Operational amplifiers and 555 timers are classified as linear integrated circuits that find application in switching circuits. A positive resistance is one in which the current is proportional to the applied voltage. The current increases linearly with increasing voltage. There is another class of devices called the negative resistance devices. In this class of devices, the current decreases with increasing voltage in certain regions of their characteristic. Therefore, these devices are said to exhibit negative resistance characteristic. Negative resistance devices find extensive use in switching applications. Some of the negative resistance devices that find application in the relaxation circuits are tunnel diodes, p–n–p–n devices and UJT. Multivibrators and sweep circuits that employ these devices are discussed in the chapter.