Overview of Power Amplifier Classes

Power amplifiers are an essential component in a huge range of devices, ranging from consumer electronics to mobile products, and many other systems. The range of frequencies power amplifiers may need to support spans from audio up to high GHz, with amplification values and power ranging widely. Instead of a discrete amplifier setup with a single transistor, designers can select power amplifier ICs that provide high-fidelity amplification in analog systems.

There is a distinct distinction between the configuration and operation of the output stages of various power amplifiers, and selecting power amplifiers can be more involved than matching up a power output and amplification value with your specs. Power amplifier classes refer to the overall operation characteristics of various power amplifier circuits, not to a specific circuit design or topology. Make sure you take the time to understand power amplifier classes when selecting components for use in an analog system.

Power Amplifier Classes

All power amplifiers are designed to increase the power of an input signal by supplying that power from some external source. The power output from the amplifier must be sufficient enough to drive the load it was designed for at the intended frequency of operation (which could be DC). When compared to voltage/current amplifiers, a power amplifier is meant to drive loads directly and it is typically employed as one of the final blocks in a signal chain. A pre-amplification step is often involved in many applications that use a voltage/current amplifier to make sure the input signal fed into the power amplifier is of the requisite strength, bandwidth, and signal-to-noise ratio (SNR).

In the simplest sense, all modern power amplifiers are built as modulator circuits. The input signal is used to modulate the power drawn from the external power source, and the supporting circuitry is designed to regulate how that power is delivered to a load.

Not all power amplifier designs are the same, and before selecting an amplifier circuit for your specific application, it is important to know the difference between various power amplifier classes. The various power amplifier classes are chosen based on the signals they are used to source and the amplifier circuit’s driving method.

Analog Power Amplifiers

Classes of amplifiers are often divided into two primary categories: analog power amplifiers and digital (PWM) power amplifiers. Power amplifiers in the first category operate by controlling the conduction angle on the output signal relative to the input signal. The conduction angle can be thought of as the duration of the output waveform for which the power amplifier transistor is in the ON state. For instance, if the transistor is ON for the entire duration of the operation, the conduction angle is 360°. Class A, B, AB, and C amplifiers fall under this category.

PWN-Driven Power Amplifiers

The second category of power amplifiers uses pulse width modulation (PWM) with a digital driver circuit to switch between ON and OFF states. These power amplifiers are often referred to as the switching amplifiers; classes D, F, G, I, S, and T fall in this category. PWM amplifier classes are not limited to those mentioned below, and there are other amplifier classes that essentially perform the same function with minor differences in configuration.

Class A

Class A power amplifiers are designed using only one switching transistor. The transistor type (BJT, IGBT, FET) depends on the intended end use application. These are linear amplifiers with high gain and 360° conduction angle. The result is high-efficiency amplification of high-frequency signals because the signal distortion level is very low as long as the transistor is operated in the linear range. The downside is reduced efficiency due to overheating (conduction losses). Since the transistor is always in the ON state, even when there is no input signal, a significant amount of heat is generated and the efficiency can be low.

Class B

Class B power amplifiers attempt to solve the heating problem in Class A amplifiers by using two complementary transistors to amplify the entire waveform. The conduction angle for each transistor is 180°, i.e., both remain in the ON state for half the duration of the input signal. One transistor conducts during the positive-half cycle of the analog waveform while the other transistor conducts during the negative half cycle.

Theoretically, Class B amplifiers can be 75% efficient, however, due to the superposition of two halves of the waveform, there is a crossover zone where there is a minor amount of distortion. This arises due to a dead zone that occurs below the rectification threshold in the transistor.

Crossover distortion occurs near 0 V as measured in the output waveform.

Class AB

As the name implies, this configuration is a blend of Class A and Class B power amplifiers. It solves the problem of reduced efficiency due to overheating. It simultaneously reduces the crossover distortion present in Class B power amplifiers through the use of a combination of diodes and resistors to provide a bias voltage. The efficiency of a Class AB amplifier is generally between 50% and 60%.

