Blog Number:-005
Hello Everybody,
Well, I wish you all will be fine.
In our last Blog, We discussed about Diode.
So, today we will discuss about Digital Logic Gates
Digital Logic Gates:-
Introduction
A Digital Logic Gate is an electronic device that makes logical decisions based on the different combinations of digital signals present on its inputs. Digital logic gates may have more than one input, (A, B, C, etc.) but generally only have one digital output, (Q). Individual logic gates can be connected together to form combinational or sequential circuits, or larger logic gate functions.
Standard commercially available digital logic gates are available in two basic families or forms, TTL which stands for Transistor-Transistor Logic such as the 7400 series, and CMOS which stands for Complementary Metal-Oxide-Silicon which is the 4000 series of chips.
Digital Logic States
The Digital Logic Gate is the basic building block from which all digital electronic circuits and microprocessor based systems are constructed from. Basic digital logic gates perform logical operations of AND, OR and NOT on binary numbers.
In digital logic design only two voltage levels or states are allowed and these states are generally referred to as Logic “1” and Logic “0”, High and Low, or True and False. These two states are represented in Boolean Algebra and standard truth tables by the binary digits of “1” and “0” respectively.
A good example of a digital state is a simple light switch as it is either “ON” or “OFF” but not both at the same time. Then we can summarise the relationship between these various digital states as being:
| Boolean Algebra | Boolean Logic | Voltage State |
| Logic “1” | True (T) | High (H) |
| Logic “0” | False (F) | Low (L) |
Most digital logic gates and digital logic systems use “Positive logic”, in which a logic level “0” or “LOW” is represented by a zero voltage, 0v or ground and a logic level “1” or “HIGH” is represented by a higher voltage such as +5 volts, with the switching from one voltage level to the other, from either a logic level “0” to a “1” or a “1” to a “0” being made as quickly as possible to prevent any faulty operation of the logic circuit.
There also exists a complementary “Negative Logic” system in which the values and the rules of a logic “0” and a logic “1” are reversed but in this tutorial section about digital logic gates we shall only refer to the positive logic convention as it is the most commonly used.
TTL and CMOS Logic Levels
| Device Type | Logic 0 | Logic 1 |
| TTL | 0 to 0.8v | 2.0 to 5v |
| CMOS | 0 to 1.5v | 3.0 to 18v |
Then from the above observations, we can define the ideal TTL digital logic gate as one that has a “LOW” level logic “0” of 0 volts (ground) and a “HIGH” level logic “1” of +5 volts and this can be demonstrated as:
Ideal TTL Digital Logic Gate Voltage Levels
Where the opening or closing of the switch produces either a logic level “1” or a logic level “0” with the resistor R being known as a “pull-up” resistor.
Now, lets discuss different Digital Logic Gates one by one.
1. 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”, 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”
The 2-input Logic AND Gate
2. Logic OR Gate
A Logic OR Gate or Inclusive-OR 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
one or more of its inputs are at logic level “1”.
logic gate that has an output which is normally at logic
level “0” and only goes “HIGH” to a logic level “1” when
one or more of its inputs are at logic level “1”.
The output, Q of a “Logic OR Gate” only returns “LOW”
again when ALL of its inputs are at a logic level “0”. In
other words for a logic OR gate, any “HIGH” input will give a “HIGH”, logic level “1” output.
again when ALL of its inputs are at a logic level “0”. In
other words for a logic OR gate, any “HIGH” input will give a “HIGH”, logic level “1” output.
The logic or Boolean expression given for a digital logic OR gate is that for Logical Addition
which is denoted by a plus sign, ( + ) giving us the Boolean expression of: A+B = Q.
which is denoted by a plus sign, ( + ) giving us the Boolean expression of: A+B = Q.
Then we can define the operation of a 2-input logic OR gate as being:
“If either A or B is true, then Q is true”
The 2-input Logic OR Gate
3. Logic NOT gate
The digital Logic NOT Gate is the most basic of all the logical
gates and is sometimes referred to as an Inverting Buffer or simply
a Digital Inverter.
It is a single input device which has an output level that is
normally at logic level “1” and goes “LOW” to a logic level “0”
when its single input is at logic level “1”, in other words it “inverts” (complements) its input
signal. The output from a NOT gate only returns “HIGH” again when its input is at logic
level “0” giving us the Boolean expression of: A = Q.
Then we can define the operation of a single input digital logic NOT gate as being:
“If A is NOT true, then Q is true”
The Logic NOT Gate Truth Table
Thank You,
Er. Rahul Kumar
Er. Rahul Kumar
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