OR gate performs logical OR operation which means outputs is logical 1 if at least one of the inputs is 1. Just like AND gate an OR gate may also have two or any more numbers of inputs but only one output. Only if all of the inputs are only in low state or logical 0 the output is low or 0 and in all other inputs conditions the output will be high or logical 1.
The logical symbol of OR gate is shown below,
Truth Table of OR Gate
From above explanation the truth table of logical OR gate can be represented as,
Logical OR gate can be of more than two inputs as we told earlier. In that case logical symbol and truth table would be,
Realization of OR Gate
Diode OR Gate
A simple two inputs OR gate can be realized by using diode as follows,
In the above circuit, if A and B are applied with 0V, there will be no voltage appears at X. When any of the inputs is given with +5V, the respective diode becomes forward biased and behaves as ideally short circuited hence this +5 V will appear at output X. +5 V means logical 1. Actually entire 5V will not appear at X, around 0.6 to 0.7 V will drop across the diode as forward bios voltage and rest of the voltage i.e. 5 – 0.6 = 4.4 V or 5 – 0.7 = 4.3 V will appear at X. This 4.4 V or 4.3 V is practically considered as logical 1.
Now if both of the inputs are given with +5 V, both diodes will be forward biased. Hence, similarly 4.4 V will appear at X.
Now if both of the inputs A and B are grounded or given 0V, There will be no voltage appears at X and hence X is considered as logical 0.
Transistor OR Gate
The OR gate can also be realized by using transistor. In this case the OR gate is referred as transistor OR gate. Two inputs such OR gate is shown below,
Now if A and B both are given with 0V, both of the transistor are in OFF condition, hence supply voltage + 5 V will not get path to the ground through either of the transistors, T1 and T2. As a result base of the transistor T3 will get enough potential to make it ON. In that condition supply + 5 V will get path to ground through resistor R′ and transistor T3. As the transistor T3 is in ON condition it will behaves as ideally short circuited, hence entire supply voltage + 5 V will drop across resistor Rʹ and X terminal (Node) will get 0V. In practice, transistor T3 will not be ideal short circuited it will have some voltage drop across it which will be around 0.6 – 0.7 V. This voltage will appear at node X and this 0.6 or 0.7 volt is considered as logical 0.
Now, if base terminal either of the transistors T1 or T2 or both are given with + 5 V, the respective transistor as both will be in ON condition. In that case supply voltage + 5 V will get path to ground through either of the transistors or both.
As a result current starts flowing to the ground from supply through this path, and entire supply voltage will drop across resistor R. So, the base of transistor T3 will not get sufficient potential to make the transistor T3 ON. Hence entire supply voltage will appear at X and the X becomes at high logical state or logical 1.
OR Gate ICs
OR gate is available in TTL and CMOS ICS, Where TTL is Transistor – Transistor Logic ICs and CMOS is Complementary MOSFET ICS. We will now discuss on the arrange of OR gate on both the ICs. In CMOS ICS. 4000 series is available in digital electronic 4071 is the CMOS IC number of inputs in each gate is two. 4075 is 3 – input OR gate and 4072 is 4 – input OR gate in CMOS ICs. A internal gate picture of 4071 can make you understand about this IC.
4071 is a 14 pin 1C as you can see where four or gates are fixed together having two inputs. Pin number 14 is tvcc where maximum SV DC supply is given which activates the IC. Pin number 7 is grounded. Pin 1 and 2 is the input for the first gate and 3 is the respective output for the first gate. Pin number 5 and 6 are inputs for the second gate whose output is pin number 4. For the third gate pin number 8 and 9 are the inputs whose output is pin number 10 for the last gate pin number 12 and 13 are the inputs whose respective output is at pin number 11. Thus how the IC looks internally for CMOS.
In TTL logic the internal arrangement differs from that incase of CMOS logics. In TTL logic 2- input OR gate IC number is 74LS32. popularly known as 7432. LS stands for low power schottky version. Again one more type of IC number is also available in market which is 74HC32 where HC stands for high speed CMOS version i.e. it has lower current consumption and wider voltage range. 74LVC32 is another low voltage CMOS version of the same. Let us have discussion on 74LS32 which is a TTL IC. In this IC there are fourteen pin. The internal gate diagram of 7432 can make your idea clear.
In this IC pin 1 and 2 are the inputs of first gate where the output is from pin number 3. Again pin number 4 and 5 are the inputs of second gate whose output is in pin 6. Pin 10 and 9 are the inputs of fourth gate whose output is at pn 8. The input of the last gate or fourth gate is pin 13 and 12 and pin 11 is its output. Pin 7 is ground and pin 14 is +vcc supply where again +Vcc supply where again +SVdC is the normal and maximum supply. One thing must be maintain at +SVDC. If the i/p voltage would be more than this it may cause damage to the IC.