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Amplifier circuits
are classified under different classes, which include the following:
classes A, B, AB and C for
analog designs, and classes D and E/F for switching designs.
Below are brief descriptions of these amplifier classes.
Class A -
100% of the input signal is used (conduction angle a = 360° or
2π)
Class A amplifiers amplify over the
entire input cycle such
that the output signal is an exact magnified copy of the input. They
are not efficient (no more than 50% efficiency is attainable), since the
amplifying device is always conducting whether or not an input
signal is applied.
Class B -
50% of the input signal is used (a = 180° or π)
A Class B
amplifier is one whose operating point is at an extreme end of its
characteristic, so that either the quiescent current or the
quiescent voltage is almost zero. If a sinusoidal input
voltage is used, the amplification of a Class B amplifier takes
place only for 50% of the cycle,
e.g., the amplifying device is switched off half of the time.
A Class B amplifier can attain an efficiency of up to 78.5%.
However, a Class B amp exhibits a higher distortion than an
equivalent Class A amp.
Class AB -
more than 50% but less than 100% is used. (181° to 359°, π < a
< 2π)
A Class AB amplifier is an amplifier that operates between the two
extremes defined for Class A and B amplifiers.
Class C -
less
than 50% is used (0° to 179°, a < π)
Class C amplifiers conduct less than 50% of the input signal,
allowing it to reach 90% efficiency but resulting in high distortion at the output.
Thus, in a Class C amp, the output current (or voltage) is zero for
more than 50% of the input waveform cycle. Some applications, such
as megaphones, can tolerate the high distortion of Class C amps.
Class C amps can also be used in tuned RF applications, since the
distortion can be significantly reduced by the tuned loads.
Class D
A class D amplifier is a power amplifier
whose power devices
are operated in on/off mode. The input signal is converted into a sequence
of pulses whose average is directly proportional to the
amplitude of the input signal. The frequency of the pulses is
typically ten or more times the highest frequency of interest in the
input signal. The output of the amplifier goes through a passive
filter to remove unwanted spectral components, resulting in an
amplified replica of the input. Power efficiency is the main advantage of a class D amplifier.
Class D amplifiers were widely used to control motors,
but they are also used
as audio amplifiers.
Class E/F
Class E/F amplifiers
are highly efficient switching power
amplifiers used at radio frequencies. Class E/F amplifiers consist
of two basic parts: 1) a 'perfect' switching device and 2) an
impedance network consisting of resistive and reactive components.
The switching device is 'on' during the zero-voltage crossing,
and off during the zero-current crossing, such that it can not have
both current flowing through it and a non-zero voltage across it at
the same time, thereby minimizing its power dissipation. The
impedance network, on the other hand, is set up such that the
'imaginary part' of the impedance is eliminated through proper
matching of complex conjugates to attain resonance, leaving behind
only its 'real part.' Thus, a Class E/F amp is very efficient
because power loss only occurs in the real part (resistive
component) of the impedance network. Classes E and F are
distinguished from each other by their resonance topology.
There are
several other classes of amplifiers not discussed in this article.
See Also:
What is an Amplifier;
Types of Amplifiers;
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