The purpose of a diode clipper is to clip off a portion of the input signal voltage above or below a selected voltage level. A diode clipper is sometimes referred to as a diode limiter.
The series diode clipper, shown in Figure 9-1, clips off the output at zero volts during the negative part of the input cycle because the diode is cut-off (reverse-biased). The output is not cut-off during the positive part of the input cycle because the diode is forward-biased. The peak output voltage slightly less than the peak input voltage during the positive part of the input cycle because of the diode forward-voltage drop (≈0.7V). Reversing the diode would allow only the negative part of the signal voltage to appear at the output.