Generally, an amplifier or simply amp, is any device that changes, usually increases, the amplitude of a signal.
An Amplifier receives a signal from some pickup transducer or other input source and provides a
larger version of the signal to some output device or to another amplifier stage. An input transducer
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-handling device). In small signal amplifiers, the main factors are usually amplification linearity and
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount
of power-handling capacity and power efficiency are of little concern. A voltage amplifier provides
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief
description of amplifier classes is provided next.
Class A:The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and
down without going to a high-enough voltage to be limited by the supply voltage level or too low to
approach the lower supply level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide
output on the positive output half-cycle and another to provide operation on the negative-output half-cycle are necessary. The combined half-cycles then provide an output for a full 360° ofoperation.
This type of connection is referred to as push-pull operation, which is discussed later in this chapter.
Note that class B operation by itself creates a very distorted output signal since reproduction of the
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually
closer to the zero base current level for better power efficiency, as described shortly. For class AB
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 of the cycle and
will operate only with a tuned (resonant) circuit, which provides a full cycle of operation for the tuned
or resonant frequency. This operating class is therefore used in special areas of tuned circuits, such
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain
a signal that varies over the full cycle (usingsample-and-hold circuitry) to recreate the output from
many pieces of input signal. The major advantage of class D operation is that the amplifier is on
(using power) only for short intervals and the overall efficiency can practically be very high, as
described next.
An Amplifier receives a signal from some pickup transducer or other input source and provides a
larger version of the signal to some output device or to another amplifier stage. An input transducer
signal is generally small (a few millivolts from a cassette or CD input or a few microvolts from an
antenna) and needs to be amplified sufficiently to operate an output device (speaker or other power-handling device). In small signal amplifiers, the main factors are usually amplification linearity and
magnitude of gain , since signal voltage and current are small in a small-signal amplifier, the amount
of power-handling capacity and power efficiency are of little concern. A voltage amplifier provides
voltage amplification primarily to increase the voltage of the input signal. Large-signal or power
amplifiers, on the other hand, primarily provide sufficient power to an output load to drive a speaker or
other power device, typically a few watts to tens of watts. In the present chapter, we concentrate on
those amplifier circuits used to handle large-voltage signals at moderate to high current levels. The
main features of a large-signal amplifier are the circuit's power efficiency, the maximum amount of
power that the circuit is capable of handling, and the impedance matching to the output device.
One method used to categorize amplifiers is by class. Basically, amplifier classes represent the
amount the output signal varies over one cycle of operation for a full cycle of input signal. A brief
description of amplifier classes is provided next.
Class A:The output signal varies for full 360° of the cycle. Figure 15.1 a shows that this requires the
Fig 15.1 Amplifier operation classes
Q-point to be biased at a level so that at least half the signal swing of the output may vary up and
down without going to a high-enough voltage to be limited by the supply voltage level or too low to
approach the lower supply level, or 0 V in this description
Class B: A class B circuit provides an output signal varying over one-half '.-: input signal cycle, or for
180° of signal, as shown in Fig. 15.1 b. The dc bias point for class B is therefore at 0 V, with the
output then varying from this bias point for a half-cycle. Obviously, the output is not a faithful
reproduction of the input if only one half-cycle is present. Two class B operations-one to provide
output on the positive output half-cycle and another to provide operation on the negative-output half-cycle are necessary. The combined half-cycles then provide an output for a full 360° ofoperation.
This type of connection is referred to as push-pull operation, which is discussed later in this chapter.
Note that class B operation by itself creates a very distorted output signal since reproduction of the
input takes place for only 180° of the output signal swing.
Class AB: An amplifier may be biased at a dc level above the zero base current level of class B and
above one-half the supply voltage level of class A; this bias condition is class AB. Class AB operation
still requires a push-pull connection to achieve a full output cycle, but the dc bias level is usually
closer to the zero base current level for better power efficiency, as described shortly. For class AB
operation, the output signal swing occurs between 1800 and 3600 and is neither class A nor class B
operation.
Class C: The output of a class C amplifier is biased for operation at Iess than 180 of the cycle and
will operate only with a tuned (resonant) circuit, which provides a full cycle of operation for the tuned
or resonant frequency. This operating class is therefore used in special areas of tuned circuits, such
as radio or communication.
Class D: This operating class is a form of amplifier operation using pulse (digital) signals, which are
on for a short interval and off for a longer interval. Using digital techniques makes it possible to obtain
a signal that varies over the full cycle (usingsample-and-hold circuitry) to recreate the output from
many pieces of input signal. The major advantage of class D operation is that the amplifier is on
(using power) only for short intervals and the overall efficiency can practically be very high, as
described next.
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