The clipper circuit is commonly used for clip the input signal.As figure shows,for positive clipper, diode connected in reverse bias.So during pisitive half cycle, diode will be in reverse bias and so current will not flowing.so positive part of the signal will be clipped by diode.but if you want some reference voltage at signal clips then you can add dc voltage supply in the series with diode as shown in figure.
For negative clipper you can change the doide's position and get the nagative part of the input signal will be clipped by the diode.
