 # Diode Protection Circuit

## Introduction

The diode protection circuit shown in Fig. 1 is designed to protect the input of a circuit from voltages higher than the Vdd voltage rail (in this example the rail is considered 3.3V), and lower than ground. Circuits like this are commonly used in digital systems that are sensitive to over voltages, where they need the input voltage of a circuit to be kept within a specific range.

This circuit can be analyzed using the following simple model of a diode:

When the diode is reversed biased (like in Fig. 2a), the voltage at node A is higher than the voltage at node B. Essentially, this means the voltage drop across the diode does not match the diode's polarity. A reverse biased diode can be thought of as an open switch because it stops all current from flowing through. A diode that is forward biased (as shown in Fig. 2b) has voltage at Node A which is lower than node B. This means the voltage drop across the diode does match the diode's polarity. In this case the diode can be thought of as a closed switch or short which allows current to pass through.

When Vin is in state 1, the voltage of Vin is between 3.3V and 0V, as displayed in Fig. 3. Both diodes are reversed biased and act like switches in the open position. In this state the voltage across the load resistor is exactly equal to Vin, which is fine because this is our ideal operating region.