Inverter (logic gate)

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Traditional NOT gate (inverter) symbol

In digital logic, an inverter or NOT gate is a logic gate which implements logical negation. It outputs a bit opposite of the bit that is put into it. The bits are typically implemented as two differing voltage levels.


Inverter truth table
Input Output
0 1
1 0

The NOT gate outputs a zero when given a one, and a one when given a zero. Hence, it inverts its inputs. Colloquially, this inversion of bits is called "flipping" bits.[1] It is equivalent to the logical negation operator (¬) in mathematical logic.

The NOT gate is one of three basic logic gates from which any Boolean circuit may be built up. Together with the AND gate and the OR gate, any function in binary mathematics may be implemented. All other logic gates may be made from these three.[2]

The terms "programmable inverter" or "controlled inverter" do not refer to this gate; instead, these terms refer to the XOR gate because it can conditionally function like a NOT gate.[1][2]

Electronic implementation[edit]

An inverter circuit outputs a voltage representing the opposite logic-level to its input. Its main function is to invert the input signal applied. If the applied input is low then the output becomes high and vice versa. 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 a 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 complementary transistors in a CMOS configuration. This configuration greatly reduces power consumption since one of the transistors is always off in both logic states.[3] 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[edit]

This schematic diagram shows the arrangement of NOT gates within a standard 4049 CMOS hex inverting buffer.

The inverter is a basic building block in digital electronics. Multiplexers, decoders, state machines, and other sophisticated digital devices may use inverters.

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).

Analytical representation[edit]

is the analytical representation of NOT gate:


If no specific NOT gates are available, one can be made from the universal NAND or NOR gates,[4] or an XOR gate by setting one input to high.

Desired gate NAND construction NOR construction
NOT ANSI Labelled.svg NOT from NAND.svg NOT from NOR.svg

Performance measurement[edit]

Voltage transfer curve for a 20 μm inverter fabricated at North Carolina State University

Digital inverter quality is often measured using the voltage transfer curve (VTC), which is a plot of output vs. input voltage. From such a graph, device parameters including noise tolerance, gain, and operating logic levels can be obtained.

Ideally, the 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 the low level. 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).

See also[edit]


  1. ^ a b Van Houtven, Laurens (2017). Crypto 101 (PDF). p. 17.
  2. ^ a b Broesch, James D. (2012). Practical Programmable Circuits: A Guide to PLDs, State Machines, and Microcontrollers. Elsevier Science. p. 19. ISBN 0323139264.
  3. ^ Nair, B. Somanathan (2002). Digital electronics and logic design. PHI Learning Pvt. Ltd. p. 240. ISBN 9788120319561.
  4. ^ M. Morris, Mano; R. Kime, Charles (2004). Logic and computer design fundamentals (3 ed.). Prentice Hall. p. 73. ISBN 0133760634.

External links[edit]