NOR gate means not an OR gate which means output of this gate is just reverse of that of a similar OR gate. We know that output an OR gate is 0 only when all inputs of OR gate is 0. But in the case of NOR gate the output is 1 only when all inputs are 0.
In all other cases, that is for all other combinations of inputs the output is 0.
Hence, the truth table of a NOR gate is shown below, It is just reverse of the AND gate
truth table which is as follows, Like OR gate, NOR gate may also have more than two inputs.
A NOR gate is also referred as universal gate. Because all binary operations can be realized by only using NOR gates.
As we know that there are only three basic operations AND, OR and NOT. Also we know that all complex binary operations can be realized by using these three basic operations. If we can prove that AND, OR and NOT operations can be realized by using only NOR gate, then we can easily say that all binary operations can be realized by using only NOR gates.
In the circuit when both A and B are given +5 V, base of the both transistors get quite a high potential to make the transistors ‘ON’. As both transistors T1 and T2 are in ON condition, supply voltage at terminal C gets path to the ground through resistor R.
Entire supply voltage ideally drops across resistor R and the output terminal of the circuit will not get any voltage and hence it will be at logical 0 state.
Practically, a transistor cannot be ideally short circuited in ON condition. There will be a voltage drop between emitter and collector of the transistor even in ON condition. So practically entire supply voltage will not drop across resistor R instead a small portion of it (0.6 V) is dropped across transistor (saturated). That is, 5 - 0.6 = 4.4 V. Hence, voltage appears at output terminal X is not practically zero instead it will be 0.6 V which is considered as logical 0.
Now, if either of inputs A and B is given with +5 V, the only corresponding transistor will be in ON condition. But in this case also supply voltage will get path to the ground through R and ON transistor and similarly the output will be in logical 0 states which is 0.6 V. Now if both of the inputs A and B are given with 0V or grounded, both transistors will be in OFF condition as in this case the base of both transistors does not have enough potential to make the transistors ON. As the supply voltage will not get nay path to the ground, the supply voltage will appear at output X, hence output is in logical 1 or high state.
Hence, in the circuit, the output is 1, only when both of the inputs are 0 and in all other conditions, the output is 0.