Class C

Class C amplifiers have the highest efficiency but the lowest linearity range compared to the other power amplifier classes mentioned above. The conduction angle in Class C power amplifiers is less than 90°. Consequently, these amplifiers are unsuitable for audio amplification because a smaller conduction angle leads to more distortion. Class C amplifiers have a tuned load that enhances one frequency while suppressing others. This makes Class C amplifiers suitable for applications like high-frequency radio signal amplification and oscillators.

Looking through the progression from Class A to Class C amplifiers, we see a steady decrease in the conduction angle of these amplifiers, as outlined below.

Class D

This is a nonlinear amplifier that uses PWM switching. Theoretically, it can achieve an efficiency of 100%. Class D amplifiers need much smaller power transformers than other amplifiers because PWM permits amplification at considerably higher frequencies. They are ideal for applications that require large power amplification in a small package, such as in power amplifiers for wireless protocols. An improved alternative to Class D amplifiers is Class T, and these two amplifier classes are often compared with each other.

Class F

Class F amplifiers use a set of harmonic resonators (high-Q parallel LC circuits) to boost output power delivered to a load and provide high efficiency. The series of harmonic resonator circuits allows the input modulating signal to generate harmonic components at multiples of some fundamental frequency. As more harmonic components are added into the amplifier’s output signal chain, the efficiency of the amplifier circuit increases (at least 90% in theory), and the output approaches a true square wave due to superposition of these harmonic components.

Class G

This class of power amplifiers is an improvement over the conventional Class AB amplifier. Class G amplifiers automatically shift between numerous power supply rails at different voltages as the input signal is varied. Class H amplifiers are also a variation of Class G amplifiers, except they use an infinitely variable analog supply rail. The use of continual switching decreases power losses in the transistor’s conduction channel.

Class I

Class I amplifiers have two sets of complementary output switching circuits stacked in a parallel push-pull configuration, similar to a bridge circuit. The basic concept is the same as in a Class B amplifier: one device is active during the positive half cycle while the other is active during the negative half cycle. At the zero-crossing point of the input signal, the switching devices turn ON and OFF simultaneously when the PWM driver duty cycle is 50%.

Class S

Class S power amplifiers are similar to Class D power amplifiers. A sigma-delta modulator is used to transform an input analog signal into a square wave, similar to a railed op-amp or a Schmitt trigger. These digital pulses are then amplified to the desired power output level. As this signal is passed to the output, a high-Q bandpass filter is then used for demodulation at the desired frequency, which will ideally leave behind a single frequency component concentrated at the bandpass filter’s resonance.

Selecting a Power Amplifier Topology

The power amplifiers outlined above could be designed from discrete components, or they may be available as ICs in a compact package. Some components are available as modules that must be mounted off-board and connected to other circuits through cables. Some of the major performance specifications include:

• Operation voltage and current, which together will give the peak/average power supplied by the amplifier
• Amplification level, which will be specified in dB
• Linear range and dynamic range; these are not the same thing in real amplifiers
• Total harmonic distortion (THD), giving the ratio of power in generated harmonics to power in the fundamental frequency
• Bandwidth, which will generally show how the amplification and ranges (linear and dynamic) vary with the input signal’s frequency

If purchasing an off-the-shelf integrated circuit, some semiconductor manufacturers will include additional safety measures built into the package. These include temperature sensing for thermal shutdown, current limiting, and ESD protection. Additional measures that are not included in the package will relate to power transfer to the load.

One final point to note regards the linear range of power amplifiers. Although power amplifiers can have large amplification, they are often run very near saturation, so there can be some harmonic generation. It is generally desired to transfer maximum power to the load, which requires impedance matching. For nonlinear loads, load-pull analysis must be used to implement impedance matching as direct matching of conjugate impedances will not match the required power across the signal chain.

When you’re ready to create your power amplifier schematics and start your PCB layout, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.

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