Editing Transistor–transistor logic (section)

==Comparison with other logic families==
{{Main|Logic family}}

TTL devices consume substantially more power than equivalent [[CMOS]] devices at rest, but power consumption does not increase with clock speed as rapidly as for CMOS devices.<ref>{{citation |author1-link=Paul Horowitz |last1=Horowitz |first1=Paul |last2=Hill |first2=Winfield |title=The Art of Electronics |edition=2nd |publisher=Cambridge University Press |year=1989 |isbn=0-521-37095-7 |page=[https://archive.org/details/artofelectronics00horo/page/970 970] |url=https://archive.org/details/artofelectronics00horo/page/970 }} states, "...CMOS devices consume power proportional to their switching frequency...At their maximum operating frequency they may use more power than equivalent bipolar TTL devices."</ref>  Compared to contemporary [[Emitter coupled logic|ECL]] circuits, TTL uses less power and has easier design rules but is substantially slower.  Designers can combine ECL and TTL devices in the same system to achieve best overall performance and economy, but level-shifting devices are required between the two logic families.  TTL is less sensitive to damage from [[electrostatic discharge]] than early CMOS devices.

Due to the output structure of TTL devices, the output impedance is asymmetrical between the high and low state, making them unsuitable for driving transmission lines.  This drawback is usually overcome by buffering the outputs with special line-driver devices where signals need to be sent through cables.  ECL, by virtue of its symmetric low-impedance output structure, does not have this drawback.

The TTL "totem-pole" output structure often has a momentary overlap when both the upper and lower transistors are conducting, resulting in a substantial pulse of current drawn from the power supply.  These pulses can couple in unexpected ways between multiple integrated circuit packages, resulting in reduced noise margin and lower performance.  TTL systems usually have a [[decoupling capacitor]] for every one or two IC packages, so that a current pulse from one TTL chip does not momentarily reduce the supply voltage to another.

Since the mid 1980s, several manufacturers supply CMOS logic equivalents with TTL-compatible input and output levels, usually bearing part numbers similar to the equivalent TTL component and with the same [[pinouts]]. For example, the 74HCT00 series provides many drop-in replacements for bipolar [[7400 series]] parts, but uses [[CMOS]] technology.  (The "T" in "HCT" stands for "TTL-compatible".  The related 74HC00 series also uses CMOS technology but is not TTL-compatible.)