# Boolean Logic Gates Buffer, NOT and, or Truth Tables YouTube Lecture Handouts

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• Behind every digital circuit is a logic gate.
• Logic gate is the building block of each digital circuit
• 7 common logic gates – XNOR, NOR, NOT, AND, OR, NAND, XOR
 Boolean Algebra Voltage State Status
• Digital logic gates can have more than one input, for example, inputs A, B, C, D etc. , but generally only have one digital output, (Q) . Individual logic gates can be connected or cascaded together to form a logic gate function with any desired number of inputs, or to form combinational and sequential type circuits, or to produce different logic gate functions from standard gates.
• A good example of a digital state is a simple light switch. The switch can be either “ON” or “OFF,” one state or the other, but not both at the same time. Then we can summarize the relationship between these various digital states as being.
 Input Output
 Input Output A

NOT gate is also known as Inverter. It has one input A and one output Y. If the input variable is A, the inverted output is known as NOT A. This is also shown as A′, or A with a bar over the top, as shown at the outputs.

If A is NOT true, then Q is true

• Logic NOT Gates are available using digital circuits to produce the desired logical function. The standard NOT gate is given a symbol whose shape is of a triangle pointing to the right with a circle at its end. This circle is known as an “inversion bubble” and is used in NOT, NAND and NOR symbols at their output to represent the logical operation of the NOT function. This bubble denotes a signal inversion (complementation) of the signal and can be present on either or both the output and/or the input terminals.
• Logic NOT gates provide the complement of their input signal and are so called because when their input signal is “HIGH” their output state will NOT be “HIGH.” Likewise, when their input signal is “LOW” their output state will NOT be “LOW.”
 Input Output A B A B
• A circuit which performs an AND operation is shown in figure. It has n input (n >= 2) and one output.
• The AND gate is an electronic circuit that gives a high output (1) only if all its inputs are high. A dot (.) is used to show the AND operation i.e.. , A. B. Bear in mind that this dot is sometimes omitted i.e.. , AB
 Input Output A B A B