Amplifier
An electronic
amplifier, amplifier, or (informally) amp is an
electronic device that increases the power of
asignal.
It does this by taking energy from a power
supply and
controlling the output to match the input signal shape but with a
larger amplitude.
In this sense, an amplifier modulates the output of the power supply.
Numerous types of electronic
amplifiers are specialized to various applications. An amplifier can
refer to anything from an electrical
circuit that
uses a single active
component,
to a complete system such as a packaged audio hi-fi amplifier.
Amplifiers are described
according to their input and output properties. They have some kind
of gain,
or multiplication factor that relates the magnitude of the output
signal to the input signal. The gain may be specified as the ratio of
output voltage to
input voltage (voltage
gain),
output power to input power (power
gain),
or some combination of current, voltage, and power. In many cases,
with input and output in the same unit, gain is unitless (though
often expressed in decibels).
For others this is not necessarily so. For example,
a transconductance
amplifier has
a gain with units of conductance (output
current per input voltage). The power gain of an amplifier depends on
the source and load impedances used
as well as its voltage gain; while an RF amplifier
may have its impedances optimized for power transfer, audio and
instrumentation amplifiers are normally employed with amplifier input
and output impedances optimized for least loading and highest
quality. So an amplifier that is said to have a gain of 20 dB
might have a voltage gain of ten times and an available power gain of
much more than 20 dB (100 times power ratio), yet be delivering
a much lower power gain if, for example, the input is a 600 ohm
microphone and the output is a 47 kilohm power amplifier's input
socket.
In most cases an amplifier
should be linear; that is, the gain should be constant for any
combination of input and output signal. If the gain is not constant,
e.g., by clipping the output signal at the limits of its
capabilities, the output signal is distorted. There are however cases
where variable
gain is
useful.
There are many types of
electronic amplifiers, commonly used
in radio and television transmitters and receivers, high-fidelity ("hi-fi")
stereo equipment, microcomputers and other electronic digital
equipment, and guitar and
other instrument
amplifiers.
Critical components include active
devices,
such as vacuum
tubes or transistors.
A brief introduction to the many types of electronic amplifier
follows.
Transistor
A transistor is
a semiconductor
device used
to amplify and switch electronic signals
and electrical
power.
It is composed of semiconductor material
with at least three terminals for connection to an external circuit.
A voltage or current applied
to one pair of the transistor's terminals changes the current through
another pair of terminals. Because the controlled (output) power can
be higher than the controlling (input) power, a transistor
can amplify a
signal. Today, some transistors are packaged individually, but many
more are found embedded in integrated
circuits.
The transistor is the
fundamental building block of modern electronic
devices,
and is ubiquitous in modern electronic systems. Following its
development in the early 1950s, the transistor revolutionized the
field of electronics, and paved the way for smaller and
cheaper radios, calculators,
and computers,
among other things.
