Positive Clippers
The shunt connected positive clipper, in which the diode conducts for positive half cycle and hence, the output is a straight line. During the negative half cycle the diode does not conduct and hence the input appears as the output.
Negative Clippers
The shunt connected negative clipper, in which the diode does not conduct for the positive half cycle and hence, the output is same as the input. During the negative half cycle, the diode conducts so the output is a straight line.