An amplifier is an electronic device that increases the power of a weak signal. It is a dual-port electronic circuit that uses electrical energy to increase the amplitude (voltage or current) of a low-power time-varying signal applied to its input terminal. It results in a greater amplitude signal at the output terminal which enhances the final power of the signal. An amplifier can be a stand-alone piece of equipment or be contained inside an audio device. Its amplifying capability is measured by its gain which is the ratio of output voltage, current, or power to input. An amp is an electrical circuit that has a power gain greater than one.
Amplifiers are used in numerous applications like wireless networks, video signal processing, microwave amplification (Klystrons) and audio systems. In this article we will focus on amplifiers used in sound systems. A sound amplifier increases the power of an audio signal such as a signal from a radio receiver, or an electric guitar pick-up. It infuses the signal with enough power to drive loudspeakers or headphones. Home sound systems, public announcement systems and musical instrument amplifiers routinely use audio power amplifiers.
A Deep Dive Into Amplifier Basics :
Types of Amplifiers :
Amplifiers can be categorized based on different criteria. One way to classify amps is by the frequency of the signal that is amplified. For instance radio frequency (RF) amplifiers amplify frequencies in the radio frequency range of 20 kHz to 300 gHz, audio amplifiers amplify audio frequencies below 20 kHz, and servo amps amplify very low frequencies close to direct current. Amps can also be classified based on their actual position on the signal chain. Preamps, for instance, come first in the chain, and provide the initial amplification, while the power amp comes last just before the signal is sent to the loudspeaker.
Amplifiers in Sound Systems :
Preamp :
In an audio device, a preamp is an electronic amplifier that amplifies a weak electrical signal at the initial stage. It is a low-power audio amplifier that strengthens the signal for advanced processing. It makes the signal powerful enough to withstand noise, and readies it for the power amp or loudspeaker. In the absence of a preamp, the final signal would be noisy and full of distortions.
Preamps are mainly used to boost signals from analog sensors like microphones, phonographs (gramophone or record player) and instrument pick-ups (a transducer that captures mechanical vibrations produced by a musical instrument) to line level. Once the preamp has amplified the signal, the power amp acts on it and further boosts its power to a level where it can drive a loudspeaker. The preamp offers voltage gain but no current gain, while the power amp provides the voltage needed for operating loudspeakers. Preamps are mostly built into the audio input of mixing consoles and sound cards. However, they are also available separately as external devices.
Power Amplifier :
A power amp is a high-power audio amplifier that increases the power of the input signal. It comes in the later stages of the signal chain. It beefs up the pre-amplified input signal and makes it robust enough to power a loudspeaker or headphone. For example, the input signal applied to the input terminal of the power amp may only be a couple of microwatts in strength but the final output signal generated at the output terminal may be a few watts in case of small electronic gadgets like clock radios, tens or hundreds of watts in a marine amplifier, a few thousand watts in a discotheque, and tens of thousands in open-air gigs like music concerts.
Before the signal hits the power amp, it goes through various stages such as pre-amplification, equalization of treble and bass frequencies, tone controls, input signal mixing and addition of electronic effects. The signal can also come from digital music players, CD players, cassette players and record players. These are low-level inputs and power amps require these inputs as they are line level. Line level is a standardized strength of audio signals needed to send analog sound between sound devices like TVs/DVD players and mixing consoles.
High-end power amps aimed at audiophiles and professional recording studios are sold as separate audio devices. Power amps used in large sound arrangements like sound reinforcement systems are also available as stand-alone units. Consumer electronics like TVs, soundbars, home cinema systems, boom boxes and clock radios have integrated power amps. The amps are built into the frame of the gadget and are relatively small in size. Another term frequently used in the amplifier market is stereo amplifier. It simply means an amplifier that has two output channels for left and right sounds.
What Are The Different Classes of Audio Amplifiers ?
Power amplifiers are classified into various classes based on the manner in which they handle the input signal. The class indicates how the amplifier processes a line level input signal and converts it to an output signal strong enough to drive a loudspeaker. It is representative of the characteristics and performance of the amplifier. It indicates the proportion of the input cycle or the amount of time in the input cycle during which the amplifier passes current. The class of a sound amplifier is depicted by alphabets. ‘A’, ‘B’, ‘AB’ and ‘C’ classes refer to analog designs, and ‘D’ and ‘E’ classes to switching designs. Here are a few common classes of amplifiers used in consumer electronics.
