Which Power Amplifier Class Should You Use?

Created: November 7, 2022
Updated: October 10, 2024

Power amplifiers are simple conceptually, but not all power amplifiers are created equal. They certainly have competing specifications and capabilities, but more important is the topology and functionality of these circuits when placed in a real system. Power amplifier functionality is divided up into classes, and there are many power amplifier classes that each function somewhat differently. There are enough power amplifier classes that it can be difficult for everyone to memorize how each amplifier functions, as well as which amplifier classes are preferable in different systems.

In this article, I’ll give an overview of the main power amplifier classes and how they can be used in different systems. As we’ll see, different types of power amplifiers are available as integrated circuits, and they work with different driving methods to provide power. Operational characteristics like packaging, correct signal reproduction, and harmonic distortion are a few of the important metrics to consider when selecting power amplifiers.

The other important point to consider is the possibility of substitutions for power amplifiers, especially in the modern environment of shortages and long lead times. Some power amplifiers are compatible with each other and can be substituted directly as long as packaging and pinouts are similar. We’ll look at substitution possibilities for the various types of power amplifiers in this article when covering power amplifier classes.

How Power Amplifiers Work

Conceptually, power amplifiers are no different from any other amplifier; you input a certain signal, and the output signal is generally more intense than the input. The input signal is amplified up to some maximum value, which may be adjustable with an I/O bank or through the strength of the input modulating signal. An example of a typical power amplifier topology is shown below.

Broadly speaking, power amplifier classes are delineated by their driving mechanism, and they are further delineated by the circuit topology. Power amplifiers can be driven with a modulated square wave signal, or with an input AC signal directly as is shown in the above image. Beyond the driving mechanism, there are several factors that differentiate power amplifier classes:

Modulation method (PWM, PFM, sigma-delta, etc.)
Power delivery topology (single supply, cascaded supplies)
Push-pull topologies
Intentional vs. unintentional harmonic generation

Power Amplifier Features

Beyond these points, power amplifiers can have a variety of structures and features built into the product. For example, certain power amplifiers have unique push-pull configurations on the output stage, while others are simple transistor amplifiers (particularly at low frequencies). Amplifiers can also use a cascaded topology to provide additional gain, or they may use a pre-amp. Beyond the basic operating requirements found in power amplifier classes, really anything is possible for adding features, ensuring signal conditioning, or providing an interface for controlling gain.

Modules or Chips?

Power amplifier products are available as chips for use in smaller devices, or as modules that can be integrated into much larger devices. Many power amplifier ICs fall into different classes and can be used in many applications, although many of the classed power amplifier products are built for audio. Most other power amplifiers that are not marketed specifically for audio and that only use a single transistor as the active element are Class A amplifiers, even if the datasheet or marketing materials do not list a specific amplifier class.

One example component is the LM4991 from Texas Instruments. This Class AB audio amplifier provides up to 3 W power output operating at logic levels (up to 5.5 V). The component can be doubled up in parallel to provide stereo output to two speakers with very low THD + N levels of distortion throughout the audio range. Some operational data and an application circuit are shown below.

Power amplifiers are also available as pre-packaged modules that include many features. Some power amplifier components or modules are meant to operate within a specific bandwidth, so there are additional circuits on the output stage that provide filtering and impedance matching, especially in high-frequency systems where RF signals are being sourced. However, you will still see power amplifier modules operating over a huge range of possible frequencies, even reaching into the GHz range. These modules are usually meant for high-power applications.

The PE15A5068 from Pasternack is a Class AB amplifier with 5 W saturation input power operating from 2 to 18 GHz. This module operates at 22 to 24 V DC with a typical gain of 37 dB. These modules can be quite expensive and are only meant for specialty RF applications in communications (e.g., military radios) or instrumentation. This particular component is built with a GaN FET, although other components could be built from III-V compound semiconductors.

AC-Driven Power Amplifiers

These power amplifiers make use of the input AC signal, and possibly some applied DC offset bias, to modulate one or more transistors. There may be a preamp in these amplifiers to bring up the input sensitivity and fully modulate the transistors.

