CMOS vs TTL Logic: Differences Explained

TTL (transistor-transistor logic) builds gates from bipolar transistors and was the standard digital family from the 1960s. CMOS builds gates from complementary MOS transistors and has since replaced it almost everywhere. The big difference is power: a CMOS gate draws almost no current when it is not switching, while a TTL gate draws current all the time. CMOS also tolerates a wide supply range, has larger noise margins and can be packed by the billions on a chip.

AspectTTLCMOS
TransistorsBipolar transistors, with multi-emitter input transistorsComplementary nMOS and pMOS transistors
Static powerHigh: current flows even when idleVery low: only leakage current
Supply voltageFixed 5 V (about plus or minus 5 percent)Wide range, from about 1 V to 15 V depending on the family
Logic levelsLow up to about 0.8 V and high from about 2.0 VAbout 30 percent and 70 percent of the supply voltage
Noise marginSmall (about 0.4 V)Large (about 30 percent of the supply)
Output swingDoes not reach the supply rails fullySwings fully between ground and the supply
InputsDraw current; an unconnected input floats highDraw almost no current, but unused inputs must be tied to a level
SpeedFast in its day (74S, 74F families)Equal or faster in modern families
Integration densityLow, because of its power drawVery high: the basis of VLSI
Typical usesLegacy systems, education and old equipmentNearly all modern digital chips, including processors and memories

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

TTL
Mostly found in legacy designs and when repairing old equipment.
CMOS
The choice for essentially every new digital design.
Mixed
Some CMOS families (such as 74HCT) accept TTL input levels, so they can connect to older TTL parts.

FAQ

Why does CMOS use less power than TTL?

A CMOS gate draws almost no current when it is not switching, because one of its two complementary transistors is always off. A TTL gate draws current all the time.

What does 74HCT mean?

It is a CMOS family with TTL-compatible input levels, so it can be connected to older TTL parts.

Do unused CMOS inputs need to be connected?

Yes. CMOS inputs draw almost no current, so an unconnected input floats and can cause erratic behavior and extra power use. Tie it to ground or to the supply.

Is TTL still used?

Rarely. It appears in legacy designs and old equipment, while nearly all modern digital chips use CMOS.

Check your understanding

Which is a typical family of logic ICs built with bipolar transistors?

  1. ATTL
  2. BCMOS 4000 series
  3. CNMOS logic
Show answer and explanation

Answer: A. TTL

TTL (transistor-transistor logic) uses multi-emitter bipolar transistors at its inputs and, as the 7400 series, became the standard for digital circuits from the 1960s. It runs from 5 V and reads inputs below about 0.8 V as low and above about 2 V as high. Because it draws current even when idle, CMOS has replaced it, but its legacy lives on in CMOS families with TTL-compatible inputs such as the 74HCT.

Why is the static (steady-state) power consumption of a CMOS inverter small?

  1. ABecause the supply voltage is high
  2. BBecause current always flows into the gate
  3. CBecause whether the input is 0 or 1, either the pMOS or the nMOS is off, so almost no current flows from supply to ground
Show answer and explanation

Answer: C. Because whether the input is 0 or 1, either the pMOS or the nMOS is off, so almost no current flows from supply to ground

In a CMOS inverter, input 0 turns the pMOS on and nMOS off, and input 1 does the reverse, so in steady state there is always an off transistor between supply and ground and almost no through current flows. Power is consumed mainly by charging and discharging load capacitance during switching: P = α·C·V²·f. With scaling, however, off-state leakage has grown, and static power is no longer negligible in advanced chips.

What does the "C" (complementary) in CMOS mean?

  1. ACircuits are built only from nMOS transistors connected in series
  2. BnMOS and pMOS transistors are combined and operate in a complementary way
  3. CBipolar transistors and MOSFETs are combined
Show answer and explanation

Answer: B. nMOS and pMOS transistors are combined and operate in a complementary way

CMOS pairs nMOS and pMOS transistors. When the input changes, one turns on and the other off, so in steady state there is no path from supply to ground and power consumption stays low. The output swings fully to the supply and ground rails, giving large noise margins. Proposed in 1963, CMOS replaced power-hungry NMOS and bipolar logic as the standard IC technology in the 1980s.

  1. In a CMOS inverter, what is the current that flows from the supply to ground while the input is switching called?
  2. In a CMOS circuit, if only the supply voltage is halved with everything else unchanged, what happens to dynamic power consumption?

Practice Logic circuits questions

Key terms explained

CMOS
A circuit style that combines nMOS and pMOS transistors in a complementary way. Almost no current flows through it in steady state, so power consumption is low.

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