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 off (0). When switch is off (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 off 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.
