In Fig. 24-8B, the output of an overdriven class-B amplifier is shown. The wave isno longer a half sine wave, but instead, it shows evidence of flat topping. The peaks areblunted or truncated. The result of this is audio distortion in the modulation on a radiosignal, and also an excessive amount of energy at harmonic frequencies.The efficiency of a circuit can be degraded by overdrive. The “flat tops” of the dis-torted waves don’t contribute anything to the strength of the signal at the desired fre-quency. But they do cause a higher-than-normal PC or PD value, which translates into alower-than-normal efficiency Pout/PC or Pout/PD.A thorough discussion of overdrive and distortion in various amplifier classes andapplications would require an entire book. If you’re interested in more detail, a good col-lege or trade-school text on radio-frequency (RF) amplification is recommended.Audio amplificationThe circuits you’ve seen so far have been general, not application-specific. With capac-itors of several microfarads, and when biased for class A, these circuits are representa-tive of audio amplifiers. As with RF amplifiers, there isn’t room enough to go into greatdepth about audio amplifiers in this book, but a couple of important characteristics de-serve mention.Frequency responseHigh-fidelity audio amplifiers, of the kind used in music systems, must have more or lessconstant gain from 20 Hz to 20 kHz. This is a frequency range of 1000: 1. Audio amplifiers446 Amplifiers24-8At A, waveshape at the output of a properly operatingclass-B amplifier. At B, distortion in the output waveshapecaused by overdrive.