What are the different amplifier classes?

Amplifiers are one of the most common circuits frequently used in electronics. There are several types of amplifiers, including:

Audio amplifiers: amplify the audio signals like music and speech signals
Radio frequency amplifiers: boost weak radio frequency signals of radios and televisions
Power amplifiers: amplify the overall power of a weak signal
Operational amplifiers: used for analog signal processing like amplification, filtering, inverting signals, and adding or subtracting signals

All amplifier circuits are categorized by amplifier classes, which indicate the overall characteristics of the circuit. This makes it easier to decide its application or suitability.

The most common are Class A, B, AB, and C amplifiers. Class D, G, H, T, etc., are less common.

In this article, we’ll cover the different amplifier classes and explain why and how amplifier circuits are categorized.

Why are there amplifier classes?
The reason is simple. Not all amplifier circuits are the same. Amplifier circuit designs have varying characteristics that have been categorized into classes for simplicity.

Often, the desired characteristics and the cost must be considered. Ideally, there should be a balance between these factors.

Fidelity: how accurately the circuit reproduces the input signal. This represents how close is the amplified signal to the source signal.
Efficiency: how much input power from the dc source is converted into output power. The amplifier circuit uses a dc source to boast the input signal and output an amplified signal. The efficiency of the circuit indicates how much power from the dc source is used to boost the input signal.
Power output: the maximum amount of power (current multiplied by voltage) the amplifier circuit can deliver to the load.

Amplifier circuits often face a trade-off between fidelity and efficiency. The amplifier classes – Class A, B, AB, and C — are arranged in ascending order of efficiency. As efficiency increases, fidelity may decrease, highlighting the practical implications of these choices.

Class A, B, AB, and C refer to linear amplifier circuits. Class D amplifiers and other classes are switching amplifiers that rely on switching the load on and off to achieve up to a theoretical 100% efficiency. Apart from linear and switching amplifiers, there are analogarithmic, instrumentation, operational transconductance amplifiers (OTA), and voltage comparators, which are specialized amplifier designs.

Linear amplifiers are easily categorized into an amplifier class by their conduction angle only. It’s fed a sinusoidal signal to measure the efficiency and fidelity of the amplifier circuit. The conduction angle is a portion of the input cycle — the load is turned on, allowing current to flow through it. The main classes of amplifiers (i.e., Class A, B, AB, and C) are easily distinguished by their conduction angle only.

How fidelity is measured
The fidelity of an amplifier indicates how accurately the circuit reproduces the input signal. A method for determining the fidelity is to consider the amplifier’s response to different frequencies across the spectrum.

For audio amplifiers, this means fidelity can be measured by inspecting the amplifier response to different frequencies of the audio spectrum (from 20 Hz to 20 KHz).

An ideal amplifier offers 100% fidelity and has a flat response. It should amplify all frequencies of the spectrum equally. If there are deviations in the flat response, there will be distortion or coloration in the output signal compared to the input signal.

To measure an amplifier’s frequency response, a signal generator feeds a pure sine wave to the circuit. The output signal is analyzed using a spectrum analyzer. It displays the amplitude of the output signal at different frequencies. Deviations from the input signal frequency response indicate distortions or coloration and represent the circuit’s limitations.

Another method for determining the fidelity of an amplifier circuit is measuring total harmonic distortion. It’s the ratio of the combined power of the harmonics generated by the amplifier to the power of the original signal.

The lower the total harmonic distortion, the less distortion or coloration will be heard in the output signal — and the higher the fidelity. To measure total harmonic distortion, a pure sine wave is fed into the amplifier. Specialized distortion analyzers measure the harmonics present in the output signal, and total harmonic distortion is calculated by comparing the harmonics’ power to the original signal’s power.

Audio amplifiers’ fidelity is typically determined by measuring the intermodulation distortion. Audio signals often consist of multiple simultaneous frequencies or tones. When these frequencies interact in the circuit, new frequencies not present in the original signal might be generated. The new frequencies are intermodulation products.

