Digital Buffer Tutorial

Digital Buffer Tutorial

Digital Buffer Definition
digital buffer

Sometimes in digital electronic circuits we need to isolate logic gates from each other or have them drive or switch higher than normal loads, such as relays, solenoids and lamps without the need for inversion. One type of single input logic gate that allows us to do just that is called the Digital Buffer.

Unlike the single input, single output inverter or NOT gate such as the TTL 7404 which inverts or complements its input signal on the output, the “Buffer” performs no inversion or decision making capabilities (like logic gates with two or more inputs) but instead produces an output which exactly matches that of its input. In other words, a digital buffer does nothing as its output state equals its input state.

Then digital buffers can be regarded as Idempotent gates applying Boole’s Idempotent Law because when an input passes through this device its value is not changed. So the digital buffer is a “non-inverting” device and will therefore give us the Boolean expression of:  Q = A.

Then we can define the logical operation of a single input digital buffer as being:

“Q is true, only when A is true”

In other words, the output ( Q ) state of a buffer is only true (logic “1”) when its input A is true, otherwise its output is false (logic “0”).

The Single Input Digital Buffer

Symbol Truth Table

digital buffer

The Digital Buffer
A Q
0 0
1 1
Boolean Expression Q = A Read as: A gives Q

The Digital Buffer can also be made by connecting together two NOT gates as shown below. The first will “invert” the input signal A and the second will “re-invert” it back to its original level performing a double inversion of the input.

Double Inversion using NOT Gates

digital buffer using not gates

You may be thinking “what’s the point of a Digital Buffer“? If it does not invert or alter its input signal in any way, or make any logical decisions or operations like the AND or OR gates do, then why not just use a piece of wire instead, and that’s a good point. But a non-inverting Digital Buffer does have many uses in digital electronics with one of its main advantages being that it provides digital amplification.

Digital Buffers can be used to isolate other gates or circuit stages from each other preventing the impedance of one circuit from affecting the impedance of another. A digital buffer can also be used to drive high current loads such as transistor switches because their output drive capability is generally much higher than their input signal requirements. In other words buffers can be used for power amplification of a digital signal as they have what is called a high “fan-out” capability.

Digital Buffer Fan-out Example

digital buffer fan-out

The Fan-out parameter of a buffer (or any digital IC) is the output driving capability or output current capability of a logic gate giving greater power amplification of the input signal. It may be necessary to connect more than just one logic gate to the output of another or to switch a high current load such as an LED, then a Buffer will allow us to do just that.

Generally the output of a logic gate is usually connected to the inputs of other gates. Each input requires a certain amount of current from the gate output to change state, so that each additional gate connection adds to the load of the gate. So the fan-out is the number of parallel loads that can be driven simultaneously by one digital buffer of logic gate. Acting as a current source a buffer can have a high fan-out rating of up to 20 gates of the same logic family.

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