The logic symbol is the same as the OR gate with an inversion bubble placed at the output side. Its function is NOT-OR. It has a similar truth table to that of a NAND gate. Its output is high only when both of the inputs are low.
Learning Objectives:
Working:
Case 1:
Input A = 0
Input B = 0
Output = 1
Case 2:
Input A = 0
Input B = 1
Output = 1
Case 3:
Input A = 1
Input B = 0
Output = 0
Case 4:
Input A = 1
Input B = 1
Output = 0
In figure 1 the waveforms or timing diagram of the NOR gate is also given. The input and output signals are represented by the pulses (a pulse of square wave). There are two inputs A and B and output is Y as shown. The output pulse falls to 0V when any one of the two inputs are high.
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| Figure 1: NOR gate – schematic symbol, truth table, logical equation and waveforms |
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| Figure 2: Simple circuit simulation results |
Truth Table Of 2-Input NOR Gate:
Logical Expression:
Y = (A + B)C
Logical NOR Gate (Explain with the help of switches) | NOR Gate Switch Logic:
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| Figure 3: Switch Model of NOR gate |
In the above figure, there is a switch model of a NOR gate. It is easy to understand the operation of the gate with the help of switches. The inputs are represented by two switches A and B. The output is represented by an LED. The LED turns ON only when both of the switches are off (open). The only path for the current to flow is from the LED. If anyone switch turns ON, the current will flow from the least resistive path.
Implementation Using Diode Logic
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| Figure 4: Diode circuit of NOR gate |
Case 1:
Both switches are off (opened), and hence both diodes are off. Current flows from V3 to LED and ground.
Case 2:
In case 2, look at the schematic. Switch S1 is ON while S2 is off. The positive terminal of the diode is connected to a 5V source and hence turns on. On the negative terminal of D1, there is a positive potential because of V3. So, there is no or very little current will flow.
Case 3:
Same as case 2.
Case 4:
Same as case 2 and case 3.
Implementation Using Transistor Logic
NOR gate is easily implemented with the help of two BJT transistors. The transistors are connected in differential pair form. It is a common emitter configuration. The output is taken from the collector while the input is at base.
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| Figure 5: BJT circuit of NOR gate |
Case 1:
Look at the schematic diagram in case 1. Both switches are off. Both transistors are off. The only path available for the current is through the LED. And Hence LED glows.
Case 2:
In case 2, S1 is on and hence Q1 is ON. The current will flow from Q1 to the ground. No current will flow through the LED.
Case 3:
Same as case 2.
Case 4:
Same as case 2 and case 3.