NAND gate (NOT-AND) is a logic gate that produces an output that is false only if all its inputs are true; thus its output is complementary to that of an AND gate. A LOW (0) output results only if all the inputs to the gate are HIGH (1); if any input is LOW (0), a HIGH (1) output results. A NAND gate is made using transistors and junction diodes.
The NAND (Not – AND) gate has an output that is normally at logic level “1” and only goes “LOW” to logic level “0” when ALL of its inputs are at logic level “1”. The Logic NAND Gate is the reverse or “Complementary” form of the AND gate.
Logic NAND Gate Equivalence
The logic or Boolean expression is given for a logic NAND gate is that for Logical Addition, which is the opposite of the AND gate, and which it performs on the complements of the inputs. Its Boolean expression is denoted by a single dot or full-stop symbol, ( . ) with a line or Overline, ( – ) over the expression to signify the NOT or logical negation of the NAND gate giving us the Boolean expression of: A.B = Q.
Transistor NAND Gate
The input is connected through resistor R2 to the transistor’s base. When no voltage is present on the input, the transistor turns off. When the transistor is off, no current flows through the collector-emitter path. Thus, current from the supply voltage (Vcc in the schematic) flows through resistor R1 to the output. In this way, the circuit’s output is HIGH when its input is LOW.
When voltage is present at the input, the transistor turns on, allowing current to flow through the collector-emitter circuit directly to the ground. This ground path creates a shortcut that bypasses the output, which causes the output to go LOW.
The Digital Logic “NAND” Gate
3-input Function
The NAND function can also have any number of individual inputs and commercially available NAND Gate ICs are available in standard 2, 3, or 4 input types.
The “Universal” NAND Gate
The Logic NAND Gate is generally classed as a “Universal” gate because it is one of the most commonly used logic gate types. They can also be used to produce any other type of logic gate function, but in practice being universal it forms the basis of most practical logic circuits.
Application
- Used in Alarm Circuits through light detection radiation
- Employed in freezer warning buzzer devices
- NAND logic gates are utilized in automatic temperature regulation circuits