AC Amplifiers:
Altering of a voltage or current signal size as it is passed through a system is called an amplitude control.An amplifier is a circuit for the amplitude control provision. Except for early relatively inefficient electromechanical amplifiers, electronic amplifier development started with the invention of the vacuum tube.Classes of amplifiers.Amplifiers are classified according to the polarity and properties of the output current or voltage. Their characteristics cover one, two, or four quadrants on the axes plane. The ac amplifiers and dc amplifiers are distinguished. The fundamental specifications of ac amplifiers are listed on their data sheets; usually they include.- small-signal and large-signal bandwidths,
- voltage and current band noise,
- harmonic distortion level,
- input and output impedances,
- current and voltage gains.
It is common knowledge that amplifiers are divided into some general classes - A, B, C, etc., depending on the type of service in which they are to be used.
A class A amplifier is one which operates in the transistor’s active region so that the output wave shapes of current are practically the same as those of the existing input signal at all times. Fig. 2.9 illustrates the typical transfer graphs of the collector current versus the base current. For the class A amplifier (Fig. 2.9,a), if the input signal is sinusoidal, the output signal is also sinusoidal. Consequently, the low clipping is the main advantage of this mode of operation. For this reason, the amplifiers of such kind are known as linear amplifiers.Low efficiency (30 to 45%) is the main drawback of the class A amplifier.
For this reason, it is commonly used in low-power applications and preamplifiers.A class B amplifier operates with a negative bias approximately equal to cutoff. Its base voltage is more negative than in the class A amplifier. Therefore, the output current is almost zero when the alternating input signal is removed or negative (Fig. 2.9,b). With a sinusoidal signal applied, the output consists of a series of half-sine waves. A bottom part of this half-wave is distorted, and the border of this distortion is called a cutoff zone.The amplifiers of such kind are known as pulse amplifiers with high clipping. Efficiency of the class B amplifier is higher (45 to 70%) than in the class A amplifier. For this reason, they are used as the balanced output stages. More often, the intermediate class AB is selected, the clipping of which is much less.
A class C amplifier operates with a negative bias essentially less than cutoff. It passes the current during the part of the positive alternation only. The output current has narrow width and its shape distortion is maximal (Fig. 2.9,c). Its high efficiency (70 to 90%) is the primary considerations at radio frequencies higher than 20 kHz. The class C amplifiers are preferable in power amplifiers with resonance load, for example, transmitters.A class D amplifier uses transistors as switches where the only modes are switch on and switch off. It is used in different switching circuits.
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