Audio Power Amplifiers: Definition, Operating Principles, and Applications

Jul 11, 2023

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Audio Power Amplifiers: Definition, Operating Principles, and Applications

The audio power amplifier, also known as a power amp, is an essential device for producing sound by reconstructing the input audio signal and delivering it to the output element. It is crucial for the reconstructed signal to maintain an ideal volume and power level while minimizing distortion. Considering that the audio frequency range typically spans from 20Hz to 20,000Hz, it is imperative for the amplifier to exhibit a good frequency response within this range. The power requirements of the amplifier vary significantly depending on the application. For instance, earphones may only require milliwatts of power, while TV or PC audio systems may need several watts. Home stereo and car audio systems typically operate in the tens of watts range, while more powerful systems for home and commercial use can deliver hundreds of watts. These high-power systems are designed to meet the sound requirements of large venues such as movie theaters and auditoriums.

1. What is Audio Power Amplifier?

The audio power amplifier is a crucial component in multimedia products and is extensively utilized in the consumer electronics industry. Linear audio power amplifiers have traditionally dominated the market due to their low distortion and excellent sound quality. However, with the proliferation of portable multimedia devices like mobile phones, tablet PCs, and notebook computers, there has been a shift towards more efficient and compact solutions. This has led to the increasing popularity of class D power amplifiers, which offer advantages such as high efficiency and small size.

The development of audio amplifiers has progressed through three generations: electron tubes (vacuum tubes), bipolar transistors, and field-effect transistors (FETs). Tube audio amplifiers are known for their warm and mellow tone, but they are bulky and suffer from drawbacks such as high power consumption, instability, and poor high-frequency response. Bipolar transistor audio amplifiers, on the other hand, offer a wide bandwidth, large dynamic range, high reliability, and long lifespan. However, they have significant static power consumption and on-resistance, making it challenging to improve their efficiency.

FET audio amplifiers, similar to electron tubes, produce a mellow tone and boast a wide dynamic range. However, they have the added advantage of small on-resistance, enabling them to achieve very high efficiency. This makes FET audio amplifiers an attractive choice for modern applications.

2. Types of Audio Power Amplifiers

There are various types of audio power amplifiers, but only four types are commonly used in audio applications: Class A, Class B, Class AB, and Class D.

2.1 Class A Power Amplifier

Class A power amplifiers operate with the transistor turned on throughout the entire cycle of the input signal. The operating point of the amplifier is set near the midpoint of the load line. This amplifier can work in a single tube or push-pull state. Due to operating in the linear range of the characteristic curve, Class A power amplifiers have low transient and alternating distortion. The circuit is simple and easy to debug. However, Class A power amplifiers require a large static current and have low efficiency. The maximum theoretical efficiency of a Class A power amplifier is 25%, so a heat sink is necessary during operation.

2.2 Class B Power Amplifier

Class B power amplifiers are composed of two complementary transistors. Each amplifier tube is turned on in one half cycle and turned off in the other half cycle of the complete signal cycle, resulting in a conduction angle of only 180 degrees. Class B power amplifiers have no static current, making them more efficient. The maximum theoretical efficiency of a Class B power amplifier can reach 78%. However, when the input signal is close to zero, crossover distortion may occur due to the critical conduction state of the amplifier tube.

2.3 Class AB Power Amplifier

Class AB power amplifiers insert two diodes into the input end of the Class B power amplifier. This allows the amplifier tube to be slightly turned on when the input is close to zero, increasing the conduction angle of each amplifier tube to be greater than 180 degrees but less than 360 degrees. Class AB power amplifiers overcome the crossover distortion of Class B power amplifiers and have an efficiency between Class A and Class B power amplifiers. However, the medium output voltage is usually far from the power supply voltage, resulting in significant power consumption in the transistor. Therefore, even well-designed amplifiers still have relatively low efficiency.

2.4 Class D Power Amplifier

Class D power amplifiers, also known as switching amplifiers, convert analog audio signals or PCM digital information into PWM (pulse width modulation) or PDM (pulse density modulation) pulse signals to control the switching of power amplifier devices. The output stage of Class D power amplifiers consists of two complementary power tubes that work in an on-off state driven by high-frequency control pulse signals. This eliminates the need for static power consumption, resulting in very high efficiency. The theoretical efficiency of Class D power amplifiers can reach 100%, but in practice, it is typically over 80%, which is 2 to 3 times higher than traditional linear power amplifiers. Class D amplifiers also resemble one-bit power digital-to-analog converters and consist of input signal processing circuits, switching signal forming circuits, high-power switching circuits (half-bridge and full-bridge), and low-pass filters (LC). These amplifiers use high-frequency switch circuits to amplify the audio signal.

