Transistor–transistor logic (TTL) is a class of digital circuits
built from bipolar junction transistors. Its name signifies that
transistors perform both the logic function (the first "transistor")
and the amplifying function (the second "transistor"); it is the same
naming convention used in resistor–transistor logic (RTL) and
diode–transistor logic (DTL).
TTL integrated circuits (ICs) were widely used in applications such as
computers, industrial controls, test equipment and instrumentation,
consumer electronics, and synthesizers. Sometimes TTL-compatible logic
levels are not associated directly with TTL integrated circuits, for
example as a label on the inputs and outputs of electronic
After their introduction in integrated circuit form in 1963 by
Sylvania, TTL integrated circuits were manufactured by several
semiconductor companies. The
7400 series by
Texas Instruments became
particularly popular. TTL manufacturers offered a wide range of logic
gates, flip-flops, counters, and other circuits. Variations of the
original TTL circuit design offered higher speed or lower power
dissipation to allow design optimization. TTL devices were originally
made in ceramic and plastic dual-in-line (DIP) packages, and flat-pack
form. TTL chips are now also made in surface-mount packages.
TTL became the foundation of computers and other digital electronics.
Even after larger scale integrated circuits made
multiple-circuit-board processors obsolete, TTL devices still found
extensive use as the glue logic interfacing between more densely
2.1 Fundamental TTL gate
2.2 TTL with a "totem-pole" output stage
3 Interfacing considerations
5 Comparison with other logic families
7.1 Analog applications
8 See also
10 External links
A real-time clock built of TTL chips around 1979.
TTL was invented in 1961 by
James L. Buie
James L. Buie of TRW, which declared it,
"particularly suited to the newly developing integrated circuit design
technology." The original name for TTL was transistor-coupled
transistor logic (TCTL). The first commercial integrated-circuit
TTL devices were manufactured by Sylvania in 1963, called the Sylvania
Logic family (SUHL). The Sylvania parts were
used in the controls of the Phoenix missile. TTL became popular
with electronic systems designers after
Texas Instruments introduced
the 5400 series of ICs, with military temperature range, in 1964 and
the later 7400 series, specified over a narrower range and with
inexpensive plastic packages, in 1966.
Texas Instruments 7400 family became an industry standard.
Compatible parts were made by Motorola, AMD, Fairchild, Intel,
Intersil, Signetics, Mullard, Siemens, SGS-Thomson, Rifa, National
Semiconductor, and many other companies, even in the Eastern
Bloc (Soviet Union, GDR, Poland, Czechoslovakia, Hungary, Romania -
for details see 7400 series). Not only did others make compatible TTL
parts, but compatible parts were made using many other circuit
technologies as well. At least one manufacturer, IBM, produced
non-compatible TTL circuits for its own use;
IBM used the technology
IBM 4300, and
The term "TTL" is applied to many successive generations of bipolar
logic, with gradual improvements in speed and power consumption over
about two decades. The most recently introduced family 74Fxx is still
sold today, and was widely used into the late 90s. 74AS/ALS Advanced
Schottky was introduced in 1985. As of 2008, Texas Instruments
continues to supply the more general-purpose chips in numerous
obsolete technology families, albeit at increased prices. Typically,
TTL chips integrate no more than a few hundred transistors each.
Functions within a single package generally range from a few logic
gates to a microprocessor bit-slice. TTL also became important because
its low cost made digital techniques economically practical for tasks
previously done by analog methods.
The Kenbak-1, ancestor of the first personal computers, used TTL for
its CPU instead of a microprocessor chip, which was not available in
Datapoint 2200 from 1970 used TTL components for its CPU
and was the basis for the 8008 and later the x86 instruction set.
Xerox Alto and 1981 Star workstations, which introduced the
graphical user interface, used TTL circuits integrated at the level of
Arithmetic logic units (ALU)s and bitslices, respectively. Most
computers used TTL-compatible "glue logic" between larger chips well
into the 1990s. Until the advent of programmable logic, discrete
bipolar logic was used to prototype and emulate microarchitectures
Fundamental TTL gate
NAND gate with a simple output stage (simplified).
