# Logic AND Gate Tutorial

A Logic AND Gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when ALL of its inputs are at logic level “1”.

The output state of a “Logic AND Gate” only returns “LOW” again when ANY of its inputs are at a logic level “0”. In other words for a logic AND gate, any LOW input will give a LOW output.

The logic or Boolean expression given for a digital logic AND gate is that for Logical Multiplication which is denoted by a single dot or full stop symbol, ( . ) giving us the Boolean expression of:  A.B = Q.

Then we can define the operation of a 2-input logic AND gate as being:

“If both A and B are true, then Q is true”

## 2-input Transistor AND Gate

A simple 2-input logic AND gate can be constructed using RTL Resistor-transistor switches connected together as shown below with the inputs connected directly to the transistor bases. Both transistors must be saturated “ON” for an output at Q.

Logic AND Gates are available using digital circuits to produce the desired logical function and is given a symbol whose shape represents the logical operation of the AND gate.

## Digital Logic “AND” Gate Types

### The 2-input Logic AND Gate

 Symbol Truth Table 2-input AND Gate B A Q 0 0 0 0 1 0 1 0 0 1 1 1 Boolean Expression Q = A.B Read as A AND B gives Q

### The 3-input Logic AND Gate

 Symbol Truth Table 3-input AND Gate C B A Q 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 Boolean Expression Q = A.B.C Read as A AND B AND C gives Q

Because the Boolean expression for the logic AND function is defined as (.), which is a binary operation, AND gates can be cascaded together to form any number of individual inputs. However, commercial available AND gate IC’s are only available in standard 2, 3, or 4-input packages. If additional inputs are required, then standard AND gates will need to be cascaded together to obtain the required input value, for example.

### Multi-input AND Gate

The Boolean Expression for this 6-input AND gate will therefore be:   Q = (A.B).(C.D).(E.F)

If the number of inputs required is an odd number of inputs any “unused” inputs can be held HIGH by connecting them directly to the power supply using suitable “Pull-up” resistors.

Commonly available digital logic AND gate IC’s include:

TTL Logic AND Gates

• 74LS08 Quad 2-input
• 74LS11 Triple 3-input
• 74LS21 Dual 4-input
CMOS Logic AND Gates

• CD4081 Quad 2-input
• CD4073 Triple 3-input
• CD4082 Dual 4-input

### 7408 Quad 2-input AND Gate

In the next tutorial about Digital Logic Gates, we will look at the digital logic OR Gate function as used in both TTL and CMOS logic circuits as well as its Boolean Algebra definition and truth tables.

• k
krishn kumar

what are logic gate

• S
Sweety

Explain how it works internally. how that output is came .how is it’s internal operation is done.

• v
v.sakana rajam

It is very usefull and easyly understand .

• a
abiyot zenebe

i need to design the register ckt block diagram using logic componet . if posible please help me ? thank you

• Nikki

3-input AND gate, this is a combination switch with on, off, but with 8 combination of possible solutions. The reset button makes it a 0 on reset again, but high on a second “on” recount. And it continuous. So there are 8 possible outcomes, with 7 upon the first count. Make sense.

• Nikki

This was helpful. I have of a picture of a transceiver. It is a control module of a firing system. Such as a module of shells to be fired on a new year’s eve countdown celebration. It now make sense. Circuitry.

• R
Rongbong Engleng

What is Boolean ?

• I
Irfan Quraeshi

Who is Boolean?

• Wayne Storr

In 1854, George Boole performed an investigation into the “laws of thought” which were based around a simplified version of the “group” or “set” theory, and from this Boolean Algebra was developed.

• A
Arnab Maity

please someone can tell me the transistor code for 2 input and gate!!!

• s
santosh kumar

sir …….tell me how to i download logic gates pdf format file ……….please given me that link

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