Class D amplifiers offer several advantages:

1) High efficiency, usually above 85%.

2) Compact size, saving space compared to analog amplifier circuits.

3) Connection without crackling noise.

4) Low distortion and good frequency response curve. They require few peripheral components, making them easy to design and debug.

3. Audio Power Amplifiers Comparisons

1) Class A, B, and AB amplifiers are analog amplifiers, while class D amplifiers are digital amplifiers. Class A amplifiers operate in a continuous conduction mode, where the output transistors are always conducting, resulting in low distortion but low efficiency. Class B amplifiers use a push-pull configuration, where each transistor handles half of the waveform, resulting in higher efficiency but potential crossover distortion. Class AB amplifiers combine the advantages of Class A and Class B amplifiers by using a small bias current to keep the output transistors partially conducting at all times, reducing crossover distortion.

2) Class B and Class AB amplifiers offer higher efficiency and less distortion compared to Class A amplifiers. This is because the output transistors in Class B and Class AB amplifiers only conduct when needed, reducing power consumption and improving heat dissipation. However, Class B amplifiers can suffer from poor switching characteristics during the transition between the transistor's on and off states, leading to distortion. Additionally, improper selection of circuit parameters in Class B amplifiers can cause alternate distortion, further affecting the audio quality.

3) Class D amplifiers, on the other hand, operate by converting the analog audio signal into a digital pulse-width modulated (PWM) signal. This PWM signal is then used to control the switching of the power amplifier, resulting in high efficiency and low distortion. Class D amplifiers have a good frequency response curve and require fewer peripheral components compared to analog amplifiers. They are known for their compact size, making them ideal for portable audio devices and applications where space is limited.

4) Currently, the Class AB amplifier and Class D amplifier are the basic circuit forms of audio power amplifiers. Class AB amplifiers are widely used due to their moderate efficiency and good audio quality. Class D amplifiers, on the other hand, have gained popularity due to their high efficiency, small size, and low distortion characteristics. DIY Class D audio amplifiers are particularly favored by audio enthusiasts for their excellent performance and compact design.

Figure 2

4 Audio Power Amplifier Circuit Architecture

The purpose of an audio power amplifier is to replicate the audio input signal on the sound output device with high efficiency and low distortion at the desired volume and power level. The frequency range of audio signals is from 20 Hz to 20000 Hz, so an audio amplifier must have good frequency response. Audio amplifiers typically consist of a preamplifier and a power amplifier.

Preamplifier:

The amplitude of the audio signal from the signal source is usually very small and cannot directly drive the power amplifier. Therefore, they need to be amplified to a certain level using a preamplifier. In addition to signal amplification, it can also have functions such as volume control, tone control, loudness control, and channel equalization.

Power Amplifier:

The power amplifier, also known as the power amp, is designed to provide enough current driving capability to the load for power amplification. Class D power amplifiers operate in a switching mode. In theory, they do not require static current and have high efficiency.

The typical topology of a Class D power amplifier circuit is shown in Figure 2: it consists of a triangle wave generator, a comparator, a power output stage, and an LC low-pass filter.

Figure 3

Figure 2 is a schematic diagram of the working principle of a Class D power amplifier. The sinusoidal audio input signal and a triangle wave signal with a much higher frequency than it are modulated by the comparator to obtain a pulse width modulation (PWM) signal, whose duty cycle is proportional to the amplitude of the input signal. It drives the output power transistors to work in a switching mode. The output of the transistors obtains an output signal with a constant duty cycle. The amplitude of the output signal is determined by the power supply voltage and has a strong current driving capability. After signal modulation, the output signal contains the input signal, the fundamental component of the modulating triangle wave, and their higher-order harmonics and combinations. After filtering by the LC low-pass filter, the high-frequency components in the output signal are filtered out, and the load receives a low-frequency signal with the same frequency as the original audio signal but with a larger amplification amplitude.

5 Typical Audio Amplifier Circuits Examples

AN7115 Audio Amplifier Circuit

Figure 4. AN7115 Audio Amplifier Circuit

Under the conditions of V=9.0V, THD=10%, and RL=8Ω, the output power of AN7115 is 2.1W, with a noise output of 3mV.

