NOT gate is a logical gate which only inverts the input digital signal. This is why, a NOT gate sometimes is referred as inverter. A NOT gate always have high or logical 1 output when its input is low or logical 0. On the other hand a logical NOT gate always have low or logical 0 output when input is high or logical 1. The logical symbol of a NOT gate is shown below,
If the input binary variable of a NOT gate is considered as A, then the output binary variable of the gate will be Ā. As the symbol of not operation is ( - ) bar. If the value of A is 1. then Ā = 0 and in opposite if the value of A is then Ā = 1. The truth table of a NOT gate hence can be represented as,
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Realization of NOT GateA NOT gate can easily be realized by using a simple bipolar transistor. The circuit of a NOT gate or transistor inverter is shown below,
Let us examine the above simple circuit by applying high input variable, i.e. +5V. At that condition the transistor T gets enough base potential to make the transistor T ‘ON’.
As soon as the transistor becomes ON, the supply voltage (+5V) at B will get a path to the earth through the resistor R. At ON condition the transistor will behave short circuited ideally, hence entire supply voltage will drop across resistor R and no voltage will appear at X and hence the output of the inverter or NOT gate will be zero. In actual case, there will be some voltage drop across collector and emitter even at ON condition, of transistor. This collector-emitter voltage is about 0.6V. So, at the above said input condition, entire supply voltage +5V will not drop across resistor instead it will be 5 – 0.6 = 4.4V. So, 0.6V is practically considered as logical zero or low. Now let us examine the condition, Where, input A = 0V i.e. base terminal of the transistor is given with 0V or grounded. At that condition, as the base of the transistor is at 0 potential, the transistor T will be in OFF condition and hence, the supply voltage will not get any path to the earth and entire supply voltage will appear at output terminal of the NOT gate high or logical 1, when input terminal A is low or logical zero.