Pull_Up & Down Resistor

When the switch is open, the voltage of the gate input is pulled up to the level of the input voltage. When the switch is closed, the input voltage at the gate goes directly to the ground. You need to use a pull-up resistor when you have a low default impedance state and wish to pull the signal to ‘high’. When the switch is open, the voltage of the gate input is pulled down to the level of the ground. When the switch is closed, the input voltage at the gate goes to Vin. Without the resistor, the voltage levels would virtually float between the two voltages.