Limiting parameters:

Vcc=13V, power consumption (without heatsink) is 1.2W, and with heatsink is 2.25W.

Operating temperature: -20℃ to 70℃, suitable for small portable radios, recorders, and audio devices as a power amplifier.

TDA2030 adopts a V-shaped 5-pin single-line plastic package structure. According to the pin shape, it can be divided into H-type and V-type. This integrated circuit is widely used in car stereos, tape recorders, and medium-power audio devices. It has the characteristics of small size, high output power, and low distortion, and comes with internal protection circuits. The circuit features are as follows:

TDA2030 Audio Power Amplifier Circuit

Figure 5. TDA2030 Audio Power Amplifier Circuit

1) Very few external components.

2) Large output power, Po=18W (RL=4Ω).

3) Uses an ultra-small package (TO-220) to increase assembly density.

4) Minimal turn-on impact.

5) Includes various protection circuits, ensuring safe and reliable operation. The main protection circuits include short-circuit protection, thermal protection, ground wire coupling disconnection, reverse polarity protection (Vsmax=12V), and load discharge voltage kickback, etc.

6 FAQ

1. What is an audio power amplifier?

An audio power amplifier, also known as a power amp, is an electronic device that amplifies low-power electronic audio signals to a level that is suitable for driving loudspeakers or headphones. It is commonly used in various sound systems, including sound reinforcement, public address systems, home audio systems, and musical instrument amplifiers such as guitar amplifiers.

2. How does an audio amplifier work?

An audio amplifier takes an input signal from a source, such as a laptop, turntable, or CD player, and amplifies it to a larger copy of the original signal before sending it to the speakers. The amplifier receives power from the mains electricity and utilizes it through the power supply within the amplifier.

3. What is the purpose of an audio power amplifier?

The main purpose of an audio power amplifier is to amplify low-power electronic audio signals, such as those from a radio receiver or electric guitar pickup, to a level that is sufficient for driving loudspeakers or headphones. By increasing the power of the signal, the amplifier ensures that the audio is audible and can be enjoyed at a desired volume level.

4. What is the most powerful audio amplifier?

One of the most powerful integrated amplifiers currently available is the McIntosh MA12000 Integrated Amplifier. It offers a whopping 350W per channel, making it McIntosh's most powerful integrated amplifier to date.

5. Do speakers require an amplifier?

Powered speakers, also known as active speakers, do not require an external amplifier. These speakers already have an amplifier built-in, which is why they are referred to as "powered speakers." Depending on the input options of the speakers, they can be connected to different audio sources without the need for a separate amplifier.

6. Does an amplifier improve sound quality?

In ideal conditions, amplifiers should amplify audio signals linearly and not technically improve or worsen sound quality. However, certain factors such as subpar amplifiers, incorrect amp settings, or unsuitable amplifier-speaker combinations may negatively impact sound quality. Amplifiers are essential for driving speakers and headphones properly, ensuring optimal performance.

7. Why would I need a power amplifier?

Some users prefer to separate the tasks of input switching and AV processing from the power delivery and connection of loudspeakers. They achieve this by using separate AV preamp/processors and power amplifiers. While this setup may result in more equipment and cable clutter, it allows for greater flexibility and customization in audio systems.

8. Which is better: a power amplifier or an integrated amplifier?

A preamplifier/power amplifier combination generally offers better audio quality. By keeping the switching circuitry separate from the amplification and using two separate power supplies, internal interference that can degrade the signal can be reduced. This setup is often preferred by audiophiles who prioritize audio fidelity.

9. What is a home audio amplifier?

A home audio amplifier, also known as an audio-visual receiver (AVR), is a powerful device that combines amplifiers and digital signal processors. It converts the output from various source devices into high-quality sound and video. In a home theater setup, the AVR serves as the central hub, managing and enhancing the audio and video experience.

10. What is the purpose of an audio amplifier?

The primary purpose of an audio amplifier is to faithfully reproduce input audio signals at sound-producing output elements, such as loudspeakers or headphones. It aims to achieve the desired volume and power levels efficiently and with minimal distortion, ensuring an accurate representation of the original audio signal.

11. What is meant by an audio amplifier?

An audio amplifier refers to any electronic device that increases the power of an electrical signal within the audio frequency range. This range is the range of vibrations that can be perceived by the human ear. Therefore, an audio amplifier is designed to amplify and enhance audio signals for improved sound reproduction.