shows that during the positive half-cycle, one transistor conducts, and during the negative half-cycle, the other transistor takes over. The waveforms overlap slightly, ensuring a continuous signal path and minimizing distortion.

Key Equations:

In Class AB operation, the relationship between the quiescent current I_q​, the peak output current I_max​, and the conduction angle α\alphaα is crucial. The DC current I_DC is given by:

This equation highlights how the biasing and conduction angle influence the average current and hence the efficiency of the amplifier.

Class B Amplifiers: High Efficiency, Low Fidelity

Class B amplifiers are known for their high efficiency, theoretically reaching up to 78.5% under ideal conditions. This class operates with each transistor conducting exactly half of the input signal, resulting in a conduction angle of π radians. However, this operation introduces significant crossover distortion due to the non-conducting phase around the zero-crossing point of the input signal.

Waveforms and Operation:

In Class B operation, as shown in the waveforms, the input signal Vg is such that each transistor conducts only when the input signal exceeds the threshold voltage V_th​. The conduction angle α is exactly π radians, with each transistor switching off completely during the other half-cycle.

The current waveform I_T displays a sharp cutoff, which corresponds to the transistors turning off when the input signal crosses zero. This sharp transition introduces crossover distortion, which is particularly problematic in low-signal applications, where the distortion becomes more noticeable.

Key Equations:

The efficiency of a Class B amplifier is one of its main advantages. The power dissipated P_diss and the output power Pout relate to the peak current I_max and the DC current I_DC as:

This equation reflects the trade-off between efficiency and linearity in Class B amplifiers. While efficiency is maximized, linearity and fidelity are compromised, making Class B suitable for applications where efficiency is more critical than signal quality.

Class C Amplifiers: Extreme Efficiency at the Cost of Linearity

Class C amplifiers push efficiency even further, often exceeding 90%. However, this comes at the cost of significant signal distortion, making Class C suitable mainly for RF applications where the signal can be reshaped using tuned circuits.

Waveforms and Operation:

In Class C operation, the input signal Vg​ is biased well below the threshold voltage V_th​, resulting in a conduction angle α of less than π\piπ radians. The transistor conducts for less than half of the input signal cycle, which leads to a highly distorted output that resembles a series of pulses.

These pulses can be reshaped in RF applications using resonant circuits to produce a sinusoidal output. The high efficiency of Class C amplifiers is achieved because the transistor is off for most of the signal cycle, dissipating minimal power.

Key Equations:

The efficiency η of a Class C amplifier can be approximated by:

This equation shows that as the conduction angle α decreases, the efficiency increases, but the linearity decreases, highlighting the trade-off in Class C operation.

Comparison and Applications

The key differences between Classes A, AB, B, and C can be summarized in terms of conduction angle, efficiency, and linearity. Class A offers the best linearity but at the lowest efficiency. Class AB strikes a balance, offering improved efficiency over Class A with acceptable linearity. Class B maximizes efficiency but suffers from crossover distortion, while Class C offers the highest efficiency with significant distortion, suitable mainly for RF applications.

Each of these amplifier classes finds its niche depending on the requirements of the application, whether it’s efficiency, linearity, or a balance between the two.

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Conclusion

Understanding the operation, waveforms, and key equations of Class AB, B, and C amplifiers provides valuable insights into their applications and trade-offs. Whether it’s for high-fidelity audio amplification, efficient RF transmission, or power-efficient applications, selecting the right amplifier class is crucial. By carefully considering the conduction angle, biasing, and desired efficiency, engineers can design amplifiers that meet the specific needs of their systems.

Learn more about this topic by taking the complete course ‘Linear RF Power Amplifier (PA) Design Theory and Principles online course – RAHRF562’. Watch the course videos for more detailed understanding. Also checkout other courses on RF system and IC design on https://rahsoft.com/courses/. Rahsoft also provides a certificate on Radio Frequency. All the courses offer step by step approach.