Difference Between CMOS and TTL


With the advent of semiconductor technology, integrated circuits were developed, and they have found its way to every form of technology involving electronics. From communication to medicine, every device has integrated circuits, where circuits, if implemented with ordinary components would consume large space and energy, is built on a miniature silicon wafer using advanced semiconductor technologies present today.

All the digital integrated circuits are implemented using logic gates as their fundamental building block. Each gate is constructed using small electronic elements such as transistors, diodes and resistors. The set of logic gates constructed using coupled transistors and resistors are collectively known as TTL gate family. To overcome the shortcomings of TTL gates more technologically advanced methodologies were designed for gates construction, such as pMOS, nMOS and the most recent and popular complementary metal oxide semiconductor type, or CMOS.

In an integrated circuit, the gates are built on a silicon wafer, technically called as substrate. Based on the technology used for gate construction, ICs are also categorized into families of TTL and CMOS, because of the inherent properties of the fundamental gate design such as signal voltage levels, power consumption, response time and the scale of integration.

More about TTL

James L. Buie of TRW invented TTL in 1961, and it served as a replacement for the DL and RTL logic, and was the IC of choice for instrumentation and computer circuits for a long time. TTL integration methods have been continuously developing, and modern packages are still used in specialized applications.

TTL logic gates are built of coupled bipolar junction transistors and resistors, to create a NAND gate. Input Low (IL) and Input High (IH) have voltage ranges 0 < IL < 0.8 and 2.2 < IH < 5.0 respectively. The Output Low and Output High voltage ranges are 0 < OL < 0.4 and 2.6 < OH < 5.0 in the order. The acceptable input and output voltages of the TTL gates are subjected to static discipline to introduce a higher level of noise immunity in the signal transmission.

A TTL gate, on average, has a power dissipation of 10mW and a propagation delay of 10nS, when driving a 15pF/400 ohm load. But the power consumption is rather constant compared to the CMOS. TTL also has a higher resistance to electromagnetic disruptions.

Many variants of TTL are developed for specific purposes such as radiation hardened TTL packages for space applications and Low-power Schottky TTL (LS) that provides a good combination of speed (9.5ns) and reduced power consumption (2mW)

More about CMOS

In 1963, Frank Wanlass of Fairchild Semiconductor invented the CMOS technology. However, the first CMOS integrated circuit was not produced until 1968. Frank Wanlass patented the invention in 1967 while working at RCA, at that time.

CMOS logic family has become the most widely used logic families due to its numerous advantages such as less power consumption and low noise during transmission levels. All the common microprocessors, microcontrollers, and integrated circuits use CMOS technology.

CMOS logic gates are constructed using field effect transistors FETs, and the circuitry is mostly devoid of resistors. As a result, CMOS gates do not consume any power at all during the static state, where the signal inputs remain unchanged. Input Low (IL) and Input High (IH) have voltage ranges 0 < IL < 1.5 and 3.5 < IH < 5.0 and the Output Low and Output High voltage ranges are 0 < OL < 0.5 and 4.95 < OH < 5.0 respectively.

What is the difference between CMOS and TTL?

• TTL components are relatively cheaper than the equivalent CMOS components. However, CMOs technology tends to be economical on a larger scale as the circuit components are smaller and requires less regulation compared to the TTL components.

• CMOS components do not consume power during the static state, but power consumption increases with the clock rate. TTL, on the other hand, has a constant power consumption level.

• Since CMOS has low current requirements, power consumption is limited and the circuits, therefore, cheaper and easier to be designed for power management.

• Due to longer rise and fall times, digital signals in CMOs environment can be less expensive and complicated.

• CMOS components are more sensitive to electromagnetic disruptions than TTL components.