What is CMOS Inverter : Working & Its Applications

The term “CMOS” stands for “complementary-symmetry metal–oxide–semiconductor” which is pronounced as “see mos”. CMOS is a type of MOSFET, where its fabrication process uses complementary & symmetrical P-type & N-type MOSFET pairs for logic functions. The main CMOS devices characteristics are consumption of low static power & high noise immunity. The inverter is accepted universally as the basic logic gate while performing a Boolean operation on a single i/p variable. A basic inverter circuit is used to accomplish a logic variable by complementing from A to A’. So, a CMOS inverter is a very simple circuit, designed with two opposite-polarity MOSFETs within a complementary way. This article discusses an overview of the CMOS inverter and its working with applications.

What is CMOS Inverter?

CMOS inverter definition is a device that is used to generate logic functions is known as CMOS inverter and is the essential component in all integrated circuits. A CMOS inverter is a FET (field effect transistor), composed of a metal gate that lies on top of oxygen’s insulating layer on top of a semiconductor. These inverters are used in most electronic devices which are accountable for generating data n small circuits.

CMOS Inverter Symbol & Truth Table
CMOS Inverter Symbol & Truth Table

CMOS Inverter Schematic Diagram

The logic element like an inverter reverses the applied input signal. In digital logic circuits, binary arithmetic & switching or logic function’s mathematical manipulation are best performed through the symbols 0 & 1. The CMOS inverter truth table is shown above. If the input logic is zero (0) then the output will be high (1) whereas, if the input logic is one (1), then the output will be low (0).

CMOS Inverter Circuit
CMOS Inverter Circuit

The CMOS inverter circuit diagram is shown below. The general CMOS inverter structure is the combination of both the PMOS & NMOS transistors where the pMOS is arranged at the top & nMOS is arranged at the bottom.

The connection of both the PMOS & NMOS transistors in the CMOS inverter can be done like this. The NMOS transistor is connected at the drain (D) & gate (G) terminals, a voltage supply (VDD) is connected at the source terminal of PMOS & a GND terminal is connected at the source terminal of NMOS. Input voltage (Vin) is connected to both the gate terminals of transistors & output voltage (Vout) is connected to the drain (D) terminals of the transistor.

It is very significant to observe that the CMOS device does not have any resistors, so it will be more power-efficient. Once the input voltage of CMOS changes between 0 to 5 volts, then both the transistors state will be changed accordingly. If we design every transistor like a simple switch that is operated through input voltage (Vin), then operations of the inverter can be observed very simply:

CMOS Inverter Operation & Working

The working of CMOS inverter is the same as other types of FETs except depends on an oxygen layer to divide electrons within the gate & semiconductor. They are designed with a power supply, input voltage terminal, output voltage, gate, drain, and PMOS & NMOS transistors which are connected to the gate & the drain terminals.

When the low input voltage is given to the CMOS inverter, then the PMOS transistor is switched ON whereas the NMOS transistor will switch OFF by allowing the flow of electrons throughout the gate terminal & generating high logic output voltage.

Similarly, when the high input voltage is given to the CMOS inverter then, the PMOS transistor is switched OFF whereas the NMOS transistor will be switched ON avoiding as many electrons from attaining the output voltage & generating low logic output voltage.

Thus, direct current supplies from the supply voltage (VDD) to the output voltage (Vout) & the load capacitor (CL) can be charged and shows that Vout = VDD. As a result, the above circuit works like an inverter.

CMOS Inverter Characteristics

The characteristics of the CMOS inverter are discussed below.

Inverter Static Characteristics or VTC

The quality of the inverter can be measured frequently by using the VTC or voltage transfer curve, which is plotted between input voltage (Vin) and output voltage (Vo). From the following static characteristics, the parameters of devices like gain, operating logic levels & noise tolerance, and noise can be obtained.

Voltage Transfer Curve
Voltage Transfer Curve

The VTC or voltage transfer curve looks like an inverted step-function that specifies accurate switching in between ON & OFF however in real devices, a gradual transition region exists. The voltage transfer curve specifies that for less input voltage Vin, the circuit generates high voltage Vout, whereas, for high input, it generates 0 volts.

The transition region slope is a measure of quality – steep slopes yield exact switching. The tolerance toward noise can be calculated by evaluating the smallest input to the highest output for every region of ON or OFF operation.

Inverter Dynamic Characteristics

The CMOS inverter dynamic characteristics are shown below. So, some of the following formal definitions of different parameters are discussed below. Here, all the percentage (%) values are the steady-state values.

Dynamic Characteristics of CMOS Inverter
Dynamic Characteristics of CMOS Inverter
  • Rise Time or tr: Rise time is the time used to increase the signal from 10% to 90%.
  • Fall Time or tf: Fall time is the time used to drop the signal from 90% to 10%
  • Edge Rate or trf : It is (tr + tf )/2.
  • The propagation delay from high to low or tpHL: The time used to drop from VOH – 50%.
  • The propagation delay from low to high or tpLH: The time used to increase from 50%- VOL.
  • Propagation Delay or tp: It is (tpHL + tpLH)/2.
  • Contamination Delay or tcd: It is the smallest time from the 50% input crossing to the 50% output crossing.

Advantages

The CMOS inverter advantages include the following.

  • The CMOS inverter’s steady-state power dissipation is negligible virtually, apart from small power dissipation because of leakage currents.
  • The VTC (voltage transfer characteristic) exhibits a complete o/p voltage swing in between 0 V & VDD, and the transition of voltage transfer characteristic is normally very sharp. Thus, the characteristics of the CMOS inverter look like an ideal inverter.
  • These inverters use electricity once they are switched ON & OFF resulting in less power consumption. As a result, these inverters generate extremely less waste heat to make them highly efficient, so used in small and delicate electronic devices.
  • These inverters include high noise immunity, which lets them block both incoming & outgoing frequency spikes.
  • These are low-cost to produce mass.

Disadvantages

The CMOS inverter disadvantages include the following.

  • As compared to other inverters, the switching speed of the CMOS inverter is high.
  • These are very difficult to fabricate due to both the transistors used on the same Silica piece.
  • It uses two transistors to make an inverter, so it uses more space on the IC as compared to the NMOS inverter.

Applications

The applications of CMOS inverters include the following.

  • CMOS inverters are used in different ICs (integrated circuits) like microprocessors, static RAM, microcontrollers, data converters, image sensors & transceivers.
  • These are found in mobile devices, digital cameras, home computers, cell phones, routers, network servers, modems & virtually each other electronic device that needs logic functions.

What is the function of a CMOS inverter?

CMOS inverters are the most frequently used flexible MOSFET inverters that are used in designing integrated circuits like CD4069UB CMOS hex inverter, CD4069UBE, CD40106BE, etc. They function through very little power loss & high speed. These inverters are used for generating data in small electronic circuits.

What is an inverter gate in CMOS?

In CMOS, an inverter gate is one type of logic gate, used to implement logical negation (¬).

Which gate is also called an inverter?

NOT logic gate is also called an inverter because NOT gate gives low output for high input and provides a high output for low input.

Why do we use P substrate in CMOS?

A P-type substrate in CMOS allows constructing n-channel transistors with no additional doping. So this is the main benefit because the doping is low, the electrons mobility is higher, the gain is higher and the transistor’s switching speed is high.

Thus, this is all about an overview of a CMOS inverter that operates at high speed and uses less power. These inverters have good logic buffer characteristics so, their noise margins within both high & low conditions are large. Here is a question for you, what P-MOS transistor?

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