Inverter (logic gate)
- For the electrical power inverter, see Inverter (electrical).
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:
| input | output |
|---|---|
| 0 | 1 |
| 1 | 0 |
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
An inverter circuit outputs a voltage representing the opposite logic-level as its input. Digital electronics are circuits that 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.
Common types include resistive-drain, using one transistor and one resistor; and CMOS, which uses two (opposite type) transistors per inverter circuit
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 logic-levels can be obtained.
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, VOH, can be a measure of signal driving strength when cascading many devices together.
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 has 14 pins, 2 of which are used for power, and the remaining 12 pins are used by the inputs and outputs of the six inverters.