BASIC KNOWLEDGE - AMPLIFIER Introduction to the Amplifier

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.

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

What’s an amplifier?

Quite simply, an amplifier is a very small electromagnetic or electrical device that increases an input signal – whether this is current, voltage or power – and delivers this amplified signal to an output circuit. An amplifier’s input/output magnitude ratio is known as “gain”, which can be expressed as current, voltage or power.

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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, as explained in a previous article, 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).

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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