- AND Gate Definition: An AND gate is a digital logic gate that outputs high only when all its inputs are high.
- Logical Operation: This gate uses logical multiplication; the output is low if any input is low, and high only if all inputs are high.
- AND Gate Circuit Diagram: Essential for understanding how AND gates can be constructed using diodes or transistors to manipulate electrical signals.
- IC Implementation: AND gates are implemented in integrated circuits like the 7408 for TTL and 4081 for CMOS, each offering multiple gates in a single package.
- Truth Table Utilization: Truth tables are crucial for visualizing the output of AND gates based on different input combinations, aiding in circuit design and troubleshooting.
What is an AND Gate?
An AND gate is a logic gate with two or more inputs and a single output. It follows logical multiplication: if any input is low (0), the output is low. The output is high only when every input is high (1). Two-input and three-input AND gates are the usual packaged forms.
Many integrated circuits use this logic. The next sections show how the output depends on each input combination.
There are two binary digits, 0 and 1. An AND gate performs binary multiplication of those digits. 0 times 0 is 0, 1 times 0 is 0 and 0 times 1 is 0. The product is 1 only when 1 is multiplied by 1.
In other words, an AND gate produces a high output only when all inputs are high. Every other input combination produces a low output. High means logical 1 and low means logical 0.
An AND gate may have two or more inputs and only one output.
2 Input AND Gate
The symbol for a two-input AND gate is logically represented as:
Where A and B represent the inputs of the gate and X represents the output. A, B and X can each be 0 (low) or 1 (high).
The logical operation of AND gate hence can be represented as:
All products of A and B can be listed in a truth table. A truth table lists the output of a digital logic circuit for every input combination. The truth table of a 2 input AND gate can be represented as:
3 Input AND Gate
A third input changes the logical symbol and the truth table of the AND gate.
The logical symbol of a 3-input AND gate is represented as:
The truth table of a 3 input AND gate is:
In digital electronics, other logic gates include NOT gates, OR gates, NAND gates, NOR gates, XOR gates, XNOR gates.
AND Gate Circuit Diagram
AND Gate Diode Circuit Diagram
Normally an AND gate is designed by either diodes or transistors.
When diodes are used to design AND gate, it is called a diode AND gate. The basic circuit of a diode AND gate is shown below
In the diode circuit, +5 V is applied at point C. If +5 V is also applied to A and B, both diodes are reverse biased, so they are OFF or open.
With both diodes OFF, no current flows through resistor R, so the voltage at C (+5 V) also appears at X. That +5 V at X is high or logical 1.
If A or B or both are at 0 V or grounded, the corresponding diode is forward biased and behaves as ON or a short. The +5 V at C then has a path through one or both diodes to ground.
Current then flows from C to ground through resistor R, so nearly all of the 5 V drops across the resistor and X is low or logical zero.
A forward-biased diode is not an ideal short. A silicon diode still has a forward voltage drop of about 0.6 V to 0.7 V.
That forward drop appears at X when the output is low, so the practical low is about 0.6 V to 0.7 V rather than 0 V. Logic circuits still treat that level as zero.
AND Gate Transistor Circuit Diagram
An AND logic gate can also be realized from the transistor AND gate. The transistor circuit diagram for an AND gate is shown below:
In this circuit, if A, B or both are grounded or at 0V, transistors T1 and T2 or both will be in the OFF state.
This is because terminal A and B are the base terminals of transistor T1 and T2 respectively. Zero base voltage makes a transistor OFF. With the path through T1 and T2 open, the base of transistor T3 still has enough potential to turn T3 ON.
The current then flows from the supply to ground through T3. As a result, the supply voltage drops across R1 and terminal X is low or logical zero.
If any of the transistors T1 and T2 are in OFF condition, the same result will come at output X as both the transistors are in series.
Now we will check what will be the logical value of X if both A and B are at high logical value. If we apply +5V at both A and B i.e. at the base of transistor T1 and T2 respectively.
This scenario turns transistors T1 and T2 ON. The supply voltage then drops across R, so the base of T3 is near zero and T3 turns OFF. The +5 V supply then appears at X, so X is logically high.
AND Gate IC 7408
The common TTL AND-gate IC is the 7408. The 7408 is a quad 2-input package: four AND gates in one chip. The internal diagram of the 7408 is shown below.
Here pins 1, 2 are the inputs of the first gate whose respective output is 3. Again 4 and 5 are the inputs of the second gate whose output is at pin 6. The inputs of the fourth are pin 12 and 13 and pin 11 is its output.
Pin 14 is VCC. The recommended SN7408 supply is 4.75 V to 5.25 V DC. The absolute-maximum voltage is 7 V; a rating of 5.2 V is not the damage threshold.
AND Gate IC 4081
In CMOS logic, that is complementary MOSFET logic, the common AND-gate IC is the 4081. It is also a quad 2-input package. The internal diagram is shown below.
Pin 1 and 2 are the inputs of the first gate whose output is in number 3. Again Pin 5 and 6 are the inputs of the second gate whose output is at Pin 4. Pin 7 is grounded. Pin 8 and 9 are the inputs of the third gate whose output is at pin 10.
Again pin 13 and 12 are the inputs of the fourth AND gate whose output is at pin number 11. Pin 14 is VDD. A CD4081B is rated for 3 V to 18 V direct current, not a 5.2 V maximum.
A supply above the part’s absolute-maximum rating can damage the IC. CMOS and TTL AND gates also use different internal transistor structures.
