Digital devices are constrained to two stable operating regions (usually voltage ranges) separated by a transition region through which the operating point may pass but not remain (see Fig. 7.1). Prolonged operation in the transition region does not cause harm to the devices, it simply means that the resulting outputs are unspecified. For the inverter shown in the figure, the output voltage is guaranteed to be greater than VOH as long as the input voltage is below the specified VIH . Note that the circuit is designed so that the input high voltage is lower than the output high voltage (vice versa for logic low voltages). This difference, called noise margin, permits interfering signals to corrupt the logic voltage within limits without producing erroneous operation. Logical conditions and binary numbers can be represented physically by associating one of the stable voltage ranges with one logic state or binary value and identifying the other stable voltage with the opposite state or value. By extension, then, it is possible to design electronic circuits that physically perform logical or arithmetic operations. As detailed examination of logic design is beyond the scope of this chapter, the reader is referred to any of the large number of digital logic textbooks. 1 Logic voltage levels. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9781315220482/58874d52-9fb3-42c4-9c16-306fdd7077ec/content/fig7_1.tif"/>