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4. Electrons and electronics
4.5 Digital electronics
Digital electronics
1. Analog signals: The electrical signals which are continuous and which can have any value over a given range are called analog signals. In an analog signal, the values of current or voltage vary continuously with time.
Digital electronics
2. Digital signal: A digital signal is a discontinuous function of time. It has two discrete values or voltage levels 0 and 1.The zero refers to some low value while 1 refers to some high value.
Digital electronics
Analog watch Digital watch
Digital electronics
3. What is logic? Logic is a form of reasoning which
tells us whether a statement is true or false when certain conditions are true or false.
Eg:1. AND operation2. OR operation3. NOT operation
Digital electronics Electrical analog
of AND operation
Digital electronics Electrical analog
of OR operation
Digital electronics Electrical analog
of NOT operation
Digital electronics
4. Logic circuit: A logic circuit is an electronic circuit, which represents a logic function.
Digital instruments use logic circuits to measure a quantity and then display it in a numerical reading form.
Digital electronics
Logic gate : A logic gate is an electronic circuit with one or more inputs and only one output. There are three basic logic gates
1. OR gate 2. AND gate and 3. NOT gate
Digital electronics
Truth table: A table which gives all the possible combinations of input variables and the corresponding output for a logic gate is called a Truth table.
Digital electronics The OR gate An OR gate has
two input signals and only one output signal. The logic symbol of a two input OR gate is shown in the figure
Output Y = A + B
Digital electronics
Digital electronics
The AND gate: The AND gate has two or more
inputs and only one output. It is a circuit whose output is one if and only if all the inputs are one.
Digital electronics
Output Y = A B
Digital electronics
The NOT gate: A NOT gate has only one input
and one output. It is a device whose output is always the complement of the input i.e. the output of a NOT gate is one when its input is zero and vice-versa.
Digital electronics
Digital electronics
UNIVERSAL GATES By combining the three basic
gates a large number of gates can be obtained. Hence these three gates are called Universal gates.
Digital electronics
NAND gate: If the output of an AND gate is
connected to the input of a NOT gate, the resultant gate so obtained is known as NAND gate. NAND is the abbreviation for NOT-AND
Digital electronics NAND gate
Digital electronics
NOR gate: When the output of an OR gate is
connected to the input of a NOT gate the resultant combination is known as a NOR gate. NOR is the abbreviation for NOT-OR.
Digital electronics
NOR gate
Digital electronics NOT gate using NAND gate
Digital electronics OR gate using
NAND gates
This is same as
Digital electronics The OR gate is the NAND gate with the
inputs inverted
Digital electronics
This same as
AND gate using NAND gate
Digital electronics
NAND-NOT = AND
Digital electronics
NOR gate using NAND gates
Digital electronics This completes the description of the
basic logic gates. These gates can be combined to make the larger circuits needed to make a computer processor. These larger circuits are multiplexors (data selectors), registers, the adder, and the logic that controls the computer.