## Friday, June 28, 2013

### OR Gate

Good evening everyone. Welcome to my blog.

So to continue my writing on logic gates I will write about OR gate this time.

What is it?

OR gate, one of the Basic logic gates is such a gate that implements logical disjunction. It behaves in a way that if only one of the Inputs is High then the Output will be High as well. In an OR gate the Output will be low only when all the Inputs are low.

Truth table:

Visual Representation:

To realize OR gate visually let's assume two persons Mr. Blue and Mr. Red divided by a river and there are two bridges across this river. Now what common sense tells us is if only one bridge is there, no matter which one is, they can meet each other. The only time they won't be able to meet is when both the bridges are unavailable. That's exactly what OR gate is. If it is for three or maybe more inputs, imagine more parallel bridges.

Symbol:

So as we have seen the Truth table and Visual Representation, let's see how it can be implemented using Diodes or Transistors.

OR Gate Using Diodes:

This Circuit is as simple as it can get. If we consider any instant when there is no input voltage for any of the inputs, A and B the resistor R will pull down the Output but if any voltage is available on either A or B or both that voltage will come across the resistor which will cause the output to be pulled up. So no Input we get a Zero on the output but for any other input we get that on the output. Adding another diode similarly will give us Three inputs.

OR Gate using Transistors:

Again let's consider a very simple circuit with two NPN transistors. If we look at the output we will see that if there is no supply to the Base of any transistor they will act pretty much like open circuit. In that scenario the 2.2K resistor will pull the output down to zero. But if you close any of the switches that are with A or B or both that will cause the transistor or transistors to conduct which will pull up the output. Again if you want another input just add another transistor similarly.

Note: If you keep the transistor base floating you might not get proper result so it will be a good idea to hook up a 10K resistor with the base of the transistor just to make sure that this circuit will work reliably.

Available OR Gates:

Or gates are available in both TTL and CMOS.

CMOS 4075: Triple 3-Input OR.
CMOS 4072: Dual 4-Input OR.