TTL inputs are the emitters of bipolar transistors. In the case of
NAND inputs, the inputs are the emitters of multiple-emitter
transistors, functionally equivalent to multiple transistors where the
bases and collectors are tied together. The output is buffered by
a common emitter amplifier.
Inputs both logical ones. When all the inputs are held at high
voltage, the base–emitter junctions of the multiple-emitter
transistor are reverse-biased. Unlike DTL, a small “collector”
current (approximately 10µA) is drawn by each of the inputs. This is
because the transistor is in reverse-active mode. An approximately
constant current flows from the positive rail, through the resistor
and into the base of the multiple emitter transistor. This current
passes through the base–emitter junction of the output transistor,
allowing it to conduct and pulling the output voltage low (logical
An input logical zero. Note that the base–collector junction of the
multiple-emitter transistor and the base–emitter junction of the
output transistor are in series between the bottom of the resistor and
ground. If one input voltage becomes zero, the corresponding
base–emitter junction of the multiple-emitter transistor is in
parallel with these two junctions. A phenomenon called current
steering means that when two voltage-stable elements with different
threshold voltages are connected in parallel, the current flows
through the path with the smaller threshold voltage. That is, current
flows out of this input and into the zero (low) voltage source. As a
result, no current flows through the base of the output transistor,
causing it to stop conducting and the output voltage becomes high
(logical one). During the transition the input transistor is briefly
in its active region; so it draws a large current away from the base
of the output transistor and thus quickly discharges its base. This is
a critical advantage of TTL over
DTL that speeds up the transition
over a diode input structure.
The main disadvantage of TTL with a simple output stage is the
relatively high output resistance at output logical "1" that is
completely determined by the output collector resistor. It limits the
number of inputs that can be connected (the fanout). Some advantage of
the simple output stage is the high voltage level (up to VCC) of the
output logical "1" when the output is not loaded.
A common variation omits the collector resistor of the output
transistor, making an open-collector output. This allows the designer
to fabricate logic by connecting the open-collector outputs of several
logic gates together and providing a single external pull-up resistor.
If any of the logic gates becomes logic low (transistor conducting),
the combined output will be low. Examples of this type of gate are the
7401 and 7403 series.
Open-collector outputs of some gates have a
higher maximum voltage, such as 15V for the 7426, useful when
driving other than TTL loads.
TTL with a "totem-pole" output stage
Standard TTL NAND with a "totem-pole" output stage, one of four in
To solve the problem with the high output resistance of the simple
output stage the second schematic adds to this a "totem-pole"
("push–pull") output. It consists of the two n-p-n transistors V3
and V4, the "lifting" diode V5 and the current-limiting resistor R3
(see the figure on the right). It is driven by applying the same
current steering idea as above.
When V2 is "off", V4 is "off" as well and V3 operates in active region
as a voltage follower producing high output voltage (logical "1").
When V2 is "on", it activates V4, driving low voltage (logical "0") to
the output. Again there is a current-steering effect: the series
combination of V2's C-E junction and V4's B-E junction is in parallel
with the series of V3 B-E, V5's anode-cathode junction, and V4 C-E.
The second series combination has the higher threshold voltage, so no
current flows through it, i.e. V3 base current is deprived. Transistor
V3 turns "off" and it does not impact on the output.
In the middle of the transition, the resistor R3 limits the current
flowing directly through the series connected transistor V3, diode V5
and transistor V4 that are all conducting. It also limits the output
current in the case of output logical "1" and short connection to the
ground. The strength of the gate may be increased without
proportionally affecting the power consumption by removing the pull-up
and pull-down resistors from the output stage.
The main advantage of TTL with a "totem-pole" output stage is the low
output resistance at output logical "1". It is determined by the upper
output transistor V3 operating in active region as an emitter
follower. The resistor R3 does not increase the output resistance
since it is connected in the V3 collector and its influence is
compensated by the negative feedback. A disadvantage of the
"totem-pole" output stage is the decreased voltage level (no more than
3.5 V) of the output logical "1" (even if the output is unloaded). The
reason of this reduction are the voltage drops across the V3
base–emitter and V5 anode–cathode junctions.
