Buffer Gate

ANSI Symbol

Functional

Truth Table

Inputs Outputs A Q 0 0 1 1

Other Gates Buffer TriBuffer NOT AND OR XOR NAND NOR XNOR Trans AOI OAI MAJ INH IMPLY NIMPLY

Other Components Plexers MUX DEMUX Encoder Decoder Pri-Encoder ALU Adder Subtractor Multiplier Divider Shifter Rotator MAC Comparator Negator Memory D latch D flip-flop SR latch JK flip-flop T flip-flop Register Register file SRAM Counter ROM CAM DRAM I/O Shift register SIPO PISO ADC DAC

A buffer, is a basic logic gate that passes its input, unchanged, to its output. It's behavior is the opposite of a NOT gate. The main purpose of a buffer is to regenerate the input, usually using a strong high and a strong low. A buffer has one input and one output; its output always equals its input. Buffers are also used to increase the propagation delay of circuits by driving the large capacitive loads.

Description

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A buffer is a very basic active device that generates an output identical to its input input. In most technologies, a buffer is made of two inverter back-to-back. One of the many purposes for a buffer is to generate weak output from non-restoring logic that was used.

Design

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Symbolic representation

Buffers are typically drown on schematics using one of a standard symbol. Below are three of the commonly found standard symbols.

ANSI	IEC	DIN	British

Implementations

A buffer can be implemented in variety of of technologies.

CMOS

A CMOS buffer gate with one input and one output can be realized as simply two inverters back to back - built out of just 4 gates.

The table on the right shows the states of the four transistors with the various inputs of A.

Buffer Gate by Transistor
A	Q1	Q2	Q3	Q4	Q
0	1	0	0	1	0
1	0	1	1	0	1

Discrete Chips

Various buffers/drivers exist chips as well for both 7400 series and 4000 series.

7400 series chips

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4000 series chips

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The 7407 is a TTL chip with 14 pins. Two pins are used for V_DD and GND, the other 12 pins are used for the 6 independent buffers. The 4050 is a CMOS Hex Buffer with 16 pins. Two pins are used for V_DD and GND, 12 pins are used for the 6 independent buffers. Pins 13 and 16 are not connected. Both chips implement the expression Q_N = A_N

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  7407 Hex Buffer/Driver

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  4050 CMOS Hex Buffer/Driver