Beatrice Lang
Electrical signals can be amplified or switched using a variety of devices, each with its own unique capabilities and applications. Transistors, for instance, are semiconductor devices composed of at least three terminals, that can amplify electrical signals by controlling the flow of current. They are a crucial component in modern electronics, playing a key role in devices such as microphones, speakers, and computers. Another device used for amplification is the amplifier, which increases the magnitude of a signal and is an essential component in thousands of devices, including telephones, music systems, and radio transmitters.
| Characteristics | Values |
|---|---|
| Devices that switch and amplify electrical signals | Transistors, Signal Amplifiers |
| Types of Signal Amplifiers | Voltage amplifiers, Strain gauge amplifiers, Guitar amplifier |
| Types of Transistors | Bipolar junction transistor (BJT), Metal–oxide–semiconductor field-effect transistor (MOSFET), Multiple-base transistor, Tunnel field-effect transistor, Diffusion transistor, Unijunction transistor, Single-electron transistors (SET) |
| Signal Amplifier example | SIRIUS LV (low voltage) signal amplifier |
| Transistor composition | Semiconductor material, usually with at least three terminals for connection to an electronic circuit |
| Transistor function | Amplifies electrical signals by controlling the flow of current between its three layers: the emitter, base, and collector |
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What You'll Learn
Transistors
There are two main types of transistors: NPN and PNP. The difference lies in whether the collector terminal draws or delivers current in the circuit. An NPN type is used when switching by input signals, while a PNP type is used when the power supply side needs to be controlled. Additionally, there are various subtypes of transistors, including bipolar junction transistors (BJTs), field-effect transistors (FETs), and metal-oxide-semiconductor field-effect transistors (MOSFETs).
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Signal amplifiers
A signal amplifier is a circuit that uses electrical power to increase the amplitude of an incoming signal voltage or current signal. It then outputs this higher amplitude version at its output terminals. The ideal signal amplifier creates an exact but larger replica of the original signal. However, a "perfect" amplifier is not possible as no circuit can perfectly and proportionately scale up all aspects of a signal past a certain point.
There are several types of signal amplifiers, each capable of conditioning different signal types. Some common signal amplifiers found in today's data acquisition systems include voltage amplifiers (low voltage amplifier, high voltage amplifier, DC voltage amplifier, AC voltage amplifier), and strain gauge amplifiers (bridge amplifier, full-bridge amplifier, half-bridge amplifier, quarter-bridge amplifier).
The SIRIUS LV (low voltage) signal amplifier is a good example of a real-world signal amplifier. Its rated bandwidth is 70 kHz. The SIRIUS LV is not a stand-alone signal conditioner but has a fully integrated ADC as well. It is a delta-sigma ADC with 24-bit resolution and built-in anti-aliasing filtering. This prevents aliased (i.e. “false”) signals caused by sampling too slowly for the incoming signals.
Transistors are another example of devices that can amplify or switch electrical signals and power. They are semiconductor devices, usually with at least three terminals for connection to an electronic circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the output power can be higher than the input power, a transistor can amplify a signal.
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Bipolar junction transistors
Transistors are semiconductor devices used to amplify or switch electrical signals and power. They are one of the fundamental building blocks of modern electronics. Bipolar junction transistors (BJTs) are a type of transistor that uses both electrons and electron holes as charge carriers.
BJTs consist of three differently doped semiconductor regions: the emitter region, the base region, and the collector region. These three terminals are labelled base (B), collector (C), and emitter (E). BJTs use two p–n junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several ways, such as changing the doping of the semiconductor material as it is grown or by depositing metal pellets to form alloy junctions.
BJTs have two types, PNP and NPN, based on the doping types of the three main terminal regions. In an NPN transistor, the n-type semiconductor is sandwiched between the two p-type semiconductors, while in a PNP transistor, the p-type semiconductor is sandwiched between the two n-type semiconductors. The arrow on the symbol for bipolar transistors indicates the p–n junction between the base and emitter and points in the direction in which conventional current travels.
BJTs can amplify a signal because a small current injected at one of its terminals can control a much larger current between the remaining two terminals. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is because the input impedance is low as it is connected to a forward-biased PN junction, while the output impedance is high as it is taken from a reverse-biased PN junction.
BJTs are used in a wide range of electronic devices such as mobile phones, televisions, and radio transmitters. They are also used in mixed-signal integrated circuits and specialised types are used for high voltage and high current switches, or for radio-frequency (RF) amplifiers.
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Metal–oxide–semiconductor field-effect transistors
MOSFETs have four terminals: the source, the drain, the gate, and the base. The substrate, source, and drain consist of either positive or negative doped semiconductors. The metal (or conductive silicon) gate terminal is separated from these by a non-conductive oxide layer. The voltage of the gate determines the conductivity of the device, and this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals.
MOSFETs are used to control the amount of electricity that can flow between the source and drain terminals based on the voltage applied to the gate terminal. This can be used as a switch to turn on and off another circuit or other transistors, forming the basis of digital logic. They can also be used to vary the amount of conductivity for analog circuits and act as signal amplifiers.
MOSFETs have a variety of applications, including in digital logic, integrated circuits, thin-film transistor (TFT) LCDs, and computer memory for both RAM and ROM. They are also used in biological and chemical sensors, as well as in modern DRAM and flash memory to store data.
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Vacuum tubes
The first vacuum tube, the diode or Fleming valve, was invented in 1904 by John Ambrose Fleming. It contains a heated electron-emitting cathode and an anode, allowing electrons to flow in only one direction through the device. This unidirectional property is what gives the diode its name, akin to a valve that permits a one-way flow of water.
The addition of one or more control grids within the tube creates a triode, tetrode, or pentode, allowing the current between the cathode and anode to be controlled by the voltage on the grids. This development enabled the creation of devices that could amplify and rectify electric signals. Multiple grids, such as in a heptode, allow signals applied to different electrodes to be mixed.
Despite being largely superseded by transistors, vacuum tubes continue to be valued in certain niche applications. In professional audio, they are favoured for their natural compression characteristics and graceful handling of signal overload. They also tend to distort signals in a musically pleasing manner, adding warmth and richness to the sound. High-power radio frequency applications still rely on vacuum tubes due to their ability to handle extremely high voltages and currents while operating at high frequencies.
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Frequently asked questions
A signal amplifier is an electronic device that increases the magnitude of a signal. It is a two-port electronic circuit that uses electric power to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output.
Some common signal amplifiers include voltage amplifiers (low voltage, high voltage, DC voltage, and AC voltage) and strain gauge amplifiers (bridge, full-bridge, half-bridge, and quarter-bridge). The guitar amplifier is another example, used to enhance the output from an electric guitar.
A transistor is a semiconductor device that can amplify and switch electrical signals. It controls the flow of current between its three layers: the emitter, base, and collector. Transistors are essential components in modern electronics, found in devices such as microphones, speakers, computers, and smartphones.
Transistors use a small input signal to control a much larger output signal. They can amplify a signal by allowing a larger output current or voltage proportional to the input. Transistors can also act as electrically controlled switches, determining the amount of current based on other circuit elements.
Transistors are used in microphones to amplify sound signals, enabling clear audio playback in devices like smartphones. In computers, they act as switches that represent binary digits for processing information. Transistors have enabled compact, efficient, and cost-effective designs in modern electronics.
