Testing Amplifiers with Square Waves
Author: J.B. Hoag
The human ear distinguishes one sound from another by the pitch and intensity not only of the fundamental, but also of the various overtones, i.e., by the shape of the composite wave-form. Although a plot of the db. gain over the audio-frequency range serves as some indication of the faithfulness of an a.f. amplifier, it does not tell the whole story. The phase shift, as will be recalled, of an R-C coupled amplifier is approximately 180° per stage. But it is not exactly 180°, nor exactly the same for all frequencies. Since some frequencies are shifted differently from others, the wave-form is not retained, even though the amplitude response is flat over the entire range. Square waves, which are rich in harmonics, offer a simple and rapid means of testing both the amplitude response and the phase shift in a single test. If a square wave is sent into the amplifier and a square wave comes out (as seen on a cathode-ray tube, for example), the wave-form has been preserved and the amplifier is a good one. It is to be noted, in choosing a suitable fundamental frequency for the test, that square waves contain only the odd harmonics, f, 3f, 5f, etc., and not the even harmonics, 2f, 4f, etc.