Class A

Class A power amplifiers are built with a single switching transistor, regardless of the type of power transistor. The intention of these amplifiers is to maximize the linear range of the device around the transistor’s Q-point so as to prevent signal distortion and harmonic generation. These amplifiers can have a high linear range when constructed properly, and they are easily built from discrete components. The downside of a Class A amplifier is heat generation because the transistor is always DC-biased into the conducting state, thus there will always be some slight conduction losses.

Class B

These amplifiers are similar to Class A, but they offer lower wasted heat through the use of two transistors operating at inverse polarities in a push-pull configuration, i.e., each transistor only sources one-half of the input signal. Each transistor is modulated OFF when the signal level passes 0 V, which then modulates the other transistor ON. The downside of these amplifiers is their dead time or dead zone; during the brief period where the signal passes 0 V, the driving signal between the two polarities will be sub-threshold, so the transistor will be OFF even if the signal level is a small, non-zero value.

Note that there is also a Class AB power amplifier, which blends the best aspects of Class A and Class B amplifier circuits with low distortion.

Class C

These amplifiers use a parallel LC circuit on the input bias terminal to provide filtering at the LC resonance. These amplifiers operate without DC bias with a single transistor, so they have strong distortion at typical operating frequencies. Therefore, they are not normally used in something in audio or in anything else requiring high bandwidth, instead, they are often used as oscillators given a sinusoidal input. The linear range of these amplifiers is also very low due to the operating point being set for these amplifiers.

PWM-Driven Power Amplifiers

These power amplifier classes are more common in digital applications due to the way in which they are driven. The required driving method can be generated from analog waveforms, or the pulse train could be synthesized with a digital processor.

Class D and S

These two amplifiers are nonlinear switching amplifiers that use modulated pulse trains and filtering to produce the desired output signal.

Class D: These amplifiers use an input sawtooth and the input signal in a comparator to generate a sigma-delta modulated signal. This signal is used to drive a push-pull amplifier circuit and the output is filtered into the audio range.
Class S: Uses a sigma-delta modulator to produce an output square wave, which is amplified and then passed through a high-Q bandpass filter to produce a sine wave.

These amplifiers operate with high efficiency and low distortion making them comparable to Class A/AB amplifiers in terms of distortion.

Class F

A Class F amplifier is designed to use a set of high-Q harmonic resonators to deliver power to a load with low resistive losses. As more resonator elements are cascaded on the output stage, the output waveform approaches a square wave with highly efficient power conversion. The generated harmonics are multiples of components in the input signal, so these amplifiers are more useful as high-power square wave generators.

Class G and H

These power amplifiers are an improvement on Class AB amplifiers with a unique power delivery scheme. Modulation in these amplifiers is achieved as the input signal forces the device to switch between multiple rail voltages during its oscillation. The difference between Class G and Class H is the difference between digital and analog supplies; Class H amplifiers use a continuously varying power supply (analog) while Class G uses a set of discrete rail voltage values.

Class I

This amplifier class operates under the same concept as a Class B amplifier, but it uses two push-pull circuits in parallel. One circuit is active during the positive half cycle while the other is active during the negative half cycle. Each side of the device switches between ON and OFF when the PWM driver duty cycle is 50% right at the zero-crossing point of the input.

Other Components For Power Amplifier Circuits

The above examples should show that power amplifier classes are more about structure and less about capabilities. This typically means that if you know how to build and use a small power amplifier circuit, the concepts can quickly scale up to larger powers or frequencies.

In any analog or RF signal chain, the amplifier will not be the only component. These systems could be loaded up with other discrete or op-amps to provide functions like filtering, or they could use specialty monolithic components when operating at high frequencies. Some of the important components needed in power amplifier circuits are linked below.

In the case where you are building an amplifier from a set of discrete components, you will need all the components shown in the above topological diagram and, eventually, to place these on a PCB. Make sure you use the best supply chain tools to find the components you need for your design.

Companies that want to implement a just-in-case supply chain strategy need access to data, insights, and a sourcing platform like Octopart to help locate and procure components. Octopart provides advanced search and filtration features to help buyers find components and up-to-date distributor pricing data, parts inventory, and parts specifications. Take a look at our integrated circuits page to find the components you need.

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