Amplifiers and Oscillators

Abstract

The last two chapters showed how transistors—bipolar and FET—work as amplifying devices, using a small current or voltage to control a much larger current. The application to a machine like a record-player, for example, is an obvious one, for the small electric signal produced by the player’s pick-up cartridge must be amplified to a sufficiently large extent to drive a speaker. A system diagram of a record-player amplifier looks quite simple (see Figure 10.1). The symbols for the cartridge, amplifier and speaker are those conventionally used. In practice the amplifier is rather complex, and breaks down into two main sections: the preamplifier, which deals with the amplification of the small signal from the cartridge; and the power amplifier, which deals with the high-power amplification necessary to drive the speaker. Audio amplifiers are covered in more detail in Chapter 13, but in this chapter we shall look at typical techniques of small-signal amplification.