Like DTL, TTL is a current-sinking logic since a current must be drawn
from inputs to bring them to a logic 0 level. At low input voltage,
the TTL input sources current which must be absorbed by the previous
stage. The maximum value of this input current is about 1.6 mA
for a standard TTL gate. The input source has to be low-resistive
enough (<500 Ω) so that the flowing current creates only a
negligible voltage drop (<0.4 V) across it, for the input to
be considered as a logical "0" (with a 0.4 V "noise margin", see
below). The output stage of the most common TTL gates is specified to
function correctly when driving up to 10 standard input stages (a
fanout of 10). TTL inputs are sometimes simply left floating to
provide a logical "1", though this usage is not recommended.
Standard TTL circuits operate with a 5-volt power supply. A TTL input
signal is defined as "low" when between 0 V and 0.8 V with
respect to the ground terminal, and "high" when between 2 V and
VCC (5 V), and if a voltage signal ranging between
0.8 V and 2.0 V is sent into the input of a TTL gate, there
is no certain response from the gate and therefore it is considered
"uncertain" (precise logic levels vary slightly between sub-types and
by temperature). TTL outputs are typically restricted to narrower
limits of between 0.0 V and 0.4 V for a "low" and between
2.4 V and VCC for a "high", providing at least 0.4 V of
noise immunity. Standardization of the TTL levels is so ubiquitous
that complex circuit boards often contain TTL chips made by many
different manufacturers selected for availability and cost,
compatibility being assured; two circuit board units off the same
assembly line on different successive days or weeks might have a
different mix of brands of chips in the same positions on the board;
repair is possible with chips manufactured years (sometimes over a
decade) later than original components. Within usefully broad limits,
logic gates can be treated as ideal Boolean devices without concern
for electrical limitations. The 0.4V noise margins are adequate
because of the low output impedance of the driver stage, that is, a
large amount of noise power superimposed on the output is needed to
drive an input into an undefined region.
In some cases (e.g., when the output of a TTL logic gate needs to be
used for driving the input of a
CMOS gate), the voltage level of the
"totem-pole" output stage at output logical "1" can be increased
closer to VCC by connecting an external resistor between the V3
collector and the positive rail. It pulls up the V5 cathode and
cuts-off the diode. However, this technique actually converts the
sophisticated "totem-pole" output into a simple output stage having
significant output resistance when driving a high level (determined by
the external resistor).
Like most integrated circuits of the period 1963–1990, commercial
TTL devices are usually packaged in dual in-line packages (DIPs),
usually with 14 to 24 pins, for through-hole or socket mounting.
The DIPs were usually made of epoxy plastic (PDIP) for
commercial-grade parts or of ceramic (CDIP) for military-grade parts.
Beam-lead chip dies without packages were made for assembly into
larger arrays as hybrid integrated circuits. Parts for military and
aerospace applications were packaged in flatpacks, a form of
surface-mount package, with leads suitable for welding or soldering to
printed circuit boards. Today, many TTL-compatible devices are
available in surface-mount packages, which are available in a wider
array of types than through-hole packages.
TTL is particularly well suited to bipolar integrated circuits because
additional inputs to a gate merely required additional emitters on a
shared base region of the input transistor. If individually packaged
transistors were used, the cost of all the transistors would
discourage one from using such an input structure. But in an
integrated circuit, the additional emitters for extra gate inputs add
only a small area.
At least one computer manufacturer, IBM, built its own flip chip
integrated circuits with TTL; these chips were mounted on ceramic
Comparison with other logic families
Main article: 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. Compared to contemporary 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
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.
Several manufacturers now 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
7400 series parts, but uses
Successive generations of technology produced compatible parts with
improved power consumption or switching speed, or both. Although
vendors uniformly marketed these various product lines as TTL with
Schottky diodes, some of the underlying circuits, such as used in the
LS family, could rather be considered DTL.
