If a digital buffer has a high fan-out rating (current source) it must also have a high “fan-in” rating (current sink) as well.
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Acting as a current source a buffer can have a high fan-out rating of up to 20 gates of the same logic family. So the fan-out is the number of parallel loads that can be driven simultaneously by one digital buffer of logic gate. 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. Generally the output of a logic gate is usually connected to the inputs of other gates. 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. 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. Then we can define the logical operation of a single input digital buffer as being: So the digital buffer is a “non-inverting” device and will therefore give us the Boolean expression of: Q = A. 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. In other words, a digital buffer does nothing as its output state equals its input state. 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.
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One type of single input logic gate that allows us to do just that is called the Digital Buffer.
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When its input signal is “LOW” its output state will NOT be “LOW”, in other words it “inverts” its input signal, hence the name “Inverter”.īut 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. So for example, when the single input to NOT gate is “HIGH”, its output state will NOT be “HIGH”. In a previous tutorial we looked at the digital Not Gate commonly called an inverter, and we saw that the NOT gates output state is the complement, opposite or inverse of its input signal.