 Inverter (logic gate)

INPUT
AOUTPUT
NOT A0 1 1 0 In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. The truth table is shown on the right.
This represents perfect switching behavior, which is the defining assumption in Digital electronics. In practice, actual devices have electrical characteristics that must be carefully considered when designing inverters. In fact, the nonideal transition region behavior of a CMOS inverter makes it useful in analog electronics as a class A amplifier (e.g., as the output stage of an operational amplifier^{[1]}).
Contents
Electronic implementation
An inverter circuit outputs a voltage representing the opposite logiclevel to its input. Inverters can be constructed using a single NMOS transistor or a single PMOS transistor coupled with a resistor. Since this 'resistivedrain' approach uses only a single type of transistor, it can be fabricated at low cost. However, because current flows through the resistor in one of the two states, the resistivedrain configuration is disadvantaged for power consumption and processing speed. Alternatively, inverters can be constructed using two complementary transistors in a CMOS configuration. This configuration greatly reduces power consumption since one of the transistors is always off in both logic states. Processing speed can also be improved due to the relatively low resistance compared to the NMOSonly or PMOSonly type devices. Inverters can also be constructed with Bipolar Junction Transistors (BJT) in either a resistortransistor logic (RTL) or a transistortransistor logic (TTL) configuration.
Digital electronics circuits operate at fixed voltage levels corresponding to a logical 0 or 1 (see Binary). An inverter circuit serves as the basic logic gate to swap between those two voltage levels. Implementation determines the actual voltage, but common levels include (0, +5V) for TTL circuits.
Digital building block
The digital inverter is considered the base building block for all digital electronics. Memory (1 bit register) is built as a latch by feeding the output of two serial inverters together. Multiplexers, decoders, state machines, and other sophisticated digital devices all rely on the basic inverter.
The Hex Inverter is an integrated circuit that contains six (hexa) inverters. For example, the 7404 TTL chip which has 14 pins and the 4049 CMOS chip which has 16 pins, 2 of which are used for power/referencing, and 12 of which are used by the inputs and outputs of the six inverters (the 4049 has 2 pins with no connection).
Performance measurement
Digital inverter quality is often measured using the Voltage Transfer Curve, which is a plot of input vs. output voltage. From such a graph, device parameters including noise tolerance, gain, and operating logiclevels can be obtained.
Ideally, the voltage transfer curve (VTC) appears as an inverted stepfunction  this would indicate precise switching between on and off  but in real devices, a gradual transition region exists. The VTC indicates that for low input voltage, the circuit outputs high voltage; for high input, the output tapers off towards 0 volts. The slope of this transition region is a measure of quality  steep (close to Infinity) slopes yield precise switching.
The tolerance to noise can be measured by comparing the minimum input to the maximum output for each region of operation (on / off).
The output voltage, V_{OH}, can be a measure of signal driving strength when cascading many devices together.
External links
References
Categories: Logic gates
 Integrated circuits
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