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Author: N.H. Crowhurst

A square wave at 4000 cycles with an amplifier response shown at the left will come out like the curve shown at the right.

Earlier we briefly discussed the problem of transient distortion. The usual cause of distortion to square waves is the way in which amplification varies at high frequencies. If the amplifier's frequency response rolls off slowly, the corners of the square wave will be rounded. If, however, the response is uniform up to the highest frequency in which we are interested, say 10,000 or 20,000 cycles, and then at some frequency after that rises up to a peak, ringing occurs. The sudden shock given by the corner of a square wave excites this peak or resonance in the frequency response and the square wave loses its square corner, developing an oscillatory waveform every time this arrives.

It is not necessary to have a detectable peak at the high-frequency end of the response to get a similar effect. If the range of frequencies over which uniform response is maintained is extended by offsetting the high-frequency loss by peaking, the overall response curve appears quite flat, and then drops off sharply, without showing a peak. The fact that a peaking circuit has been used to extend the response produces the same effect as a peaked overall response curve - every time the corner of the square wave hits it, ringing results. For this reason, an amplifier with a sharp rolloff also produces ringing on square waves. This is true whether the sharp rolloff is produced by this kind of synthesis or, even more important, by feedback adjustments.

Ringing can cause other troubles in an amplifier in addition to unwanted oscillation on the corners of a square wave. For example, the oscillation may cause grid current to flow, when otherwise the amplifier would be well within its safe limit, and this, in turn, can initiate other troubles.

The original response shown in the diagram at the left can be corrected by using a peaking compensation. The peak creates makes a square wave to generate ringing as shown at the right.

Last Update: 2010-11-03