NOT Gate:
This is the most basic gate, with one input and one output. It produces T output if the input is ‘0’ and vice-versa. That is it produces an inverted version of the input at its output. This is why it is also known as an inverter.
The Boolean expression is given by; Y = (\overline{A})
(\overline{A}) means NOT A meaning which is not A. (∴ A = 0, Y = (\overline{0}) = 1) and (A = 1, Y = (\overline{1}) = 0) . The truth table is given as follows
| A | Y –(\overline{A}) |
| --- | --- |
| 0 | 1 |
| 1 | 0 |
The sign for NOT gate is shown in the figure. It is also represented by an electrical circuit. When switch is on (1) then Y (bulb) is (0). When switch is (0) then Y is lighted (1). Figure shows NOT gate sign.
For NOT gate transistor is used. A simple circuit is shown in the figure, in which an NPN transistor is used. The base B of the transistor is connected with the terminal A. Emitter E is earthed and collector C is connected to the positive end of direct current VCC = (5 V). Y is the potential difference of collector C relative to potential difference.
When, input terminal A is earthed then VA = 0 is also earthed. In this the base current is zero, when emitter current is zero the collector current is also zero, whereas the transistor is in cut *** state.
Now if relative to earth + 5 V voltage is used (meaning A – 1) then base-emitter junction is forward biased. Now base current, emitter current and collector current all are present. If the values of RVB and RVC are chosen in such a way the collector current is high then transistor is in saturation state. In this state, the potential drop is + 5 V and Y = 0.
