Transistor–transistor logic

Transistor–transistor logic is a logic family built from bipolar junction transistors. Its name signifies that transistors perform both the logic function and the amplifying function, as opposed to earlier resistor–transistor logic and diode–transistor logic.
TTL integrated circuits were widely used in applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, and synthesizers.
After their introduction in integrated circuit form in 1963 by Sylvania Electric Products, 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 TTL devices were originally made in ceramic and plastic dual in-line package and in flat-pack form. Some TTL chips are now also made in surface-mount technology packages.
TTL became the foundation of computers and other digital electronics. Even after Very-Large-Scale Integration CMOS integrated circuit microprocessors made multiple-chip processors obsolete, TTL devices still found extensive use as glue logic interfacing between more densely integrated components.

History

TTL was invented in 1961 by 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. The first commercial integrated-circuit TTL devices were manufactured by Sylvania in 1963, called the Sylvania Universal High-Level Logic family. 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.
The 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. 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 in the IBM System/38, IBM 4300, and IBM 3081.
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 1971. The Datapoint 2200 from 1970 used TTL components for its CPU and was the basis for the 8008 and later the x86 instruction set. The 1973 Xerox Alto and 1981 Star workstations, which introduced the graphical user interface, used TTL circuits integrated at the level of arithmetic logic units 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 under development.

Implementation

Fundamental TTL gate

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 transistor's collector 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 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.
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 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. 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. Some advantage of the simple output stage is the high voltage level of the output logical "1" when the output is not loaded.

Open collector wired logic

A common variation omits the collector resistor of the output transistor, making an open-collector output. This allows the designer to fabricate wired 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, 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 15 V for the 7426, useful when driving non-TTL loads.

TTL with a "totem-pole" output stage

To solve the problem with the high output resistance of the simple output stage the second schematic adds to this a "totem-pole" output. It consists of the two n-p-n transistors V₃ and V₄, the "lifting" diode V₅ and the current-limiting resistor R₃. 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.
When V₂ is "on", it activates V₄, driving low voltage 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 of the output logical "1". The reasons for this reduction are the voltage drops across the V₃ base–emitter and V₅ anode–cathode junctions.

Interfacing considerations

Like DTL, TTL is a current-sinking logic since a current must be drawn from inputs to bring them to a logic 0 voltage level. The driving stage must absorb up to 1.6 mA from a standard TTL input while not allowing the voltage to rise to more than 0.4 volts. The output stage of the most common TTL gates is specified to function correctly when driving up to 10 standard input stages. 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 V_CC, 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". 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 V_CC 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 later than original components. Within usefully broad limits, logic gates can be treated as ideal Boolean devices without concern for electrical limitations. The 0.4 V 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, the voltage level of the "totem-pole" output stage at output logical "1" can be increased closer to V_CC by connecting an external resistor between the V4 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.