NOT Gate or Inverter

NOT gate (Inverter)
The NOT gate is a gate with only one input and one output. It is so called, because its output is complement to the input. It is also known as inverter. Animation shows the logic symbol for NOT gate. The Boolean expression to represent NOT operation is Y = A.
The NOT gate can be thought of like an electrical circuit as shown in animation. When switch A is closed, input is high and the bulb will not glow (i.e) the output is low and vice versa.
Here a transistor in CE mode, which is used as NOT gate.
When the input A is high, the transistor is driven into saturation and hence the output Y is low. If A is low, the transistor is in cutoff and hence the output Y is high. Hence, it is seen that whenever input is high, the output is low and vice versa. The operation of NOT gate is shown in Table.