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An
electronic amplifier is a device
that magnifies or increases the voltage, current, or power of a
signal. An amplifier accomplishes this by taking additional
power from a power supply, and producing an output signal that is an
exact copy of the input signal, but of a higher amplitude. The
ratio of the output signal to the input signal is referred to as the
'gain' (G)
of the amplifier. Thus, an amplifier that outputs a voltage signal
Vout that is a magnified copy of the input voltage signal Vin has a
gain of G, wherein G = Vout / Vin.
An amplifier can be designed to
magnify the voltage of a signal (voltage
amp), the current of a signal (buffer amp), or both the voltage and
current of a signal (power amp). Electronic amplifiers can operate
using either
a single-sided power supply (a voltage “rail” or “bus” that's either
positive or negative) or a double-sided or balanced power supply,
which has both a positive and a negative supply rail aside from the ground.
If the
output waveform of an amplifier is not a perfect copy of the input
signal, then the amplifier is said to exhibit
distortion.
One type of distortion is linear distortion, which causes an output
signal to have a shape that's different from that of the input
signal. Good design of the amplifier circuit, proper selection
of circuit components, and correct biasing (which is discussed in
the next paragraph), will minimize distortion.
An
electronic amplifier needs what is known as an electrical 'bias' in
order to function. The
bias
of an amplifier is the method by which its active devices (usually
transistors) are powered up and excited in order to attain the
desired amount of gain with minimum distortion. This usually
entails setting the DC component of the output signal midway between
the maximum voltages available from the power supply.
It is
common to see amplifiers that consist of
multiple stages
connected in series to attain higher gains. Each stage of the
amplifier may be a different type of amplifier to meet the
requirements of each stage. For instance, the first stage might be a
Class A stage, the output of which is fed into a class AB push-pull
second stage, which then drives a class G final output stage. This
design takes advantage of the strengths of each amplifier class at
each stage while minimizing weaknesses. Refer to the
definitions of the different classes
of amplifiers.
An
amplifier's output signal may be of different phase or polarity as
the input signal.
A
non-inverting
amplifier maintains equal phase relationship or polarity between the
input and output waveforms.
An
emitter follower is a type of this amplifier, indicating that
the signal at the emitter of a transistor follows the phase of the input signal.
An
inverting amplifier
produces an output that is of opposite polarity or 180 degrees
out-of-phase with the input signal.
There are
many different ways to classify
amplifiers. Amplifier classifications include the
following: 1) by function; 2) by frequency range; 3) by common
terminal; 4) by type of load; 5) by coupling, etc.
See Also:
Types of Amplifiers;
Amplifier Classes;
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