TTL vs CMOS: two logic families, and how to choose

TTL (transistor-transistor logic) and CMOS (complementary metal-oxide-semiconductor) are the two classic families of logic chips. TTL builds its gates from bipolar transistors and runs from a 5 V supply: the 74LS00, a low-power Schottky TTL quad NAND, is specified for 4.75 V to 5.25 V on TI's datasheet. CMOS builds its gates from pairs of NMOS and PMOS transistors: the 74HC00, the high-speed CMOS version with the same logic and pinout, runs from 2 V to 6 V and draws much less power, and 4000-series CMOS such as the CD4011 runs from 3 V to 18 V. Modern chips are almost all CMOS. Same logic, different transistors: choose by supply voltage, power and what you are connecting to.

What each family is good at

TTL defined the classic 7400 series, so many textbooks, tutorials and classic projects are written around its 5 V chips. CMOS wins on power: in any steady state exactly one of its two transistor networks conducts, so it draws almost no current except while switching, as the CMOS power lesson explains, and its wide supply range suits batteries. The one habit CMOS demands is to tie unused inputs to a supply rail, since TI's 74HC datasheets warn that a floating CMOS input can drift and draw current.

Both live in digiwleea

digiwleea simulates logic at the CMOS switch level: you build NOT, NAND and NOR from NMOS and PMOS transistors, the way 74HC and 4000-series chips are made. The chip pages run the same logic for both families with their datasheet pinouts: the 7400 NAND chip (74LS00 and 74HC00), the CD4011, the 74LS47 decoder and the 74HC595 shift register. It models logic values, not voltages or currents, so supply ranges and power are on the datasheets, not in the simulation.

Side by side

	74LS (TTL)	74HC (CMOS)	4000 series (CMOS)
Transistors	Bipolar	NMOS and PMOS	NMOS and PMOS
Supply	5 V (4.75 V to 5.25 V)	2 V to 6 V	3 V to 18 V
Static power	Higher	Very low	Very low
Example chip	74LS00 quad NAND	74HC00 quad NAND	CD4011 quad NAND

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More comparisons: Digital logic concepts compared

Signed vs unsigned Bit vs byte Fixed point vs floating point KB vs KiB Cache mapping compared ASCII vs Unicode Open drain vs push-pull Ladder logic vs logic gates Moore vs Mealy MOSFET vs BJT Active high vs active low Relay vs transistor

See for yourself: build a logic gate from transistors in the simulator, then follow the path from sand to an 8-bit CPU.

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Frequently asked

What is the difference between TTL and CMOS?

TTL gates are built from bipolar transistors and run from a 5 V supply; CMOS gates are built from complementary NMOS and PMOS transistors, draw almost no power when not switching and, depending on the family, run from about 2 V to 18 V.

Is 74HC TTL or CMOS?

74HC is CMOS: high-speed CMOS with the same logic functions and pinouts as the classic 74-series TTL chips, running from 2 V to 6 V.

Is 74LS TTL?

Yes. 74LS is low-power Schottky TTL, built from bipolar transistors, for a 5 V supply (4.75 V to 5.25 V for the 74LS00 on TI's datasheet).

Why should unused CMOS inputs be tied high or low?

A floating CMOS input can drift between logic levels and draw extra current, so TI's 74HC datasheets say unused inputs must be terminated to VCC or GND.