OR Gate

Output of an OR gate is devised by logical disjunction (v)-mathematical logic. The output of an OR gate attains “OFF” signal only if both of the inputs of a two input OR basic gate are “OFF”. We will mathematically represent the output of an or gate Y as ; Y=A+B. In the language of sets you can say that it is like union of the two inputs A and B i.e. the output of an OR gate is “ON” if any of the two inputs are is “ON” and also if both of the inputs are “ON”. There are two symbols for OR gate. These are American symbols and IEC symbols. There is also a symbol less used in the profession which is the DIN symbol. We can device an OR gate using a transistor through TTL. TTL stands for transistor transistor logic. There is a vast application of OR gate as CMOS. We can device an OR gate out of the CMOS.

TTL(TRANSISTOR-TRANSISTOR LOGIC):-

We are going to reach the output of an OR gate using 3 transistors. We know that transistors can be of two types based on the order of the type of semiconductor used. They are;

1. n-p-n transistor
2. p-n-p transistor

For this purpose we are going to 3 n-p-n transistors. We know that there are three terminals in a transistor. They are base, collector and emitter. Their voltage and orientation are very crucial if you want to take output of an OR gate i.e. Y=A+B using transistors.

But before that, let me introduce a concept which is working regions of a transistor. A transistor can work in two regions:

1. Saturation region:- Where C-E path becomes a closed switch if base voltage is high.
2. Cut-off region:- Where C-E path becomes open switch if base voltage is low.

Now let us see how we are going to device output of an OR gate. It involves the following three steps:-

1. Put three n-p-n transistors horizontally one after another. And number them 1,2 and 3 from left to right
2. Take the base of the transistor 1 and 2 as your base.
3. Connect the E terminal of the 3 transistors and earth them together.
4. Connect C terminals of 1 and 2 together and name the common potential as X.
5. Connect the common potential to a voltage( say VCC) via a resistor.Also connect it to the base of transistor 3.
6. Finally connect the VCC to the C of the transistor 3 via resistor.

Now the potential difference between the C and E terminals of the transistor 3 is your final output

 Y= A+B.

CMOS TECHNOLOGY: APPLICATION OF OUTPUT OF AN OR GATE:-

CMOS stands for Complementary Metal-Oxide Semiconductor. It is located in the motherboard of the CPU. It is used  to restore the data in the computer memory and keep them in order even after the power off of the computer. It is like a small RAM. Its storage is 256 bytes. It stores data from BIOS settings. It includes time and date and some system settings.

There are two types of data in this context:-

1. Volatile data:- Which isn’t erased when the computer is turned off.
2. Non-volatile data:- Which is deleted when the computer is turned off. CMOS data falls in this category.

We use CMOS batteries to deal with the non-volatility of the CMOS data. This CMOS battery is also one of the applications of the two input OR basic gate.

OR GATE OUT OF CMOS:-

Two steps are involved in devising OR gate out of CMOS.

1. Draw a NOR gate.
2. Connect an inverter to that NOR gate connecting the NOR gate to the inverter will result in making it inverse of NOR gate i.e. OR gate.

This is the one way to design OR gate out of CMOS.It involves the following steps:-

1. Draw the pull-up , pull down networks.
2. Connect the resultant output to the inverter. Inverted output is same as the output of an OR gate i.e. Y=A+B.

CONCLUSION:-

We have seen so far  the conversion of  the two input OR basic gate into the technologies building the modern century. TTL is used as a mini computer processor.  We have also seen CMOS inverters making the non-volatile CMOS memory into a volatile one. For better understanding one is advised to see the circuit diagrams of TTL-OR gate circuit, and CMOS OR gate inverter circuit. At the end there’s something to lit your curiosity. TTL OR gate is 10 times faster than RTL(Resistor-transistor logic). Now look for what RTL is and in what context TTL is faster than RTL and how?