The Integrated Circuits or ICs used in the computers are usually manufactured in different techniques and these ICs are classified according to their features and manufacturing process. Two most popular types of such chips are CMOS and TTL.
These two chips are significantly different in several aspects making each of them function differently and this article discusses them to make you more knowledgeable.
You will also find some additional information which will help you to select one of the two technologies very easily.
In This Article
- The CMOS chips are much more efficient both in terms of operation and power in comparison to the TTL chips due to the higher density of logic gates and transistors that have lower current requirements.
- The CMOS technology is much more modern because it uses MOSFETs that help in performing the logic functions in a much better way than the TTL technology which is older and uses Bipolar Junction Transistors in the chips.
- The components that make the CMOS chips are much more expensive than those used in making the TTL circuits. However, the smaller size of these chips makes them much less expensive at the system level.
- The CMOS chips usually come with much higher propagation delays than the TTL chips.
- The much longer rise and fall time of the CMOs chips simplifies the digital signals and makes them less expensive and the chips more energy efficient at the same time.
The 27 Differences Between CMOS and TTL
1. Full Form
TTL is the acronym for Transistor-Transistor Logic and the full form of CMOS is Complementary Metal Oxide Semiconductor.
2. Invented In
The CMOS technology is much newer technology, being invented in 1968.
In comparison, the TTL technology is a bit old technology that was invented in the year 1963.
TTL chips are made up of Bipolar Junction Transistors or BJTs that run at 5 volts at a frequency of up to 25 MHz while performing both logic and amplifying functions. It is therefore named Transistor-Transistor Logic.
On the other hand, CMOS uses both symmetrical and complementary pairs of the N-type and P-type Metal Oxide Field Effect Transistors or MOSFETs to support N-channel and P-channel devices respectively and are therefore called ‘complementary’ and it helps it to perform the logic functions.
4. Power Consumption
The TTL consumes much more power than CMOS due to the fact that it is made up of bipolar transistors that require some amount of current to be turned on. These chips consume more power as compared to the CMOs even when they are at rest.
On the other hand, CMOS transistors consume less power because the field-effect transistors already have an electric field at the gate that is enough for the conduction of the semiconductor.
Therefore, in theory, it does not need to draw any additional current apart from a tiny amount of leakage current of the gate which is usually of picoamps or nanoamps, especially at slower speeds.
5. Power Consumption Rate
The rate of power consumption however can increase in a CMOS chip much faster in comparison to the power consumption rate of the TTL circuits and can reach up to 10 nanowatts by a single logic gate.
On the other hand, one logic gate of the TTL circuit can consume up to 10 megawatts of power.
The outputs of TTL are either pull ups or totem poles which can swing within 0.5 volts of the rails only.
However, the output currents of TTL are much higher than the CMOS. On the other hand, the output currents are much lower for CMOS than TTL.
CMOS logic is perfect for miniaturization due to their smaller requirements for current that allows packing millions of transistors in a small area which does not raise the current consumption impractically. However, CMOS is not as rugged as TTL.
On the other hand, TTL does not allow such miniaturization due to its higher need for current. However, TTL is more rugged than the CMOS.
The Field Effect Transistors of the CMOS provide isolation among them on the basis of the thin silicon oxide layer present between the channel and the gate.
This layer of oxide is usually nanometers thick and also comes with a very low breakdown voltage which seldom goes over 20 volts even in the case of the high power FETs. All these facts make CMOS much more vulnerable to overvoltage and electrostatic discharge.
On the other hand, this is not the case with the TTL chips which make them quite resilient and enduring when it comes to electrostatic discharge and overvoltage. However, the TTL chips are more robust than the CMOS chips when it is at rest.
The inputs of CMOS are more often than not pulled up, down or they are grounded when they are left floating. This is because they build up charges slowly which results in specious changes in the output state.
On the other hand, the TTL chips do not experience such issues in most of the cases because usually the inputs are transistor based. This is less sensitive to noise and behaves more like a diode, all due to its lesser impedance.
Usually there are a few specific types of logic gates in CMOS such as the CMOS Inverter, CMOS NOR, CMOS NAND, and CMOS Operational Amplifiers.
