NEET Physics Notes Modern Physics-Electronic Devices-Logic Gates
Logic Gates
A logic gate is a digital electronic circuit which follows a logical relationship, between its input and output.
Three Basic Logic Gates
On the basis of types of operations, i.e. OR, AND, NOT etc, there are three basic logic gates.
These gates with their detailed descriptions are given below:
OR Gate
It has two inputs, one output.
Logic symbol of OR gate is shown in fig.(a). Its Boolean expression is Y = A + B (being read as Y equals A OR B).
An OR gate can be realised by using two p-n junction diodes as shown in fig. (b).
AND Gate
It also has two inputs, one output.
Boolean expression of AND gate is Y = A.B (being read as Y equals A AND B).
Truth table of AND gate
Logic symbol of AND gate is shown in fig. (a) and the actual circuit arrangement used to realise AND gate in practice has been shown in fig. (b).
NOT Gate
It has an one input and one output.
Boolean expression of NOT gate 
(being read as NOT A equals Y).
Truth table of NOT gate
The logic symbol and actual circuit for realisation of NOT gate have been shown in above figure.
Combination of Logic Gates
Some common combinations obtained from three fundamental gates OR, AND and NOT are as follow
NAND Gate
If the output of AND gate is joined to input of a NOT gate, then the combination is a NAND gate.
Boolean expression of NAND gate is 
Its logic symbol is shown in above figure.
If both the inputs of a NAND gate are joined together, it will behave as a NOT gate as shown in figure. Two/Three NAND gates may be joined, so as to have the same effect as a single AND gate/OR gate.
NOR Gate
If the output of OR gate is joined to input of a NOT gate, then the combination is a NOR gate.
Boolean expression of NOR gate is
. Its logic symbol is shown in figure.
If both the inputs of a NOR gate are joined together, it will behave as a NOT gate as shown in figure.
Two/ Three NOR gates may be combined, so as have the same effect as a single OR/AND gate.
XOR Gate
The logic gate, which gives high output (1) when either input A or input B, but not both of them, are high is called exclusive OR gate, the XOR gate. If both inputs A and B are either high (1) or low (0), then the output is zero.
XOR gate can be obtained by having a combination of 2 NOT gates, 2 AND gates and 1 OR gate as shown in fig. (a). The logic symbol is shown in fig. (b).
Boolean expression of XOR gate, is
Transistor as a Switch
Transistors are commonly used as electronic switches, both for high power applications, such as switched mode power supplies, and for low power applications, such as logic gates.
In a grounded emitter transistor circuit, such as the light switch circuit shown, as the base voltage rises the emitter and collector currents rise exponentially. The collector voltage drops because of the collector load resistance. If the collector voltage were zero, the collector current would be limited only by the light bulb resistance and supply voltage.
The transistor is then, said to be saturated. It will have a very small voltage from collector to emitter. The transistor provides current gain, allowing a relatively large current in the collector to be switched by a much smaller current into the base terminal. The ratio of these currents varies depending on the type of transistor and even for a particular type, varies depending on the collector current.
In any switching circuit, values of input voltage would be chosen, such that the output is either completely OFF or completely ON. The transistor is acting as a switch, and this type of operation is common in digital circuits.