At A, connection of two diodes to act as an ac limiter. At B, illustration of sinewave peaks cut off by the action of the diodes in an ac limiter.
In previous sections you learned that a diode will not conduct until the forward-bias voltage is at least as great as the forward breakover voltage. There’s a corollary to this: a diode will always conduct when the forward-bias voltage reaches or exceeds the forward breakover voltage, when the device is conducting current in the forward direction. In the case of silicon diodes this is approximately 0.6 V. For germanium diodes it is about 0.3 V, and for selenium diodes it is about 1 V.
This phenomenon can be used to advantage when it is necessary to limit the amplitude of a signal, as shown in above figure. By connecting two identical diodes back-to-back in parallel with the signal path (A), the maximum peak amplitude is limited, or clipped, to the forward breakover voltage of the diodes. The input and output waveforms of a clipped signal are illustrated at B. This scheme is sometimes used in radio receivers to prevent “blasting” when a strong signal comes in.
The downside of the diode limiter circuit, such as the one shown in above figure, is the fact that it introduces distortion when clipping occurs. This might not be a problem for reception of digital signals, for frequency-modulated signals, or for analog signals that rarely reach the limiting voltage. But for amplitude-modulated signals with peaks that rise past the limiting voltage, it can cause trouble.