Amplifiers

1. We are finally ready to apply the knowledge we gained in previous sections about electrons, vacuum tubes and tran­sistors to some practical matters.¹ In the following sections 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 must be familiar with the circuits that make them up.

2. In this section 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. As a matter of fact the ratio of the output voltage to the input voltage is called the amplification or gain of the amplifier.

3. 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.

4. When a number of amplifiers are hooked up in series so that the output of one serves as the input to the next am­plifier stage, the function of the early stages is usually to build up the voltage to a high level, while the last stage builds up the power to a level sufficient to operate an output device. Audio amplifiers amplify electrical ac signals that have a frequency range corresponding to the range of human hearing, or from about 20 to 15,000 cycles per second.