27.1: Theory Overview

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Typical audio amplifiers utilize one or more small signal class A stages to achieve sufficient voltage gain which then feeds a class B power stage connected to the load (normally a loudspeaker). The stage preceding the power section is referred to as the driver stage or simply the driver. The driver is often directly coupled instead of coupled via a capacitor. This maximizes gain and reduces component count.

A typical loudspeaker exhibits a nominal 8 \(\Omega\) impedance. As such, it demands considerable current. The job of the class B follower is to create a good match to this low impedance and produce sufficient current and power gain to drive it effectively. The voltage gain comes from the prior stages. If any of the amplifier stages clip the waveform, the loudspeaker will reproduce the distorted wave. This distortion can be clearly audible and produce a signal that sounds fuzzy or harsh. Loudspeakers can also be used as microphones (although the quality will not be as high as that achieved with a properly designed microphone). In this experiment, a loudspeaker will be used as a microphone to inspect the waveshapes produced by the human voice; waveshapes that are potentially far more complex than simple sine waves.