Class A amplifier :
These amplifiers are also known as linear amps. They are the most basic kind of audio amps. They consume a lot of power and generate enormous amounts of heat. Class A amps take in the input signal and keep the transistors running throughout the entire waveform of the signal. This results in the entire waveform being reproduced as it was applied to the input terminal. The benefit of this kind of amplification is excellent sound quality and realistic audio reproduction. However, the amp uses more energy, heats up quickly, and requires constant ventilation. It does not have a standby mode or energy-saving features. It stays on as long as the sound device (soundbar, stereo system) is on. Class A amps have an efficiency rate of 20%. If it puts out 20 watts, it generates approximately 80-100 watts in heat. Hence, they are not very efficient.
Class B amplifier :
A Class B amp is also known as a push-pull amplifier. The signal in a Class B amp is divided into a positive and negative waveform, and each half of the wave is processed by a different circuit. The transistors (output devices) switch on and switch off depending on what part of the signal they are handling. To perform this unique task, a Class B amp requires at least two transistors per channel. Each transistor amplifies one half of the waveform which means that the two transistors are never on at the same time. At any given point in time, half the output devices are off and the other half are on. Due to this reason, Class B amps consume less power, and are cooler and more efficient than Class A amps. But, Class B amps have average sound quality. Because they do not reproduce the entire soundwave in one go, there are more distortions which affect the overall sound.
Class AB amplifier :
A Class AB amplifier is a combination of the previous two classes. It is one of the commonest types of audio amplifiers found in home stereo systems. It incorporates the best features of Class A and Class B amps. It has the sound reproduction capability of a Class A amp and the energy-saving features of a Class B amp. A Class AB amp functions like a Class A amp when the output levels are low. Its distinctive operation minimizes distortions at the point where the waveform crosses over from the positive to the negative side and back. It generates a moderate amount of heat and does not waste power.
Class G and Class H amplifiers :
Class G and Class H amps are basically Class AB amps with different voltage rails. The voltage rail is the amount of voltage generated or permitted by the power source.
A Class G amplifier uses multiple rails of power supply with different voltages. It acknowledges the need for additional power when the signal demands it. For example, in peak voltage level when the output signal crosses the main voltage level, the amplifier automatically switched to a different rail which supports higher voltage or load. As a result it is able to achieve a greater Class AB efficiency, allowing for larger peaks as and when required, without raising the voltage rail across the system. A Class H amp is similar to a Class G amp except it controls the voltage at the input terminal. It regulates the supply rail voltage as required and allows for handling of peaks. Class G and Class H amps have accurate and realistic sound reproduction. They use less energy and generate less heat.
Class D amplifier :
Class D amps are not digital amps in the purest sense of the term. They are highly efficient and are mainly used as subwoofer amps. They are also known as non-linear switching amps due to the manner in which they switch output devices in a rapid succession, typically at least twice per waveform. Class D amps are around 80% efficient, but you do have to deal with distortions and fluctuating sound quality. A lot of research is currently underway on Class D amps. Their quality is steadily improving and a lot of brands are incorporating Class D amps in desktop systems and home stereo receivers. Please note that Class D amps may be analogue or digitally controlled. The result, however, is always analogue.
Class C amplifier :
Class C amps have the highest efficiency of all the amps mentioned so far. But when it comes to linearity, they fare very poorly. They have a greater audio distortion level, and hence, are not suitable for audio applications. They are mostly used in high frequency sine wave oscillators and RF amps.
How Does An Amplifier & A Speaker Work ?
The primary function of an amplifier is to accept a low-power electrical signal and convert it to a high-power amplified signal. There are two kind of amplifiers in audio systems. The first is the preamp. It enlarges the signal just up to the point where it can be accepted by the power amp. The power amp is the second point of amplification. It takes in the pre-amplified signal and further expands it so it can drive a loudspeaker.
The working principle of an audio amp is fairly simple. It has 3 basic connections: an input, an output and a power source. It starts when a signal is fed to the input terminal from a source, generally a music player, a smart phone, a CD or a DVD player. The amp enlarges this signal and sends the amplified replica to the output terminals that lead to the speakers. The energy required to perform this task is derived from a 110 volt wall socket or a battery. The alternating current (AC) from the wall receptacle is converted to direct current (DC) in the power supply unit of the sound amplifier. In AC current, the electrons flow in both directions while in DC current it flows in one direction. The electric current from the wall socket or battery is then channeled into a transistor which is a variable resistor. The transistor acts like a valve regulating the amount of current passing though the amp based on the size of the input signal from the source. The input signal stimulates the transistor to lower its resistance and allow current to flow. A small signal causes a small amount of current to flow while a larger signal causes more current to flow. The frequency of the input signal also has a direct impact on the way the transistor functions. For example, a 100 Hz signal will cause the transistor to open and close 100 times per second. In other words, the transistor controls both the amplitude and the frequency of the electrical current routed to the speaker. This, in a nutshell, is how an audio amplifier works.
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