Logic AND Gate
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 2input logic AND gate as being:
“If both A and B are true, then Q is true”
2input Transistor AND Gate
A simple 2input logic AND gate can be constructed using RTL Resistortransistor 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.Digital Logic “AND” Gate Types
The 2input Logic AND Gate
Symbol

Truth Table


2input
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 3input Logic AND Gate
Symbol

Truth Table


3input
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 4input packages. If additional inputs are required, then standard AND gates will need to be cascaded together to obtain the required input value, for example.
Multiinput AND Gate
The Boolean Expression for this 6input 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 “Pullup” resistors.
Commonly available digital logic AND gate IC’s include:
TTL Logic AND Gates
 74LS08 Quad 2input
 74LS11 Triple 3input
 74LS21 Dual 4input
CMOS Logic AND Gates
 CD4081 Quad 2input
 CD4073 Triple 3input
 CD4082 Dual 4input
7408 Quad 2input 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.