In digital logic, an inverter is a logic gate which inverts the digital signal driven on its input. It is also called NOT gate. The truth table of the gate is as follows:

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).

Electronic implementation

An inverter circuit outputs a voltage representing the opposite logic-level to its input. Inverters can be constructed using a single NMOS transitor or a single PMOS transitor coupled with a resistor. Since this 'resistive-drain' approach, uses only a single type of tranistor, 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 consuption and processing speed. Alternately, inverters can be constructed using two complimentary transistors in a CMOS configuration. This configuration greatly reduces power consuption since one of 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 and the 4049 CMOS chip each have 14 pins, 2 of which are used for power/referencing, and the remaining 12 pins are used by the inputs and outputs of the six inverters.

Performance measurement

Ideally, the voltage transfer curve (VTC) appears as an inverted step-function - 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

• CMOS Hex Inverting Buffer/Converter from Texas Instruments
• Datasheet: CMOS Hex Buffer/Converter

References