On the other hand, there are also a few specific types of TTL such as Standard Transistor-Transistor Logic, Schottky Transistor-Transistor Logic, Advanced Schottky Transistor-Transistor Logic, Fast Transistor-Transistor Logic, Low Power Transistor-Transistor Logic, and High Power Transistor-Transistor Logic.
The components used to make the CMOS are usually much more expensive than the components used to make the TTL chips.
The size of the CMOS chips is much smaller than the TTL chips which however makes them less expensive at the system level.
13. Propagation Delays
Though both CMOS and TTL chips have propagation delays present in them it is usually much higher in the CMOS chips often ranging between 20nS to 50nS on average as opposed to the 10nS propagation delays of the TTL chips.
14. Rise and Fall Times
The CMOS chips usually have a much higher rise and fall times which make the digital signals less expensive and simpler. The longer rise and fall times of these chips however make them more energy efficient as well.
On the other hand, the rise and small times of the TTL chips is not as long as in the CMOS chips thereby affecting their digital signals in a different way and making them less energy efficient.
15. Voltage Level Range
The voltage level ranges between 4.75 volts and 5.25 volts in the TTL chips.
On the other hand, in the case of the CMOS chips the voltage level ranges between 0 to 1/3 VDD and 2/3 VDD to 1 VDD at low and high levels respectively.
The CMOS technology is more favored because it is more economical as compared to the TTL logic.
This is due to its low current requirements that make it much easier to design the circuits and ensure optimal power management.
17. Electromagnetic Disruptions
The CMOS components can cause electromagnetic disruptions due to their higher sensitivity in comparison to the TTL components.
The CMOS allows much lower noise during transmission in comparison to the TTL chips.
On the other hand, the TTL chips do not come with the same noise immunity as the CMOS circuits and therefore allow higher noise during transmission.
19. Standard Output Number
The TTL chips allow connecting a maximum of 10 standard loads under standard operation conditions to the output of the gate which is the fan-out.
On the other hand, there can be as many as 50 of such standard loads connected in the CMOS.
20. Standard Input Number
As for the number of standard inputs which is a fan-in that can be connected to the gate of TTL is approximately 12 to 14.
However, in comparison the same number comes down to 10 only in the case of CMOS chips.
21. Basic Gates
In the construction of the TTL chips the NAND gate is used as the basic gate but, in comparison, in the CMOS circuits both the NAND gate and NOR gates are used.
The design of the CMOS is considered to be much simpler. On the other hand, the TTL chips typically come with a very complex design.
23. Number of Parts
In a CMOS chip, one single logic gate may contain as little as two Field Effect Transistors.
On the other hand, in a TTL chip there is a substantial number of additional components required such as the resistors.
24. Vulnerability to Damages
The CMOS chips and its components are more vulnerable to physical damages as well as damages due to electrostatic discharge in comparison to the TTL circuits and its parts.
25. Replacement Logic
There are a few particular CMOS chips that come with TTL logic which can be used as a replacement for the TTL chips. On the other hand, the TTL chips do not come with any CMOS logic.
The CMOS chips are much more efficient than the TTL circuits due to the different types of technologies used in making these two types of chips.
27. Logic Gate Density
The density of logic gates in the TTL chips is less as compared to the CMOS chips.
The greater density of logic gates of the CMOS circuits offers it the biggest advantage in operation.
Which is Better – CMOS or TTL?
There are a lot of ways, both in features and performance, in which the CMOS has outmoded the TTL chips. Considering the improved performance it is safe to say that there is no real benefit of using a TTL chip anymore these days in any given way.
You will get a few TTL chips these days though with the input levels being standardized to some extent.
So, it is not uncommon to have a TTL output stage but the CMOS seems to be the clear winner in almost any and every way.
Right from the design of the CMOS chips to the benefits offered by them in terms of power and performance, they are far superior than the TTL chips which is why these are used extensively in modern devices and designs.
Most importantly, the easy availability of these low power CMOS chips make them more economical to use than the TTL chips.
However, these chips are quite delicate and susceptible to damages and therefore need proper attention and care while using them.
So, if you really have to choose, always go with the Complementary Metal Oxide Semiconductor chips.
After going through the differences between TTL and CMOS chips entailed in this article, it must be very clear to you by now that the CMOS chips are far superior and can be used easily and effectively as the right replacement for the Transistor-Transistor Logic Integrated Circuits.