To measure intermodulation distortion, two or more pure tones at different frequencies are fed to the amplifier circuit, and the intermodulation products are considered. Intermodulation distortion is calculated as the ratio of intermodulation products’ power to the original signal’s power.

Yet another method for determining the fidelity of an amplifier circuit is calculating the signal-to-noise ratio (SNR). To measure the SNR, a specific signal level is fed to the amplifier while the input signal is kept turned off. Then, the output noise is measured, and the SNR is calculated as the ratio of signal level to noise level expressed in decibels (dB). A high SNR indicates high fidelity and there’s less noise/distortion in the output signal.

How amplifier’s efficiency is measured
The efficiency of an amplifier circuit is calculated as the ratio of average output power (average output power of an ac signal delivered by amplifier to the load or speakers) to the average input power (average input of dc power consumed by the amplifier from the source, battery, or mains).

The efficiency is expressed as percentage and calculated according to the following equation.

Efficiency (%) = (Output power / Input power) x100

To measure the efficiency of the average ac power at the output of the amplifier and the average input dc power from the supply, ac and dc power meters are used, respectively. Alternatively, the voltage and current are measured at the output of the amplifier. The input from the supply and the product of voltage and the current provide the output and input power at the amplifier.

The amplifier classes
Amplifiers are mainly classified as Class A, B, AB, and C amplifiers, which represent the ascending order of efficiency. Other classes, such as Class D, H, G, and T, are less common. As far as linear amplifiers are concerned (frequently used in audio and power amplification), the amplifier circuit always falls between Class A and Class C.

Class A amplifiers
Class A amplifier has a conduction angle of 360 degrees. These amplifiers offer high linearity and gain but at the cost of efficiency. The efficiency of a Class A amplifier is typically limited to 15~30%.

The amplifier circuit often has an input stage that pre-amplifies the weak signal from a music player or microphone. The circuit’s output stage consists of one or more transistors that amplify the input signal and deliver it to a speaker or load. The transistor(s) in the output stage are biased so they’re always in a conducting state.

The circuit may use coupling capacitors to block dc from the supply and let only the audio signal pass through. These amplifiers have high fidelity and output an amplified signal almost identical to the input signal. However, as the transistors in the output stage are always conducting, the efficiency is limited to 15 to 30%.

Most of the power from the source is wasted in heat dissipation. As there is a constant current flow through the transistors all the time, resulting in heat, the maximum output power of the amplifier is also limited.

Class B amplifiers
Class B amplifiers have a conduction angle of 180 degrees, offering relatively high efficiency without compromising fidelity. The efficiency of a Class B amplifier is typically 70%. The fidelity is not quite as good as a Class A amplifier because increased efficiency comes at the cost of introducing crossover distortions. Due to crossover distortions, these amplifiers are only suitable for directly amplifying audio signals if there’s an additional circuit to mitigate crossover distortions.

Class B amplifiers are still used in public address systems where the fidelity of the audio output is less important. They’re often used for radio frequency amplification since they don’t require absolute fidelity, but efficiency is critical.

A pre-amplification circuit is sometimes used at Class B’s input stage to boost a weak input signal. At the output stage, two complimentary transistors (NPN and PNP) are arranged in a push-pull configuration. The NPN transistor conducts and amplifies the positive cycle of the input signal, and the PNP transistor conducts and amplifies the negative cycle of the input signal.

Each transistor perfectly conducts for 180 degrees of the output waveform, alternating one after the other. The output waveforms are then combined to drive the output device. But there’s a catch behind the perfect alternation of the two transistors. At the zero point crossing of the waveform, there is a dead band in the range from -0.7 to 0.7 V. Each transistor requires a base voltage of 0.7 V to turn on. At this dead band, the output signal gets distorted because neither transistor is in a conducting state.

