Designing NOT gate using Transistor

A NOT gate has a single input and a single output. The definition of NOT gate states that if the input is 1 then the output is 0 and vice versa. The structure of the NOT gate is shown below.

The truth table of NOT gate is shown below.

Now to understand the working of NOT gate, you must understand how the transistor works as a switch. The transistor is shown in the figure below. A transistor is a 3 terminal device. The terminals are named as collector, base, and emitter as labeled below. To simply put, collector and emitter are the two ends of the switch. The closing and opening of this switch are controlled by supplying a small current ib to the base. When the base current (ib) is supplied to the transistor, the acts as a closed switch and the current flows from collector to emitter. When there is no base current, the transistor acts as an open circuit.

Having understood the working of the transistor as a switch let us understand the circuit diagram of NOT gate which is shown below.

Construction:- Vin is the input voltage. R2 is the base resistance. R1 is collector resistance. Q1 is and PNP transistor. VCC is a supply voltage of 5 volts. Vout is the output voltage.

Working:-
Case 1:- Vin = 0 V
In this case, as the input voltage is 0, there is no base current supplied to the transistor. thus the transistor acts as an open switch. Therefore, the path for current to flow from collector to emitter is blocked out. The only path left for the current flow is through Vout. Hence the output voltage is HIGH(5 V).

Case 2:- Vin = 5 V
In this case, since the input voltage is 5 v, the base current is supplied to the transistor. Thus the transistor acts as a closed switch and current flows from collector to the emitter to the ground. Therefore Vout is LOW(0 V).

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