at one frequency is different than it is at another frequency, the output will be distorted, in the sense that it will not have the same shape as the input waveform. This alteration in wave shape is called amplitude distortion. Figure 10-3 shows the distortion that results when a triangular waveform is passed through an amplifier having an inadequate frequency response. In this example, the high-frequency components in the waveform fall beyond the upper cutoff frequency of the amplifier, so they arc not amplified by the same amount as low-frequency components. It can be seen that knowledge of the frequency response of an amplifier is important in determining whether it will distort a signal having known frequency components. The bandwidth must cover the entire range of frequency components in the signal if distortion amplification is to be achieved. In general. “jagged” wave-forms and signals having abrupt changes in amplitude, such as square waves and pulses, contain very broad ranges of frequencies and require wide-bandwidth amplifiers. An amplifier will also distort a signal if it causes components having different frequencies to he shifted by different times. For example, if an amplifier shifts one component by I ms, it must shift every component by 1 ms. This means that the phase shift at each frequency must be proportional to frequency. Distortion caused by failure to shift phase in this way is called phase distortion. In most amplifiers, phase distortion occurs at the same frequencies where amplitude distortion occurs, because phase shifts are disproportional to frequency outside the mid-band range.
Amplitude and phase distortion should be contrasted with nonlinear distortion, which we discussed in an earlier chapter in connection with the nonuniform spacing of characteristic curves. Nonlinear distortion results when an amplifier’s gain depends on signal amplitude rather than frequency. The effect of nonlinear distortion
is to create frequency components in the output that were not present in the input signal. These new components are integer multiples of the frequency components in the input and are called harmonic frequencies. For example, if the input signal were a pure l-kHz sine wave, the output would be said to contain third and fifth
harmonics if it contained 3-kHz and 5-kHz components in addition to the fundamental. Such distortion is often called harmonic distortion.