Variations of and successors to the basic TTL family, which has a
typical gate propagation delay of 10ns and a power dissipation of
10 mW per gate, for a power–delay product (PDP) or switching
energy of about 100 pJ, include:
Low-power TTL (L), which traded switching speed (33ns) for a reduction
in power consumption (1 mW) (now essentially replaced by CMOS
High-speed TTL (H), with faster switching than standard TTL (6ns) but
significantly higher power dissipation (22 mW)
Schottky TTL (S), introduced in 1969, which used
Schottky diode clamps
at gate inputs to prevent charge storage and improve switching time.
These gates operated more quickly (3ns) but had higher power
dissipation (19 mW)
Low-power Schottky TTL (LS) – used the higher resistance values of
low-power TTL and the Schottky diodes to provide a good combination of
speed (9.5ns) and reduced power consumption (2 mW), and PDP of
about 20 pJ. Probably the most common type of TTL, these were used as
glue logic in microcomputers, essentially replacing the former H, L,
and S sub-families.
Fast (F) and Advanced-Schottky (AS) variants of LS from Fairchild and
TI, respectively, circa 1985, with "Miller-killer" circuits to speed
up the low-to-high transition. These families achieved PDPs of 10 pJ
and 4 pJ, respectively, the lowest of all the TTL families.
Low-voltage TTL (LVTTL) for 3.3-volt power supplies and memory
Most manufacturers offer commercial and extended temperature ranges:
7400 series parts are rated from 0 to
70 °C, and 5400 series devices over the military-specification
temperature range of −55 to +125 °C.
Special quality levels and high-reliability parts are available for
military and aerospace applications.
Radiation-hardened devices (for example from the SNJ54 series) are
offered for space applications.
Before the advent of VLSI devices, TTL integrated circuits were a
standard method of construction for the processors of mini-computer
and mainframe processors; such as the DEC
VAX and Data General
Eclipse, and for equipment such as machine tool numerical controls,
printers and video display terminals. As microprocessors became more
functional, TTL devices became important for "glue logic"
applications, such as fast bus drivers on a motherboard, which tie
together the function blocks realized in VLSI elements.
While originally designed to handle logic-level digital signals, a TTL
inverter can be biased as an analog amplifier. Connecting a resistor
between the output and the input biases the TTL element as a negative
feedback amplifier. Such amplifiers may be useful to convert analog
signals to the digital domain but would not ordinarily be used where
analog amplification is the primary purpose. TTL inverters can
also be used in crystal oscillators where their analog amplification
ability is significant.
A TTL gate may operate inadvertently as an analog amplifier if the
input is connected to a slowly changing input signal that traverses
the unspecified region from 0.8 V to 2 V. The output can be
erratic when the input is in this range. A slowly changing input like
this can also cause excess power dissipation in the output circuit. If
such an analog input must be used, there are specialized TTL parts
Schmitt trigger inputs available that will reliably convert the
analog input to a digital value, effectively operating as a one bit A
to D converter.
7400 series integrated circuits
Positive emitter-coupled logic
Positive emitter-coupled logic (PECL)
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Wikimedia Commons has media related to TTL.
Fairchild Semiconductor. An Introduction to and Comparison of 74HCT
CMOS Logic (Application Note 368). 1984. (for relative
ESD sensitivity of TTL and CMOS.)
Texas Instruments logic family application notes
TTL NAND and AND gates from Lessons In Electric Circuits by Tony
Depletion-load NMOS logic
Depletion-load NMOS logic (including HMOS)
Diode–transistor logic (DTL)
Direct-coupled transistor logic (DCTL)
Emitter-coupled logic (ECL)
Gunning transceiver logic (GTL)
Integrated injection logic
Integrated injection logic (I2L)
Resistor–transistor logic (RTL)
Transistor–transistor logic (TTL)
Current mode logic / Source-coupled logic (CML/SCL)