Amplifiers
We are ready to apply the knowledge we gained about electrons, vacuum tubes, and transistors to some practical matters. We shall consider a variety of circuits employing electron tubes and transistors. Circuits are combinations of tubes or transistors with other components, such as resistors, capacitors and inductors, and form the basic building blocks of electronic systems: radio, automatic computer and so on. To understand the systems, you should be familiar with the circuits that make them up.
We shall discuss amplifier circuits, or more specifically, audio amplifiers. An amplifier is an electron tube or transistor circuit, which builds up an ac signal applied to its input. It is called a voltage amplifier if the magnitude of the output voltage from the amplifier is considerably greater than that of the input voltage. In fact, the ratio of the output voltage to the input voltage is called amplification or gain of the amplifier.
There are also so-called power amplifiers. These are similar to voltage amplifiers, except that their main purpose is to supply a considerable amount of power i.e. voltage times current to the output or load circuit, although the ac. input signal may not draw any grid current and, hence, the input power may be zero.
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