Explain the operation of a NOT gate and give its truth table.

NOT gate : A NOT gate is a logic gate with one input and one output. It gives output high
(1) if the input is low (0) and vice versa.

Operating of NOT gate :
i) When input A= 0 : When A (the base of the transistor) is connected to ground, the base of transistor Q₁ becomes negative. This makes base-emitter junction reverse biased so that the base current and hence collector current is Zero. Therefore output voltage is +V volts. Hence the output is high i.e., y=1
ii) When input A=1 : If sufficiently large positive voltage is applied at A, the Base- Emitter junction and collector - Base junction become forward biased and transistor comes in the saturation mode under this condition maximum collector current flows and output voltage becomes Zero. Hence output is low i.e., y=0