This crossover distortion makes the Class B amplifier unfit for directly amplifying audio signals. The output stage of Class B is often coupled with the load or speaker by a transformer to minimize distortion. Class B amplifiers work fine at radio frequency amplification. The crossover distortion of Class B amplifiers has minimal effect in RF applications.

Class AB amplifiers
A Class AB amplifier combines Class A and Class B design elements. It biases its output transistors to conduct for slightly more than half of the input signal cycle, reducing crossover distortion present in Class B while maintaining reasonable efficiency compared to Class A. These amplifiers provide an efficiency of 50~70%.

The input stage of a Class AB amplifier might have a pre-amplification circuit to boost a weak input signal. At the output stage, two complimentary transistors, one NPN and one PNP, are arranged in a push-pull configuration like in a Class B amplifier. Although there’s no dc base bias in Class B amplifiers, in Class AB, the transistors are biased to conduct slightly more than 180 degrees of the waveform. A series diode or a resistor might provide the bias.

The transistors in a Class AB amplifier are biased to remain on even at zero input, which is a critical aspect of their operation. The transistors amplify their respective halves (slightly more than half waveform) of waveforms, which are then combined before being delivered to the load or speaker. This biasing technique effectively minimizes crossover distortion, albeit at a slight compromise to the efficiency of the Class B design.

Often, coupling capacitors are used at the amplifier’s output stage to block dc and let the output ac signal pass to the load or speaker. Due to a nearly perfect balance between fidelity and efficiency, class AB design is most widely used for audio power amplifiers.

Class C amplifiers
Class C amplifiers generally have a conduction angle of around 90 degrees. This amplifier design is highly efficient, with an efficiency of around 90% or more, but it offers the poorest fidelity. While Class A, B, and AB are considered linear amplifiers — with the phase and amplitude of the input and output signals linearly related — Class C amplifiers are considered non-linear. Due to poor fidelity, Class C amplifiers are never used in audio applications.

Class C amplifiers are constructed with a single transistor biased to conduct for a short span around the peak of either a positive or negative cycle. Usually, a tuned circuit (like an LC circuit) is connected to the collector or drain of the output transistor. The LC circuit converts the amplifier’s output pulses into complete waveforms. The output signal is a distorted version of the input signal, but it contains the desired frequency component due to the tuned circuit’s filtering effect.

Other amplifier classes
While Class A, B, AB, and C amplifiers are the most common, some other less common amplifier classes exist. These high-efficiency designs use pulse width modulation (switching) or RLC circuits to further increase the efficiency of the amplifier. These amplifier classes range from Classes D to T.

Class D amplifiers: this non-linear amplifier design relies on switching the load on and off by a PWM signal to achieve maximum efficiency. Theoretically, Class D amplifiers can provide 100% efficiency. Practically, they offer more than 90% efficiency.

Class F amplifiers: use multiple harmonic resonators at the output stage, tuned to specific input signal frequencies. The amplifier is controlled by a square or sine wave to maximize efficiency.

Class G amplifiers: rely on rail switching to minimize power consumption. The amplifier circuit uses multiple power supply rails of different voltages, to which it switches according to the level of the input signal. This way, switching over to relevant power supply rails reduces the overall power consumption.

Class I amplifiers: combine some elements of Class D switching amplifiers with features to improve linearity and reduce distortion. They use two sets of complementary output switching devices, often MOSFETs operating in a push-pull configuration, similar to a bridge circuit. Instead of simple on/off switching in the Class D design, these amplifiers apply an interleaved PWM technique to minimize distortion while achieving high efficiency as Class D amplifiers.

Class S amplifiers: the input analog signal is first converted to a digital square waveform, amplified, and then demodulated at the output stage by a band-pass filter.

Class T amplifiers: have a switching amplifier design developed and trademarked by Tripath Technologies. The design uses a feedback loop that takes the signal directly from the switching node (i.e., the output of the transistors instead of the filtered output). It improves linearity and reduces distortion compared to some Class D amplifiers. The control signals for the switching transistors are computed using either digital signal processing (DSP) or fully analog techniques.