Logic gate

A logic gate performs a logical operation on one or more logic inputs and produces a
single logic output. The logic normally performed is Boolean logic and is most
commonly found in digital circuits. Logic gates are primarily implemented electronically
using diodes or transistors, but can also be constructed using electromagnetic relays,
fluidics, optical or even mechanical elements. A Boolean logical input or output always takes one of two logic levels. These logic levels can go by many names including: on / off, high (H) / low (L), one (1) / zero (0), true (T) / false (F), positive / negative, positive / ground, open circuit / close circuit, potential difference / no difference.

A logic gate takes one or more logic-level inputs and produces a single logic-level output.
Because the output is also a logic level, an output of one logic gate can connect to the
input of one or more other logic gates. Two outputs cannot be connected together,
however, as they may be attempting to produce different logic values. In electronic logic
gates, this would cause a short circuit.


In electronic logic, a logic level is represented by a certain voltage (which depends on the
type of electronic logic in use). Each logic gate requires power so that it can source and
sink currents to achieve the correct output voltage. In logic circuit diagrams the power is
not shown, but in a full electronic schematic, power connections are required. The simplest form of electronic logic is diode logic. This allows AND and OR gates to be
built, but not inverters, and so is an incomplete form of logic. To build a complete logic
system, valves or transistors can be used. The simplest family of logic gates using bipolar
transistors is called resistor-transistor logic, or RTL. Unlike diode logic gates, RTL gates
can be cascaded indefinitely to produce more complex logic functions. These gates were
used in early integrated circuits. For higher speed, the resistors used in RTL were
replaced by diodes, leading to diode-transistor logic, or DTL.


Electronic logic gates differ significantly from their relay-and-switch equivalents. They
are much faster, consume much less power, and are much smaller (all by a factor of a
million or more in most cases). Also, there is a fundamental structural difference. The
switch circuit creates a continuous metallic path for current to flow (in either direction)
between its input and its output. The semiconductor logic gate, on the other hand, acts as
a high-gain voltage amplifier, which sinks a tiny current at its input and produces a lowimpedance voltage at its output. It is not possible for current to flow between the output
and the input of a semiconductor logic gate.