BASIC KNOWLEDGE - AMPLIFIER Introduction to the Amplifier

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ROHM Semiconductor GmbH

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CISSOID S.A.

From giving an electric guitar its satisfying crunch to boosting signals in radios so that broadcasts can be listened to around the world, amplifiers are a critical component of any device that reproduces sound signals.

This article explains how they work, the different types, and their applications.

What’s an amplifier?

Before we dive deeper in the world of amplifiers, we would like to answer the following question: What does amplify mean? An amplifier is an electronic device designed to increase the magnitude of an input signal, whether it be current, voltage, or power, and deliver this amplified signal to an output circuit. The ratio of the amplifier's output magnitude to its input magnitude is termed "gain," which quantifies the amplification effect and can be expressed in terms of current, voltage, or power. Importantly, gain can also be represented logarithmically in decibels (dB), especially when discussing power amplification. While amplifiers can vary significantly in size based on their application and power requirements — ranging from compact units in portable electronics to large-scale systems for concert halls or broadcasting — they are fundamental in enhancing signal strength across various electronic and communication devices.

What does an amplifier do?

The signal that’s amplified is used to power the system to which the amplifier is connected while minimizing distortion (or “noise”). An amplifier typically contains a transistor, which is based on a semiconductor material with the ability to conduct variable amounts of electric current. The basic steps are as follows:

• The amplifier draws charge from a power supply

• Current is conveyed across the transistor to an output circuit

• The output circuit converts the increased output signal into an equal voltage

• The amplified voltage powers the system

Two further components usually found in an amplifier circuit are diodes and rectifiers. A diode is a two-terminal semiconductor device whose purpose is to conduct current in one direction only and prevent it from flowing back. Meanwhile, a rectifier is a circuit that converts alternating current (AC) into direct current (DC).

The different types of amplifiers and their applications

There are various amplifier types, differentiated by their function, application, and the types of signals they amplify. Here are some basic categories:

• Audio Amplifiers: These amplify low-frequency signals that are audible, typically in the range of about 20 Hz to 20 kHz. They are used in audio devices like speakers, stereo systems, and headphones.

• RF Amplifiers (Radio Frequency): These amplifiers are used for higher frequency signals, typically in the radio frequency range, and are found in transmitters, receivers, and wireless communication systems.

• Operational Amplifiers (Op-Amps): Highly versatile integrated circuits used in a wide variety of electronic circuits to amplify, filter, integrate, differentiate signals, and more.

• Power Amplifiers: Designed to deliver high power output to drive loads like speakers or motors. They are classified into different classes based on their efficiency and the shape of their output signals (e.g., Class A, B, AB, and D).

• Instrumentation Amplifiers: Designed for applications requiring high precision, stability, and high input impedance, such as medical devices and sensor signal processing.

• Microwave Amplifiers: Specialized in amplifying signals in the microwave frequency range, including satellite communications and radar.

• Guitar Amplifiers: Specifically designed for amplifying the signals from electric or acoustic guitars, with a particular focus on tone and volume.

An amplifier for every situation

While there are many different types of amplifier, the most common are:

• Transistors: this category can be further broken down into point-contact, bipolar junction (BJT) and metal-oxide-semiconductor field-effect (MOSFET) transistors

• Operational amplifiers: an integrated circuit that amplifies voltage with a high level of efficiency

• Vacuum tubes: mostly replaced by integrated circuits, but still used in UHF systems, high-power radio and radar applications due to its strong output

• Instrument amplifiers: for use with musical instruments

• Distributed amplifiers: found in oscilloscopes due to their ability to split an input current and amplify the separate segments

BASIC KNOWLEDGE - MOSFET VS. BJT

What’s the difference between MOSFET and BJT?

Passive, active, bridged and paralleled

When shopping for a new sound system, the terms “active” and “passive” often arise. The difference between the two is simple: in an active setup, the system has a built-in amplifier, while in a passive one the system requires an external amplifier, which is connected by speaker wire. Amplifiers can also be connected in series to increase the amount of power available. Usually used in professional audio systems, a bridged configuration involves two channels of a stereo amplifier being fed the same signal, while a loudspeaker (the bridge) is connected between those two channels to create a mono amplifier with twice the power. For paralleled setups, multiple amplifiers send the same signal to both speakers in a stereo system for increased output drive.

Here, there and everywhere

Well-known applications for amplifiers include stereo systems, hearing aids, wireless communications and television receivers. Amplifiers can also take the form of electromagnetic devices known as transformers, which boost voltage while keeping it constant. Transformers are found in everything from power stations through to laptops and phones, in which they regulate the voltage used to charge the battery to prevent the latter from becoming overloaded.

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