Inverter (logic gate)

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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 non-ideal 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]).


Electronic implementation

NMOS Inverter

PMOS Inverter

Static CMOS Inverter

Schematic of a Saturated-Load Digital Inverter

An inverter circuit outputs a voltage representing the opposite logic-level to its input. Inverters can be constructed using a single NMOS transistor or a single PMOS transistor coupled with a resistor. Since this 'resistive-drain' 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 resistive-drain configuration is disadvantaged for power consumption and processing speed. Alternatively, inverters can be constructed using two complimentary 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 NMOS-only or PMOS-only type devices. Inverters can also be constructed with Bipolar Junction Transistors (BJT) in either a resistor-transistor logic (RTL) or a transistor-transistor 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).

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