Some systems require a voltage limiter to protect the input circuit. Designers can connect diodes (Fig. 1) to VCC and Gnd to create a simple solution. This simplicity, however, comes at a disadvantage: the input circuit can see voltages within a range of Gnd – VDup to VCC + VD.
If the circuit is sensitive and can handle only voltages between Gnd and VCC, the circuit in Figure 2 is a better alternative. The improved limiter includes circuitry to generate two voltage power supplies:
VP = VCC – VD
which can sink current, and:
VN = Gnd + VD
which can source current.
The circuit assumes that VD = VBE. In this case, the protection diodes at the input will clamp the input voltage at no more than VCC and no less than Gnd.